The basic task of a solid-state power supply is to safely and reliably supply the required electrical energy to the load. For electronic equipment, power supply is its core component. In addition to requiring the power supply to supply high-quality output voltage, the load also puts forward higher requirements for the reliability of the power supply system.
IGBT is a widely used device with self-shutdown capability, high switching frequency, and is widely used in various solid-state power supplies. But it can be easily damaged if not properly controlled. It is generally believed that there are two main reasons for IGBT damage: one is that the IGBT exits the saturation region and enters the amplifying region, which increases the switching loss; the other is that the IGBT is short-circuited, resulting in a large transient current, which damages the IGBT. The protection of IGBT usually adopts the method of fast self-protection, that is, when a fault occurs, the IGBT drive circuit is turned off to realize desaturation protection in the drive circuit; or when a short circuit occurs, the IGBT is quickly turned off. The short-circuit protection of different IGBTs can be divided into Uge monitoring method or Uce monitoring method according to different monitoring objects. The principles of the two methods are basically similar, and both use the phenomenon that Uge or Uce will also increase when the collector current IC increases. When Uge or Uce exceeds Uge sat or Uce sat, the drive circuit of IGBT is automatically turned off. Because Uge is basically unchanged when a fault occurs, while Uce changes greatly, and when desaturation occurs, Uge changes are small and difficult to grasp, so in practice, Uce monitoring technology is generally used to protect IGBTs. The IGBT protection circuit studied in this paper realizes the protection of the IGBT by monitoring the tube voltage drop Uce when the IGBT is turned on.
Using the IGBT short-circuit protection circuit introduced in this paper can achieve fast protection, and at the same time, it can save the Hall current sensor required to detect the short-circuit current and reduce the cost of the entire system. Practice has proved that the circuit has a relatively large practical value, especially in the application of low DC bus voltage, the circuit has broad application prospects. The circuit has been successfully applied in a certain type of high frequency inverter.
1 How short-circuit protection works

Figure 1(a) shows the H-bridge PWM conversion circuit working in the PWM rectification state (this figure is the equivalent circuit under the input of sine wave positive and half-wave, the two IGBTs of the upper half-bridge are not drawn). 1(b) is the driving signal of the two high-power devices of the lower half-bridge and the related device waveforms. Now take the positive half-wave working process as an example for analysis (for three-phase PWM circuit, the analysis process and conclusion of PWM circuit are basically similar in rectification, inverter working state or single-phase DC/DC working state).
In the circuit shown in Figure 1, in the positive half cycle of the commercial power supply Us, the high-frequency driving signal shown in Ug2.4 is added to the gates of the two IGBTs in the lower half bridge to obtain the tube voltage drop waveform UT2D. Its working process is analyzed as follows: At the time of t1~t2, under the action of the driving signal, T2 and T4 are turned on (actually, T2 is turned on, and T4 is in the freewheeling state), and the current through the inductor LS increases under the action of Us. The tube voltage drop waveform that rises exponentially as shown in UT2D in Figure 1(b) is formed on the T2 tube. The tube voltage drop is the voltage drop generated by the on-state current on the bulk resistance of the IGBT when it is turned on; at t2 At ~t3 time, T2 and T4 are turned off. Since there is energy storage in the inductor LS, the diodes D2 and D4 continue to flow under the action of the inductor LS, forming the tube shown in the shaded part of UT2.D in Figure 1(b). voltage drop waveform, and so on. The analysis shows that in order to detect the value of the tube voltage drop when the IGBT is turned on, the tube voltage drop of the IGBT when the IGBT is turned on at the time of t1~t2 should be reserved, and the tube voltage drop of the IGBT detected at the time of t2~t3 should be reserved. The value of , that is, the tube pressure drop waveform shown in the shaded part of UT2.D in Fig. 1(b) is eliminated. Because the switching frequency of IGBT is relatively high, and there is a large switching noise, sufficient consideration should be given when designing the sampling circuit.

