JP2007150003A - Insulated signal transmission circuit - Google Patents

Abstract

An insulating signal transmission circuit having a simple configuration is provided that suppresses a decrease in current conversion efficiency due to a steady increase in a current flowing through a light-emitting diode of a photocoupler and enables stable transmission of a PWM signal or the like.
In A insulated signal transmission circuit for transmitting insulates signal by a photo coupler, a light-emitting diode D constant current circuit 1 of a negative feedback type connected in series with _the optocoupler 2 and the switching element Q The constant current circuit 1 has a transistor Tr ₁ as a constant current control element that is in a saturated state when the switching element Q is turned off and flows a constant current when the switching element Q is turned on. Q flowed through the transient current to the light emitting diode D ₁ when is turned on, almost to flow through the constant steady current smaller amplitude than subsequent to the transient current.
[Selection] Figure 1

Description

  The present invention relates to an insulating signal transmission circuit that electrically insulates and transmits a PWM signal from a control circuit thereof, for example, in order to drive semiconductor switching elements of upper and lower arms of an IPM (Intelligent Power Module). is there.

An electric motor drive system applied to recent vehicle equipment is composed of a power source, a buck-boost converter, and an inverter as shown in FIG. 5 for the purpose of improving efficiency and saving energy.
Here, the power supply 10 is configured by a power supply voltage from an overhead wire or a battery connected in series. The step-up / down converter 20 boosts the voltage of the power supply circuit (for example, 280 [V]) to a voltage suitable for driving the three-phase electric motor 40 (for example, 750 [V]) when driving the vehicle, and for braking the vehicle. Then, the voltage (eg, 750 [V]) generated from the electric motor 40 serving as a generator is stepped down to the voltage of the power supply circuit (eg, 280 [V]) to perform a power regeneration operation.

When the vehicle is driven, the inverter 30 changes the vehicle speed by turning on and off the switching element so that a current is passed through each phase of the electric motor 40 by the voltage boosted by the step-up / down converter 20 and controlling the switching frequency. I am letting.
When the vehicle is braked, the switching element is turned on / off in synchronization with the AC voltage generated in each phase, so that a so-called rectification operation is performed, the AC voltage is converted into a DC voltage, and the power circuit is regenerated.

FIG. 6 shows a detailed configuration of the buck-boost converter 20.
The converter 20 includes a reactor 21, a capacitor 22, switching units 23 and 24, and control circuits 25 and 26 that control the switching units 23 and 24, respectively. Here, the switching parts 23 and 24 of the drive system of recent vehicle equipment are composed of IGBTs 231 and 241 and diodes 232 and 242 respectively connected in reverse parallel to each other as shown in the figure.

The principle of the step-up / step-down operation of the converter 20 will be described below, and the current waveform of the reactor 21 during boosting is shown in FIG.
A. Step-up operation (1) IGBT231 is a result on (conductive), the reactor 21 (inductance value L) plus the current I flows, the energy of the ^LI 2/2 is accumulated.
(2) When the IGBT 231 is turned off (non-conducting), a current flows through the diode 242 of the switching unit 24, and energy stored in the reactor 21 is sent to the capacitor 22.

B. If step-down operation (1) IGBT241 is turned on, a current I flows in the reactor 21, the energy of the ^LI 2/2 is accumulated.
(2) When the IGBT 241 is turned off, a current flows through the diode 232 of the switching unit 23, and the energy stored in the reactor 21 is regenerated to the power supply 10.

By changing the on-duty of the IGBTs 231 and 241 (the ratio of the conduction period to the switching period of the IGBTs 231 and 241), it is possible to adjust the voltage of the step-up / step-down voltage. Can be obtained.
[Formula 1]
On-duty [%] = V _L / V _H
V _L : Power supply voltage V _H : Voltage after boosting

However, in practice because of the like change or a change in power supply voltage of the load, monitors the voltage V _H after buck, which is controlling the on-duty of IGBT231,241 so that the target value.
FIG. 8 is a block diagram of an IPM (Intelligent Power Module) for a step-up / step-down converter, which is roughly composed of an upper arm switching unit 240, a lower arm switching unit 230, and a control circuit 250.

