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In order to protect IGBT devices effectively during an Over-Current condition, one needs to focus on

two factors.

First, one needs to detect the type of Over-Current mode and shut the system down. In both Line-to-

Line and Shoot Through mode, the short-circuit current flows from and to the DC bus capacitors

(Figures 1 and 3). Therefore, a shunt resistor in the ground path can detect these Over-Current

conditions. However in the Ground Fault mode, the current flows from the AC line input, through the

positive DC bus and high side IGBT, to the earth ground (Figure 2). Therefore, the Ground Fault

condition has to be detected either with a shunt resistor on the positive bus line or by detecting the

current in the output lines.

Second, one needs to look at the AC drive architecture. Protection circuitry needs to be built in a manner

that does not disrupt the drive system. For instance, Figure 4 shows a typical Floating Ground Refer-

ence Architecture in which the micro-controller (

µC) is on the Floating Ground reference.Therefore, any

Over-Current detection circuit needs to be isolated. Alternate architecture may require reduced or no

isolation.

2-2 Traditional Methods of Over-Current Detection

Method 1

One can detect the Line-to-Line Short and Shoot Through currents by inserting a Hall Effect sensor

or a linear opto isolator across the shunt resistor. The device should be in series with the negative

DC bus line. For Ground Fault protection, an additional Hall Effect leakage current sensor could be

placed either on the AC line input or on the DC bus. The protection circuit is then implemented by

using fast comparators. The output of these comparators is ‘OR’d with the micro-controller or PWM

generator to initiate the shutdown of the gate signals.

Method 2

If located in the motor phase output, each Hall Effect sensor uses 2 comparators. This is done

because both positive and negative current polarities flow during a Line-to-Line Short condition.

Another important consideration is the total propagation delay for shutdown. The delay time associ-

ated with the optical isolators in the gate drive and the Hall Effect sensor is typically more than 2

microseconds. Therefore, regardless of protection circuitry implementation, this delay should be

added to the circuit delay, before matching it with the IGBT short circuit duration time. As shown in

Figure 4, the configuration requires 2 Hall Effect sensors and/or opto isolators in addition to the

protection circuit. The protection circuitry comprises of comparators (2), voltage references, capaci-

tors, and resistors.

Method 3

Another protection method is to use an IGBT de-saturation circuit. This discrete circuit can be con-

structed in the secondary side of the opto gate driver. (Opto isolated devices with in-built de-satura-

tion circuits can also be found in the market). This circuit detects voltage build-up across the collec-

tor and emitter while the device is fully on. If the voltage exceeds a specified limit, the associated

gate signal is shut off. A discrete circuit would require a comparator with voltage reference, a high

voltage diode, and various resistors and capacitors.