Problems with SMPS with TL494, again :)

[OhMy]

Hi!

A couple of months ago, when I was thinking of making something in a SMPS area, I bought some TL494 chips. Now I got to know, that they aren't the best, but still it's better to use them, not just leave unburned .
But as usual, I can't find any suitable circuit or create one by myself. So combination of different schematics takes place.
The biggest problem is to find a good gate drive circuit for driving mosfets. Found this, but somewhere read about it, that it's not made right. Then fount this one- AC-DC converter SG3525 Full Bridge.jpg.
Next I did my best at combining them together and the result is in SMPS_494.GIF. I'm quite sure, that there are a lot of mistakes, so, please, help correcting them.
I already made a PCB for it for testing, but it doesn't work (surprise...). The 494 isn't oscillating, as far as I can measure it with a DMM. I have no oscilloscope. Pins 8 and 11 stay at +12V and even smoke isn't coming out yet .

Please, any help would be appreciated.

 

[KrisBlueNZ]

The only problems I can see immediately are:

1. The diodes across the main MOSFETs should be rated for more than 1A.

2. The current "limit" is actually an overcurrent trip.

3. The overcurrent detector may not act quickly enough to protect the main MOSFETs if a direct short is placed on the output. You may need to monitor the primary current as well.

The characteristics of the drive transformer are going to be important. You need to specify the core shape, size, material and gap (or A_L value).

There are probably other issues. I have little experience of forward converters.

If the TL494 isn't producing any drive, check that the oscillator is running (a scope would be very helpful for this) and check that the feedback and overvoltage/overcurrent input voltages are what they should be.

 

[OhMy]

Hi, Kris, thanks.

1. OK, will try to find better substitutes;
2. Yes, this supply is meant to be this way, thats why there is the SCR to keep the IC shut down;
3. OK, probably you're right, but for now it doesn't really matter .

The gate drive is wound on an unknown toroid with cross section of 50mm^2, with 17 turns of three isolated and twisted together wires.

The voltages of 494 can be seen in attached image, measured from the IC's ground.
Frequency at pins 8 and 11 is 0Hz, but, when I touch pin 15 with a metal tweezer, the frequency appears as 28.5kHz with about 80% duty. Touching pin 16 makes unstable frequency around 10kHz with unknown duty ratio. So there is something wrong with the open inputs of this internal comparator.

 

[KrisBlueNZ]

OK, the pin 3 voltage is too high. One of the error amps is pulling it upwards. The first error amp has the right voltages going into it, so it must be the second one, on pins 15 and 16 as you suspected. Since you're not using that error amp, you have to disable it by tying pin 16 to 0V and feeding pin 15 with a positive voltage such as Vref from pin 14.

Give that a try... but I would use some kind of current limiting in the output stage just in case something is wrong. It's easy to make the MOSFETs lose their magic smoke, and there's no way to put it back in!

BTW I'm concerned about your comment that that the drive transformer is wound on an "unknown" toroid. You might as well wind it on a bit of rotten wood you found in the garden! You might be lucky. Let's hope so!

 

[OhMy]

Been a bit busy playing with the other SMPS lately...

Yes, Kris, your advice seems to be right about the pins 15 and 16, because now I was able to see the TL494 oscillate at about 46kHz, but strangely at 53.7% duty cycle according to my DMM.
But the GDT driving transistors got noticeably warm, and my aux power supply wasn't able to sustain the 12V, it dropped to just 7V.

BTW I'm concerned about your comment that that the drive transformer is wound on an "unknown" toroid. You might as well wind it on a bit of rotten wood you found in the garden!

Me too, but it is, what it is. Today searched eBay for toroid ferrite ring prices and had another heart attack ...
I think I'll have to wind an EE type GDT, at least the chances of having a ferrite instead or iron powder will be greater.

 

[Rleo6965]

Iron powder core was not suitable for high frequency voltages. Iron powder cores mostly use in noise filter inductor. Use ferrite core for your GDT.