The upper arm switching unit 240 includes the above-described IGBT 241 and diode 242, resistors 2401 and 2402, IGBT protection circuit 2403, and gate drive circuit 2404. The lower arm switching unit 230 includes the above-described IGBT 231 and diode 232. , Resistors 2301 and 2302, IGBT protection circuit 2303, gate drive circuit 2304, and _VH detection circuit 2305. Here, the V _H detection circuit 2305 includes a triangular wave generator 2306, a voltage dividing circuit 2307, a level adjustment circuit 2308, and a comparator 2309.

Further, the control circuit 250 includes a low-pass filter 254 to which a PWM signal output from a photocoupler 253 described later is input, a V _H comparator 255 that compares an output signal of the low-pass filter 254 and a step-up / step-down command value, The gate signal generator 256 generates a gate signal according to the output.
Since the upper arm switching unit 240, the lower arm switching unit 230, and the control circuit 250 need to be electrically insulated from each other, they are configured to transmit and receive signals using photocouplers (or pulse transformers) 251 to 253. ing.

Here, since the photocoupler is smaller and cheaper than the pulse transformer, it is recently being used for vehicle equipment.
9 shows, for example, the configuration of an IGBT driving IC (the IGBT protection circuit 2303 and the gate driving circuit 2304 in FIG. 8 or the IGBT protection circuit 2403 and the gate driving circuit 2404 in FIG. 8) provided at the subsequent stage of the above-described photocouplers 251 and 252. 261 is a temperature signal comparator, 262 is a current signal comparator, 263 is a logic circuit, 264 and 265 are MOSFETs, 266 is a binarization circuit, 267 is a logic circuit, and 268 is an IGBT drive circuit. .

The IGBT driving IC has the following typical functions.
・ Current discharge to the input terminal (input terminal for the PWM signal output from the photocoupler) and suction function (the function to improve the noise resistance of the IGBT drive IC)
-Binarization function of PWM signal output from photocouplers 251 and 252-Gate drive function of IGBT-Overcurrent and overheat protection function of IGBT

Among the above functions, the function of discharging current to the input terminal and sucking current from the input terminal is intended to improve noise resistance. As shown in FIG. 10, while the voltage level of the input signal does not fall below the threshold value _VINL. The current I _INH is discharged from the input terminal to the outside as an ejection current, and does not reach the threshold value _VINL unless a larger amount of current is drawn as a pre-stage circuit (photocoupler). On the other hand, when the voltage level of the input signal is below the threshold value V _INL, stop the discharging of the current I _INH, until a voltage level of the input signal exceeds the threshold value V _INH, drawn into the current I _INL is from the input terminal as a sink current Therefore, the threshold voltage _VINH is not reached unless a larger amount of current is discharged as the pre-stage circuit (photocoupler).

FIG. 11 is a diagram showing the V _ce (collector-emitter voltage) -I _c (phototransistor collector current) characteristic using the current _If flowing through the light-emitting diode of the photocoupler as a parameter. In order to increase the collector current in order to improve the noise resistance of the IC, it is necessary to increase the current _If .
On the other hand, FIG. 12 shows the lifetime deterioration characteristic of the current conversion efficiency of the photocoupler, and the current conversion efficiency (CTR: I _c / I _f ) tends to decrease in a short time as the current _If increases. is there.

As another prior art, in the light emitting diode driving circuit used in the optical communication system, an invention in which a light emitting diode emits and extinguishes at high speed without applying a voltage signal having a large amplitude is disclosed in Patent Document 1 below. Are listed.
In this prior art, by increasing the amplitude of the drive current waveform of the light emitting diode at the beginning, the stepped waveform is made smaller thereafter, so that the light output waveform of the light emitting diode is high-speed and has no overshoot rise characteristic. It is what you have.

Japanese Patent Laid-Open No. 10-93143 (paragraphs [0014] to [0017], [0037] to [0045], FIG. 5, FIG. 6 etc.)

