Applied to the driver stage of an IGBT inverter, this can be done by continually
measuring that the power supply has been turned off, i.e. 0V is present, while there is
also a measurement signal that indicates that the driver stage is out of operation. The
conceptual design of such a circuit is shown in
shows a specific
application, which is used in many driver stages with integrated under-voltage
monitoring (UVLO) as a functional block for "SafeStopping".
If the power supply of this type of driver stage has been turned off by such measures,
the actual supply voltage of the individual IGBT drivers falls below the UVLO threshold,
after a certain delay. The length of the delay depends on the total capacity of any buffer
capacitors present and the actual load of the individual drivers. As soon as the voltage
falls below theUVLO threshold, the driver actively switches off the output andwith it the
IGBT with the remaining energy (the voltage is still in the range of about 10V) and thus
blocks further control of the IGBT internally. A fault signal, i.e. the presence of a UVLO
detection, notifies the fault analyser (microcontroller) of this state
. This fault signal,
together with the knowledge of the actual value of the supply voltage, realises two
independent information channels to implement a "SafeStop".
Once the gate of the IGBT is disconnected from any voltage supply, it can no longer
switch on. However, a gate emitter pull down resistor is absolutely essential as any
transients in the DC-bus can push energy into the gate via the IGBT Miller capacity,
whichmight turn on the IGBT.
Conceptual implementation of "SafeStop"
Different microcontrollers are usually used for the safety function and the actual control of the semiconductors.
The conceptual circuit presented here does not show this.