Power transistor (GTR) is a kind of high voltage and high current resistantBipolar junction transistor(Bipolar Junction Transistor BJT), so it is sometimes called Power BJT.But its driving circuit is complex and its driving power is high.
GTR and ordinary bipolar junction transistors work in the same way.
Chinese name
Power transistor
Foreign name
Giant Transistor
Alias
Power BJT
Literal translation
giant transistor
Characteristics
High withstand voltage, high current, good switching characteristics
GTR is a current controlled bipolar double junction high power, high back voltagePower electronic devicesIt has self turning off capacity and was produced in the 1970s. Its rated value has reached 1800V/800A/2kHz, 1400v/600A/5kHz, 600V/3A/100kHz.It not only has the inherent characteristics of transistor saturation voltage reduction, short switching time and wide safe working area, but also increases the power capacity. Therefore, the circuit composed of it is flexible, mature, with small switching loss and short switching time. It is widely used in power supply, motor control, general inverter and other circuits with medium capacity and medium frequency.The disadvantages of GTR are large driving current, poor ability to withstand surge current, and easy to be damaged by secondary breakdown.In switching power supply and UPS, GTR is gradually replaced by power MOSFET and IGBT.Its symbols and ordinaryNPN transistorSame.
product mix
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Power transistor (GTR for short), also known as BJT (Bipolar Junction Transistor).The names GTR and BJT are equivalent, and the structure and working principle are the same as those ofpower transistorVery similar.GTR consists of three semiconductor layers and two PN junctions.And small powertriodeSimilarly, there are two types: PNP and NPN. GTR usually uses NPN structure.[1]
working principle
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In power electronics, GTR mainly works in the switch state.GTR usually operates in positive deviation (Ib ＞ 0), and large current is conducted;When reverse bias Ib<0=, it is in the cut-off state.Therefore, apply a pulse driving signal with enough amplitude to the base of GTR, and it will work in the on-off state of conduction and cut-off.
Product features
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L Output voltage
Can be usedPWMTherefore, the output voltage is a strong pulse sequence whose amplitude is equal to the DC voltage.
2 Carrier frequency
Due to the long opening and closing time of power transistors, the allowable carrier frequency is low, and the upper limit carrier frequency of most inverters is about 1.2~1.5kHz.
3 Current waveform
Because the carrier frequency is low, the higher harmonic component of the current is large.These high-order harmonic currents will form eddy currents in the silicon steel sheets, and make the silicon steel sheets vibrate and generate noise due to the electromagnetic force generated between them.Because the carrier frequency is located in the area where the human ear is more sensitive to sound, the electromagnetic noise of the motor is strong.
4 Output torque
Because the components of higher harmonics in the current are large, the output torque on the motor shaft at 50Hz is slightly smaller than that at power frequency operation.
Basic characteristics
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(1) Static characteristics
The common emitter connection can be divided into three working areas:
① Cut off area.In the cut-off area,iB≤0，uBE≤0，uBC＜0，Only leakage current flows through the collector.
② Zoom in area.iB ＞0，uBE＞0，uBC＜0，iC =βiB。
③ Saturation zone.IB ＞ Ics/β, uBE ＞ 0, uBC ＞ 0, iCS is the collector saturation current, and its value is determined by the external circuit.
Conclusion: Both PN junctions are forward biased, which is the characteristic of saturation.When saturated, the tube voltage drop uCE between collector and emitter is very small, which is equivalent to switching on. At this time, although the current is large, the loss is not large.When the GTR just enters saturation, it is critical saturation. If iB continues to increase, it is supersaturation. When it is used as a switch, it should work in the deep saturation state, which is conducive to reducing uCE and the loss during conduction.
