CN101227778A - A self-excited oscillation high-power LED constant current drive circuit - Google Patents

Abstract

The invention provides a self-excited oscillation type high-power LED constant current drive circuit, which has low cost, high reliability, high stability and high drive efficiency; the circuit includes a rectification filter circuit, a switch circuit, a constant voltage constant current output circuit, a transformer , self-oscillating pulse width modulation signal generation circuit, current feedback blocking circuit, rectification and filtering circuit to connect the mains to rectify the DC and output it to the switching circuit for self-oscillating high-frequency switching, and then through the transformer to stabilize the voltage and constant current The high-frequency voltage of the output circuit is converted into a DC voltage to drive the LED, and at the same time the sampling current signal is generated by the current feedback blocking circuit to control the operation of the switching circuit; The components form a self-excited oscillation structure, drive the switch circuit and combine the current feedback to realize the constant current drive of the high-power LED.

Description

A self-excited oscillation high-power LED constant current drive circuit

technical field

The invention relates to a high-power LED drive circuit, in particular to a self-excited oscillation high-power LED constant current drive circuit.

Background technique

LED is the abbreviation of English light emitting diode (light-emitting diode), its basic structure is a piece of electroluminescent semiconductor material, placed on a shelf with leads, and then sealed with epoxy resin around it. LED has many advantages, such as long life, short start-up time, firm structure, low energy consumption, high safety in use, no ultraviolet radiation, etc. In addition, its production can achieve mercury-free, which is of great significance to environmental protection and energy conservation.

Due to the many advantages of LEDs compared with traditional lighting equipment, research efforts in this area have been greatly strengthened in the world, especially the research and development and production of high-power LEDs (above 1W). With the gradual expansion of the market size of high-power LEDs, and began to expand to the field of general lighting, such as household lighting, flashlights, flashlights, car headlights and so on. The lighting industry and lighting market are huge industries both in my country and in the world. According to authoritative forecasts, around 2010, LED will replace traditional incandescent and fluorescent lamps on a large scale.

A single LED needs to be driven by a DC low-voltage power supply, and the power supply voltage is between 1V-24V. Due to the large driving current of high-power LEDs, compared with low-power LEDs, it is more necessary to consider the load capacity and efficiency of the drive circuit. Traditional low-power LEDs Generally, it can be driven by connecting digital logic level and current limiting resistor, but this method cannot drive high-power LED. If high-power LEDs are used in the field of general lighting, a special driving circuit is necessary. Since the forward volt-ampere characteristic of the LED is very steep, the luminous brightness changes with the magnitude of the current. If the voltage fluctuation increases slightly, the current will increase to the extent that the LED will be burned. When the application environment changes or the LED ages, its load will also change. If constant voltage control is used, the current will be different from the set constant current value. Since the most critical parameter of LED light emission is the current flowing through it, constant current control should be used. Especially for high-power LEDs, the current usually reaches a level above 1A. In order to stabilize the working current of the LED, improve the efficiency of the drive circuit, and ensure the normal and reliable operation of the LED, various high-power LED drive circuits have emerged as the times require.

The simplest is to connect a current-limiting resistor in series after the rectification and voltage stabilization of the mains, but this cannot achieve constant voltage and constant current drive. When the voltage fluctuates greatly, it will cause damage to the LED, and the brightness of the light will be inconsistent. A traditional solution is to use a power frequency transformer to step down the mains power first, and then rectify and filter it into a DC voltage to drive the LED. If higher stability is required, a linear voltage regulator will be used to stabilize the voltage. ; However, the power frequency transformer is relatively heavy and occupies a large area. In addition to the low efficiency of the linear voltage regulator, it cannot meet the constant current requirements, so this method is rarely used now. In addition, there are also ordinary switching power supplies used as drivers for high-power LEDs, but the general regulated power supply is relatively expensive, has a large area, and lacks constant current control. There is also a solution that is widely used at present, which uses a dedicated high-power LED driver chip, uses the principle of a switching converter, and supplements certain peripheral components to form a constant-voltage constant-current driver. Although the use of an integrated circuit chip can reduce the circuit area, but The price of various high-power LED driver chips is high, and the quality is uneven. The LED driver chips with internal integrated switch tubes have limited power capacity and weak load capacity, and cannot drive some higher-power LEDs.

Contents of the invention

The invention provides a self-excited oscillation type high-power LED constant current drive circuit with low cost, high reliability, high stability and high drive efficiency. This circuit does not use any driver chip, and only uses less ordinary discrete components such as resistors, capacitors, and triodes to form a self-excited oscillation structure, drive the switching circuit, and combine current feedback to realize the constant current drive of high-power LEDs. . Moreover, by adjusting the resistance value of the external current sampling resistor, the luminous brightness of the LED can be changed.

The present invention adopts following technical scheme:

A self-excited oscillation high-power LED constant current drive circuit, including a rectification filter circuit, a switch circuit, a regulated constant current output circuit, a transformer, a self-excited oscillation pulse width modulation signal generation circuit, a current feedback blocking circuit, and an AC mains circuit. After the rectification and filtering circuit, it is converted into a high-voltage DC signal and sent to the switch circuit. The switch circuit performs pulse width modulation on the high-voltage DC signal to obtain a high-frequency AC signal and transmits it to the transformer. The secondary output winding of the transformer transmits the signal from the primary winding. The high-frequency alternating voltage is output to the constant-voltage constant-current output circuit, and the high-frequency alternating voltage is rectified and filtered by the constant-voltage constant-current output circuit to output a direct-current voltage to drive the LED, and the direct-current voltage is used as a current sampling signal Output to the current feedback blocking circuit to generate a blocking signal and output the signal to the switch circuit to suppress the pulse width modulation signal, control the operation of the switch circuit, and the positive feedback winding of the transformer outputs the induced positive feedback voltage to the self-excited oscillation pulse The width modulation signal generating circuit generates a pulse width modulation signal and outputs it to the switch circuit, and the signal controls the switch circuit to generate high-frequency alternating voltage.

