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Isolated DC-DC convertor

Posted: Tue Oct 20, 2015 7:54 pm
by Jan Lichtenbelt
I need to control a MOSFET. But the MOSFET must be separated powered. There are commercial isolated DC-DC convertors available, like the e.g. SN6501 see figure. But why using this IC when a microchip is available?
with Vin=5V
with Vin=5V
SN6501.jpg (13.3 KiB) Viewed 6368 times
Switching the gate of the MOSFET by means of a transformer, like in the figure, seems a bad idea to me. That will not be fast enough. But the microchip can countinously produce a 50% PWM which can be transformed to the 5V needed for the gate. The switching of the gate can be done by means of an optocoupler, powered by this 5V (at the collector of the optocoupler). The current will be extremely low.

My question is: should it be possible to use the PWM as source for the transformer?

With kind regards

Jan Lichtenbelt

Re: Isolated DC-DC convertor

Posted: Tue Oct 20, 2015 9:17 pm
by Steve001
HI jan,

In short yes you can not sure why you want to do this when there are plenty of pwm ic around and also dc dc convertor "bricks" , that save on the programming and other circuits that are required that do it all in a single module or an 8 pin ic (mostly)

I have had a go making my own dc dc convertor a couple of times and found it to be a minefield. I would recommend a dc dc brick type from traco power they are very good devices and the hard work is done for you just drop it in and away you go

attached is some guidance notes on switch mode psu's that may be of some use

if you look on ON SEMICONDUCTOR web site they have lots of reference designs for switch mode psu's


Re: Isolated DC-DC convertor

Posted: Wed Oct 21, 2015 2:34 pm
by Jan Lichtenbelt
Hi Steve,

Your e-mail activated me to look more in detail what is possible. On internet one can find two solutions which can switch a MOSFET powered separately and with no additional power for the gate:
1) optical MOSFET driver
2) optical MOSFET
photo MOSFET.jpg
photo MOSFET.jpg (7.92 KiB) Viewed 6330 times
The last one has off coarse my prefference. But for 100A peak current there is only one available SOM04200, which is rather large (5 cm) and slow. The driver versions are small and much faster. Up to now I found the APV1121 ( I will try this one. I will keep you informed.

Kind regards

Jan Lichtenbelt