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Re: Oscillator circuit for solid-state TC
Greetings
>> Now that I'm working at 100kHz I may go back to the 3524 for
>> that side too.
>..or perhaps not. 3524 has a very low output current rating. It means
>the mosfets will be charged and discharged slowly. You may also
>experience some of the stuff I did: parasitic drain source capasitance
>did cause spurious turn-ons ie. the driver was at low state and
>still during the switching fets did turn on! It helped a lot when
>I moved to a system where gate had -3.3V when both fets should have
>been off.
Yes I should have explained that the driver board I've put together uses
dedicated mosfet driver chips - tc4422cpa. These have an output
current capability of 9A. easily driving 4 fets each. I use a 10 ohm
resitor to each gate and a pair of schottkey diodes to clamp the
chip output to the 0v, 11v rails. They can be switched from CMOS
so the output capability of the pwm chip is not an issue. They aren't
that expensive, 4 quid - thats 6 dollars for those without real money :)
The plan is to drive 2 or more of these boards from the PWM. Not just
to switch more power but also to use shorter more intense bursts -
higher peak power - to move towards spark gap performance :)
Because I use a two board design it's fairly easy to swap the PWM
boards. I'm going to keep my 500w system intact and not canibalise
it. I've got an earlier PWM board with a 3524 on it, I was going to
reuse it and see how it goes. It also used buffer chips and was
abandoned for the wrong reasons.
I've redesigned the current limiting around an opto-isolator accross
a resistor on the high side. I've bought three non-inductive
resistors (0.22ohm film in a to220 case) - more expensive than
the driver chips!!
Rsense------47ohm----47ohm-- LED---Rsense
between the resistors
is a 3.9v 1W zener (protection) to Rsense bottom.
47ohm resistors prevent overload. The LED is
part of the H11N2, the other side is a schmit
trigger - 300ns max response time. Should come
on at about 10A per 0.22ohm resistor. I may need
a voltage divider as well.
The schmit will also drive an indicator LED via a monostable.
There will be a blanking period to allow for the initial
switch on surge.
It might even work - unless someone knows better.
On the output transformer I've decided to use 2 ec70 cores
side by side. I've taken 2 plastic bobbins and saw off
all the plastic on one side of each, leaving 2 blank
plastic faces, hot glued them together to give a fairly
solid double bobbin - holding the cores less than
1/8" apart. (it is the joined pairs that are slightly
apart - there is no core gap)
ie =IOI= + =IOI= gives =IOIIO=
A triple or quad would also be possible - if I get
ambitious.
have fun
Alan Sharp (UK).