The present invention has the following advantages:

1, the drive circuit of the present invention only uses common discrete components and parts such as less resistance, electric capacity, triode, constitutes a kind of self-excited oscillation structure, drives switch MOS tube, therefore does not need special PWM (pulse width modulation) chip, greatly The cost of the circuit is saved, and the chip is prone to failure under conditions such as large climate change and mechanical vibration, while the discrete device is less affected by the environment, so the reliability of the circuit is also improved.

2. The present invention adds a group of positive feedback windings to the transformer, cooperates with the integral structure of resistors and capacitors to generate self-excited oscillation, controls the conduction and cut-off of the triode, thereby drives the power MOS tube to perform high-frequency switching, so that the switch circuit can work normally , transfer the energy of the primary side to the secondary side through the high frequency transformer. Because the switching circuit structure is adopted, the efficiency of the entire driving circuit is greatly improved compared with the traditional linear voltage stabilizing structure, and can reach more than 80%.

3. The output part of the present invention adopts current mode sampling feedback, and forms a blocking circuit through ordinary resistors, capacitors, triodes, and photocouplers. When the output current is higher than the set constant current value, the blocking circuit stops the switching circuit Work, when the output current just drops below the constant current value, the blocking circuit stops working, and the switching circuit continues to work; in this way, the output LED working current can be in a constant current state with only a small ripple; thus, constant current output can be realized , so that high-power LEDs work more reliably, emit light uniformly, and prolong the service life.

4. The circuit of the present invention can further adjust the brightness of the LED. Since the brightness of the LED is proportional to the current flowing, the brightness of the LED can be adjusted by adjusting the external current sampling resistor, which is convenient and reliable. Since the opening condition of the blocking circuit is that the base of the triode reaches 0.7V, the 0.7V is the voltage drop on the current sampling resistor. If the resistance value is changed, the current flowing through the sampling resistor at the 0.7V voltage drop can be changed, that is, the LED luminescence is adjusted. constant current setting value.

5. The input and output parts of the circuit of the present invention are isolated by a transformer and a photocoupler, and the drive output of the LED has no direct electrical connection with the high-voltage mains, so that the interference is small, the reliability is improved, and the safety of the circuit is also enhanced. .

6. The circuit of the present invention also provides the working voltage for the optocoupler and the triode by adding the auxiliary winding of the transformer, and does not need an external power supply, which saves the circuit area and cost.

7. The input circuit of the present invention uses inductance, capacitance, and resistance to form a transient suppression circuit to prevent damage to the subsequent circuit by surge voltage and current. At the same time, this structure also provides a filtering function, which can improve the power factor and help limit line Carrier noise and noise generated by the power supply are filtered to improve reliability.

8. The price of the existing high-power LED driver chips is high, and the quality is uneven. The LED driver chips with internal integrated switch tubes have limited power capacity and weak load capacity, and cannot drive some higher-power LEDs; and This circuit adopts an external switch tube, which can be freely selected according to the actual power demand, and can drive LEDs with higher power.

Description of drawings

Fig. 1 is a structural block diagram of the present invention.

Fig. 2 is a schematic circuit diagram of the present invention.

FIG. 3 is a schematic circuit diagram of an embodiment of the present invention.

Fig. 4 is a test waveform (1) diagram of an embodiment of the present invention.

Fig. 5 is a test waveform (2) diagram of the embodiment of the present invention.

Detailed ways

The purpose, circuit structure and advantages of the present invention will be further described below through specific embodiments of the present invention in conjunction with the accompanying drawings.

As shown in FIG. 1 , the present invention includes a rectification filter circuit 1 , a switch circuit 2 , a self-excited oscillation pulse width modulation signal generation circuit 3 , a constant voltage constant current output circuit 4 , a current feedback blocking circuit 5 and a transformer 6 .

The rectifying and filtering circuit 1 converts the AC mains power into a relatively stable high-voltage DC signal, and outputs it from the output terminal of the high-voltage DC signal to the input terminal of the high-voltage DC signal of the switch circuit 2, as the energy supply of the whole circuit; the switch circuit 2 will receive the The DC high-voltage signal is converted into a high-frequency AC signal after pulse width modulation by a high-frequency switch, and the energy is transferred to the primary winding a of the transformer 6 through its high-frequency AC signal output terminal; at the same time, the switch circuit 2 receives its signal input terminal The pulse width modulation signal generated by the self-excited oscillation pulse width modulation signal generating circuit 3 through self-excited oscillation or the blocking signal transmitted by the current feedback blocking circuit 5 is used to control the switch of the switch S1; the transformer 6 is provided with a primary winding a , positive feedback winding b, secondary output winding c and auxiliary power supply winding d; the secondary output winding c of the transformer 6 outputs the sensed high-frequency alternating voltage to the high-frequency alternating voltage input of the regulated constant current output circuit 4 After being rectified and filtered, the DC voltage is output from the DC voltage output terminal of the constant voltage output circuit 4 to drive the LED, and the DC voltage is output to the DC voltage input terminal of the current feedback blocking circuit 5, and the voltage is stabilized at the same time The current sampling output terminal of the constant current output circuit 4 outputs the current sampling signal to the current sampling signal input terminal of the current feedback blocking circuit 5; the positive feedback winding b of the transformer 6 outputs the sensed positive feedback voltage to the self-excited oscillation pulse width The positive feedback voltage input terminal of the modulation signal generation circuit 3; the auxiliary power supply winding d of the transformer 6 transmits the high-frequency alternating voltage sensed to the auxiliary high-frequency alternating voltage input terminal of the current feedback blocking circuit 5, and the current feedback blocking circuit 5 The current sampling signal is processed, and when the current exceeds the set constant current, the blocking circuit is started, and the power switch MOS transistor S1 in the switch circuit 2 is forcibly turned off, so that the LED current is stabilized at the constant current set value.

