Switching Power Supply Startup Circuit Problem?

Question

I'm repairing a Switching Power Supply. It has HIGH voltage DC circuit @ 320VDC. There are two "chopper" supplies. The 12VDC is the primary output. The 20VDC is for the chopper circuit, a TL494 chip. All semi test fine with Ohm meter. There is no Vcc on the TL494 when plugged in so the chopper never runs. I can "jumpstart" the TL494 by momentarily applying external 9VDC to Vcc and Grnd. Supply runs fine w/o battery after that until unplugged.
Brand: HANNY Model: FX-101
No schematic or tech support available. The problem is the initial startup. How does HANNY design the startup for their switching supplies? Is it a mini linear supply? A starting resistor? A starting cap? Self resonance? What do I look for? Checked power resistors, all in spec. What can I look for? Supply is only five months old, likely not dried caps. Ideas?

Answer 1

I'm not familiar with this particular supply, but no Vcc on the pulse width modulation chip would normally cause things to not run. There has to be a source of power to Vcc somewhere that is missing. The data sheet for the TL494 shows that Vcc (pin 12) should normally be 7 to 40 VDC. The total supply current is roughly 5 to 10 mA for this chip. It may be that it uses a voltage divider from the 320 VDC input to power it. Look for two resistors tied to the Vcc pin (or missing or burned out).

You might find something in the data sheet that matches up to your circuit as well...hard to diagnose without more info or seeing it myself.

Answer 2

Yep, the TL494 (as with any IC) is not going to function without Vcc and Grnd.

Vcc for this chip is min 7 volts and max is 41 Volts DC.

Typical is 15 Volts, so I would trace the Vcc pin for the TL494 back to it's source and see what's up.

Could be: No source voltage, bad feed-through, cracked clad, solder short, contamination...etc, etc.

Answer 3

Here's a wild idea. If the power supply is only 5 months old, how about returning it for a warranty repair?