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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Recently my friend gave me his mini fridge to fix it for him. When i took it apart i noticed it is a peltier and a smps that outputs 12v at 6 amps.
After exploding the voltage doubler and a couple fuses and an NTC resistor, i charged the 12v filter cap, and it started running (i also used a large DC motor as a load). After further inspection i noticed that there is a diode to supply 12v from the 12v rail to Vcc on the TL494. I do not really understand this board too well. It uses a halfbridge, but im not so sure how the gates are driven. This PSU seems to work the same way an ATX PSU does. It has a ferrite transformer for the 12v output, and then a smaller one which seems to do all kinds of things. It must be a GDT, but i don't understand how it is driven. It seems that it is driven by the bridge, yet also provides gate drive.
Does anyone know what normally provides the voltage to pull up Vcc? This SMPSs tl494, and all other low voltage components are galvanically isolated from mains, so Vcc is not charged with a resistor from mains. Im thinking this smaller transformer has something to do with it, but it does not oscillate until i charge up the 12v supply. Ideas?
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
There is one other approach I've come across several times. This is a capacitor from mains to bootstrap the power supply. It feeds some current to charge up the supply rail until the oscillator starts and the output comes up which then takes over powering the controller.
My suggestion is draw a schematic of the whole power supply. It will be much easier to figure out how it works then, and you will probably learn a few things.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Some power supplies have a small auxiliary Switch mode PSU using a small surface mount ic that contains just about everything needed, typically in a four to eight lead package.
With designs that use a power factor correction pre-converter they sometimes place, an auxiliary winding on the boost inductor.
On small PSUs a couple of resistors from the positive of the DC bus are typical, but you say there aren't any with your design.
But your best bet is just to work your way through the circuit starting from the mains input side.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
James wrote ...
There is one other approach I've come across several times. This is a capacitor from mains to bootstrap the power supply. It feeds some current to charge up the supply rail until the oscillator starts and the output comes up which then takes over powering the controller.
My suggestion is draw a schematic of the whole power supply. It will be much easier to figure out how it works then, and you will probably learn a few things.
That is what i wanted to do, but that would require pulling the whole 12v supply up to 12v, which includes the peltier which is a 6 amp load. And then, whenever the fridge was cool enough, the 12v supply would be turned off...
If i cannot figure it out i will just use some smps from a computer and use an internal op amp in the tl494 to control the temperature.
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Avanti Mini fridge PSU.
