Design and Application of High Power Transistor Driving Circuit-EEWORLD

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Abstract: This paper introduces the design of base drive circuit for high power transistor (GTR), analyzes the requirements and design method of base drive circuit, and gives a practical drive circuit.

Keywords: high-power transistor; base drive circuit; analysis; design

1 Introduction

As the core component of the inverter circuit, high-power switching devices are generally divided into three types, namely bipolar, unipolar and hybrid. Bipolar types include GTO, GTR, SITH, etc.; unipolar types include power MOSFET, SIT, etc.; hybrid types include IGBT, MGT (MOS gate transistor), etc. The operating status and safety of these high-power devices directly determine the performance of the frequency converter and inverter, and a good driving circuit is an important guarantee for the safe and reliable operation of the switching device. This article focuses on the base drive circuit of GTR.

The Giant Transistor (Giant Transistor—GTR), also known as the giant transistor, is a three-layer bipolar fully controlled high-power high reverse voltage transistor. It has the advantages of self-shutdown capability, easy control, low saturation voltage drop and relatively wide safe operating area, and has been used in many power conversion devices.

In power electronic devices, GTR mainly works in the switching state. GTR is a current-controlled device, that is, after injecting current I _B into its base , the collector can obtain the amplified current IC, and the current amplification factor is evaluated by h _FE . For GTR working in the switching state, the key technical parameters are the reverse withstand voltage V _CE and the forward conduction current IC. Since GTR is not an ideal switch, when it is saturated and turned on, there is a tube voltage drop V _CES , and there is a leakage current I _CEO when it is turned off ; in addition, the switch conversion process has an on time t _on . (including delay time t _d and rise time tr ₎ , and a turn-off time t _off (including storage time t _s and fall time t _f ). Therefore, when using GTR, its collector power consumption PC and junction temperature T _jm should also be given enough attention.

2 Base drive circuit design principles

The GTR base drive circuit and performance directly affect the working condition of the GTR. Therefore, the following two points should be considered when designing the base drive circuit: optimal drive mode and automatic and fast protection.

The so-called optimized drive is to control the switching process of GTR with an ideal base drive current waveform in order to increase the switching speed and reduce the switching loss. The ideal base drive current waveform is shown in Figure 1. As can be seen from Figure 1, in order to speed up the turn-on time and reduce the turn-on loss, the forward base current requires not only a steep leading edge at the beginning of the turn-on, but also a certain time of overdrive current I _B1 . The base drive current I _B2 in the conduction stage should keep the GTR just in the quasi-saturation state in order to shorten the storage time t _s . In general, the value of the overdrive current I _B1 is selected to be about 3 times the quasi-saturation base drive current value I _B2 , the overdrive current waveform leading edge should be controlled within 0.5μs, and its width should be controlled at about 2μs. When turning off the GTR, the reverse base drive current I _B3 should be larger in order to speed up the extraction speed of carriers in the base region, shorten the turn-off time, and reduce the turn-off loss. In practical applications, I _B3 = I _B1 or larger is often selected. This base drive waveform is generally achieved by a speed-up circuit and a Baker clamp circuit.

Figure 1 Ideal base drive current waveform

In addition, the driving circuit of GTR should also have a self-protection function so that the base drive signal can be quickly and automatically cut off in the fault state to avoid damage to GTR. There are many types of protection circuits, which can be appropriately selected according to the different requirements of devices and circuits. In order to increase the switching speed, an anti-saturation protection circuit can be used; to ensure that the power consumption of the switching circuit itself is low, a desaturation protection circuit can be used; to prevent the base from being underdriven and causing the device to be overloaded, power supply voltage monitoring protection can be used. In addition, there are pulse width limiting circuits and overvoltage, overcurrent, overheating and other protection circuits to prevent GTR damage.

There are many forms of base drive circuits, which can be summarized into three obvious trends:

1) In order to increase the working speed, the anti-saturation Baker clamp circuit is used as the basic circuit;

2) Continuously improve and expand automatic protection functions;

3) Continuously improve and perfect the opening and closing speeds.

3 Base drive circuit example

3.1 Circuit composition and function

The following is a practical and efficient self-protection base drive circuit, which can not only maintain the GTR working in a quasi-saturation state, but also provide fast and reliable protection for the GTR overload, preventing the GTR from entering the amplification area. In addition, it can improve the switching characteristics of the GTR, shorten the switching time, reduce the driving power, and improve the driving efficiency. The specific circuit is shown in Figure 2. It is mainly composed of a signal isolation circuit, a desaturation detection circuit, a control signal synthesis circuit and an adaptive output circuit with reverse bias. The signal isolation circuit is composed of a photocoupler BD _to achieve electrical isolation between the logic control circuit and the drive circuit; the desaturation detection circuit is composed of a diode D6 _and a voltage comparator A1 _. When the collector-emitter voltage VCE of the GTR is higher than a certain specified _value , the voltage comparator A1 _outputs an overload protection signal. The control signal synthesis circuit is composed of a transistor V1 _. Its function is to superimpose the normal switch drive signal and the desaturation prohibition signal and send them to the output stage. The adaptive output driver stage with reverse bias is composed of transistors V ₃ , V ₄ , diodes D ₇ , D ₈ , D ₉ , capacitor C ₂ and other components. Its function is to increase the switching speed and generate a reverse bias driving waveform.

