Electronic – Switching power supply output goes to zero when load is attached

circuit analysispower supplyswitch-mode-power-supplytransformer

I built a SMPS, my first one. It is essentially this one: https://www.infineon.com/dgdl/DN-ServerStandby.pdf?fileId=db3a304412b407950112b418a93b266b, which is a Design Note from about 20 years ago, by an Infineon engineer. The only changes I made were to add the necessary AC mains input section, with a fuse, two X2 capacitors, common mode choke in a pi-filter configuration, bridge rectifier, and filter capacitor. I used all new components.

Here's what it does: with no load the output is around 4.5-5.0 Vdc, varying somewhat irregularly. If I had to guess I would say it fluctuates between 4.5 and 5V, with a period of around 1 sec. When I place a load on it, the outuput drops to zero. I've tried 8, 16 and 4R dummy loads. I also attached an LED in series with a 200R current limiting resistor, and the LED pulsated between dim and off, again with a period of around 1 second.

I have a basic understanding of how SMPSs work, but I have no idea how to troubleshoot. Everything seems ok on the input side, around 157Vdc going where it's supposed to go. I tested the feedback circuit by placing a variable dc voltage on the output terminals (with input disconnected) and when the voltage was below 5v across the output the voltage across the LED in the optocoupler was well below the 1.3v forward voltage in the data sheet. As soon as the voltage hit 5v the drop across the optocoupler LED rose to 1.3v. This is exactly what the feedback circuit is supposed to do isn't it?

The project called for making one's own transformer. Maybe I did a crummy job with that and so the transformer's the problem? How critical are things like the ferrite material and parameters like A_L? The plans called for an E20/10/6 core of N67 material with a 1.0 mm gap, with A_L = 60nH. I couldn't find N67 anywhere but I got an E20/10/6 with N87 and .25mm gap. I expanded the gap, the data sheet says that with a 1.0mm gap A_L =60nH. But I might not have made the gap big enough, maybe only .5mm, which would give A_L=103nH. Would that make a difference?

I have read many sites on line about trouble shooting SMPS, but I can't find anything specific about the condition of the output dropping to zero when a load is connected.

Thanks for any suggestions.

Okay, here's the followup: I rewound the transformer from scratch, no real change in behaviour. First I increased the gap in the core to exactly 1.0mm which is what the design called for. Then I rewound it. I am certain I wound it correctly this time. I am unsure about the type and amount of insulation between the windings. The design calls for "one layer of Makrofol" between windings. I'm not even sure what that is but I Googled it and didn't seem so easy to get. I used the standard yellow polyester film adhesive tape that seems ubiquitous on high frequency transformers – it is 1.0 mil base, measures about 2.0 mil thick with the adhesive, and I put two layers between each winding. Does that seem right?

With a load of 4R using dummy resistors, the output pulses about once a second. Between pulses the output is 0 V and each pulse is somewhere between around .2V and 1.3V. With no load, the output is fluctuating between 4.3V and 5.0V, going up and down with a period of about a second – same as before.

I have checked my circuit over and over again.

Answers to frr:

I used the following diodes:D1 – MBR745G, D2 – 1N4148, D3 – 1N4937T, D4 – Vishay BZX55C18-TAP. The electrolytics are Wurth alum. can caps, e.g mouser.com/ProductDetail/710-860010575013, and there are various film and ceramics caps, all parts are new. I do have a scope – it's a Soar MS-3015 that I bought at a junk yard for $25. Older, analog, no info on the web about it but It actually works, sort of. It's 2 channel, 2MHz. If I use the scope on an SMPS do I need an isolation transformer? Probes are the Cat II 1000V that came with my Fluke 115. Are those safe?

Update: Ok, I have attempted to measure the inductance of the primary winding on the transformer. I don't have an LCR meter so I had to use one of the many ad hoc methods (put unknown inductor in tank circuit with known capacitor and measure resonant frequency, etc. etc.) I finally settled on the simple method of using my signal generator and an accurate known resistor, because it gave me the most consistent results and is in the ballbpark of the few known inductance coils I own ( I have a number of coils that I have salvaged out of junk of unknown inductance.)

I measured 458uH. The Infineon design note calls for a primary inductance
of 435uH. Assuming my inductance measurement isn't off, isn't that pretty close? This doesn't surprise me really since I followed the instructions for the transformer build carefully. So I am persuaded by Rohat's comment that it seems the controller chip is shutting down because it's entering overload-protection mode, but I have no idea why.

