What you need to know about IGBT protection circuit design

1 Introduction

IGBT (Insulated Gate Bipolar Transistor) is a new type of power electronic device that uses MOS to control transistors. It has the characteristics of high voltage, large current, high frequency, and low on-resistance, so it is widely used in the inverter circuit of the frequency converter. However, due to the poor overcurrent and overvoltage resistance of IGBT, it will be damaged once an accident occurs. For this reason, it is necessary to carry out relevant protection for IGBT. Starting from practical applications, this article summarizes the relevant issues and various protection methods of overcurrent, overvoltage and overheating protection, which is highly practical and has good application effects.

2 Overcurrent protection

Manufacturers have strict restrictions on the safe working area provided by IGBTs, and the time that IGBTs can withstand overcurrent is only a few microseconds (SCR, GTR and other devices can withstand overcurrent for tens of microseconds), and the overcurrent resistance is small, so the first thing to pay attention to when using IGBTs is overcurrent protection. The reasons for overcurrent are: damage to transistors or diodes, malfunctions caused by control and drive circuit failures or interference, short circuits caused by wrong connection of output lines or insulation damage, short circuits at the output end to the ground and damage to the motor insulation, short circuits in the bridge arm of the inverter bridge, etc.

There are two cases for overcurrent detection protection of IGBT:

(1) There is no protection function in the drive circuit. At this time, an overcurrent detection device should be set in the main circuit. For small-capacity inverters, the resistor R is generally connected in series in the main circuit, as shown in Figure 1 (a), and the voltage across the resistor is used to reflect the current; for large and medium-capacity inverters, due to the large current, a current transformer TA (such as a Hall sensor, etc.) is required. The current transformer is connected in series in the main circuit like a series resistor, as shown by the dotted line in Figure 1 (a); the second is connected in series on each IGBT, as shown in Figure 1 (b). The former only uses one current transformer to detect the total current flowing through the IGBT, which is economical and simple, but the detection accuracy is poor; the latter directly reflects the current of each IGBT, with high measurement accuracy, but requires 6 current transformers. The current signal detected by the overcurrent outputs a blocking signal to the control circuit through the optocoupler, thereby turning off the triggering of the IGBT to achieve overcurrent protection.

Figure 1 IGBT overcurrent detection

Abstract: This paper discusses the related issues of IGBT overcurrent protection, overvoltage protection and overheat protection, and summarizes various protection methods from practical applications. These methods are highly practical and have good protection effects.

1 Introduction

IGBT (Insulated Gate Bipolar Transistor) is a new type of power electronic device that uses MOS to control transistors. It has the characteristics of high voltage, large current, high frequency, and low on-resistance, so it is widely used in the inverter circuit of the frequency converter. However, due to the poor overcurrent and overvoltage resistance of IGBT, it will be damaged once an accident occurs. For this reason, it is necessary to carry out relevant protection for IGBT. Starting from practical applications, this article summarizes the relevant issues and various protection methods of overcurrent, overvoltage and overheating protection, which is highly practical and has good application effects.

2 Overcurrent protection

Manufacturers have strict restrictions on the safe working area provided by IGBTs, and the time that IGBTs can withstand overcurrent is only a few microseconds (SCR, GTR and other devices can withstand overcurrent for tens of microseconds), and the overcurrent resistance is small, so the first thing to pay attention to when using IGBTs is overcurrent protection. The reasons for overcurrent are: damage to transistors or diodes, malfunctions caused by control and drive circuit failures or interference, short circuits caused by wrong connection of output lines or insulation damage, short circuits at the output end to the ground and damage to the motor insulation, short circuits in the bridge arm of the inverter bridge, etc.