As shown in Figure 2, the described rectification filter circuit 1 comprises diodes D1, D2, D3, D4, capacitors C1, C2, resistor R1, inductance L1; diodes D1, D2, D3, D4 form a full bridge rectifier, diode D3 cathode The anode of diode D1 is connected, the cathode of diode D4 is connected to the anode of diode D2, and the two connection points are connected to the AC mains; the cathodes of diodes D1 and D2 are connected to one end of the inductor L1 as the rectified DC high voltage output of the mains, and the anodes of diodes D3 and D4 Connected to ground, as the rectified input terminal ground; inductance L1 and resistor R1 are connected in parallel, one end is connected to the positive pole of capacitor C1 at the same time, the other end is connected to the positive pole of capacitor C2, and the other ends of capacitors C1 and C2 are both grounded to form a transient suppression and EMI filter circuit; The positive pole of the capacitor C2 is used as the high-voltage DC signal output terminal of the rectification and filtering circuit 1 .

The switch circuit 2 includes a diode D5, a capacitor C3, resistors R2, R3, R4, R5, and a power switch MOS tube S1; one end of the resistor R2 is used as the high-voltage DC signal input end of the switch circuit 2, and the other end is connected to one end of the resistor R3. The other end of R3 is connected to the gate of the power switch MOS transistor S1 and one end of the resistor R4, the other end of the resistor R4 is grounded, the resistors R2, R3, and R4 constitute the starting circuit of the power switch MOS transistor S1, and the intersection of the resistors R2 and R3 is used as a switch The pulse width modulation signal input terminal of circuit 2; the source level of the power switch MOS transistor S1 is grounded, and the drain is connected to the opposite end of the primary winding a of the transformer 6, and also connected to the anode of the diode D5; the resistor R5 and the capacitor C3 are connected in parallel, and one end Connect to the cathode of diode D5, and the other end is used as the high-frequency AC signal output end of switch circuit 2 to connect to the same-named end of primary winding a of transformer 6. Resistor R5, capacitor C3 and diode D5 form an RCD buffer.

Self-excited oscillation pulse width modulation signal generating circuit 3 includes diode D6, capacitors C4, C5, resistors R6, R7, R8, R9, transistor Q1, resistor R6 connected in series with capacitor C5, and the other end of resistor R6 is used as a self-excited oscillation pulse The positive feedback voltage input end of the wide modulation signal generating circuit 3 is connected to the same-named end of the positive feedback winding b of the transformer 6, and the other end of the capacitor C5 is connected to the collector of the triode Q1 and the cathode of the diode D6 to act as a clamp, and the triode Q1 The emitter of the diode D6 and the anode of the diode D6 are grounded, and the collector of the triode Q1 is used as the pulse width modulation signal output terminal of the self-excited oscillation pulse width modulation signal generating circuit 3, and the pulse width modulation signal input terminal of the switch circuit 2, and the current feedback block The blocking signal output terminal of circuit 5 is connected, one end of the resistor R9 is grounded, the base of the transistor Q1 is connected to the other end of the resistor R9 and one end of the resistor R8, the other end of the resistor R8 is connected to one end of the capacitor C4 and one end of the resistor R7, and the capacitor C4 The other end of the resistor R7 is grounded, and the other end of the resistor R7 is used as the positive feedback voltage input end of the self-oscillating pulse width modulation signal generating circuit 3 . Resistors R7, R8, R9 and capacitor C4 form a driving circuit for the base of transistor Q1.

The voltage-stabilizing constant-current output circuit 4 includes a diode D7, a voltage-stabilizing diode D8, capacitors C6, C7, and C8, a resistor R10, and a variable resistor RV1; the anode of the diode D7 is used as the high-frequency alternating voltage input of the voltage-stabilizing constant-current output circuit 4 Terminal, connected to the opposite end of the secondary output winding c of the transformer 6, the cathode of the diode D7 is used as the DC voltage output terminal of the constant voltage output circuit 4, and connected to the anode of the LED load; after the capacitor C6 is connected in series with the resistor R10 It is connected in parallel with diode D7 to act as a buffer; the positive terminal of capacitor C7 is connected to the cathode of diode D7, and the other end is connected to the output ground, which acts as a filter energy storage; one end of capacitor C8 is connected to the cathode of diode D7, and the other end is connected to the cathode of LED to filter high frequency Clutter function; the cathode of the Zener diode D8 is connected to the cathode of the diode D7, and its anode is connected to the cathode of the LED load, which acts as overvoltage protection; one end of the variable resistor RV1 is used as the current sampling output terminal of the constant voltage and constant current output circuit 4 to connect to the LED cathode. The other end is connected to the output ground for current sampling. The current feedback blocking circuit 5 includes a diode D9, a capacitor C9, resistors R11, R12, R13, R14, R15, an optocoupler OP1, transistors Q2, Q3; the anode of the diode D9 is used as the auxiliary high frequency of the current feedback blocking circuit 5 The alternating voltage input terminal is connected to the opposite end of the auxiliary power supply winding d of the transformer 6, the cathode of the diode D9 is connected to the positive pole of the capacitor C9, and the negative pole of the capacitor C9 is grounded, and the two rectify the high-frequency alternating voltage provided by the auxiliary power supply winding d The filtered DC voltage is used as an auxiliary power supply to supply the photocoupler OP1 and the transistor Q2 to work; one end of the capacitor R13 is used as the current sampling signal input terminal of the current feedback blocking circuit 5, and is connected to the DC voltage output terminal of the constant voltage constant current output circuit 4 , the other end is connected to the collector of Q3 and the anode of the light-emitting diode of the photocoupler OP1, which acts as a current limiter; the base of the triode Q3 is used as the current sampling input terminal of the current feedback blocking circuit 5 and connected to the current sampling of the constant voltage output circuit 4 At the output end, the emitter of the triode Q3 and the cathode of the light-emitting diode of the photocoupler OP1 are grounded, one end of the resistor R14 is connected to the positive pole of the capacitor C9 and one end of the resistor R15, and the other end of the resistor R14 is connected to the collector of the phototransistor of the photocoupler OP1. Act as a current limiter; the emitter of the phototransistor of the photocoupler OP1 is grounded, the resistor R15 acts as a current limiter, and the other end is connected to the collector of the transistor Q2, and the base of the transistor Q2 is connected to one end of the resistor R12 and the other end of the resistor R11 One end of the resistor R11 is connected to the collector of the phototransistor of the photocoupler OP1, the other end of the resistor R12 is grounded to the emitter of the transistor Q2, and the collector of the transistor Q2 is used as the blocking signal output end of the current feedback blocking circuit 5 and connected to the switch circuit 2 When the LED current reaches the set constant current value, S1 in circuit 2 is forcibly closed to achieve the constant current function.