In response to frr's comments about PCB layout, here is some info about my pcb layout:

Pcb Layout: I originally copied the schematic and the PCB layout for the 5V 20W “Server Standby” board, but then I needed to add an AC input section with a fuse, EMI filter, rectifier and filter cap. I discovered another Infineon Design Note at https://www.infineon.com/dgdl/DN-ChargerAdapter40W.pdf?fileId=db3a304412b407950112b418a3ee265f
Which is for a 5V 40W SMPS, almost the same as the first one, but bigger with more power. I used that as a basis for the input section and then I noticed the board layout was different and seem more compact, so I copied that. So that’s where I got the layout in math.hunter.cuny.edu/thompson/pcb_1.png
and math.hunter.cuny.edu/thompson/pcb_2.png.
The red is the front copper layer, the blue is the back. The second image shows the copper pour filled in, the first image only has it outlined.

Yes, that is a huge copper plane covering the entire front side of the board. (BTW it’s a home-made board). There are two grounds in the circuit, one for the input side, one for you output side. In the schematic they are labeled “GNDPWR” and “GND”, and are only connected by a Y cap. The big front copper plane isn’t connected to anything, it’s isolated.

Let me see if I can remember why I did this. The Infineon design was for a one-sided board (copper on the back, components on the front) and there was jumper wire. I had a piece of two-sided copper clad that I wanted to use up, and I had been wanting to make a two-sided board, so I did that. There’s only one trace on top (where the jumper was). I had to make a home-made “via”. The board is all through hole and all the components are on top. I wanted to minimize the amount of etching I guess, so I left all that copper on top. It didn’t occur to me that this might not be a good idea for some reasons.

Problem Solved: All fixed, the power supply works great. I can't thank all of you enough for your help. There was a bad solder connection on the board where R8 joined the input pins to the controller chip. I was pursuing Rohat's analysis, which was absolutely correct, that the chip was not getting powered by the auxiliary winding. I tried reducing the value of R8 by attaching another resistor onto it in parallel, but that didn't work. I replaced D2 with a large 3A fast Schottky diode I had, but that didn't work. I was getting ready to take out the transformer and rewind it (again!) when I found the faulty solder joint.

Here are pics if you're curious:
http://math.hunter.cuny.edu/thompson/pic1.jpg

http://math.hunter.cuny.edu/thompson/pic2.jpg

http://math.hunter.cuny.edu/thompson/pic3.jpg

Yes I know, it's a home-made board and it looks like crap, but it's actually not that badly made. It was my first attempt at making a two-sided board and the registration of the pads between the sides was not perfect. I was aware of this and tried to compensate and thought I had addressed any issues. But with this particular joint its plain to see what happened. The hole from the top missed the center of the pad on the bottom and the solder joint didn't make good contact with the lead. My eye just didn't catch that one bad joint.

Here is V_out for various values of dummy load, steady as a rock:

R_L (ohms) | V_out (V)

  _____|________   

no load | 5.000

  16   | 4.993

  8    | 4.987

  4    | 4.975

When I finished the project and it didn't work, I was disappointed of course, but I also knew that I would learn a lot more from the troubleshooting process, which I did. Thank you in particular Andy for the information about transformers, frr for all the info about components, pcb layout, grounds and isolation, and Rohat for explaining in detail clearly how the circuit works and identifying the problem.

Best Answer

Well, at first I thought that the converter enters into over-load protection. But after reviewing the OP's schematic, I'm pretty sure that the controller chip is not getting enough supply. And the behavior looks like so.


Here's what normally should happen:

Once the converter gets energized, the 22u capacitor will start charging up through two series 680k resistors. Once the voltage across the 22u cap hits the turn-on threshold of the chip, the chip turns the internal MOSFET on and a ramp current starts to flow through it. Once the peak of the current hits the limit value (determined by 1V/0R82), the chip turns the MOSFET off. After the MOSFET turns off, a non-zero voltage is developed across the secondary (and thus the aux winding). Henceforth, the output voltage regulation is done via the chip and the feedback network.

As long as the chip regulates the output, the aux winding supplies the controller chip.


Let's look at what is possibly happening:

The 22u capacitor charges up through a resistance of 1M36 (2x 680k). The start-up threshold for the controller is about 13V. So it takes about 1.2 seconds for the controller chip to start-up:

$$ \mathrm{ V_C=V_S(1-e^{\frac{-t}{RC}}) \\R=1.36M\Omega \\C=22\mu F \\V_C=13V \\V_S=230\sqrt2 \approx325V \\\therefore t\approx1.2s. } $$

If the whole circuit is supplied from 120Vac then the start-up time will be 2.4 seconds.

If the controller chip can't get enough supply through the aux winding then the charge stored by the 22u capacitor will be consumed by the controller chip quickly. So the voltage across the capacitor will drop and once it hits the turn-off threshold (8.5V), the controller will stop.

Once the controller chip stops, it'll try to start through the start-up resistors and this will take another 1.2 seconds. Then the whole cycle starts afresh.


So, the problem may come from at least one of the following:

  • the aux winding (maybe it needs more number of turns or maybe it's direction is wrong).
  • 6R8 resistor (maybe it needs to be decreased).
  • 4148 switching diode (maybe it's broken).