Hi all
I am charging up a capacitor that has a very large internal resistance (its a gate of an IGBT/MOSFET) through an external resistor of about 1.5Ohms.
You can see the current and voltage across this resistor (top graph) and the voltage on the capacitor (lower graph) in untitled.jpg.
What I want to do is find a way of evaluating the charge on the capacitor after about 250-300ns. The internal resistance of the gate capacitor is high - about 2ohms and changes about 2mOhm/C. So using just the voltage across it isn't sufficient. I want to accurately measure the charge accumulated in the gate after 300ns at different temperatures, which will roughly be dependent on this change in internal resistance as all other parameters are stable in comparison.
What I have thought of doing, is integrating the voltage seen across the gate resistor. Obviously the proper way to measure charge is to know the current and time, correct? But this voltage will be proportional to the current flowing into the gate.
This is done with 2 opamps - 1 differential amplifier with gain 1, and then an integrator. I have attached an LTSpice pic. Obviously I do not get the charge on the capacitor, but I do get a voltage at the output of the integrator that appears to be proportional to the charge on the capacitor, at least until the opamp saturates.
I attach the circuit and waveforms, does anyone have any comments for improvements and whether this is OK? My main questions are whether its OK to have an opamp swinging from +ve saturation to -ve saturation constantly, or whether I need another component to periodically discharge the capacitor in the integrator opamp?
I am also concerned about when I put this output voltage into an ADC. Its rising almost 30V in 300-500ns. I am worried about the sample and hold time óf the ADC. If there are small fluctuations then the reading could be invalidated. If I increase the integration capacitance it slows the rise time but I get decreased sensitivity at the output - and my goal is measuring the variation of charge with temperature.
Lastly, does anyone have any other ideas to do this? Other ways would be actually mirroring the current through the gate resistor into an external capacitor of similar size to the gate and measuring the voltage on this. But when I try to simulate this (putting transistor in the current path with a current mirror) it severly effects the switching of the MOSFET. Or perhaps you could mirror the current with a gain of 0.1x into a capacitor 1/10th of the size to reduce the power consumption of the measurement? I dont know how you would do this.
I hope Ive explained everything well.
Any help appreciated