The working process and principle of the circuit of the present invention:

The circuit of the invention can work normally as long as it is connected to the AC mains. Since the input voltage required by the switch circuit 2 is direct current, the AC mains must be rectified and filtered first, and this function is completed by the rectified and filtered circuit 1 . The AC mains is input by a full-wave rectifier bridge composed of diodes D1-D4, rectified into a full-wave voltage, and then converted into a high-voltage DC after passing through a transient suppression and EMI filter circuit, and input to the switch circuit 2. This filter circuit structure helps to limit the line-loaded noise and filter out the noise generated by the power supply; in addition to the filter function, the transient impedance of the inductor L1 is very high, and the combination with the resistor R1 can provide the necessary series impedance to limit the transient surge. Inrush current to protect the subsequent circuit. In addition to filtering, capacitors C1 and C2 also function as energy storage.

The power switch MOS transistor S1 performs high-frequency switching under the control of the pulse width modulation signal, modulates the DC high voltage into a high-frequency alternating voltage on the primary winding a of the transformer 6, and uses the transformer to transfer energy to the secondary output winding. When the power switch MOS transistor S1 is turned on, the DC high voltage-primary winding a-power switch MOS transistor S1-ground constitutes a loop, the current flows through the primary winding a, and the energy is stored on the primary winding a. At this time, the terminal with the same name is positive. The opposite end is the drain voltage of the power switch MOS transistor S1, which is close to 0V. According to the nature of the transformer, the same-named end of the secondary output winding c is also positive at this time, and the opposite-named end is negative, the diode D7 reverses and cuts off, and the LED is powered by the energy stored on the capacitor C7. When the power switch MOS tube S1 is turned off, the primary winding a generates an induced potential, the terminal with the same name is negative, and the terminal with the same name is positive, then the secondary output winding c is sensed, and the terminal with the same name becomes negative, and the terminal with the same name is positive. The diode D7 conducts forwardly, and the energy is transferred from the primary winding a to the secondary output winding c through the transformer, and the current of the secondary output winding c flows into the energy storage capacitor C7 to charge it and supply power to the LED at the same time. Because when the power switch MOS transistor S1 is turned off, the reflected voltage of the secondary output winding c is induced on the primary winding a, and the voltage on the drain of the power switch MOS transistor S1 is DC high voltage plus the reflected voltage, if considering the winding leakage inductance peak, etc. Factors, it is possible to exceed the withstand voltage value of the power switch MOS tube S1 and damage it. The diode D5, the capacitor C3 and the resistor R5 form an RCD buffer, which is used to reduce the peak voltage generated when the power switch MOS transistor S1 is turned off, and protect the power switch MOS transistor S1. When the diode reverses and cuts off, a high-voltage spike will also be generated on it, and the capacitor C6 and the resistor R10 act as buffers to reduce the spike voltage and protect the secondary rectifier tube D7. Resistors R2, R3, and R4 form the start-up circuit of the power switch MOS transistor S1. When the mains power is first connected, the circuit as a whole has not yet started to operate. At this time, the DC high voltage is directly used to turn on the power switch MOS transistor S1 after voltage division. The resistor R3 also acts as a current limiter for the gate of the power switch MOS transistor S1. The capacitor C8 in the output circuit filters out the high-frequency noise components of the output voltage. If in some state, the output LED drive voltage is too high instantaneously, the Zener diode D8 acts as a regulator to protect the LED from damage. The current passing through the variable resistor RV1 is the same as that of the LED, and its voltage drop can represent the magnitude of the current, which is used as a sampling of the current and provided to the current feedback blocking circuit 5 .

