Hi, I have been trying to drive 50A 1us pulses through an LED using the circuit attached.

When the supply voltage to the LED is below 35V, the FET's drain voltage is able to be switched to ground quickly and the rise speed of the current increases with increasing supply voltage. However, when the supply voltage goes above 35V, for example 45V, the FET's drain voltage can only be switched to 10V (45V - 35V). So the extra voltage cannot be used to overcome the inductance but dropped across the FET instead.

Is it because the high initial drain voltage limits the turn-on speed of the FET? If so, how can I improve it? Thank you very much in advance.

 

What are the voltage levels of the trigger pulse?

 

What are the voltage levels of the trigger pulse?

The trigger is initially at 3.3V so the opamp outputs 0V. Then the trigger goes to 0v, the opamp turns the FET on. The trigger level can be adjusted to get an accurate current level.

 

And what is the FET and what is the op-amp and what are its power supplies?

 

And what is the FET and what is the op-amp and what are its power supplies?

The FET is PSMN8R2-80YS. I can't remember exactly the opamp model, but it has slew rate of 120V/us and output capability of 100mA. The supply is single 9V.

 

Do find out what the op-amp is.
The RDSon for that FET is specified at 10V so with only a 9V supply, and a volt of that lost across the current sense resistor and the op-amp output is unlikely to go all the way up to the positive supply, there may not be enough voltage to fully turn on the FET.

 

Do find out what the op-amp is.
The RDSon for that FET is specified at 10V so with only a 9V supply, and a volt of that lost across the current sense resistor and the op-amp output is unlikely to go all the way up to the positive supply, there may not be enough voltage to fully turn on the FET.

But the RDSons at 5V and 10V Vgs only have less than 5mohm difference, does that have a big effect?