As described above, in order to improve the noise resistance of the IGBT driving IC, if the collector current (drawing current) of the photocoupler, and thus the current _{If of the} light emitting diode, is constantly increased, the current conversion efficiency is reduced within a short time. As a result, the PWM signal for driving the IGBT may not be transmitted accurately.
The prior art described in Patent Document 1 changes the drive current waveform of the light emitting diode in a stepwise manner, but requires a plurality of differentiating circuits as the drive circuit of the light emitting diode, and the circuit configuration becomes complicated. No mention is made of means for improving the current conversion efficiency and transmission accuracy of the photocoupler.

  Accordingly, an object of the present invention is to provide an isolated signal transmission circuit that enables stable transmission of signals by suppressing a decrease in current conversion efficiency due to a steady increase in current flowing through a light-emitting diode of a photocoupler, and having a simple configuration. Is to provide.

In order to solve the above problems, the invention described in claim 1 is an insulated signal transmission circuit that insulates and transmits a signal using a photocoupler.
A negative feedback constant current circuit connected in series with the light-emitting diode of the photocoupler and a switching element;
The constant current circuit is:
A constant current control element that is saturated when the switching element is turned off, and that allows a constant current to flow when the switching element is turned on;
When the switching element is turned on by a signal to be transmitted, a transient current is passed through the light-emitting diode via the constant current control element, and thereafter, a substantially constant steady current having an amplitude smaller than the transient current is applied. The light-emitting diode is made to flow through the constant current control element.

The invention described in claim 2 is the invention according to claim 1,
The constant current circuit includes a constant voltage element, and the constant current control element is a transistor that is forward-biased by the constant voltage element and becomes saturated when the switching element is turned off.

The invention described in claim 3 is the invention according to claim 1,
The constant current circuit includes an operational amplifier, and the constant current control element is a transistor that is forward-biased by the operational amplifier and is saturated when the switching element is turned off.

The invention described in claim 4 is, in claim 2,
The constant voltage element is a Zener diode or a shunt regulator.

The invention described in claim 5 is any one of claims 1 to 4,
The switching element is an FET or a transistor.

According to the present invention, when a switching element connected in series to a light-emitting diode of a photocoupler is turned off, a constant current control element such as a transistor in a negative feedback constant current circuit is kept saturated and PWM to be transmitted. When the switching element is turned on by a signal or the like, the current of the light emitting diode is steadily increased for the purpose of improving the noise resistance of the IGBT driving IC by flowing a transient current having a larger amplitude than that of the subsequent steady state. This eliminates the need to prevent the current conversion efficiency of the photocoupler from decreasing over time and signal transmission errors.
Further, the negative feedback type constant current circuit can be easily realized by a transistor, a Zener diode, a resistor, etc., and there is an effect that the circuit configuration can be provided simply and inexpensively.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram showing a first embodiment of the present invention. This signal transmission circuit is for isolating a PWM signal by a photocoupler and transmitting it to the gates of the IGBTs of the upper and lower arms of the IPM. The negative feedback type constant current circuit 1, the photocoupler 2, the pulse generator 3, The switching element Q and resistors R _g , R _d , and _RL are included.

Constant current circuit 1, between a series circuit of a pnp transistor Tr ₁ as resistor _{R c} and a constant current control element connected to a power source (+ Vcc _1), wherein the power supply (+ Vcc ₁₎ and the base of the transistor Tr ₁ And a resistor _Rz connected between the base and the ground.
Further, a light emitting diode D ₁ , a resistor R _d , and a switching element Q such as an n-channel MOSFET are connected in series between the collector of the transistor Tr ₁ and the ground.
The gate of the switching element Q, PWM signal from the pulse generator 3 via a resistor R _g is input. Further, the collector of the phototransistor Tr ₂ in the photocoupler 2 is connected to a power source (+ Vcc ₂₎ via a load resistor _{R L.}
In the following description, the resistor _{R g, R d, R L} , R c, a resistance value of _{R z,} as with each of the symbols _{R g, R d, R L} , R c, as _{R z} [Ω] To do.