The pulse width modulation signal for driving S1 in the switch circuit 2 is generated by a self-oscillating pulse width modulation signal generating circuit 3 . When the power switch MOS transistor S1 starts to turn on, the positive voltage is induced at the end of the positive feedback winding b with the same name, which is added to the gate of the power switch MOS transistor S1 through the path of resistor R6, capacitor C5, and resistor R3, making it more conductive Pass, for positive feedback. At the same time, the positive feedback voltage charges the capacitor C4 through the resistor R7. When the voltage on the capacitor C4 is not enough to make the base of the transistor Q1 reach 0.7V-1V, the transistor Q1 is turned off without affecting its conduction. The resistor R8 is a three-stage The current-limiting resistor driven by the base of tube Q1, and the resistor R9 plays the role of protecting the base. When the capacitor C4 is continuously charged and the base of the triode Q1 reaches 0.7V-1V, the triode Q1 will be turned on, and the gate of the power switch MOS transistor S1 will be pulled to a low level to turn off the switch tube. When the power switch MOS tube S1 is turned off, the secondary output winding c outputs energy, the voltage of the positive feedback winding b is reversed, and its terminal with the same name is induced as a negative voltage, the diode D6 is turned on at this time, and the collector of the transistor Q1 is clamped at The potential of a diode voltage drop lower than the ground, that is -0.7V, maintains the off state of the power switch MOS transistor S1. At this time, since one end of the capacitor C5 connected to the collector of the triode transistor Q1 is at approximately zero potential, and the other end is at negative voltage, the capacitor C5 is charged. After the secondary winding c releases energy, the voltage of each winding of the transformer 6 tends to 0 level, that is, the positive feedback voltage output terminal of the positive feedback winding b tends to 0V. Since the voltage of the capacitor C5 cannot change abruptly, the capacitor C5 and the resistor The end connected to R3 is a positive voltage, which will prompt the power switch MOS transistor S1 to turn on again. After turning on for a while, it will be accelerated by positive feedback. Repeat the above steps. This is a process of self-excited oscillation, so that A high frequency pulse width modulation control signal is obtained. During the switching process of the power switch MOS transistor S1, the turn-on time is determined by the charging time of the capacitor C4, and the turn-off time is equal to the time when the secondary output winding c releases energy every cycle.

If the circuit does not add a feedback adjustment module, the output voltage will always rise, and the constant current cannot be stabilized. Therefore, the present invention adds a current feedback blocking circuit 5 to ensure that the circuit is a constant voltage and constant current output. The current flowing through the variable resistor RV1 is the same as that flowing through the LED, and its voltage drop can indicate the magnitude of the output current. When the voltage drop of the variable resistor RV1 is less than 0.7V, that is, the base-emitter voltage of the triode Q3 is less than 0.7V, the triode Q3 is turned off, the output voltage passes through the current-limiting resistor R13, and a current flows through the light-emitting diode of the optocoupler OP1. According to the characteristics of the optocoupler, the current also flows through the phototransistor. Auxiliary power supply winding d, diode D9 and capacitor C9 form an auxiliary power supply to supply power for the triode Q2 and the phototransistor of the optocoupler OP1 when they are working. Resistors R15 and R14 are current limiting resistors. Since the phototransistor of the optocoupler OP1 conducts and flows current, its collector is pulled to the ground potential, that is, the base of the triode Q2 is at a low level and does not conduct. At this time, the current feedback blocking circuit 5 is disconnected from the switch circuit 2. Play a blocking role. When the output current flowing through the LED reaches the set constant current value, the sampling value of the variable resistor RV1 is equal to 0.7V, the triode Q3 is turned on, most of the current passes through the triode Q3, the current flowing through the light-emitting diode is basically 0, and the photosensitive triode The upper current is also basically 0, and its collector potential is no longer pulled to a low level. The auxiliary power supply is applied to the base of the triode Q2 through the divided voltage of the resistors R11, R14 and R12, and it is turned on. The gate of the power switch MOS transistor S1 is forced to be low and turned off, which plays a blocking role. The switch circuit 2 Stop transmitting energy, the output voltage and current will drop. When the output current decreases slightly and the sampling value is slightly lower than 0.7V, the triode Q2 will be turned off immediately, and the blocking effect will be removed. The power switch MOS tube S1 will continue to switch and transmit energy. , the voltage and current start to rise back to the set stable value, and so on. Therefore, the driving voltage and current output to the LED are basically constant, with only a small ripple, which belongs to constant current control. In addition, changing the resistance value of the variable resistor RV1 can change the current value when the voltage drop is 0.7V, that is, changing the constant current setting value of the driving LED current, because the brightness of the LED is proportional to the current flowing , so that the brightness of the driven LED can be changed by adjusting the resistance value of the variable resistor RV1.

Since the voltage drop of different LEDs is not the same, and the number of LEDs connected in series is different according to actual requirements, the present invention can generate different driving voltages by modifying the design of the transformer, changing the turn ratio of the primary and secondary windings, and replacing a small number of components. The output can adapt to various design requirements.

Below in conjunction with accompanying drawing 4,5 illustrate advantage of the present invention with the data of a specific embodiment (referring to Fig. 3):

To drive two high-power LEDs connected in series, the constant current driving current is 1A when each LED is normally lit, and the voltage drop of a single tube is 4.5V, then the output voltage of the driving circuit is required to be 9V, 1A, and the current is kept constant, and the current ripple Wave range is within plus or minus 5%.

Input: 220V AC mains

Output: 9V, 1A constant current, 9W, driving two LEDs in series

The parameter selection of the rectifier bridge mainly considers the withstand voltage value and current capacity. For a 220V application system, considering the voltage peak under special circumstances, a rectifier bridge with a withstand voltage of 1000V is generally selected. The current is determined according to the input current of the actual circuit, and a certain margin must be left. For the LED drive circuit of the present invention, under the condition of maximum power application, the input current will not exceed 1A, so a 2A rectifier bridge can be selected, such as KBP210.

The parameters of capacitors C1, C2, and inductor L1 should be determined according to the actual filtering requirements and circuit tests. The following formula can be roughly referred to: L=Z/(2π×fc), C=1/(2π×fc×Z), where Z is the impedance of the filter input or output at the cutoff frequency, and fc is the filter cutoff frequency, which is related to the design requirements. The capacitor R1 is connected in parallel with the inductor L1 to help suppress the surge current and play a protective role, generally 10K ohms.

