Electrical – SMPS transformer issues

powerpower supplyswitch-mode-power-supply

I have searched the web thoroughly and find very little info on smps transformers. How to take them apart … bare bones operations in Power supplies etc. I have a smps that still doesn't work after replacing ALL burnt components. So I'm thinking the transformer bit it as well. AFter blue ring testing seems the thing is toast. Took it apart to rebuild.
Noted as much as I could and remembered the rest.
This transformer is pretty simple … 2 primaries and two secondaries … primary one (pins 1 & 2 ) is from the mains measuring approx 165V and is part of the main switching transistor to produce +18V and -18V on it's secondary which of course has a center tap on secondary for the split supply.
The 2nd primary is for the 5V supply for the digital supply. It's pins 8 & 9 on the schematic … it's primary input voltage is the same as the 1st primary on pins 3 & 4.
So when I took it apart the winding ratio's didn't make any sense for the respective ins and out voltages.

Primary 2 … pins 3 & 4 was approx 44 winds @ 28 awg. It's secondary was 4 winds @ 23/24 awg. This one sorta makes sense. This primary was the innermost wind … it's secondary was the outermost wind.

The 2nd primary was only 2 winds (possibly 1 wind) and it's secondary winds were not counted but was at least say 20-25 winds. Insulating tape was used between all winds but not sure how thick each layer was so subsequent winds after the 1st weren't known exactly.

So problem is I need secondary voltages of +18 and -18 on the 1st primary and secondary then need 5v from 2nd primary and secondary … according to what I know about turn ratios they don't line up.

After rebuild 1 I had 5v supply working but found out that priamry one should be attached to pins 3 & 4. Got that fixed … haven't finished the rebuild at that point. BUT the windings for second primary and secondary don't make sense for +-18V One or two winds for primary and say 20-30 winds dual for split secondary. +-18V

Any help would be appreciated

Best Answer

It seems to me that you are confusing some things. The two primary windings are on the LEFT in the schematic, pins 1-2 and 3-4. The two on the RIGHT, are secondary windings, pins 5-6-7 and 8-(9).

The 2nd primary is for the 5V supply for the digital supply. It's pins 3 & 4 on the schematic

It doens't work that way. In a transformer, all the coils influence each other. If I understand the schematic correctly, the primary coil that 'does all the work' (power goes IN) is the Pin 1-2 winding. The 3-4 primary winding is for feedback and power for the primary switching transistor/circuit. So although it's on the primary side, power doesn't go IN, but OUT. I'd call it a primary winding, because it's on the live side of things. But maybe to call it that is wrong, and it should be called a secondary winding? I'd be interested to learn how that works actually. I guess it depends on how you look at it.

primary one (pins 1 & 2 ) is from the mains

Primary 1 ... pins 3 & 4 was approx 44 winds

Let's stop the confusion, and call Pins 1-2 Primary 1 (P1), and pins 3-4 Primary 2 (P2). Pins 5-6-7 Secondary 1 (S1) and Pins 8-(9) Secondary 2 (S2). Maybe you should edit your post to match...

So count the windings of P1. Then count the windings of S1. This should be something like 9:1:1 (for 50% duty cycle at 165V). So for every 9 winding on P1, S1 Pin5-7 has 1 winding and Pin 6-7 has 1 winding (2 windings in total). Of course there are guestimates, the actual amount depends on the desing of the SMPS. Also take note that the direction of the turns matters!! The turn direction is marked by a dot in the schematic.

Then for S2, you still compare with P1. This should be something like 33:1 (again, for 50% duty cycle). Update: Apparently this SMPS has quite a low duty cycle at 165VDC, see corrected values at UPDATE below.

I don't know about your broken E-coil, but I'd say to glue it is a long shot, and you'd be better off with a new transformer... But I might be wrong.

Also note that not all broken parts will seem 'burned'. Usually the main switching transistor blows, taking more parts along with it. You can't always find the faulty parts by eyesight.

I hope you'll be able to fix it. Just make sure you insulate the primary windings from the secondary ones properly. Use the right tape for it, as your safety depends on it!

Hope this helps. Good luck!

