JPH0254697B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a load current balance circuit that can easily balance the collector currents (load sharing currents) flowing through each transistor when switching a common load current using a plurality of transistors at the same time. It is something.

When a transistor is used as a switch element to switch a load current, a plurality of transistors may be used because the current capacity or internal power loss is insufficient if a single transistor is used. In such a case, if the terminals (collector, emitter, base) of the transistors are simply connected in parallel and used, the current flowing through each transistor will be unbalanced due to the difference in characteristics between the transistors. For this reason, conventionally, for example, a circuit as shown in FIG. 1 was used to balance the currents flowing through each transistor.

FIG. 1 shows a circuit in which DC power supplied from a DC power source E _i is switched by a switch circuit S driven by a drive signal from a drive pulse generation circuit P, and is supplied to a load Z _L .

If you want to obtain an appropriate DC voltage as the load voltage applied to the load Z _L , the main transformer, rectifier circuit, smoothing circuit, etc. for step-up or step-down are included in the load Z _L , and you can obtain it by using them. Considering this, the switch circuit S itself may be a circuit as shown in FIG.

In FIG. 1, P is a drive pulse generation circuit;
A ₁ to An are drive circuits for switching the transistors Q ₁ to Qn, respectively, and each has a drive transformer DT, and once the transistors Q ₁ to Qn are turned on by the on signal from the drive winding N ₄ , The current winding N ₂ ( _the current remaining after subtracting the base current flowing through the winding N ₁ from the emitter current, that is, the collector current flows through the winding N 2 ), and the collector current is positively fed back by the base winding N ₁ . This constitutes a so-called current feedback type drive circuit that supplies a base current.

In this circuit, the unbalanced collector current caused by variations in characteristics between each transistor is balanced by the balancing current transformers (BT ₁₂ to BTn ₁ ), and the load sharing current of each transistor is
i ₁ ~in are approximately balanced. However, in this conventional circuit, not only are the same number of balancing current transformers as switching transistors required, but also two balancing current transformers are inserted into the load circuit seen from the transistor, which increases the wiring inductance. This increases the surge voltage between the collector and emitter when the transistor is turned off. In addition, if there are variations in the storage time of transistors, the surge voltage between the collector and emitter of the transistor that turns off first and has a short storage time will be especially large, causing an imbalance in the voltage waveform at turn-off. Therefore, use a transistor with a margin in terms of withstand voltage. There is a need to.

As described above, the conventional circuits have the disadvantage of high costs due to the necessity of a large number of balancing current transformers and the necessity of using transistors with sufficient withstand voltage.

The present invention has been made in order to eliminate the drawbacks of the prior art as described above, and therefore, an object of the present invention is to eliminate the need for many balancing current transformers and to provide a sufficient voltage margin. It is an object of the present invention to provide a load current balance circuit for parallel-connected switching transistors that does not require the use of certain transistors and is therefore inexpensive.

In order to achieve the above object, the present invention provides a load current balance circuit for parallel-connected switching transistors, in which a drive transformer having at least three windings is prepared for each of the transistors, and the base of each transistor is and Emitsuta,
The first winding of the corresponding drive transformer is connected, one end of the second winding of the corresponding drive transformer is connected to the emitter of each transistor, and the other end of the second winding is connected from the collector of each transistor to the emitter of each transistor. All the circuits leading to the ends are connected in parallel, and a current proportional to the sum (load current) of the collector currents of the respective transistors is caused to flow through the third winding of each drive transformer, and the second winding The base current flowing through the first winding decreases when the current increases, and the base current flowing through the first winding increases when the third winding current increases. It is characterized in that each third winding is wound around the drive transformer.

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 2 is a circuit diagram showing one embodiment of the present invention.

In this embodiment, a driving transformer is used instead of the balancing current transformers (BT ₁₂ to BTn ₁ ) shown in Fig. 1.
The DT is characterized by having a collector current negative feedback winding for correcting unbalanced current.

In the drive transformer DT shown in Figure 2, _N2 is used as a negative feedback winding, and _N3 is a winding that positively feeds back the sum of the collector currents of all transistors to the base of each transistor to supply the base current after turn-on. Use as a line. When the drive circuit is configured in this way, the current flowing through the _N3 winding (collector current sum of all transistors _o 〓 ^K=1 ik) is positively fed back to the base winding _N1 , but the current flowing through the _N2 winding of each transistor is The collector current is negatively fed back to m of n transistors.
Base current after turn-on of the th transistor
i _B (m) has a value expressed by the following formula.

i _B (m)=N ₃ 〓〓ik−N ₂ im/N ₁ ……(1) where im: collector current of the m-th transistor From the above equation (1), the following can be seen. If the collector current im of the m-th transistor increases under a certain load ( _o 〓 ^K=1 ik=constant), the base current _B (m) of that transistor decreases, and conversely, the collector current
If im decreases, base current i _B (m) increases.

