An IGBT (Insulated-Gate Bipolar Transistor) has a MOS (Metal-Oxide Semiconductor) gate structure, and in order to charge and discharge this gate while switching, gate current (drive current) must flow. These gate charge dynamic input characteristics depict the electric load required to drive the IGBT and are used to compute variables such as average drive voltage and driving electric power.
A drive circuit, in principle, has a forward bias power supply that alternately switches the upper and lower arm IGBT in a half bridge topology . The current used to charge and discharge the gate during this switching is known as the driven current.
The estimated calculation for the drive current peak value or IGP is:
For different IGBT, the internal gate resistance Rg varies. You have to refer to the respective IGBT’s datasheet for the internal gate resistance Rg values.
The average value of the drive current IG, on the other hand, can be determined using the gate charge characteristics as follows:
Consequently, it’s critical to configure the drive circuit’s output stage in order to conduct this approximate current flow (IGP and IG). Furthermore, assuming the drive circuit’s power dissipation loss is totally consumed by the gate resistance, the drive power (Pd) required to drive the IGBT is shown in the following formula:
As a result, a gate resistance capable of charging this estimated capacity is required. Make certain that the drive circuit is designed in such a way that the above-mentioned drive current and drive power are supplied properly.
Setting an On-Off Timing Delay
In order to prevent short circuits in inverter circuits and other similar circuits, an on-off timing “delay” (dead time) must be set. The upper and lower arms are both “off” during the dead time. In general, the dead time should be set to be longer than the IGBT switching time (toff max.).
For example, increasing RG increases switching time, hence dead time must be raised as well. Other driving circumstances and temperature variables must also be taken into account. Short dead durations should be avoided since the heat generated by the short circuit current may damage the IGBT module in case a short circuit issue will take place in the upper or lower arms.
As a result, for IGBT power modules, a dead time of greater than 3usec is advised. Appropriate dead time, on the other hand, should be determined by the validation of a practical machine.
Short Circuit (Overcurrent) Protection
In the event of a short circuit, the collector current of the IGBT rises first, and then the C-E voltage spikes. Depending on the device’s properties, the collector current can be kept at or below a particular level during the short-circuit, but the IGBT will still be subjected to a strong load, that is high current and high voltage.
As a result, this problem must be addressed as quickly as feasible. The amount of time permitted between the commencement of a short circuit and the current being shut off, on the other hand, is restricted by the IGBT power module’s short circuit withstand capability or the time it takes for the short-circuit current to start until the module is destroyed.
As a result, when the IGBT is short-circuited, a significant current is required to be cut off within the IGBT’s short circuit withstand capabilities. The withstand capability is affected by the gate to emitter voltage VGE, collector to emitter voltage VCE,, and/or the junction temperature Tj.
In general, the greater the junction temperature and the higher the supply voltage, the lower the withstand capability get.
Short-Circuit Modes And Causes
| Short Circuit Mode | Cause |
| Arm short circuit | Transistor or diode destruction Faulty control/drive circuit or noise induce malfunction |
| Short in output circuit | Miswiring or dielectric breakdown of load |
| Ground fault | Miswiring or dielectric breakdown of load |
Short-Circuit (Overcurrent) Detection
As previously indicated, when there’s a short-circuit, the IGBT must be turned off as soon as feasible. As a result, the period between overcurrent detection and complete circuit shut-down in each circuit must be as short as possible.
Because the IGBT shuts down very quickly, if the overcurrent is turned off with an ordinary drive signal, the collector-emitter voltage will grow owing to the inductive kick, and the IGBT may be killed by overvoltage (RBSOA destructions). As a result, it is recommended that while cutting off the overcurrent, the IGBT be switched off carefully (Soft turn-off).
Overvoltage Protection
The current change rate (di/dt) is quite high at turn-off or during FWD reverse recovery due to the high switching speed of the IGBT module. As a result, the circuit wire inductance to the module can create a large turn-off surge voltage (V=L(di/dt)).
As an example, using the IGBT’s waveform at turn-off, we will introduce the causes and methods of their suppression, as well as illustrate a concrete example of a circuit (using an IGBT and FWD together).
Overvoltage Suppression Methods
The following are many strategies for suppressing turn-off surge voltage, which causes overvoltage:
a. Add a protection circuit (snubber circuit) to the IGBT to manage the surge voltage. In order to bypass high frequency surge currents, use a film capacitor in the snubber circuit, ideally as close to the IGBT as possible.
b. Reduce the di/dt value by adjusting the IGBT driving circuit – VGE or RG.
c. To lower the effective inductance of the wiring, place the electrolytic capacitor as close to the IGBT as possible. Make use of a low impedance capacitor.
d. Use thicker and shorter wires to reduce inductance in the main and snubber circuit wiring.Using laminated copper bars in the wiring is also highly effective.