JPS6352671A - Resonant converter - Google Patents

Abstract

PURPOSE:To keep resonance conditions nearly constant and to control the no-load or light-load output voltage, by assuring the supply of given currents to the main switching elements as well as to the inductance and capacitance related to no-load or light-load resonance. CONSTITUTION:The output from a DC source is chopped by main switching elements 2 and 6 to produce an AC voltage across the secondary winding 18 of a main transformer 3 which connects resonance elements. This output voltage is rectified by rectifiers 7 and 8 on the output side, so that the prescribed DC voltage is obtained. The inputs of the rectifiers 7 and 8 are connected to the primary winding 21 of an auxiliary transformer 20. The secondary winding 22 of this auxiliary transformer 20 is connected to the DC power source 1 through a rectifier 23. The surplus power is returned to the DC power source 1 through the rectifier 23.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a resonant converter that achieves high efficiency, low noise, and high frequency in a resonant switching power supply or the like.

"Prior Art" Conventionally, a basic circuit as shown in FIG. 12 has been used as a switching power supply using a resonant converter. This circuit runs from the DC power supply (1), through the first FET (2), to the transformer (3), to the first capacitor (4), and from the DC power supply (1) to the second capacitor (5), and the transformer ( 3), alternately turns on and off in the direction of the second FET (6), converts it into AC, and connects it to the rectifier (7), (8), and the capacitor (9).
(10) and is smoothed by a reactor (11) to obtain an output voltage v0 between output terminals (1, 2) and (13). This output voltage v0 is detected and amplified by the detection amplification circuit (32) and
ET, the output voltage V by controlling the oscillation frequency of the second FET (2) and (6). is under control.

``Problems to be Solved by the Invention'' In such conventional circuit configurations, the resonance conditions vary widely depending on the load (1) For example, when the output is no-load, the controlled oscillation frequency must be significantly lowered. In some cases, the frequency is below the audible frequency, producing noise, and the output ripple also increases significantly.

The present invention has been made to solve the above-mentioned problems, and provides a main transformer having a resonant circuit element by chopping the DC power supply by switching the main switching elements. In a resonant converter, an AC voltage is obtained at the secondary winding of the auxiliary transformer, and this AC voltage is rectified by an output rectifier to obtain a predetermined DC voltage.
The secondary winding of this auxiliary transformer is connected to the DC power supply side via a rectifier. If the AC voltage on the side becomes abnormally high, excess power is returned to the DC power source via the auxiliary transformer and rectifier to clamp it below a certain value. Therefore, by guaranteeing a constant current passing through the main switching element and the inductance and capacitance involved in resonance during no load or light load, the resonance conditions are kept approximately constant. The output voltage at light loads is effectively suppressed, and the controlled frequency only needs a small range of change, making overall control easy.Furthermore, the range of change on the secondary side of the main transformer is also small, making it suitable for magnetic amplifiers, etc. This makes it possible to use this method, and this has the effect of making it possible to have a multi-output configuration.

"Example" Embodiments of the present invention will be described below based on the drawings.

In Figure 1, (1) is a DC input power supply, which is coupled to the midpoint (15) of the primary winding (14) of the main transformer (3), Volume a(14)-
A diode (1
6), MO9O9 type first FET as the first main switching element
) is inserted, and the other end and the DC input power supply (]) are inserted.
A series circuit including a diode (17) and an MO5 type second FET (6) as a second main switching element is inserted between the negative electrodes of the switch. Both ends of the secondary winding (18) of the main transformer (3) are coupled via a capacitor (19) and coupled to one point via rectifiers (7) and (8), and this coupling point and two
The intermediate points of the next winding are connected to output terminals (12) and (13) via a smoothing filter circuit consisting of capacitors (9) and (10) and a reactor (11).

A primary winding (21) of an auxiliary transformer (20) is connected to the AC part on the input side of the rectifiers (7) and (8), and a secondary winding (22) of this auxiliary transformer (20) is a rectifier circuit (23)
is coupled between both ends of the DC input power source (1) via the DC input power source (1).

(24) is a power supply IC commercially available as MB 3759, and transistors (25) and (26) which are turned on and off alternately in this power supply IC (24) are the first and second FETs (2), respectively. (6). Further, a capacitor (27) is inserted between the intermediate point (15) of the main transformer (3) and the negative electrode side of the DC input power source (1).