Since the drain of the power switch MOS transistor S1 will withstand a high voltage of about 500V during the shutdown process, considering a certain safety margin, generally choose a MOS transistor with a withstand voltage of 600V or 700V. In addition, the peak current flowing through the primary winding a of the transformer 6 should be taken as a standard, and the power switch MOS transistor S1 that can withstand the peak current should be selected. For most high-power LED drive circuits, the peak current is generally between 1A and 5A depending on the power.

Resistor R3 is the current-limiting resistor driven by the gate of the power switch MOS transistor S1, and is used to limit the current when the gate of S1 is charged and discharged, generally about 10 ohms. Resistors R2 and R4 are connected between the DC high voltage and the ground to provide gate drive voltage for the start-up of the power switch MOS transistor S1. , for power MOS tubes, generally the driving voltage is 10V ~ 15V. That is, Vdc×R4/(R2+R4)=10V-15V, and Vdc is the value of DC high voltage. In addition, the resistance values of resistors R2 and R4 should not be too small, otherwise, the power consumption will be relatively large when the circuit is working normally, generally tens of K ohms to hundreds of K ohms. Since the resistance value of resistor R3 is relatively small, it can be ignored in the process of voltage division.

The basic formula for the selection of resistor R5 is $\mathrm{<figure-callout\; id="5"\; label="R"\; filenames="S2008100206526E00011.png,S2008100206526E00021.png"\; state="\{\{state\}\}">R</figure-callout>}5=\frac{{{\mathrm{<figure-callout\; id="3"\; label="V\; C"\; filenames="S2008100206526E00011.png,S2008100206526E00021.png"\; state="\{\{state\}\}">V</figure-callout>}}_C}^2}{\frac{1}{2}{L}_{\mathrm{lk}1}{i_{\mathrm{peak}}}^2\frac{V_{C3}}{V_{C3}-{\mathrm{nV}}_o}{f}_{\mathrm{the\; s}}},$ Where V _C3 is the voltage across C3, generally

is 2 to 2.5 times of nV _o , L _lk1 is the leakage inductance of the primary winding a, i _peak is the peak current on the primary winding a, n is the turn ratio of the primary winding a to the secondary output winding c, V _o is the output voltage , f _s is the switching frequency of the power switch MOS tube S1. The basic formula for the selection of capacitor C3 is $\mathrm{<figure-callout\; id="3"\; label="C"\; filenames="S2008100206526E00011.png,S2008100206526E00021.png"\; state="\{\{state\}\}">C</figure-callout>}3=\frac{{\mathrm{<figure-callout\; id="3"\; label="V\; C"\; filenames="S2008100206526E00011.png,S2008100206526E00021.png"\; state="\{\{state\}\}">V</figure-callout>}}_C}{{\mathrm{\&Delta;V}}_{C3}{R5f}_{\mathrm{the\; s}}},$ Among them, ΔV _C3 is the ripple of V _C3 , which is generally 5% to 10%. The reverse withstand voltage value of the diode D5 is greater than the withstand voltage value of the power switch MOS transistor S1, and a fast recovery diode capable of flowing 1A current is generally used.

The output rectifier diode D7 generally uses a fast recovery diode or a Schottky diode. In order to prevent the peak interference caused by the reverse recovery current, the maximum current should be 30% higher than the set LED drive constant current value. Capacitor C6 generally takes 3×Cout, and Cout is the output capacitance of diode D7, and the following formula is used to calculate resistance R10 $R$$10=\frac{1}{{2\mathrm{\&pi;f}}_d{C}_{\mathrm{out}}},$ f _d is the resonant frequency of the leakage inductance of the winding c and the output capacitance of the diode D7 when the RC snubber circuit is not added. The calculation formula of the RC snubber circuit is used as a reference, and the optimal selection needs to be carried out through testing. Capacitor C7 is the output capacitor, and its minimum calculation formula is, $C7=\frac{t_{\mathrm{off}}{I}_{\mathrm{DC}}}{V_{\mathrm{pp}}},$ Among them, t _off is the turn-off time of diode D7, I _DC is the LED load current, and V _pp is the peak-to-peak value of the maximum output ripple voltage required. Capacitor C8 is used to eliminate high-frequency clutter of the output voltage, generally 1uF. Diode D8 is a zener diode to prevent output overvoltage. Generally, the regulated value is selected to be 10% higher than the normal output LED drive voltage. The variable resistor RV1 is used as a current sampling resistor, which must ensure that the voltage drop is 0.7V when the required constant drive current is reached, that is, $\mathrm{<figure-callout\; id="1"\; label="RV"\; filenames="S2008100206526E00011.png"\; state="\{\{state\}\}">RV</figure-callout>}1=\frac{0.7}{I_{\mathrm{DC}}}.$

When the capacitor C5 is self-oscillating in the circuit, the voltage at its two ends should be basically kept constant. Since the oscillation frequency is high, the capacitance value can be selected as 50nF. The resistor R6 is the current-limiting resistor when the positive feedback winding b charges the capacitor C5. Generally, Choose 100 ohms. Transistor Q1 has no special requirements, just choose an ordinary NPN transistor such as S9013. The most commonly used 1N4007 is selected as the clamping diode D6. Resistor R8 is a current-limiting resistor for driving the base of transistor Q1, generally 100 ohms. In order not to affect the normal operation of the circuit, the protection resistor R9 generally takes a larger value, between 50K ohms and 100K ohms. Resistor R7 and capacitor C4 form an RC charging structure. When the upper end of capacitor C4 is charged to 0.7V-1V, transistor Q1 will be turned on. The charging time is the pulse width of the pulse width modulation signal generated by the self-excited oscillation circuit. The relationship is: $u_c=u_0{e}^{-\frac{t}{\mathrm{RC}}}+u_{\mathrm{the\; s}}(1-e^{-\frac{t}{\mathrm{RC}}}),$ Among them, u _c is the voltage on the capacitor C4, U ₀ is the initial voltage on the capacitor C4 when the power switch MOS transistor S1 is just turned on in each cycle, which is generally a negative value in this circuit, and U _s is the RC charging voltage, that is, positive feedback The voltage generated by winding b, R and C are the values of resistor 7 and capacitor C4 respectively, and t is the charging time. When u _c =0.7V-1V, t=the set pulse width, and U ₀ and U _s are known, the value of RC can be calculated.

