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Switching power supply

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Hi all,

I thought about building a power supply for a thermoelectric (peltier) cooler element, which maximum power is 122W @ 15,7V (~7,8A).
I have a toroidal transformer whose ratings are 300VA and 2x35VAC, so dropping the voltage from about 40VDC (or something, I don't know the  exact voltage which comes from the transformer after rectifying&filtering) to around 12-14 volts would most propably produce tremendous amount of heat to the linear pass transistor(s)...  (40V-13V) * 6,5A = 175,5 W of heat if i calculate correctly. That could make good heater though :)

So I started planning a switching power supply. TL494 might be good choice as a controller IC, so i downloaded the datasheet and started designing with EAGLE...

Only problem is that the circuit presented in the datasheet has output voltage of +5VDC and current capability of 200mA.
I understand that the output voltage and current are both "sensed" by the TL494's on-board operational amplifiers, which control the PWM "chopping" process... changing resistor values might do the trick!

I think that by changing the resistor values, switching transistor, output coil, capacitors and the current sense resistor would solve my problem.

Would someone give me some advice ???

TL494's page: http://www.onsemi.com/site/products/summary/0,4450,TL494,00.html


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Welcome to this community.

When rectifying and filtering 32VAC you end up with about 50VDC and this is to much for the TL494.

Ante ::)

Thank you very much! :)

Yes, i was aware that after rectifying and filtering the voltage might be too high for the IC to handle. However I found this picture in the datasheet, which is the solution:


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Then I would change the TIP 32A for a MJ 4502 or similar. The inductor and diode should be higher rated (10A+) and the voltage and current feedback resistors changed to give you the desired output. A bit larger caps is necessary and remember heavy wiring for the input / output path.

Ante ::)

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I made the circuit with Eagle and modified it a bit.

  • I have some MJ15004 transistors, so I'll propably use one of those
  • Diode is ultrafast type, rated at 200V/8A
  • Most propably I'll take the inductor from an old PC power supply

I only hope that the Hfe of MJ15004 is high enough (about 25)... maybe I should try darlington connection?

If you notice any errors, please tell me and I'll try to fix ;)


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OK, time to flood the forum again with my pictures ;D

Well, actually it's the same circuit, with a soft-start feature that should limit inrush current when starting. The board measures about 10,5 x 7,5 cm, and I designed it with the soft-start components in place.

If I ever get this thing to work, could this be added to the "projects" -section....? ::)
If you will take just one more Powersupply circuit! ;)


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Hello vainajala

I encourage you to finish it and then i will surrelly add it under the project section with your name and e-mail. You have done a great work so far. When you finish it contact me using the contact page and it will be shown under projects section right away!


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If you can fit a heatsink on the TO3 your home free.

Ante ::)

Heatsinking of the MJ15004 (in TO-3) is a problem, should I do a mofidied version with, for example, parallel-connected two or three BD912's (in TO-220)? Because TO-220 is way more easier to heatsink than the TO-3 in my opinion...

If I end up using the BD912's, I don't think that the TL494 has current capability to drive paralleled transistors. So driver transistors might be a mandatory addition in that case...


Should I just stick with the original design?
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Perhaps you could leave it as it is and later make a new REV?


That's OK, if you say so!
You know, I'm just trying to make it "even better" and easier to use... ;)

I posted a picture about a thing which confuses me. I hope you can answer.

What do you think about the R12's value? Is it correct? The TL494 needs supply current of 15mA MAX. My transformer gives about 45VDC, so can I calculate resistor value like this?:
(45V-39V)/0,015A=400 ohms?

Would be glad for help! :)


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I have now built the powersupply and tested it a little bit. I unfortunately can't test it thoroughly, because I don't have a variable DC-supply. I'll try it tomorrow with two car batteries in series, resulting 24V.

I did some tests with a car battery charger as a mains supply, trying some 12V bulbs and small fans as a load. It did work quite nicely, and it lowered the output voltage to about 8-9V (charger's voltage is about 18V when not loaded).

I also measured volts and amperes & calculated effiency "n":

  • With 21W car bulb: n=74%
  • small 12V/100mA fan: n=68%
  • 55W car halogen: n=46%

(charger's max. current sets the limit here, it's only 4A)

It definitely needs more testing, fine-tuning and of course heatsinking, but if it works after that, I think it will be worth of it :)


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Some testing with 30VAC 300VA transformer...

I have made some changes to the device: I put transistor and diode into heatsinks, and added a trimpot to adjust the voltage.

The 150R base resistor heats up very quickly! I changed it to a 11W 150R power resistor and it got up to 70


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Is the PNP transistor used as a switch? Normally in a PNP use of a transistor as a switch, the value of R2 = Supply Voltage / ( Maximum Current Required / Minimum HFE * 1.3 ) You can find the HFE from your data sheet on the transistor.
Resistor R3 is not essential to the circuit but is generally used for stability and to insure that the transistor switch is completely turned off. This resistor insures that the base of the transistor
does not go slightly negative which would cause a very small amount of collector current to flow. The value of this resistor is not critical but a value about 10 times R2 is normally chosen.
I also noticed that you have connected the collectors (C1 and C2) of the TL494 chip to the base of the PNP transistor instead of connecting the TL494 emitters (E1 and E2) to the PNP. Is this what you meant to do? The data sheet shows C1 and C2 connected to VCC.
Hope this is helpful.


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Try to use a 2.2k for R3. As far as I can see the emitters and collectors of the TL 494 is correctly connected. With the change of R3 the efficiency should improve a bit and the heat in R2 will be reduced. Have you access to a scope? I would be useful to see how Q1 performs and possibly make some minor changes to improve the efficiency.

Ante ::)

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The 150 ohm resistor heats because it has pulses of nearly 40V across it. ON Semi's application circuit shows it being used with an input voltage of only 10V, where 150 ohms will dissipate an average of only about 1/4W. You have a very high supply voltage.
The data sheet for your MJ15004 does not spec hFE beyond a collector current of only 5A, where its hFE is guaranteed to be only 25, and will be much less at higher currents. Even a 150 ohm resistor won't supply it with enough base current for only 5A collector current. I think that a lower value (and higher power rating) base resistor is needed, or else a darlington arrangement. A power MOSFET would be perfect here.
TI (who invented the TL494), have a teriffic application note showing a complimentary darlington output stage that is operating safely from a 32V supply and outputting 10A. Its base resistor is reasonable. This note is here:

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