On Fri, 19 Jul 2002, Roman Black wrote:

>Richards, Justin P wrote:
>>
>> Yes, you are correct.  I had not considered that current was going to
>> flow in the opposite direction.  The drawing I made sort of obscured
>> this.  I can think of a way to eliminate this but then the uneven demand
>> placed on the batteries would be increased and as suggested present more
>> problems than it has solved.
>
>
>You suggested using 400v fets, that won't work.
>You have +340v dc and -340v dc supplies, essentially,
>switching into a +/- 340v peak wave. The max fet volts
>will be +340v to -340v. You need *at least* 680v
>insulated fets.

That's the second reason why you use a buck regulator followed by a H
switch polarity inverter.

>Now look at typical HV switcher, like a TV h-output
>stage. Normally about 110v dc regulated input, at
>about 100W or about 1A. This will use a 1500v 6A
>MINIMUM rated switching transistor. To switch a
>100v rail! And they are still somewhat unreliable,
>even with snubbers and damping all over the place
>keeping the max volts down. Continually switching
>large voltages is very hard on semis. It will be
>cheaper and more reliable on the semis to switch
>100A at 36v than to switch 10A at over 300 volts.

They are somewhat reliable, and I say this on the grounds that the vast
majority of TVs out there did NOT visit your shop(s) within their useful
life. The high specs on the LOPT driver are due to the way you need to
drive the H deflection yoke (it has to be half-resonant or else you do not
get the required current to deflect the beam in a 120 degree crt).

I could never understand why they used high frequency H deflection and low
frequency V deflection. The other way around would save 25% or more
energy, cost, etc. (the H:V ratio is 4:3 in TV).

Peter

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