Note that (28) and (29) are the linear cage inductance (Ll) inherent between both ends of the secondary winding (18) of the main transformer (3).

The operation of the circuit shown in FIG. 1 as described above will be explained.

The circuit shown in FIG. 1 can be replaced with an equivalent circuit as shown in FIG. In this circuit, the first. 2nd FET (2) (6
) is limited, and a gap is provided in the main transformer (3), the excitation impedance of which is represented by . The secondary winding (18) has an L-type of beam-cage inductance (28) (29), and a resonance state as shown in FIG. 3 exists between this and the C-type of the capacitor (19). Incidentally, this resonance is of the so-called parallel resonance type, but first we will explain its basic operation.

In FIG. 3, (a) is the drain of the first FET (2),
The waveform of the source-to-source voltage VQt, (c) shows the exciting current Iφ flowing through the exciting impedance 0, (d) shows the resonance current Ω between the beam cage inductance L1 and the capacitor (19) C1, and (b) shows the exciting current Iφ. This is a combination of the current flow φ and the resonant current IQ. Ir in (e) is a capacitor (19
) is the resonance current due to C1 and (L1+L, ), and the current I oac is the current supplied to the load side.

The secondary current of the main transformer (3) is a combination of the above. (f) is the capacitor (19
).

It is obvious in a resonant circuit that when there is no load or a light load, the resonant voltage Vc due to the leakage inductance L1 and the capacitor (19) 01 increases abnormally, and this is represented by the voltage VcQ in (g). . At this time, the primary and secondary winding ratios of the auxiliary transformer (20) are adjusted so that the output voltage of the output side rectifier (23) of the auxiliary transformer (20) is equal to the input voltage ■1 at Vch (f). With this configuration, the wave height of the voltage VcQ shown in (g) is clamped at the voltage Vch value shown by the dotted line. The electric current shown in (h) is the current flowing into the auxiliary transformer (20), and the solid line is close to zero 1 when the load is heavy, and the dotted line is when the load is light. Furthermore, depending on the turns ratio of the auxiliary transformer (20), (g
) can be freely changed.

In this way, the capacitor (19
), a constant burden current is ensured between both ends of the line, and the resonance conditions do not deteriorate significantly, ensuring stable resonance.

In the circuit shown in Fig. 1, the parallel resonance condition requires inductance and capacitance, and the inductance L0 is due to the gap between the cores of the main transformer (3), and the inductance L1 is due to the glue cage inductance of the secondary winding (18). Also, the capacitance C1 can be partially or completely filled by the stray capacitance of the secondary winding (18).

If there is a change in the resonance conditions between the inductance L1 and the capacitance C1, and the characteristics become as shown in Figure 4, F E
The current I9 flowing through T (2) and (6) flows backwards, significantly reducing its efficiency. So, to prevent that backflow,
In order to maintain resonance conditions even if there are slight changes in conditions,
Rectifiers (16) (17) are inserted between both ends of the primary winding (14) of the main transformer (3) in Figure 1 and the first and second FETs (2) and (6), respectively. Ru. Output side rectifier (
7) If the output side of (8) is a complete choke input, as shown in Figure 5 (b) and (e), Ioac will be divided at the time when Vch is zero, and noise will be introduced to the output side at the time of this division switching. Occur. Therefore, as shown in FIG. 1, a small capacitor (9) is inserted between the output side of the output side rectifiers (7) and (8) and the other output terminal (13). Then, as shown in FIGS. 5(d) and 5(e), switching between the positive and negative voltages of Vch can be performed when Ioac is zero (t□), and the above-mentioned disadvantage is eliminated.

The circuits in Figures 9 and 10 are output rectifiers (7) (8).
) A saturable reactor (30) (3'l) is inserted between the input side of the main transformer (3) and the secondary side (18) end of the main transformer (3), and the final output is detected and amplified by the detection amplification circuit (32). and rectifier (3
3) Said saturable reactor (30) via (34)
(31) to operate as a so-called magnetic amplifier, but in these circuits, the voltage supplied from the main transformer (3) is affected by the load as shown in Figures 3(f) and (g). If it fluctuates greatly, the voltage burden on the saturable reactors (30) and (31) for magnetic amplifiers increases, and sometimes the voltage V becomes uncontrollable at no load.
Clamping the auxiliary transformer (20) according to the present invention is extremely effective when output control is performed using a magnetic amplifier as described above.