Since the output current of the auxiliary power supply is small, the output rectifier diode D9 generally adopts a 1A fast recovery diode, and the size of the filter capacitor C9 is only 1uF. Optocoupler OP1 adopts PC817B with a current transmission ratio of 1:1. Transistors Q2 and Q3 mainly work in on-off and off-states, and general NPN-type triodes can be used, such as S9013. Resistor R13 is to make the optocoupler OP1 work normally, it must meet $\frac{V_0}{R13}=5~10\mathrm{mA},$ V _o is the output LED driving voltage. Resistor R14 is the current-limiting resistor of the phototransistor in the optocoupler OP1. Since the current transfer ratio of the optocoupler OP1 is 1:1, the current flowing through the phototransistor and the light-emitting diode is similar, so there is $\frac{V_{\mathrm{the\; s}}}{R14}=5~10\mathrm{mA},$ V _s is the output voltage of the auxiliary power supply. Resistor R15 is the current-limiting resistor between the collector of triode Q2 and the auxiliary power supply, which is about 80K ohms. Resistor R11 is a current-limiting resistor driven by the base of transistor Q2, and its value is 100 ohms. The resistance value of the resistor R12 should be selected to ensure that the voltage division between it and the resistors R11 and R14 is enough to make the base-emitter of the transistor Q2 conduct, that is $\frac{R12}{R12+R11+R14}{V}_{\mathrm{the\; s}}>0.7V.$

When designing transformer 6, the turns ratio between the windings has the following relationship: for a 220V system, the high voltage DC after rectification and filtering is about 310V, and the output voltage of the positive feedback winding b is generally 20V, so as to ensure that the power switch MOS tube S1 is effectively turned on , so the turn ratio between the primary winding a and the positive feedback winding b is na:nb=310:20, about 15-16. The turn ratio of the secondary output winding c to the primary winding a is different according to the output voltage V _o . In order to prevent the drain peak voltage from being too high when S1 is turned off, the general requirement is $V_{\mathrm{DC}}+\frac{\mathrm{na}}{\mathrm{nc}}{V}_o<500V,$ V _DC is the high voltage direct current after rectification and filtering. The turn ratio of the auxiliary power supply winding d and the secondary output winding c is the voltage ratio of the two. $\frac{\mathrm{nc}}{\mathrm{nd}}=\frac{V_o}{V_{\mathrm{the\; s}}},$ Auxiliary power V _s is generally 10V ~ 15V. After the turns ratio is determined, it is also necessary to pay attention to selecting the appropriate number of turns (determining the inductance) and wire diameter during design to avoid saturation of the transformer core or excessive temperature rise.

According to the above basis, the parameters of each component in this example are calculated as follows:

Resistor R1: 10KΩ; Resistor R2: 990KΩ; Resistor R3: 6.8Ω; Resistor R4: 56KΩ;

Resistor R5: 130KΩ; Resistor R6: 100Ω; Resistor R7: 7.5KΩ; Resistor R8: 100Ω;

Resistor R9: 56KΩ; Resistor R10: 10Ω; Resistor R11: 100Ω; Resistor R12: 3.3KΩ;

Resistor R13: 1KΩ; Resistor R14: 1KΩ; Resistor R15: 80KΩ;

Sliding rheostat RV1: 0~1Ω (0.7Ω when it is normally on);

Capacitor C1: 5.6uF; Capacitor C2: 10uF; Capacitor C3: 2.2nF; Capacitor C4: 3.3nF; Capacitor C5: 50nF;

Capacitor C6: 2.2nF; Capacitor C7: 1000uF; Capacitor C8: 1uF; Capacitor C9: 1uF;

Rectifier bridge D1～D4: KBP210; Diode D5: HER108; Diode D6: 1N4007; Diode D7: SF22;

Zener diode D8: 9.8V Zener tube; Diode D9: SF14; Inductor L1: 100uH;

NPN transistor Q1, Q2, Q3: S9013; power switch MOS tube S1: 1N60; photocoupler OP1: PC817B;

Transformer 6: the turns ratio of each winding na:nb:nc:nd=14:1:1:2, the primary winding inductance is 2.12mH

Result analysis:

Referring to FIG. 4, Vo and Io are the voltage and current output by the circuit to drive the LED respectively, and Vgs1 is the pulse width modulation signal on the gate of the power switch MOS transistor S1. When the self-excited oscillation occurs, the gate of the power switch MOS transistor S1 is controlled by the pulse width modulation signal, and the switching circuit performs high-frequency switching, and the energy is transferred to the secondary output through the transformer. It can be seen that when the pulse width is modulated, due to energy transfer, the output voltage and current gradually increase. When the output current Io reaches a certain set value, which is about 1.05A in the figure, the blocking circuit starts to work, forcing the The gate of the power switch MOS transistor S1 is set low, turn it off, the switch circuit stops transmitting energy, and the secondary output current and voltage gradually decrease. When Io drops to a certain value, which is about 0.98A in the figure, the blocking circuit stops functioning. The gate of the power switch MOS transistor S1 continues to be controlled by the pulse width modulation signal, Vo and Io start to rise again, and so on, and finally ensure that the output current Io has only a small ripple, which is basically a stable current of 1A, and the circuit functions as a constant current. Drive the LED function.