In other words, this negative feedback operation functions as a load sharing function (balance of collector currents i ₁ to in) of each transistor. Here, N ₃ =2/nN ₂ (2) where n is the number of parallel connections, the total amount of current feedback to the base winding N ₁ can be made exactly the same as in the case of FIG.

Next, let us consider the case where there is variation in storage time among transistors and the turn-off characteristics of the m-th transistor are different compared to the others.a If the storage time is short and the turn-off is fast, the collector current of the m-th transistor is faster than the collector current of the other transistors. Even if the collector current im of a transistor becomes o, the collector currents of other transistors are fed back as the base current i _B (m) as shown in equation (1) above, which prevents the collector current im from quickly reaching o. effect occurs.

b When the accumulation time is long and the turn-off is slow In the above equation (1), if the collector current other than the collector current im of the m-th transistor is o, the base current i _B (m) is i _B (m) = (N ₃ −N ₂ ) im/N ₁ . From equation (2), since N ₃ ≦N ₂ (n≧2), the base current i _B (m) becomes o or a negative current, and the base current immediately becomes o or a reverse bias current, and the collector current im This produces the effect of quickly turning o.

In other words, the circuit according to the present invention not only balances currents in the saturated ON state of a plurality of switching transistors used in parallel, but also balances variations in turn-off characteristics.
Even in the case of an inductance load, the voltage waveforms at turn-off of each transistor can be balanced, so the practical effect is very large.

FIG. 3 is a circuit diagram showing another embodiment of the present invention. The embodiment shown in FIG. 2 shows a circuit that is improved from the circuit shown in FIG. 2 for the case where a large number of parallel circuits are used.

In the circuit shown in Figure 2, the positive feedback winding of each drive transformer
N ₃ is connected in series, through which the load current flows directly. Therefore, when a large number of transistors are used in parallel, the wiring inductance of the main circuit becomes large, which causes a large surge voltage to be generated when the transistors are turned off.

In the circuit shown in Figure 3, the current flowing through the _N3 winding is the load current (total collector current _o 〓 ^{K = 1} ik), which is once passed through the current transformer CT.
This reduces the wiring inductance of the main circuit by converting the current into a small current through the transformer to prevent a large load current from flowing directly through each drive transformer, and also reduces the current in the _N3 winding of the drive transformer, making it easier to manufacture.

In this case, the relationship between the turns ratio of the drive transformer DT and the current transformer CT for load current conversion is N ₃ = (N ₂ ′/N ₁ ′) 2/nN ₂ ……(3), as shown in Figure 2. You can get exactly the same effect. However, N ₁ ′ and N ₂ ′ are the respective numbers of turns of the primary winding and secondary winding of the current transformer CT.

In Fig. 3, diode D ₁ is connected to current transformer CT
As described above, the circuit according to the present invention uses the winding through which the collector current of the current feedback drive transformer flows for negative feedback operation for collector current balance.
Furthermore, by providing a new winding so that a current proportional to the total collector current is positively fed back as a base current, the switching operation current and voltage of the plurality of transistors are balanced.

Therefore, the load current balance circuit according to the present invention is as follows:
It can be easily applied to parallel use of any number of transistors without using a balancing current transformer as in the past, so in circuits that use transistors as switch elements, the capacitance of the switch circuit can be reduced by parallel connection. Large-capacity chopper circuit that can be easily expanded, DC-AC conversion inverter,
It has great effects when applied to DC-DC converters, etc.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional example of a load current balance circuit for parallel switching transistors.
3 are circuit diagrams each showing an embodiment of the present invention. Symbol explanation, Ei...DC input voltage, E...power supply for drive circuit, _ZL ...load, _Q1 to Qn...switching transistors used in parallel, Q...driver transistor, _A1 to An...transistor _Q1 to Qn drive circuit,
BT ₁₂ ~ _{BTn 1} ... Balance current transformer, P... Drive pulse generation circuit, DT... Drive transformer, R... Resistor,
D, D ₁ ...diode, CT...current transformer, S...switch circuit section.

Claims (1)

[Claims] 1 A load current balance circuit for parallel-connected switching transistors, in which a drive transformer having at least three windings is prepared for each of the transistors, and a corresponding drive transformer is provided between the base and emitter of each transistor. A circuit connecting the first winding of the transistor, connecting one end of the second winding of the corresponding drive transformer to the emitter of each transistor, and connecting the collector of each transistor to the other end of the second winding. are all connected in parallel, and a current proportional to the sum of the collector currents of the respective transistors (load current) is caused to flow through the third winding of each of the drive transformers, so that the second winding current increases. Each of the first to third windings is configured so that, when the base current flowing through the first winding decreases, and when the third winding current increases, the base current flowing through the first winding increases. A load current balance circuit comprising a wire wound around the drive transformer.