There are two cases for overcurrent detection protection of IGBT:

(1) There is no protection function in the drive circuit. At this time, an overcurrent detection device should be set in the main circuit. For small-capacity inverters, the resistor R is generally connected in series in the main circuit, as shown in Figure 1 (a), and the voltage across the resistor is used to reflect the current; for large and medium-capacity inverters, due to the large current, a current transformer TA (such as a Hall sensor, etc.) is required. The current transformer is connected in series in the main circuit like a series resistor, as shown by the dotted line in Figure 1 (a); the second is connected in series on each IGBT, as shown in Figure 1 (b). The former only uses one current transformer to detect the total current flowing through the IGBT, which is economical and simple, but the detection accuracy is poor; the latter directly reflects the current of each IGBT, with high measurement accuracy, but requires 6 current transformers. The current signal detected by the overcurrent outputs a blocking signal to the control circuit through the optocoupler, thereby turning off the triggering of the IGBT to achieve overcurrent protection.

Figure 1 IGBT overcurrent detection

(2) The drive circuit is equipped with a protection function. For example, HR065 of Japan Inda, EXB840-844 of Fuji Electric, M57962L of Mitsubishi, etc. are integrated circuits (called hybrid drive modules) that integrate drive and protection functions. Their current detection uses the characteristic that the forward conduction tube voltage drop Uce (ON) is proportional to the collector current Ie under a certain forward gate voltage Uge. The size of Ie is determined by detecting the size of Uce (ON), and the product has high reliability. Different models of hybrid drive modules have different output capabilities, switching speeds, and du/dt tolerances. When using them, they should be appropriately selected according to actual conditions.

Since the overcurrent protection critical voltage action value of the hybrid drive module itself is fixed (generally 7-10V), there is a problem of coordination with the IGBT. The commonly used method is to adjust the number of diodes V connected in series between the IGBT collector and the drive module, as shown in Figure 2 (a), so that the sum of the on-state voltage drops of these diodes is equal to or slightly greater than the difference between the overcurrent protection action voltage of the drive module and the on-state saturation voltage drop Uce (ON) of the IGBT.

Figure 2: Coordination between hybrid drive module and IGBT overcurrent protection

The above method of adjusting the overcurrent protection action point by changing the number of diodes is simple and practical, but the accuracy is not high. This is because the on-state voltage drop of each diode is a fixed value, so the voltage between the driver module and the IGBT collector c cannot be continuously adjusted. In actual work, there are two ways to improve it:

(1) Change the type and number of diodes. For example, when the on-state saturation voltage drop of the IGBT is 2.65V and the critical action voltage value of the overcurrent protection of the driver module is 7.84V, the sum of the on-state voltage drop of the entire diode should be 7.84-2.65=5.19V. At this time, 7 silicon diodes and 1 germanium diode are selected in series, and the sum of their on-state voltage drops is 0.7×7+0.3×1=5.20V (the silicon tube is regarded as 0.7V and the germanium tube is regarded as 0.3V), which can better achieve coordination. (2) Combining diodes with resistors. Due to the differences in the on-state voltage drop of diodes, it is difficult to accurately set the critical action voltage value of the IGBT overcurrent protection with the above improved method. If 1 to 2 diodes are replaced by resistors, as shown in Figure 2 (b), accurate coordination can be achieved.

In addition, due to the overlapping of control signals of two IGBTs on the same bridge arm or the long delay of the switch device itself, the upper and lower IGBTs are directly turned on and the bridge arm is short-circuited. At this time, the current rise rate and surge impact current are very large, which is very easy to damage the IGBT. For this reason, the bridge arm interlock protection can also be set, as shown in Figure 3. In the figure, two AND gates are used to interlock the drive signals of the two IGBTs on the same bridge arm, so that the working state of each IGBT is a constraint condition for whether the drive signal of the other IGBT can pass. Only after one IGBT is confirmed to be turned off, the other IGBT can be turned on, which strictly prevents the occurrence of overcurrent caused by arm bridge short circuit.

