# High Impedance Load Driving with Mosfet as Common Source Configuration

I am just trying a design ac switch with mosfets which are connected as common source. Mosfets will drive 16.2A load in normal conditions. When I try to drive 10 ohm load with 162V (peak, 400hz), as you can see in picture 1, there is no problem in load voltage. ( Green wave : Load Voltage, Red wave : Gate-Source Voltage)

I just try to increase load to 100k, I saw the almost 162V on load without gate firing. What is the reason of this ? Leakage current, drain-source parasitic capacitance etc.. and How can we handle this problem ? Is this really problem ? In addition, frequency is affecting the load voltage in off-state. I see almost nothing on load when I used 162VDC source in off-state of mosfets. It just happens with AC sources.

By the way, I saw the same thing on real circuit, not just a simulation. I am waiting your kind comments, Thank you,

$$\I_{DSS}\$$ is the drain source leakage current when the gate is tied to the source and, for the IXFX90N30 at 25 degC this is 100 uA. Of course that 100 uA will appear as an AC signal but, assuming the peak is 100 uA, into 100 kohm it produces a voltage peak of 10 volts. This is part of the story.

Then the bigger part of the story is drain source capacitance ($$\C_{OSS}\$$). This is quoted as 1800 pF. There is also $$\C_{RSS}\$$ and this is quoted as 700 pF and, because you have gate and source connected by a voltage source both capacitors are regarded as in parallel. At 400 Hz, this reactance of these capacitors is 159 kohm.

So, this reactance and the 100 kohm form a potential divider that attenuate 5.5 dB at 400 Hz. This is a reduction of the input voltage from 162 volts to 86 volts. It looks like your peak voltage is more like 140 volts so there is a discrepancy but your model in your simulation may not be accurately reflecting what the data sheet says. Please check your model.

Is this really problem ?

It depends on what you are trying to achieve. If you are wanting to switch a lamp on and off then it's not a problem. If you want to control the voltage to a high impedance load then it is a problem.