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Re: Oscillator circuit for solid-state TC
Greetings all,
Thanks for the comments Harri - helpful as always.
>Uhum.. 11V rail? I figured all out like this: gate-source capasitance
>is charged through the resistor you have between gate and driver.
>Current flowing will be then (11V-Vgs)/R. When the cap has been
>charged the current will decrease exponentially. I thinks that's
>why most people go for a 15V drive. With irf740 Vgs of somewhere
>between 6V and 10V will start to have little effect. However,
>at high powers you certainly want to squeeze everything out and
>have the fet as conducting as possible. And quickly!
Yes I started with 12v but when a fet blew 140v came through
the gate, blew the driver chip, blew the other fet, sent the +12v rail
sky high blowing the PWM chip, the other driver chip, and the FETs
on the other side - then when everything had blown the fuse went :)
Now I isolate each driver chip from the 12v supply via a diode.
11v is less work for the driver chip but you're right I had only consider
that 11v is more than the max Vgs of 10v - I'll do some sums.
>And that's not expensive? Wow.. what if you blow a pair of fets,
>say $3-4 each. If blown fets take the driver with them it will
>add extra $12 there. That's why I like using low-cost pwm drivers
>with driver onboard. It keeps the costs to a mininum is does not
>cost that much to be replaced if something goes wrong.
The 3825 is L9 ($13) over here - dearer than the driver chips.
The new driver chip has survived a FET blowing without
taking the other FET with it. Though I haven't made any smoke
for a while. I smoked a lot of FETs trying to drive conventional
primaries.
>However, that does not solve the usual problem. I had lot of trouble
>fitting all the turns and insulators on the core winding area. I never
>managed to get all the turns I had wanted to. Feeding multiple
>transformers with secundaries in series will help. Of couse it helps
>to have bigger core becouse smaller number of primary turns will do.
>However, heating problems are more easily solved in multiple
>transformer solution.
Yes - it helps though, fewer turns means that you can use heavier wires.
I'm half way through winding a double transformer - still using silicone.
but I checked out epoxy resins - 10kv per mm and conduct heat 6 times
better.
I'm now using 16 of 0.4mm enameled wire twisted together for the primary's.
I reckoned that the skin depth at 100kHz was 0.2mm so there didn't seem to
be any point in going to thinner wire. Unless there's something I missed.
In theory the windings should have less than 1/2 the AC resistance of the
single transformer.
I considered a current transformer I was unsure because it would be
working with pulsed DC rather than AC. I'll see how the opto checks
out when I get time.
Have fun,
Alan Sharp (UK)