In the above configuration, for operating the transistor Tr ₁ as a constant current control element, when the _Vcc 1 _-V z the base potential by the Zener diode ZD _{(V z} is the Zener voltage) is fixed to become a value, the emitter potential of the transistor Tr ₁ is _{, Vcc 1 -V z + V be} (V be is the base - emitter voltage) becomes.
Now, at the time of steady operation (current _{I f} flowing through the light emitting diode _{D 1} is the steady state), such that the voltage drop of the series connected resistors _{R c} to the transistor Tr ₁ becomes _V z _{-V BE,} that is, the transistor By performing a negative feedback operation so that the emitter potential of Tr ₁ becomes Vcc ₁ −V _z + V _be , the current _If is expressed by the following Equation 2.
[Formula 2]
_{I f = (V z -V be} ) / R c
At this time, the switching element Q simply performs an on / off switching operation, and plays the role of causing the current _If to flow intermittently in accordance with the PWM signal from the pulse generator 3 as an input signal.

The above description is of the case where the current I _f flows constantly, the switching element Q is at the time of transition to turn on from the off state, the operation as described below.
First, the switching element Q is off, that is, when the current _{I f} is zero, the emitter potential of the transistor Tr ₁ is Vcc _1, the base potential is _Vcc 1 -V _z. Therefore, the base of the transistor Tr ₁ - the junction between the emitter and the forward bias is applied, the transistor Tr ₁ is a so-called saturation state (V _ce ≒ 0.3 V).
When the switching element Q is turned on in this state, the transistor Tr ₁ does not perform a current control operation, and therefore, a current _If having a magnitude of approximately Vcc ₁ / (R _c + R _d ) flows through the light emitting diode D ₁ . Note that the exact value of the current _If in the transient state is expressed by Equation 3.
[Formula 3]
_{I f = (V cc1 -V ce} -V f) / (R c + R d + R on)
V _ce : saturation voltage between the emitter and the collector of the transistor Tr ₁ (≈0.3 [V])
R _on : ON resistance of switching element Q V _f : Forward voltage of light-emitting diode D ₁

Amplitude of the current I _{f (transient} current) becomes a value larger than the current of the steady state, the base potential of the transistor Tr ₁ is fixed as described above, the magnitude of the current I _f initially It cannot be maintained and immediately decreases to a constant current value.
FIG. 2 shows an observation waveform of the transient state of the current _If in the circuit of FIG. The upper waveform in the left and right diagrams of FIG. 2 is the PWM signal waveform output from the pulse generator 3, and the lower waveform is the waveform of the current _If . As apparent from FIG. 2, about 20 [mA] flows as a transient current during a period of about 1.6 [μs] after the switching element Q is turned on, and thereafter, about 14.8 [mA] smaller than this flows. A constant current flows in synchronization with the PWM signal.
The amplitude of the above transient current can be adjusted by the value of the resistance _Rd in FIG. 3, resistance by reducing the value of R _d is an example of increasing the transient current value, in this example, by supplying a transient current to instantaneously about 46 [mA].

As described above, according to this embodiment, to drive the light emitting diode D ₁ of the photo-preparative coupler 2 with the Zener diode ZD and the transistor Tr ₁ and the like negative feedback type of the constant current circuit 1 and the switching element Q such as consisting of and so, the switching element Q is the base of the transistor Tr ₁ is turned off - by previously kept constantly saturated transistors Tr ₁ to forward bias the emitter, the light emitting diode D ₁ by turning on the switching element Q At the moment when the current starts to flow, the transient current _{If according} to Equation 3 described above is passed, and then the current _If can be made a constant current by negative feedback operation.
Therefore, there is no need to steadily increase the current _If in order to improve the noise resistance of the IGBT driving IC, and it is possible to prevent the current conversion efficiency of the photocoupler from decreasing with time and the PWM signal transmission error. it can.
In this embodiment, the Zener diode ZD is used as the constant voltage element in the constant current circuit 1, but a shunt regulator may be used. It is also possible to use a FET in place of the transistor Tr _1.

Next, FIG. 4 is a block diagram of the main part showing a second embodiment of the present invention.
In this embodiment, the light emitting diode is connected to the switching element Q such as a p-channel MOSFET on the anode side of the _{D 1,} the cathode npn type in side transistor Tr ₁ ', an operational amplifier OP, resistors _R b, negative feedback type consisting of _{R i} This is different from the first embodiment in that the constant current circuit 1A is connected, and the other configurations are the same.

