The AM Forum

Hello everyone,
Recently built another class e rig with UCC35701-based pwm modulator but I'm having some issues with increasing current draw when warming up. Initially thought I had a bad doorknob cap, but turned out to be the modulator itself drifting.
During warmup it increases the carrier pwm duration.
When cold, setting the carrier to lowest setting gives a duration of 35% on and gradually increases to 50% and beyond, until distorsion sets in.
Verified both the voltage to the chip which is steady at 13.2v, and also the isolated gate driver voltage which also is steady at 13.2v.
Could it be so simple that I'm using bad capacitors and/or resistors on the pcb? Anyone else had this problem?
BTW, this is based on the PUWMA pwm modulator, can add a snip of the schematic if anyone interested
//SA2CLC

I use the same circuit and have never noticed that, most weird!

Can you swap out the chip?

IIRC there's only one cap and res (CT&RT) used for timing but still that's not your issue as is the % that's changing and that's set with a preset voltage on the VSCLAMP pin.

I'd monitor the preset voltage that sets your carrier level, if that's steady and the duty cycle still alters I'd change out the chip.

J.

Well, in hindsight everything is simple.
Turned out to be a leaky audio coupling capacitor feeding dc to the VSCLAMP. Replaced with a film cap, and now it works
//SA2CLC

That's good it wasn't the chip.  I've used that device for many years, and it's always been rock stable.

Not the cheapest chip in the world, either.

The puwma is a decent pwm.  I built one, quite a few moons ago


--Shane
KD6VXI