UPDATE: So the correct number of winds should be:

P1 (pin1,2) has 44 winds
P2 (pin3,4) has 2 winds
S1 (pin5,6,7) has unknown winds
S2 (pin8,9) has 4 winds

Up1:Us2 = 165:5 = 33:1
P1:S2 = 44:4 = 11:1
33/11=factor 3

Up1:Us1 = 165:18 = 9:1
divided by factor 3 gives you:
P1:S1 = 3:1

Since P1 has 44 winds, 44/3=14.7 (approx. 15). That would mean that S1 should have 15 winds for 18V (so 2x 15winds for 2x 18V -or- 15 winds between pin6-7 and 15 winds between pin7-5)

You can read more on how it works here

About turn direction:

Look at the dot in the schematic. If you connect the start of the wire to the dot, start turning in one direction, say CLOCKWISE. Then for the other coils, also connect the start of the wire to the dot and start turning in the SAME direction - in this case I chose CLOCKWISE. This way you'll do the direction right. You can also choose to turn CCW, but then you should do the other coils also CCW.

IMPORTANT
The dot on P2 is in the wrong position. The dot in the schematic should be on pin4, and NOT on pin3!

About dot notation:

Yeah, the dots... there's so much confusion about them, and I've looked at many schematics online only to find so many dots in the wrong place.

P1 is the coil that creates the magnetic field. The + is connected to the dot, so current flows from the dot terminal to the non-dot terminal. Remember that.
S1 is a coil that does the opposite. It turn the magnetic field into a current. Here, the current flows in the SAME direction, so from dot to non-dot. To make that happen, the + must be at pin 6.
Same for S2. Current flows from pin9 to pin8, so the + must be at pin 8. That matches with D8 and D10, otherwise nothing would happen.
For the + to be at pin3 of P2, current must flow from pin4 to pin3. So the dot must be at pin4.

Related Solutions

Electronic – Hand winding smps transformer

1) Winding direction doesn't matter as long as you keep it consistent. If you are adapting a lathe or drill as a winder, the winding direction will be determined by the machine.

2) If the small windings are at approximately the same potential they can share a layer, otherwise you must provide sufficient insulation between them. Sometimes you may have to separate layers to reduce capacitance between them, or interleave them to reduce leakage inductance, but that is too complex to discuss here.

3) Winding coils side by side is simpler and provides better voltage isolation especially when the sections are different bins on the bobbin. That is essential for high voltage supplies and makes mains voltage safety approval easier.

However it increases the leakage inductance, and that matters for high frequency applications like SMPS. The only reasonable way to know if you can do this is to wind one that way and measure the leakage inductance. If it's out of spec (or even close to it) wind another in layers and compare them.

Why the voltage drop is so high in the Half-Bridge SMPS

Is your transformer secondary 12 turns total or 12 turns on each side of the center tap? If it's the former, that's why you can only get 30 V under load. I calculate it this way:

Input voltage is 220 V_RMS full-wave rectified to about 310 V_DC.

This means that your half-bridge is driving the transformer with a voltage whose peak is half of this, or 155 V.

The 33:12 transformer is going to turn this into a peak voltage of about 56 V.

If the secondary is center-tapped, then you're only hitting the rectifiers with a peak of 28 V.

As for the excessive rise at low loads — well, that's why lots of SMPS specify a minimum load. It's actually quite difficult to design an efficient one that also has a huge dynamic range. One problem might be excessive leakage inductance (i.e., less than perfect coupling) in your transformer.

EDIT: Since I can't put this drawing in the comments, I'll add it here. Your transformer drive waveform always needs to be symmetric. At 50% duty cycle, it should look like a square wave, with a small amount of "crossover distortion" created by the dead time:

  --------            --------
 |        |          |        |
 |        |          |        |
 |        |          |        |
 |        |          |        |
           |        |          |        |
           |        |          |        |
           |        |          |        |
           |        |          |        |
            --------            --------

But at lower duty cycles, it still needs to be symmetric, with longer "off" periods between the alternating pulses. It should look like this:

  --                  --
 |  |                |  |
 |  |                |  |
 |  |                |  |
 |  |                |  |
-    ------    ------    ------    --
           |  |                |  |
           |  |                |  |
           |  |                |  |
           |  |                |  |
            --                  --

This is the sort of waveform that the drivers on the SG3525 are designed to produce.

Best Answer

Related Solutions

Electronic – Hand winding smps transformer

Why the voltage drop is so high in the Half-Bridge SMPS

Related Topic