Follow-up of Using IR21531 as a simple 1 channel gate driver

After doing the above question (I'm very sorry, beacuse my question seems a XY one), I'll try to explain better what I believe that I want (current circuit below):

I'm doing a space-saving design for a LED driver. On it, I only have a 78V PSU and a 3.3V PSU.

The module drive with PWM a LED strip, but the control signal will be of 3.3V and the MOSFET needs more than 10V to trigger. How can I get let's say 15V for driving the gate?

I've seen some vague words on some electronics forums saying that I could use a zener diode. How could I use the zener diode?

I'm open to suggestions. Also I could use a optoisolated transistor along with the zener.

Update: The below circuit seems to be well done, but with more than 50% of PWM duty cycle, the lamp starts blinking and fading as if it have a capacitor.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ That circuit won't work (more later) but what are the LEDs i.e. what spec (spec.pdf data sheet and no links to ebay etc.) are they and, are you aware that forcing a fixed voltage across 1 or more LEDs in series is a recipe for disaster unless there is a current limiting mechanism. \$\endgroup\$ – Andy aka Dec 18 '15 at 13:29
  • \$\begingroup\$ @Andyaka: The V1 supply is a constant current power supply at 300mA \$\endgroup\$ – Helio Dec 18 '15 at 13:40
  • \$\begingroup\$ Then don't confuse folk by showing it as a constant voltage. Nobody here is a mind reader. \$\endgroup\$ – Andy aka Dec 18 '15 at 13:42
  • \$\begingroup\$ @Andyaka: What should be? i.stack.imgur.com/t3iai.png \$\endgroup\$ – Helio Dec 18 '15 at 13:48
  • \$\begingroup\$ The current source is back to front but I'd just add a note saying it's current limited to 300 mA. \$\endgroup\$ – Andy aka Dec 18 '15 at 13:51

This answer was made before the OP decided to scrap his original circuit (following what I'd said) thus rendering part of my answer obsolete. The OP should consider that evolving his questions following advice in answers is problematic. Regards the new circuit drawn, it is full of mistakes and won't work effectively when those mistakes are corrected. Note to the OP - if you persist in evolving your question, don't expect any more help from me. Neither should you take it on yourself to edit my answer.

Original answer: -

Firstly, if the power supply can produce 200V then the IRF630 is a terrible choice - it's maximum voltage rating is only 200V and you need clearance - I'd go for a MOSFET with 300V ratings.

Secondly, the solution is best addressed by putting the MOSFET at the bottom of the chain of LEDs. Now its source connects to ground and driving from a 3V3 signal is easy except that the IRF630 won't be great at turning on with a 3V3 signal. It might work but, given its inadequate voltage rating I'd look for a better device. Here's what I'm thinking: -


simulate this circuit – Schematic created using CircuitLab

So try looking at Fairchild (for example) to find a suitable MOSFET

  • \$\begingroup\$ I've removed the 200V part because it was for simplifying, it works at 78V. \$\endgroup\$ – Helio Dec 18 '15 at 14:16
  • \$\begingroup\$ When I switch the gate directly at 3.3V, the leds are dimming well, but with a very low brightness. \$\endgroup\$ – Helio Dec 18 '15 at 14:17
  • \$\begingroup\$ Could you kindly read the update of my question? \$\endgroup\$ – Helio Dec 18 '15 at 14:17
  • \$\begingroup\$ The zener and opto transistor are backwards. When that is fixed the mosfet will overheat. I think you need to start thinking about not evolving this question any more. \$\endgroup\$ – Andy aka Dec 18 '15 at 16:23
  • \$\begingroup\$ @Andy: What does "o/c" mean? Also, which mosfet would you use for M1? \$\endgroup\$ – zeffur Dec 18 '15 at 16:54

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