The switching characteristics of the freewheeling diode that protects the IGBT 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 in 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 in diC/dt in the IGBT module. By using a specially designed and optimized CAL (Controllable Axial Lifetime) diode with soft recovery function, the reverse peak current is small, so that the on-current of the IGBT in the bridge is small.
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 turns on. When the signal is low, the P-channel MOSFET turns on, producing a two-transistor push-pull output configuration. The output stage of the MOSFET can have one or two outputs. Depending on whether the output stage has one or two outputs, a solution for symmetrical or asymmetrical gate control with one or two gate resistors (on, off) can be implemented.
Figure 3 R G(on) /R G(off) connection
Calculation of gate resistance
For low switching losses, no IGBT module oscillation, low diode reverse recovery peak current and maximum dv/dt limit, the gate resistance must reflect the best switching characteristics. Usually in applications, IGBT modules with high current ratings will be driven with smaller gate resistors; similarly, IGBT modules with low current ratings will require larger gate resistances. In other words, the resistance values ​​given in the IGBT data sheet must be optimized for each design. The gate resistance value is specified in the IGBT data sheet. However, the optimum gate resistance value is typically between the values ​​listed in the IGBT data sheet and twice its value. The value specified in the IGBT data sheet is the minimum value; twice the rated current can be safely turned off under specified conditions. In practice, due to the difference between the test circuit and the various application parameters, the gate resistance value in the IGBT data table is often not necessarily the optimum value. The approximate resistance value mentioned above (ie twice the data table value) can be used as a starting point for optimization to reduce the gate resistance value accordingly. The only way to determine the ultimate optimal value is to test and measure the final system. It is important to minimize parasitic inductance in the application. This is necessary to keep the IGBT turn-off overvoltage within the specified range of the datasheet, especially in the event of a short circuit. The gate resistance determines the gate peak current IGM. Increasing the gate peak current will reduce turn-on and turn-off times as well as switching losses. The maximum value of the gate peak current and the minimum value of the gate resistance are determined by the performance of the driver output stage, respectively.
Design, layout, and troubleshooting
In order to withstand the large loads that occur in applications, 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 a parallel connection of resistors. This creates a redundancy, and if one of the gate resistors is damaged, the system can run temporarily, but the switching losses are large. Choosing the wrong gate resistance can cause problems and undesirable results. If the selected gate resistance value is too large, the loss will be too large and the gate resistance value should be reduced. The switching performance in the entire application should be borne in mind. Excessive gate resistance values ​​can cause the IGBT to operate in linear mode for long periods of time during switching, eventually causing gate oscillations. However, if the power dissipation and peak power capability of the resistor are insufficient, or if a non-surge resistor is used, the gate resistor will overheat or burn out. During operation, the gate resistor has to withstand a continuous pulse flow. Therefore, the gate resistor must have a certain peak power capability. Using very small gate resistors results in higher dv/dt or di/dt, but can also cause EMI noise.
Excessive inductance in the application (DC link) or the use of a small off-gate resistor will result in a larger di/dt, resulting in excessive IGBT voltage spikes. Therefore, the inductance should be minimized or the gate resistance of the shutdown gate should be increased. To reduce voltage spikes during short circuits, a soft-shutdown circuit can be used, which turns the IGBT off more slowly. The distance between the gate resistor circuit and the IGBT module should be as short as possible. If the wiring between the gate resistor and the IGBT module is too long, a large inductance will be generated on the gate-emitter channel. Combined with the input capacitance of the IGBT, the line inductance will form an oscillating LC circuit. This oscillation can be attenuated simply by shortening the wiring or by using a larger gate resistance than the minimum value (RG(min) ≥ 2 √ (Lwire/Cies).
Reference reading
<1> http://
<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 International
<6> M. Hermwille, "IGBT Driver Calculation", Application Note AN-7004, SEMIKRON International
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