(2) Dynamic characteristics
Output characteristics of GTR common emitter connection
Figure 1
Figure 1 GTR Switch Characteristics
The leakage current of GTR is very small when it is turned off, and the saturation voltage drop is very small when it is turned on.Therefore, the loss of GTR is very small in both on and off states, but the current and voltage are both large in the switching process of off and on, so the loss is also large in the switching process.When the switching frequency is high, the switching loss is the main part of the total loss.Therefore, shortening the opening and closing time is of great significance for reducing loss, improving efficiency and improving operation reliability.
main parameter
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(1) Maximum operating voltage
(2) Maximum allowable current of collector ICM
(3) Maximum allowable dissipative power of collector PCM
(4) Maximum working junction temperature TJM
Secondary breakdown and safe working area
(1) Secondary breakdown
The secondary breakdown is an important factor affecting the safe and reliable operation of GTR.When the collector voltage of GTR rises to the breakdown voltage, the collector current increases rapidly. The first breakdown is avalanche breakdown, which is called primary breakdown.After a breakdown occurs, as long as Ic does not exceed the limit corresponding to the maximum operating dissipated power, GTR will not be damaged and its operating characteristics will not change.However, in practical applications, it is often found that if the current is not effectively limited when the primary breakdown occurs, Ic will suddenly rise sharply when it reaches a critical point, accompanied by a sudden drop in voltage. This phenomenon is called secondary breakdown.The methods to prevent secondary breakdown are as follows: ① The actual working voltage ratioReverse breakdown voltageMuch lower. ②There must be voltage and current buffer protection measures.[1]
(2) Safe work area
Figure 2
The working area composed of DC limit parameters ICM, PCM and UCEM is the primary breakdown working area, and the PSB (secondary breakdown power) composed of USB (secondary breakdown voltage) and ISB (secondary breakdown current) is an unequal power curve.In order to prevent secondary breakdown, GTR with enough power shall be selected. The maximum voltage actually used is usually much lower than the limit voltage of GTR.
Figure 2 GTR Safe Work Area
Figure 3
Figure 3 GTR base drive current waveform
Drive protection
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1. GTR base drive circuit
(1) Requirements for base drive circuit
① Realize electrical isolation between main circuit and control circuit.
② When conducting, the base forward drive current shall have a sufficiently steep front and a certain magnitude of forced current to accelerate the opening process and reduce the opening loss.
③ During the GTR conduction period, the base current should make the GTR in the critical saturation state, which can not only reduce the conduction saturation voltage drop, but also shorten the turn off time.
④ When the GTR is turned off, the reverse base current of sufficient magnitude shall be provided to the base to speed up the turn off speed and reduce the turn off loss.
⑤ It shall have strong anti-interference ability and certain protection function.
(2) Base drive circuit
Figure 4
Figure 4 Practical GTR Drive Circuit
2. Integrated drive
The integrated driving circuit overcomes the shortcomings of many common circuit components, complex circuits, poor stability and inconvenient use, and also increases the protection function.
3. Protection circuit of GTR
The switching frequency is high, and fast melting protection is invalid.Generally, buffer circuit is used.There are mainly three types of RC buffer circuit, charge discharge type R, C, VD buffer circuit and stop discharge type R, C, VD buffer circuit, as shown in Figure 5.
Figure 5
(a) (b) (c)
Figure 5 Buffer circuit of GTR
The RC buffer circuit shown in Figure 5a is only applicable to small capacity GTR (current below 10A).The charge discharge type R, C and VD buffer circuits shown in Figure 5b are used for high-capacity GTR.The R, C, and VD buffer circuits of the blocking discharge type shown in Figure 5c are more commonly used in high-capacity GTR and high-frequency switching circuits, and their biggest advantage is that the loss generated by buffering is small.
circuit analysis
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Figure 6
Figure 6 shows a three-phase bridge PWM inverter circuit. The power switching device is GTR, and the load is inductive.From the perspective of circuit structure, three-phase bridge PWM frequency conversion circuit can only use bipolar control mode, and its working principle is as follows:
The three-phase modulated signals urU, urV and urW are sine waves whose phases differ by 120 ° in sequence, while the three-phase carrier signals share a triangle wave uc changing in positive and negative directions, as shown in Figure 6.U. The control methods of phase V and phase W self turning off switch devices are the same. Now take phase U as an example: in each section of urU>uc, the upper bridge arm power transistor V1 is given a turn on driving signal, and the lower bridge arm V4 is given a turn off signal, so the output voltage of phase U is uUN '=Ud/2 relative to the neutral point N' of the DC power supply Ud.At urU
