How to perform High-side switching of N-channel IGBT (STGW20NC60VD) if required gate-voltage is greater than the rated max gate voltage?

[Moh]

I am building an PV module I-V tracer that makes use of a capacitive load. In order to trace/record the current and voltage characteristics of the solar panel I am measuring the voltage and current that develops over the capacitive load as it charges from 0V to the open-circuit voltage(Voc) condition of the PV module. I am using a switching scheme to achieve this charging and discharging of the capacitive load using the STGW20NC60VD. The first switch is turned on to connect the PV module to the capacitive load. This connection is maintained until the capacitor charges to Voc where it is then discharged through a resistor by closing switch 2.

I am having a problem with switch 1. The configuration for switch 1 is such that the capacitive load is connected to the emitter of the IGBT, the source is connected to the PV rail and the gate is used to switch it on an off. I am aware that for a high-side configuration like this, that a voltage greater than the gate-threshold voltage + the voltage over the capacitive load, is required at the gate to turn on the IGBT. This voltage however will need to be Vge(th)+Voc, roughly 53V, which is much greater than the allowable gate voltage (+-20V). I have been struggling to find an alternative igbt with a maximum allowable gate voltage that will allow this voltage.

Please can someone assist me with how I can perform switching for this functionality? 

[Peter BENSCH]

Welcome, @Moh​, to the community

If you use an IGBT as a high-side switch and it is connected as you have described (a small circuit sketch would certainly be more suitable, hand-drawn is sufficient), then the emitter is at the level of the output voltage or capacitive load. In this case you need a gate voltage of Vge(th)+Voc to switch on the IGBT, but this is only the level with respect to GND (when the load is connected to GND).

Since the IGBT is not connected to GND and does not see any GND, it is only interested in the voltage difference between the gate and the emitter as Vge, which is / must be below the permissible limit.

The problem is rather different: how do you shift the control voltage Vge to the level of Voc?

Regards

/Peter