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Reflected Power
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From: Harri Suomalainen [SMTP:haba-at-cc.hut.fi]
Sent: Tuesday, February 03, 1998 1:51 PM
To: tesla-at-pupman-dot-com
Subject: Re: Reflected Power
>>Directional coupler and pulse-width/rate modulation. The
directional
Directional couplers for "low frequency" range are not fun to design.
Tent to get fairly high size etc.
>coupler
>I had great fun with driving a convenional primary/ secondary
arrangement
>with a flyback. Lots of blown FETs due to overvoltage - from the
reflected
>amplified voltage. The other problem is that the current going
through the
>FETs can be double the meter reading as reflected power goes via the
>reverse diodes into the power supply caps and is recycled.
OK, you're talking about flyback still I assume. Solution is simple:
if the energy
stored in the coil is *not* taken by the load (coil) it will be
returned usually by
a clamp winding back to the caps. If there is too much current
recycled you
definately are storing too much energy in the transformer. Reduce the
energy
by altering the gap.
Then there is the other case too: when fet conducts it acts like a
forward mode
SMPS. Reducing duty cycle for the forword mode is no good. Therefore
it is
probably best to run at near 50% duty cycle and make the transformer
store
suitable amount of energy for the flyback mode like time (time when
fet is not
conducting).
You could look into some combined flyback/forward converter examples
in
literature for more info. You might also be interested to look into
class-E
SMPS designs. Class-E case happens to be very close to driving a coil
in "flyback".
>3) use a halfbridge or a full bridge to drive the primary with
reversed
>output diodes to clamp the output to the supply rails.
I'd stick to those too. High-power flyback design is just too big.
>I'm not sure about transorbs - I think these can only take a limited
>number of hits before they blow.
I think transorbs are more like zeners, not destroyed eventially by
pulses
like MOVs are.
--
Harri Suomalainen mailto:haba-at-cc.hut.fi
We have phone numbers, why would we need IP-numbers? - a person in a
bus