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Posted on 14 February 2019

Protection and monitoring functions for IGBT and MOSFET modules

 

 

 

 

 

 

 

To protect MOSFET or IGBT modules in the event of malfunction/errors, the use of different efficient, quick-response protection functions in the driver is recommended; for example, overcurrent and short circuit protection, protection from excessive drain-source or collector-emitter voltage, gate overvoltage protection, overtemperature protection and monitoring of the gate control voltages VGG+ and VGG-.

Block diagram of a high-performance IGBT bridge arm driver circuit

Figure 1. Block diagram of a high-performance IGBT bridge arm driver circuit with TOP/BOTTOM interlock, protection, and monitoring functions

With reference to Figure 1, the integration of protective functions in the driver is explained below.

Overcurrent and short circuit protection

The current signal can be generated as an analogue signal (measured via e.g. shunt, current probe, RDS(on) of the driven power MOSFET or sense-source/sense-emitter cells) or as exceeding the maximum rating (desaturation of the IGBT). As soon as an error has been detected by comparing the actual value to a defined maximum rating, an error memory is set (ERROR status) either on switch potential already or - in the case of potential isolated sensors - in the primary circuit of the driver, which will block the power transistors until the RESET signal is triggered.

If the error memory is integrated on the secondary side, the error memory status signal will be transmitted to the primary side by a potential isolated unit. If potential isolating, high-precision current sensors are used (as for example in SKiiP), their output signal may serve as the actual value for control loops.

Gate overvoltage protection

In contrast to the protection functions described so far, the gate protection might have to limit the gate voltage periodically without the presence of an actual error that would require the power transistors to be turned off. For this reason, there is no connection to the error memory.

Protection from excessive drain-source or collector-emitter voltage

Voltage limitation at the main terminals of a power transistor can be performed by the transistor itself (avalanche-proof MOSFET), by passive networks or by an active circuit, which implements a defined partial turn-on of the transistor in case of overvoltage.

Optional basic protection - although this is not able to detect switching peak voltages and other fast overvoltage peaks - which is integrated into the SKiiP driver as static DC bus voltage monitoring is also available. A "quasi"-potential-isolated sensor will indicate the actual DC bus voltage value, transmit it to the main control circuit in the form of an analogue actual value and set the error memory to ERROR as soon as the limiting value is exceeded. In addition, a brake chopper buffer may be used to protect the DC link capacitors from active feedback loads.

Overtemperature protection

The temperature of the power transistor chips, the temperature around the chips, and the heatsink temperature can be determined using certain calculation methods. If the sensor is isolated, the temperature signal (e.g. voltage) may also be transmitted to a main control circuit. A threshold switch on the primary side will set the error memory to ERROR as soon as a limiting value is exceeded.

Supply undervoltage protection for the gate control voltages VGG+ and VGG-

If the gate control voltage drops considerably, the secondary control, protection, and transmission functions may fail. Moreover, this will also mean that the power transistors can no longer be fully controlled or blocked.

In order to detect this critical state in time, either one of the control voltages or the internal driver power supply has to be monitored. In the event of an error or malfunction, the error memory is set to ERROR.

 

For more information, please read:

Types of Faults for IGBT and MOSFET Modules

Fault Current Detection and Reduction in Converter Circuits

Overload and Short Circuit Behavior of IGBTs and MOSFETs

Overvoltage Limitation for Power Transistors

 

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