I would only take this route if you're very comfortable with circuit analysis, not because it's particularly dangerous, but because of the value of the load.
Assuming no fets in series, if you have 3 leds in a string, and 85V source, each diode will drop ~28.33V. (85V divided by 3). Naturally if you connected just that, the string would draw more than 350mA.
Putting n-mos mosfet's in series with the chain (connected to current mirror) is adding a 4th voltage, Vds (voltage across drain-source). In order to operate the leds at 26.7V, the mosfet's Vds will have to drop the remaining voltage.
3* 26.7V = 80.1V.
Vds must be the difference:
85V - 80.1V =
4.9V
the power lost by the fet in this state = 4.9V * 0.350A = 1.715W
Assuming the voltage drop across the leds in the chain goes down from 80.1V to 75V (very drastic voltage droop. Sort of a worst case scenario).
Vds must be:
85 - 75V = 10V
and the power lost by fet = 10V * 0.350A = 3.5W
In order for Vds to be as low as possible (and thus wasted power from fet lowest), the natural current draw without the fets should be lower than your current mirrors reference current. So if you want 350mA, you'd have to design the strings so they naturally pulled 350mA on their own, then limited it at say 370mA.
You should try LTSpice. It's free and pretty good at doing simulations (for sanity checks on your math and component selection). Btw, my latest design has transformers to the mains for safety. I was just designing the supply without it.
Hi churchhaze,
I am by no way good in electricity and circuit analysing, hence I just hope to understand a bit what you did.
Imagining I want to power 10 strings at 350mA with a driver that give 3,5A it would know be possible because you claim that this mofset will regulate the voltage in each line.
So Let say I have 3 vero 10 on each string, making 30vero in total with only one driver at 3,5A and 85V (has to be >3x26,7V)
If I understand well, I will put a mofset on each string, I choose it so that it is just a bit upper than 26,7V. It will consume almost nothing as long as the voltage stay under 26,7V and if the voltage want to go upper than 26,7V it will transform the voltage drop in heat.
But then you try to take ac current directly? For the sake of simplicity I would rather just buy a big ass driver from meanwell and power plenty of led as Mrflux did. But protected with your way...
Tell me if it sound possible or if i didn't understand anything.
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