Using gate resistors to control the switching of IGBTs

1 Introduction

Power semiconductor devices for control, regulation and switching purposes require higher voltages and larger currents. The switching action of power semiconductor devices is controlled by the charging and discharging of the gate capacitance. The charging and discharging of the gate capacitance is usually controlled by the gate resistor. By using a typical +15V control voltage (V _G(on) ), the IGBT is turned on, and when the negative output voltage is -5V to -15V, the IGBT is turned off. The dynamic performance of the IGBT can be adjusted by the gate resistance value. The gate resistance affects the switching time, switching losses and various other parameters of the IGBT, from electromagnetic interference EMI to the rate of change of voltage and current. Therefore, the gate resistor must be selected and optimized very carefully according to the parameters of the specific application.

2 Influence of gate resistance RG on IGBT switching characteristics

The setting of the IGBT switching characteristics can be influenced by the external resistor RG _. Since the input capacitance of the IGBT changes during switching and must be charged and discharged, the gate resistor determines the charging and discharging time by limiting the amplitude of the gate current (IG ₎ pulse during turn-on and turn-off (see Figure 1). Due to the increase in the gate peak current, the turn-on and turn-off times will be shortened and the switching losses will be reduced. Reducing the resistance values ​​of RG _(on) and _RG(off) will increase the gate peak current. When reducing the gate resistor value, it is necessary to consider the current rise rate di/dt generated when large currents are switched too quickly. The presence of stray inductance in the circuit generates large voltage spikes on the IGBT, the effect of which can be observed in the waveform diagram of the IGBT when it is turned off, shown in Figure 2. The shaded area in the figure shows the relative value of the turn-off losses. The transient voltage spike on the collector-emitter voltage may damage the IGBT, especially in the case of short-circuit turn-off operation, because the di/dt is large. Vstray can be reduced by increasing the value of the gate resistor _. Therefore, the risk of the IGBT being destroyed due to overvoltage is eliminated. Fast turn-on and turn-off result in higher dv/dt and di/dt, respectively, thus generating more electromagnetic interference (EMI), which may cause circuit failure. Table 1 shows the effect of different gate resistance values ​​on di/dt.

Figure 1. On, off/gate current

Figure 2 IGBT shutdown

Table 1 Change rate/characteristics

3. Impact on the switching characteristics of the freewheeling diode

The switching characteristics of the freewheeling diode are also affected by the gate resistance and limit the minimum value of the gate impedance. This means that the turn-on switching speed of the IGBT can only be increased to a level compatible with the reverse recovery characteristics of the freewheeling diode used. The reduction of the gate resistance not only increases the overvoltage stress of the IGBT, but also increases the overvoltage limit of the freewheeling diode due to the increase of di _C /dt in the IGBT module. By using a specially designed and optimized CAL (controlled axial life) diode with soft recovery function, the reverse peak current is reduced, thereby reducing the conduction current of the IGBT in the bridge circuit.

4 Design of driver output stage

The driver output stage of the gate drive circuit is a typical design that uses two MOSFETs configured in a totem pole form, as shown in Figure 3. The gates of the two MOSFETs are driven by the same signal. When the signal is high, the N-channel MOSFET is turned on, and when the signal is low, the P-channel MOSFET is turned on, resulting in a configuration of two device push-pull outputs. The output stage of the MOSFET can have one or two outputs. Based on this, a solution for symmetrical or asymmetrical gate control with one or two gate resistors (on, off) can be realized.

Figure 3 Connection of _RG(on) /RG _(off)

5 Calculation of gate resistance

For low switching losses, no IGBT module oscillation, low diode reverse recovery peak current and maximum dv/dt limitation, the gate resistor must reflect the best switching characteristics. Usually in the application, the IGBT module with a large rated current will be driven with a smaller gate resistor; similarly, the IGBT module with a small rated current will require a larger gate resistor. In other words, the resistor value given in the IGBT data sheet must be optimized for each design. The IGBT data sheet specifies the gate resistor value. However, the optimal gate resistor value is generally between the value listed in the data sheet and twice it. The value specified in the IGBT data sheet is the minimum value. Under the specified conditions, twice the rated current can be safely turned off. In practice, due to differences in test circuits and individual application parameters, the gate resistor value in the IGBT data sheet is often not necessarily the optimal value. The approximate resistor value mentioned above (i.e. twice the data sheet value) can be used as a starting point for optimization to reduce the gate resistor value accordingly. The only way to determine the optimal value is to test and measure the final system. It is important to minimize the parasitic inductance in the application. This is necessary to keep the turn-off overvoltage of the IGBT within the specified range in the data sheet, especially in short-circuit conditions. The gate resistor determines the gate peak current IGM. Increasing the gate peak current will reduce the turn-on and turn-off times, as well as the switching losses. The maximum value of the gate peak current and the minimum value of the gate resistor are determined by the performance of the driver output stage.

6 Design, Layout, and Troubleshooting

In order to withstand the large loads that occur in the application, the gate resistor must meet certain performance requirements and have certain characteristics. Due to the large load on the gate resistor, it is recommended to use resistors in parallel. This will create a redundancy, and if one gate resistor is damaged, the system can temporarily operate, but the switching losses are large. Selecting the wrong gate resistor may cause problems and undesirable results. If the selected gate resistor value is too large, the losses will be too large and the gate resistor value should be reduced. Too high a gate resistor value may cause the IGBT to operate in linear mode for a long time during switching, eventually causing gate oscillation. However, in case the power dissipation and peak power capacity of the resistor are not enough, or non-surge protection resistors are used, the gate resistor will overheat or burn out. During operation, the gate resistor has to withstand continuous pulse currents, so the gate resistor must have a certain peak power capacity. Using a very small gate resistor will result in higher dv/dt or di/dt, but it may also cause EMI noise.

Too much inductance in the application (DC link) or too small a gate resistor will result in a larger di/dt and thus too large an IGBT voltage spike. Therefore, the inductance should be minimized or the gate resistor value should be increased. To reduce the voltage spike in the event of a short circuit, a soft turn-off circuit can be used, which turns off the IGBT more slowly. The distance between the gate resistor and the IGBT module should be as short as possible. If the connection between the gate resistor and the IGBT module is too long, a large inductance will be generated in the gate-emitter path. In combination with the input capacitance of the IGBT, this line inductance will form an LC oscillating circuit. This oscillation can be damped simply by shortening the connection or by using a gate resistor larger than the minimum gate resistor value RG ( _min) ≥ ^2√Lwire /Cies.

References

[1] http://www.semikron.com
[2] Application Manual Power Modules, SEMIKRON International
[3] M. Hermwille, "Gate Resistor – Principles and Applications", Application Note AN-7003, SEMIKRON International
[4] M. Hermwille, "Plug and Play IGBT Driver Cores for Converters", Power Electronics Europe Issue 2, pp. 10-12, 2006
[5] P. Bhosale, M. Hermwille, "Connection of Gate Drivers to IGBT and Controller", Application Note AN-7002, SEMIKRON nternational
[6] M. Hermwille, “IGBT Driver Calculation”, Application Note AN-7004, SEMIKRON International. ■