Figure 2 Base drive circuit

3.2 Working Principle of Driving Circuit

When the input signal Vin _is at a high level, the optocoupler is cut off, point B is approximately equal to the power supply voltage, point A is _{the voltage division level of R3 and R4, then VB}_> VA , the output terminal C of the voltage comparator is at a low level, the transistor V1 _is_cut_off , V2 _is turned on, V3 _and V4 _are cut off, and thus GTR is cut off.

_{When the} input signal Vin changes from high level to low level, the output of the optocoupler changes from cutoff to on. C1 _is charged through R8 and D3 _. Taking advantage of the fact that the voltage at both ends of the capacitor cannot change suddenly, the base potential of V2 also becomes zero, V2 _is_cut off, V3 _and_V4 are _turned on, and the GTR is quickly saturated and turned on through the acceleration network _{C2 and R12} ; _when the GTR is turned on, its VCE decreases accordingly, D6 _is_turned on, so that the potential of point B is clamped at VB = _VCE < VA , and the output terminal _{C
of the}_voltage comparator A1 _becomes high level, so that V1 _is turned on, and the base potential of V2 _is maintained at the ground potential; V2 is kept _off , and V3 _and V4 _are turned on. At the same time, the conduction of V1 provides a discharge circuit for _C1 , so that the voltage at both ends of the capacitor _C1 drops _to zero , preparing for the next work.

When Vin changes from low level to high level, the output stage of the optocoupler changes from on to off, making D1 _on and D2 _off_, making V _B > V _A again , point C outputs low level, V1 _is off, V2 _is on, and the reverse bias circuit composed of _C2 , V5 _, D10 _, D9 _, etc. makes GTR turn off quickly, and D6 _is turned off at the same time. The above working process will be repeated in the next cycle.

The working principle of the reverse bias drive circuit is as follows: When V4 _is turned on, GTR is also turned on. The relatively large charging current of the acceleration capacitor _C2 provides an overdrive current to the GTR base, and the maximum current is only limited by _{the
resistance value}_of R11 . After the charging is completed, it enters the conduction stage. The base current of GTR is determined by R11, R12 and D8. At this time, C2 is charged with _a positive voltage on the left and a negative voltage on the right. When V4 _is turned off and V5 _is turned on, capacitor _C2 discharges _through the CE junction of V5 _→ D10 _→ D9 _→ C2 . _The reverse bias voltage of GTR is equal to the conduction voltage drop of D10 _,_which is about 0.7V, causing GTR to turn off quickly.

3.3 Working Principle of Protection Circuit

During normal operation, D6 is _turned on, making V _B = V _CE ; if GTR is overloaded or exits saturation due to other reasons, V _CE rises to V _B = V _CE

3.4 Device requirements for drive circuits

First of all, the requirement for the optocoupler is a high-speed optocoupler. This is because for the bridge inverter circuit, the dead time t _Δ (between 15 and 20 μs) should be set between the upper and lower complementary control signals of the same bridge arm. The ordinary optocoupler has a long switching time, generally between (4 and 6 μs), and the delay time of the post-stage driver is as long as about 10 μs, and the opening and closing time may be unequal, so that the normal dead time cannot be guaranteed. In order to work safely and reliably, a high-speed optocoupler must be selected, and the total delay of the post-stage driver must be strictly limited to within 5 μs. For example, the high-speed optocoupler 6N137 in Figure 2 can meet the system requirements.

Secondly, the optocoupler is required to have strong anti-interference ability. This is because during the switching process of the GTR, the potential of point P jumps (Figure 3). For example, when GTR1 is turned on or D ₁ is continuous, point P is equal to point M; and when GTR2 is turned on or D ₂ is continuous, point P is equal to point N. The jump speed of the potential at point P is determined by the reverse recovery time of the diode. For small and medium power three-phase asynchronous motor inverters, the d v /d t at point P will reach several thousand volts per second. If the optocoupler has weak anti-interference ability. The jump of the potential at point P will be coupled through the internal parasitic capacitance of the optocoupler, forming an interference pulse in the drive circuit, causing the GTR to malfunction and fail to work normally.

Figure 3 Connection relationship between GTR main circuit and drive circuit

4 Conclusion

The driving circuit is the basis for the safe operation of GTR. Careful design of the driving circuit and careful selection of driving circuit components and parameters are an important part of ensuring the reliable operation of the whole machine. In recent years, this type of special module driving circuit (such as UAA4002) has been widely used to make the operation of GTR safer and more reliable. Practice has proved that the set of reverse bias adaptive driving circuit designed in this paper has a simple structure and reliable performance, and can meet the general driving requirements of GTR inverters.

Reference address：Design and Application of High Power Transistor Driving Circuit

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