According to the above analysis, under normal circumstances, the value of the tube voltage drop Uce(sat) when the IGBT is turned on is relatively low, usually less than the rated value of the data Uce(sat) given in the device manual. However, if the H-type bridge converter circuit fails (such as the "shoot-through" phenomenon in which the upper and lower IGBTs on one side of the bridge arm are turned on at the same time), then there will be a ratio between the C and E poles of the lower IGBT at this time. The normal value is much larger tube voltage. If the tube voltage drop value at the time of the fault can be detected quickly, it can be used as the basis for the protection of the IGBT, so that the IGBT can be effectively protected.

2 Design of short circuit protection circuit

From the analysis of the circuit shown in Figure 1, the principle circuit diagram of the IGBT short-circuit protection circuit can be obtained. IC4 and its peripheral devices constitute a gate logic circuit, IC5 and its peripheral devices constitute a filtering and amplifying circuit, IC2 and its peripheral devices constitute a threshold comparison circuit, and IC1 and its peripheral devices constitute a holding circuit. Under normal circumstances, the outputs of IC2D, IC2C and CD4011 connected to the cathodes of D1, D2 and D3 are all high level, and the output state of IC1 will not change. Suppose for some reason, when sending a drive signal to T2, the tube voltage drop of the left half-bridge lower tube T2 of the H-type bridge PWM conversion circuit is abnormally increased (set the level value to "high"), that is, UT2- If the voltage of terminal d rises abnormally, the high-level UT2-d is added to the cathode of D8 through R2; at the same time, the high-level driving signal sent to T2 is also added to the cathode of diode D5. For IC2C, its inverting input terminal is high level, if the level value is greater than the threshold level value of the non-inverting input terminal, then IC2C output is "low". The "low" level is added to the R input terminal of the RS flip-flop IC1 through D2, so that the output level of the output terminal Q is reversed, and an IGBT fault alarm signal is sent to the control system. If the output of IC2D is "low" due to the abnormal rise of the tube voltage drop of the lower tube T4 of the right half-bridge, the "low" level will be added to the R input of the RS flip-flop IC1 through D5 to make its output Q The output level of the switch is reversed, and an IGBT fault alarm signal is sent to the control system. The filtering and amplifying circuit composed of IC5A, IC5C and their peripheral devices preprocesses the voltage signal describing the voltage drop of the IGBT tube sent by the gating circuit, and sends it to the adder composed of IC5B for operation processing. If the output level of the adder is greater than the threshold level determined by R22 and R32, the third input terminal of the R terminal of the RS flip-flop IC1 will be "low", and an IGBT fault alarm signal will also be sent to the control system. By changing the threshold level determined by R22 and R32, the physical meaning represented by the third alarm signal can be flexibly changed, so that the protection circuit can be flexibly designed. Terminals T4-d and T2-d are respectively connected to the collectors of T4 and T2, and T4-G and T2-G are respectively connected to the driving signals of IGBT devices T4 and T2. Special attention should be paid to the circuit design, D8, D5, D9, D4 must use fast recovery diodes

3 Simulation and experimental results

When the PWM converter shown in Figure 1 works in the single-phase high-frequency rectification mode, the circuit is simulated and researched using the PSPICE simulation software. The simulated waveform is equivalent to the signal waveform observed at pin 7 of IC5B in the circuit. The simulation results show that the detection circuit can quickly and effectively detect the tube voltage drop when the lower tube of the PWM converter is turned on. The waveforms shown in Figure 3 are the relevant waveforms detected during actual circuit operation. In the figure, the 1# channel shows the given waveform of the single-phase high-frequency rectifier inductor current, and the 2# channel shows the actual detected working waveform of the 7th pin of IC5B in the circuit. Comparing Figure 2 and Figure 3, it can be concluded that the detection circuit can quickly and effectively detect the tube voltage drop when the IGBT is turned on, thereby effectively protecting the IGBT.

Figure 4 shows the actual detected current flowing in the PFC inductor and the waveform of the protection circuit operation when the IGBT is overcurrent.

The actual operation result of the circuit proves that the IGBT short-circuit protection circuit introduced in this paper can effectively protect the IGBT, with low cost and reliable operation. Practice has proved that the circuit has a relatively large practical value, especially in the application of low DC bus voltage, the circuit has broad application prospects. The circuit has been successfully used in a certain type of 3KVA high frequency inverter.