On Mon, 17 Feb 2003, Dmitriy A. Kiryashov wrote:

*>Hi Peter and guys.
*>
*>What is the most difficult part with PWM type of DC-AC with transformer
*>converter design? Either regular PWM or some sort of magic sine waves.
*>(minimal efforts to receive optimal output sine wave with minimal
*>distortions)

The load dump condition. Assume a load is attached and suddenly turns off,
f.ex. at voltage zero crossing. If the UPS reactive elements contain X
Joules and are not unity damped these X Joules will become voltage that
keeps rising until it finds a place to put some current through. The usual
PI filter, VDRs, spark arrestors, and fuses at the input of any circuit
connected to mains are there to prevent this, and sometimes succeed. Now
if you drive a transformer with relatively clean sine (class B or better)
there will be no load dump (well, a little) even if there is no feedback
loop since the transformer output impedance is hard-coupled to the driver
stage. The voltage will not rise (well, a little). If the transformer is
driven with a switch then even if there is feedback the energy in the
transformer will become a fearsome spike and thefeedback circuit can do
nothing to stop it. There are several ways to deal with this (like dynamic
crowbars etc) but they add complications to the high voltage side.

*>Negative feedback in addition thru say some opto couple to make sure that
*>output AC signal is sine and required voltage and frequency.
*>
*>Probably main problem is to drive high current into primary windings
*>but with on/off type control it will actually save you on heatsink
*>and will increase? converter efficience as well.
*>
*>Anybody was successfull with doing this type of design with PIC ?
*>(different than class B)

Not with PIC, with logic circuits (divide by 4 and gates, driving
00-nothing 01-one phase 10-nothing 11-other phase), after a scheme from a
book. The secondary had to be damped with a string of neon lamps to
prevent it from doing stupid things when not loaded. It had a capacitor
across it. The voltages that appear when undamped are quite enough to
break down the insulation of just about any transfomer and will arc over
5mm spark gaps easily.

Peter

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