Figure 3 IGBT bridge arm direct short circuit protection

3 Overvoltage protection

When the IGBT is turned off from the on state, the current Ic suddenly decreases. Due to the effect of the stray inductance and load inductance in the circuit, a very high surge peak voltage uce=L dic/dt will be generated at both ends of the IGBT c and e. In addition, the IGBT has a poor overvoltage tolerance, which will cause the IGBT to break down. Therefore, its overvoltage protection is also very important. Overvoltage protection can be carried out from the following aspects:

(1) Reduce the stray inductance in the circuit as much as possible. As a module designer and manufacturer, you should optimize the internal structure of the module (such as using layered circuits, reducing the effective loop area, etc.) to reduce parasitic inductance; as a user, you should optimize the main circuit structure (using layered wiring, shortening the connection lines as much as possible, etc.) to reduce stray inductance. In addition, add more low-resistance and low-inductance decoupling capacitors to the entire circuit to further reduce the line inductance. All of these have a good effect on directly reducing the IGBT turn-off overvoltage.

(2) Use an absorption circuit. The function of the absorption circuit is to absorb the energy released in the inductor when the IGBT is turned off to reduce the turn-off overvoltage. There are two commonly used absorption circuits, as shown in Figure 4. Figure (a) is a charge-discharge absorption circuit, and Figure (b) is a clamping absorption circuit. For the selection of components in the circuit, in actual work, the capacitor c uses a high-frequency, low-inductance coil-wound polyethylene or polypropylene capacitor, or a ceramic capacitor with a capacity of about 2 F. A larger capacitance will better suppress the surge peak voltage, but too large a capacitance will be limited by the discharge time. The resistor R uses an oxide film non-inductive resistor, and its resistance value must be determined to meet the requirement that the discharge time is significantly less than the main circuit switching cycle. It can be calculated as R≤T/6C, where T is the main circuit switching cycle. The diode V should use a soft characteristic buffer diode with a low forward transition voltage and a short reverse recovery time.

(3) Increase the gate resistance Rg appropriately. Practice has shown that increasing Rg slows down the switching speed of the IGBT and can significantly reduce the switching overvoltage spike, but it increases the switching loss and increases the heat generation of the IGBT, so overheat protection should be carried out. The principle of selecting the Rg resistance value is: when the switching loss is not too large, choose a larger resistance as much as possible. In actual work, select Rg=3000/Ic.

Figure 4 Absorption circuit

In addition to reducing the overvoltage between c and e as mentioned above, some protection elements can be set between g and e to prevent gate charge accumulation and gate-source voltage spikes from damaging the IGBT. The circuit is shown in Figure 5. The function of resistor R is to discharge the gate accumulated charge, and its resistance value can be 4.7kΩ; two reverse-connected Zener diodes V1 and V2 are to prevent gate-source voltage spikes from damaging the IGBT.

Figure 5 Protection against gate charge accumulation and gate-source voltage spikes

4 Overheat protection

The power loss of IGBT mainly includes switching loss and conduction loss. The former increases with the increase of switching frequency and accounts for the main part of the total loss; the latter is the product of the average current controlled by IGBT and the power supply voltage. Since IGBT is a high-power semiconductor device, the power loss makes it generate more heat (especially when Rg is selected to be too large), and the junction temperature of IGBT cannot exceed 125℃. It is not suitable to work at a high temperature for a long time. Therefore, appropriate heat dissipation measures should be taken for overheating protection.

Heat dissipation is generally achieved by using a radiator (including ordinary radiators and heat pipe radiators), and forced air cooling can be performed. The structural design of the radiator should meet the following requirements: Tj = P△ (Rjc + Rcs + Rsa) < Tjm where Tj is the operating junction temperature of the IGBT.

P△－Power loss

Rjc - Junction-Case Thermal Resistance vkZ Electronic Data Network

Rcs - Shell - Heat sink thermal resistance

Rsa - heat sink - ambient thermal resistance

Tjm－IGBT maximum junction temperature

In actual work, we use a combination of ordinary radiators and forced air cooling, and install a temperature switch on the radiator. When the temperature reaches 75℃~80℃, the shutdown signal of SG3525 stops PMW from sending control signals, so that the driver blocks the switch output of IGBT and shuts down for protection.