As its operation, the constant current circuit 1A turns on and off the transistor Tr ₁ ′ so that the potential v _i at one end of the resistor R _i coincides with the reference potential v _r when the current _If flowing through the light emitting diode D ₁ is steady. To perform constant current control.
In addition, during a transition in which the switching element Q is turned on from the off state, since the current _If does not flow when the switching element Q is off, a forward bias is applied between the base and emitter of the transistor Tr ₁ ′ by the operational amplifier OP. A voltage is applied, and the transistor Tr ₁ ′ is saturated. When the switching element Q is turned on in this state, a transient current _If having a magnitude of approximately Vcc ₁ / (R _d + R _i ) flows according to the same principle as described above. This current has a larger value than that in the steady state, and the current flowing through the transistor Tr ₁ ′ is decreased by the negative feedback operation by the resistor R _i and the operational amplifier OP, and thereafter, is stabilized at the steady value.

Also in this embodiment, the switching elements Q on to flow a relatively large transient current I _f is the moment the current starts to flow through the light emitting diode D _1, thereafter a substantially constant value by the negative feedback operation the current I _f it can be, it is possible to eliminate the need for increasing the current I _f constantly to prevent transmission errors over time decreases and the PWM signal of the current conversion efficiency of the photo-coupler 2.

  In each of the above-described embodiments, the case where the PWM signal is transmitted using the photocoupler 2 has been described. However, the present invention can also be applied to transmission of an unmodulated pulse signal or a PFM (pulse frequency modulation) signal. .

1 is a circuit diagram showing a first embodiment of the present invention. It is a wave form diagram of PWM signal and current _{If in} a 1st embodiment. It is a wave form diagram of PWM signal and current _{If in} a 1st embodiment. It is a circuit diagram which shows 2nd Embodiment of this invention. It is a block diagram of a vehicle drive system. It is a block diagram of the buck-boost converter in FIG. It is a figure which shows the current waveform which flows into a reactor at the time of pressure | voltage rise operation | movement in FIG. It is a block diagram of IPM for buck-boost converters. It is a block diagram of IGBT drive IC provided in the back | latter stage of a photocoupler. It is a wave form diagram of the input voltage and input terminal current of IC for IGBT drive. The current _{I f} of the photocoupler is a diagram illustrating the parameters and the _V ce -I _c characteristics (output characteristics). It is a figure which shows the lifetime deterioration characteristic of the current conversion efficiency of a photocoupler.

Explanation of symbols

1, 1A: constant current circuit 2: Photocoupler 3: pulse generator Tr _1, Tr ₁ ': transistor Tr _2: phototransistors D _1: light emitting diode (LED)
Q: switching element ZD: Zener diode R _c , R _z , R _d , R _g , R _i , R _L : resistance

Claims (5)

In an insulated signal transmission circuit that insulates and transmits signals with a photocoupler,
A negative feedback constant current circuit connected in series with the light-emitting diode of the photocoupler and a switching element;
The constant current circuit is:
A constant current control element that is saturated when the switching element is turned off, and that allows a constant current to flow when the switching element is turned on;
When the switching element is turned on by a signal to be transmitted, a transient current is passed through the light-emitting diode via the constant current control element, and thereafter, a substantially constant steady current having an amplitude smaller than the transient current is applied. An insulated signal transmission circuit, wherein the light-emitting diode is passed through the constant current control element. In the insulated signal transmission circuit according to claim 1,
The constant current circuit includes a constant voltage element,
The insulated signal transmission circuit according to claim 1, wherein the constant current control element is a transistor that is forward biased by the constant voltage element and becomes saturated when the switching element is turned off. In the insulated signal transmission circuit according to claim 1,
The constant current circuit includes an operational amplifier,
The insulated signal transmission circuit according to claim 1, wherein the constant current control element is a transistor that is forward biased by the operational amplifier and becomes saturated when the switching element is turned off. In the insulated signal transmission circuit according to claim 2,
The insulated signal transmission circuit, wherein the constant voltage element is a Zener diode or a shunt regulator. In the insulated signal transmission circuit according to any one of claims 1 to 4,
An insulating signal transmission circuit, wherein the switching element is an FET or a transistor.