Referring to FIG. 5, Vn2 is the voltage of the positive feedback winding b, Vc4 is the voltage at the upper end of the capacitor C4, and Vgs1 and Vds1 are signals on the gate and drain of the power switch MOS transistor S1, respectively. In the process of self-excited oscillation, the power switch MOS transistor S1 starts to conduct when the gate is high, Vds1 is about 0V, and the positive feedback winding b is at a high level of 20V at this time, and the capacitor C4 is charged through the resistor R7, and the voltage at the upper end of the capacitor C4 gradually When it is charged to 0.7V-1V, the NPN transistor Q1 starts to conduct, pulls the gate of the power switch MOS transistor S1 to low level, and turns off the power switch MOS transistor S1. At this time, the primary winding a is reversed, Vds1 is a high voltage of about 450V, the positive feedback winding b is also reversed to about -10V, and C4 is reversely charged to about -3V. After the discharge of the secondary output winding c is completed, the power switch MOS transistor S1 is turned on again, and the self-excited oscillation circuit is formed according to the above-mentioned steps.

Claims (3)

1. A self-excited oscillation type high-power LED constant current drive circuit is characterized in that: comprise rectification filter circuit (1), switch circuit (2), constant voltage constant current output circuit (4), transformer (6), automatic The excitation pulse width modulation signal generation circuit (3), the current feedback blocking circuit (5), the AC mains is converted into a high-voltage DC signal by the rectification filter circuit (1) and sent to the switch circuit (2), and the switch circuit (2) will The high-voltage DC signal is pulse-width modulated to obtain a high-frequency AC signal and transmitted to the transformer (6), and the secondary output winding (c) of the transformer (6) transfers the high-frequency alternating voltage from the primary winding (a) It is output to the constant voltage and constant current output circuit (4), and the high frequency alternating voltage is rectified and filtered by the constant voltage and constant current output circuit (4) to output a DC voltage to drive the LED, and the DC voltage is used as a current sampling The signal is output to the current feedback blocking circuit (5) to generate a blocking signal and output the signal to the switch circuit (2) to suppress the pulse width modulation signal, to control the operation of the switch circuit (2), and the positive feedback winding of the transformer (6) (b) Output the induced positive feedback voltage to the self-excited oscillation pulse width modulation signal generation circuit (3) to generate a pulse width modulation signal and send it to the switch circuit (2), and the signal controls the switch circuit (2) to generate high-frequency alternating Voltage. 2. A self-excited oscillation type high-power LED constant current drive circuit according to claim 1, characterized in that: said self-excited oscillation type pulse width modulation signal generating circuit (3) includes a diode (D6), a capacitor (C4, C5), resistors (R6, R7, R8, R9), triode (Q1), the resistor (R6) is connected in series with the capacitor (C5), and the other end of the resistor (R6) is used as a self-oscillating pulse width The positive feedback voltage input end of the modulating signal generating circuit (3) is connected with the terminal of the same name of the positive feedback winding (b) of the transformer (6), and the other end of the capacitor (C5) is connected with the collector of the triode (Q1) and the diode (D6 ), the emitter of the triode (Q1) and the anode of the diode (D6) are grounded, and the collector of the triode (Q1) is used as the pulse width modulation signal output terminal of the self-excited oscillation pulse width modulation signal generating circuit (3), and The pulse width modulation signal input terminal of the switch circuit (2) is connected to the blocking signal output terminal of the current feedback blocking circuit (5), one end of the resistor (R9) is grounded, and the base of the triode (Q1) is connected to the other end of the resistor (R9). One end of the resistor (R8), the other end of the resistor (R8) is connected to one end of the capacitor (C4) and one end of the resistor (R7), the other end of the capacitor (C4) is grounded, and the other end of the resistor (R7) is used as a self-excited oscillation The positive feedback voltage input terminal of the pulse width modulation signal generating circuit (3). 3. A self-excited oscillation type high-power LED constant current drive circuit according to claim 1, characterized in that: said current feedback blocking circuit (5) comprises a diode (D9), a capacitor (C9), a resistor ( R11, R12, R13, R14, R15), photocoupler (OP1), triode (Q2, Q3), the anode of diode (D9) as the auxiliary high-frequency alternating voltage input terminal transformer of the current feedback blocking circuit (5) (6) At the opposite end of the auxiliary power supply winding (d), the cathode of the diode (D9) is connected to the positive pole of the capacitor (C9), the negative pole of the capacitor (C9) is grounded, and one end of the capacitor (R13) is used as a current feedback blocking circuit (5 ) of the current sampling signal input terminal is connected with the DC voltage output terminal of the constant voltage output circuit (4), and the other terminal is connected with the collector of the triode (Q3) and the anode of the light-emitting diode of the optocoupler (OP1), and the triode (Q3 ) as the current sampling input terminal of the current feedback blocking circuit (5) is connected to the current sampling output terminal of the constant voltage output circuit (4), the emitter of the triode (Q3) and the light emitting diode of the photocoupler (OP1) The cathode is grounded, one end of the resistor (R14) is connected to the positive electrode of the capacitor (C9) and one end of the resistor (R15), and the other end of the resistor (R14) is connected to the collector of the phototransistor of the photocoupler (OP1), and the photocoupler (OP1) The emitter of the photosensitive transistor is grounded, the other end of the resistor (R15) is connected to the collector of the transistor (Q2), the base of the transistor (Q2) is connected to one end of the resistor (R12) and the other end of the resistor (R11), and the resistor (R11) ) is connected to the collector of the phototransistor of the photocoupler (OP1), the other end of the resistor (R12) is grounded to the emitter of the transistor (Q2), and the collector of the transistor (Q2) is used as the current feedback blocking circuit (5). The signal output terminal is connected to the pulse width modulation signal input terminal of the switch circuit (2).

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