Electronic – Constant current source with shunt fet

constant-currentcurrent-sourceled-drivermosfetshunt

I'm want to create a project with a LED strobe with IR LEDs.

As LEDs the Osram SFH 4249-UV will be used in strings of 15 LEDs and 32 strings parallel.

This results in a Vf of 34.5 – 45 V and a If of 32 A in an 0.3 ms flash.

The pulse frequency will be between 1 and 100 Hz and the current is most likely to be between 0.5 and 1 A.

Due to the fact that I'm not experienced with current sources created with shunt fets I read there is a high possibility of oscillation. I'm looking into using the CPH6442-TL-W from ON semiconductor since this one is capable of a drain current > 1A with Vds of 50V at 1 ms.

  • How can I prevent oscillation and where do I have to look out for selecting a opamp?
  • how does a fet shunt current source work?
  • Does is switch really fast on and off or does it increase the Rds?

I was thinking of the following circuitry:

schematic

Update: thank you all for the ideas and recommendations. I have found a BUK6D43-60EX mosfet that would be sufficient but when I calculate the maximum power I get with a thermal impedance of 6 K/W a junction temperature of 135°C at a ambient temperature of 60°C (T-junction = T-ambient + R-ja * (Vcc-Vled-Vsense) *If). This seems really insufficient and generating a lot of heat. I know I can switch te mosfet for a BJT or IGBT but I'm not so familiar with these components other than I know a BJT would have a much higher error since the base current also goes though the sense resistor.

Best Answer

I can't comment on your post (I don't have enough reputation) but I can't answer you question either because I don't really understand what u are trying to achieve.

If your whole purpose is to create an LED strobe I would not build it that way at all. You got 32 Amps and your 3V3 controlling circuit seems to be directly connected to your working circuit. You should always make sure ur controlling circuit is galvanically isolated from any heavy electronic load.

Instead of MOSFETS you should use optocouplers I would even use 1 optocoupler for every String of LEDs they cost like 3 Bucks and are even cheaper if you buy them in bulk.

After you got that part you can pulse the input of the optocouper with about 3 - 20mA (depending on the optocoupler) using an uC or if you want to build a pulse generator from scratch you can just connect the output of the pulsegenerator to the input of the Optocoupler. If you want to dimm LEDs I would just use PWM tbh.

EDIT Okay I get it you want to dimm it analogish you can always smoothen the pwm Signal using a capacitor. That way you could limit the Voltage and if you use a poti as Protective resistor you can adjust your max current aswell just keep in mind you need a 2nd Resistor serial to the Poti so you won't be able to go near 0 Ohm.

Concept Sketch

This is just a concept and I didn't calculate the values at all.

oscilloscope If you are really into OPAMPS and analog stuff you can go with that schematic of yours. But tbh I was so happy when I graduated that I never have to touch a freakin OPAMP any more. You can always use digital components.