# Constant current source with shunt fet

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:

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.

• I can't wrap my head around your fourth and fifth sentences: "If of 32 A" vs "current between 0.5 A and 1A" are a contradiction, right? – Marcus Müller Feb 22 '19 at 8:36
• It must be designed for 1A per string with the possibility of dimming to 0.5A – BobLee Feb 22 '19 at 8:45
• But where do you see the potential for oscillations? – Marcus Müller Feb 22 '19 at 8:49
• electronics.stackexchange.com/questions/313826/… Because of the Ciss – BobLee Feb 22 '19 at 8:58
• So, that's the circuit you're planning to use? Because: that thing oscillates at ca 5MHz, your pulses are 0.3 ms long, so I'm not sure that you'll ever see the full oscillation materialize; in the end, that'd depend on the driver. – Marcus Müller Feb 22 '19 at 9:01

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.

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

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.

• FYI: Answers that aren't answers are removed. Try to earn reputation so that you can comment. Breaking the rules will only hinder your ability to earn reputation (That's part of why reputation exists). – Bort Feb 22 '19 at 13:42
• Thank you for your comment. If I would do it this way I would lose the ability to limit the current "constantly" while it is demanded. I don't want to turn it on and off I just want it on during the duration of the pulse. So I want an analogue dimming instead of the PWM dimming. – BobLee Feb 22 '19 at 14:17
• @raaVGO Vc is used to set desired pulse current. tHIS IS BUFFERED THEN DIVIDED BY 8k2/3k3 to form a lower ref voltage. This is again buffered and used to control a constant current source withe the 1 Ohm sense resistor. The Vsense vltage for the current source is turned on only when Vpulse is high. – Russell McMahon Feb 22 '19 at 18:32
• @Raavgo; you do realise that at the physical level, all components are analog, right? (At least until you get to planck levels) – Peter Smith Mar 2 '19 at 15:24
• @PeterSmith of course you can break them down to analog components but what is the point of inventing the wheel over and over again? I can build a buck converter from scratch or I just buy one for 50 cents. The level of abstraction was invented to make development easier and faster. – Raavgo Mar 2 '19 at 15:56