Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 29

resistors are connected between the driving signal and the gate of the IGBT. Moreover, zener
didoes are connected across the gate and the emitter of the IGBT to prevent excessive driving
voltage. If the driving signal does not offer an off bias with a negative voltage to speed up the
turn-off process, then ZD ge 2 is not needed. The selection of the turn-on and turn-off resistors is
determined by the ratings of the IGBT. Generally, R on > R off should be recommended to avoid

a false turn-on of the IGBT caused by the Miller effect. Because the collector of the IGBT
is connected to a high voltage, a resistor R ge is connected across the gate and the emitter of
the IGBT to avoid a false trigger caused by external high-voltage interference. This resistance
should not be too small. Otherwise the gate voltage would not be high enough to trigger the
IGBT and the peak voltage on the collector would also be high. Normally, this resistor is
placed as close as possible to the gate and the emitter of the IGBT with R ge =

10 k (FUJI

2004).
The ground of a driving circuit is connected to the emitter of the driven IGBT. Due to the
line leakage inductance, there is an induced voltage between the ground-connected point and
the emitter, especially in high-current applications because of the high di/dt during switching
(POWEREX 2000). Therefore, the ground point of the gate signal should be connected to
the emitter of the IGBT, as close as possible. Moreover, an off bias with a negative voltage
V EE should always be adopted. Figure 1.30(a) shows a circuit that is suitable for low-current
six-pack devices, in which the power switches are integrated with minimal inductance on the
negative bus and low di/dt. However, this circuit has a ground loop problem for high-current
applications because the ground of the driving circuits is far from the emitters of the switches.
Figure 1.30(b) shows a circuit with a common power supply for the driving circuits but with
separate capacitors for each power switch, where the emitter of the switch is connected to the
ground of the corresponding capacitor. This is suitable for modules rated up to 200 A. Figure
1.30(c) shows a circuit with isolated power supplies for switches with a common ground. This
is recommended for IGBT modules rated 300 A or more. Because isolated power supplies are
adopted, the ground loop problem is avoided.

1.3.2.4 Snubber Circuits

Power semiconductor switches are the main devices for an inverter and should be operated
under safe working conditions. Snubber circuits should be placed across these devices to
protect them and improve the system performance. The main functions of snubber circuits
include (Severns n.d.):

1. Shaping the load line to keep it within the safe operating area (SOA).
2. Reducing or eliminating voltage or current spikes.
3. Limiting di/dt and dv/dt.
4. Reducing total losses due to switching.
5. Reducing EMI by damping voltage and current ringing.
6. Transferring power dissipation from the switch to a resistor or a useful load.

Table 1.2 shows three typical snubber circuits, together with the main features, for individual
switches. Some snubber circuits can also be connected as close as possible between the
collector of the upper IGBT and the emitter of the ground-connected IGBT to form lump