- Jan 1, 1970
I fully agree with Mr. Hill's assessment regarding 120us being much slower
than desireable. Really though the situation looks even worse than that
In fact. After looking at the circuit I am led to believe that the circuit
would be an excellent way to celebrate the new year. That is, power it up
as the clock strikes twelve and it should make for an impressive fireworks
display to ring in the new year.
Move that resistor R1 from gate to ground to gate to source of the two
MOSFETs. I think this is a very critical and mandatory change. Imagine
what happens when you try to turn on the MOSFETs. The source rises up to
200V above ground potential. In order for the MOSFET to be on the gate
should be at 10V higher than the source. So in other words the gate is at
210V above ground potential when everything is on. But wait a minute. R1
is between gate and ground. So lessee here. 210V/22k = 9.5mA. Wait a
minute. The only source of current for the gate is through the
optoisolator. But Winfield Hill already determined with a CTR of around 20%
it will only conduct something like 3mA (under bad conditions, the 4N25 will
likely have somewhat better CTR in most cases). Hmm...
3mA < 9.5mA. That is, there is no way this is going to work right. The
drain cannot reach 200V or else the gate resistor will suck away too much
juice, and the gate will no longer keep rising even though bootstrapped.
The solution to this problem is quite simple. Just move the resistor from
gate to ground to gate to source of the MOSFETs.
Still though, the drive is much too weak and too stressful on the MOSFETs.
Maybe it works, but it could be much better without much extra trouble.
Using a much higher CTR optocoupler and using a much smaller value resistor
would be two easy ways to make things better.
I suggest getting a very high gain optocoupler (maybe a photodarlington, but
be weary of the extra voltage loss), and then using a gate-source pull down
resistor of something like 1k.
Have I been abandoned? Why no more replies? Have I said something
to offend anyone?