The circuit in Figure 11 is an amplification circuit (32) that detects the final output voltage.
Amplify the detection with a suitable insulating element such as a photocoupler (35
), the period of the trigger pulse is controlled by this output, and this trigger pulse is used as input to operate a one-shot multivibrator that outputs a substantially constant conduction time width, and the output is used to control the period of the trigger pulse. 1. 2nd FET
(2) This is a resonant converter that controls the final output voltage by controlling the switching frequency in (6). This is a general method of output control for resonant converters, but it is also the main method in this type of circuit configuration. By coupling the auxiliary transformer (20) to the alternating current section of the secondary winding (18) of the transformer (3), the required control of the frequency is significantly reduced, and the ripple is also negligible, eliminating the problems of the previous ones. Greatly improved.

In the above embodiments, the basic resonant circuit was explained using a parallel resonant circuit, but the present invention is not limited to this, and as shown in FIG. , a series resonant circuit as shown in FIG. 7, a resonant circuit as shown in FIG. It is clear that their resonance conditions can be best maintained.

"Effects of the Invention" Since the present invention is configured as described above, by guaranteeing a constant current passing through the main switching element and the inductance and capacitance involved in resonance during no load or light load, the resonance conditions can be kept almost constant. This effectively suppresses the output voltage during no-load or light-load conditions, and simplifies overall control since the range of frequency change to be controlled is small. Since the variation width on the next side is also small, it is possible to use magnetic amplifiers, etc.
This has the effect of enabling a multi-output configuration.

[Brief explanation of the drawing]

Fig. 1 is an electric circuit diagram showing a first embodiment of a resonant converter according to the present invention, Fig. 2 is an equivalent circuit diagram of Fig. 1, Fig. 3 is a waveform diagram of each part, and Fig. 4 shows changes in resonance conditions. Figure 5 is a waveform diagram before and after improvement by choke input, Figure 6, Figure 7, Figure 8, Figure 9, Figure 1.
0 and 11 are electrical circuit diagrams showing examples of the present invention implemented in different circuits, and FIG. 12 is a conventional circuit diagram. (1)...DC input power supply, (2)...1st FET,
(3)...Main transformer, (6)...Second FET, (7
) (8)... Rectifier, (12) (13)... Output terminal, (14)... Primary winding, (], 6H17)...
- Rectifier, (18)...Secondary winding, (19)...Capacitor, (20)...Auxiliary transformer, (21)...
11th winding, (22)...secondary winding. (23)... Rectifier circuit, (24)... Power supply IC1
(32)...detection amplifier circuit.

Claims (6)

[Claims] (1) The DC power source is chopped by switching the main switching elements to obtain an AC voltage in the secondary winding of the main transformer having a resonant circuit element, and this AC voltage is rectified by the output rectifier to produce a predetermined DC voltage. In the resonant converter, a primary winding of an auxiliary transformer is coupled to the input side of the output rectifier, and a secondary winding of the auxiliary transformer is coupled to the DC power supply side via the rectifier. A resonant converter characterized by: (2) The resonant converter according to claim 1, wherein the resonant circuit elements include the excitation inductance, leakage inductance, and stray capacitance of the main transformer. (3) A resonant converter according to claim 1 or 2, wherein a backflow prevention rectifier is inserted between the primary winding of the main transformer and the main switching element. (4) A capacitor is inserted between the output side rectifier coupled to the secondary winding of the main transformer and the other output terminal. resonant converter. (5) A saturable reactor is inserted between the secondary winding of the main transformer and the input side of the output rectifier, and the saturable reactor is operated as a magnetic amplifier by detecting the final output. A resonant converter according to the first, second, third or fourth range. (6) Detect the output corresponding to the final output voltage via an insulating element, control the period of the trigger pulse, use this trigger pulse as an input signal to operate a one-shot multivibrator with a substantially constant conduction time width, and The resonant converter according to claim 1, 2, 3, 4, or 5, wherein the final output voltage is controlled by controlling the switching frequency of the main switching element using the output.