- Why would the drain to source current of MOSFETS suddenly increases.
- What can be done to resolve it?
I am aware that there is supposed to be an increase in current as soon as the gate voltage of the MOSFET reaches the threshold voltage, but I would like to determine whether the behavior of my electronic load is typical because the increase seems too sudden given my minimal increase in voltage.
Introduction to setup and circuit
I am building an electronic load that uses three parallel IRF640 to control the current. It is for testing our solar panel strings so the voltage applied to it is approximately 100V at open circuit and it will receive a current of at most 2.5A. This is within the maximum values of the MOSFET. I have addressed the maximum power problem/heating of the MOSFETS by adding 20 ohms 400 Watts in series to the drain of the MOSFET, so it does not heat up by a noticeable amount at any point. The MOSFETS are also attached to a heat sink to minimize heating further.
Here is my circuit (I'm unable to add images directly)
The circuit behaves as expected at around 40Vds. As I slowly increase the voltage applied to the gates of the MOSFETs, the drains to source current steadily increases.
The problem occurs whenever I increase the voltage across the drain to source. Initially the current would increase as expected in earlier tests, but as I reach 4.1V or somewhere around that number, the drain to source current suddenly jumps from a mere 0.7A to 3A (maximum current of supply).
Here's an example of my tests at 80v Vds
Vgs -- Ids
0 ---- 0
1V ---- 0A
3V ---- 0A
3.2V -- 0.1A
3.4V -- 0.2A
3.6V -- 0.3A
3.8V -- 0.5A
4.0V -- 0.7A
4.1V -- 3A <--- As you can see here. It suddenly jumps up at the tiny increase in gate voltage