I’ll try one more time as clear as I know how. I am looking at the primary winding of a transformer with a previously described tester. This tester generates a fast pulse of an unknown period at an amplitude of up to 600mv. It then stops the pulse and then examines the ring cycles resulting from that pulse. Normally there are lots (relative term here) of ring cycles on the board I’m testing. The other day a serendipitous discovery was made. We found that heating the rectifiers above 45C caused the rings to be greatly diminished. My original question was “Any idea what the diodes are doing that would cause this? “ My hope was to find an alternative rectifier that doesn’t exhibit this effect (at least up to around 55C). That’s why I asked if I should switch to SiC. Why do I care? Because with a number of these devices the transformers have failed with burned out windings. Maybe there is cause & effect. Maybe not.

I shouldn’t have initially called it a Q test. I should have said ring test. The maker of the tester calls it a Ring Tester for High Q Inductive components. I work two jobs so I don’t have much time to be as careful in my wording as I’d like. Sloppy on my part.

As for being at sea and pressured by management, I am that management and the only pressure is that which I apply to myself.

20ga wire isn't a lot of current for a transformer primary. But what is driving the primary is most likely the origin of the problem, or they used the wrong type of transformer for the application.

 

Normally there are lots (relative term here) of ring cycles on the board I’m testing. The other day a serendipitous discovery was made. We found that heating the rectifiers above 45C caused the rings to be greatly diminished. My original question was “Any idea what the diodes are doing that would cause this? “ My hope was to find an alternative rectifier that doesn’t exhibit this effect (at least up to around 55C). That’s why I asked if I should switch to SiC. Why do I care? Because with a number of these devices the transformers have failed with burned out windings. Maybe there is cause & effect. Maybe not.
f.

A ring test is supposed to be done with the transformer removed from the circuit. I'm surprised it rang with a circuit on the secondary, but your tester probably doesn't put out enough voltage until the diodes are heated up and the forward junction voltage drops. Then it loads the secondary and the ring cycles are dampened.

 

One rather important question does not seem to have been asked or answered. "Whatis the purpose of the circuit, to which we are only shown a small portion of the whole package. Is this a power supply or a part of some instrumentation scheme?? Why is "transformer Q an issue? Is it a problem, even??
What has not been pointed out yet is that an unterminated winding with only internal capacitance has a rather HIGH "Q" and is quite likely to ring when pulsed from a high impedance source.
Since that is not likely to be open circuited in the actual application, why does it even matter??
Also, consider this as another example of showing us only a part of the circuit that has already been selected as the problem. In addition, it appears that the tester used did not apply enough voltage to bias the diodes into conduction, which probably would have changed the transformer waveform a bit.
Another consideration is that since heating the diodes just a bit, really, changed things quite a bit, that these diodes are not at all suitable for the application. Aside from current capacity and reverse breakdown voltage, there are several other properties that can matter, such as diode switching time, and reverse leakage versus temperature, and even junction resistance versus temperature. That information may not even be available for some brands of diodes.

Many years back I had a problem with a tube-type audio amplifier that developed a slight mains buzz when I used silicon diodes to replace the selenium rectifiers. Ultimately I found that the diodes, after carrying the forward current for half a cycle, did not instantly switch off when the voltage passed thru zero. Instead they continued to conduct until the reverse voltage increased to almost a volt, before switching off suddenly. so I was getting very fast one volt spikes at the input to the filter. The fix was a low value series resistor and a small ceramic disk capacitor to common following each of the two diodes. (It was a full-wave center-tapped 300 volt secondary) My point being that diode switching off can be an issue, especially at a higher frequency.

I have seen products that were not suitable for any application under any condition, no matter how low the price.
One more thought, relative to the transformers burning up, is that it is entirely possible that there are simply not enough turns of wire in any of the windings. That happens when cost cutting is done by those unqualified to make any engineering decisions, such as purchasing types. It may also be that, if these are custom transformers for the application, that a bit of damage happens during production or assembly.

 

My brilliant and capable technician used to refer to a group of us engineers as: "the deaf and dumb leading the blind and the brain damaged". It seems eerily apropos.

 

My brilliant and capable technician used to refer to a group of us engineers as: "the deaf and dumb leading the blind and the brain damaged". It seems eerily apropos.

It's what we called, a snark hunt.

 

It's what we called, a snark hunt.

My design revision problems have always been caused by purchasing department individuals. Either going for a similar item at a cheaper price, or changing vendors to one who had no clues.
But the very worst was the purchasing team that sent half of the orders thru the one guy's girl friend, who charged a 20% service charge to place the order. Eventually it killed the company. That is a long sad tale for a different thread, rather off topic, I suspect.

 

My design revision problems have always been caused by purchasing department individuals. Either going for a similar item at a cheaper price, or changing vendors to one who had no clues.
But the very worst was the purchasing team that sent half of the orders thru the one guy's girl friend, who charged a 20% service charge to place the order. Eventually it killed the company. That is a long sad tale for a different thread, rather off topic, I suspect.

There is always a modicum of space for a good war story.

 

What we are missing is a lot of data: first, what are the current and voltage requirements of the DC load from this bridge rectifier supply, not only the portion shown, but also the other connections that are not shown what they connect to. In addition we really do need to see the circuit feeding the transformer primary.

As in many electrical circuits, the burning or overheating of one component is caused by the incorrect function of some other component. In this case, the TS has already decided that the sole problem is the transformer. Given that we have no hint as to how much power is passing thru we are unable to provide a reasonable suggestion for a fix.
Certainly switching to 1N4007 diodes could avoid any PRV breakdown issues, if that is the problem.

 

The enamel appears to sometimes be burning off leaving shorted turns. I

which transformer winding is doing this?

you could go to a better diode like this one: https://mou.sr/4ah6Rg5 But its not going to fix the problem.

But I think its not designed well considering they are full wave rectifying a switcher oscillator which is not normally done. Because the transformer has to dissipate power in both cycles, and a 15Khz half wave ripple is going to be just as easy to filter as a 30Khz signal.
Normal practice is to half wave rectify with like a .33 ohm resistor before the diode that is the fuse device for the secondary.

The ring test didn't show anything but a low impedance load on the test (1-3 rings) instead of an open circuit (5-8 rings) diodes forward voltage drop varies with temperature regardless of the diode. They even dedicate a chart for this in datasheets.

 

L.S. Makes a very good point, which is that feeding a transformer with other than a quite balanced wave AC voltage is going to have magnetic saturation problems unless the transformer is designed for that application. And the output of a single transistor switching circuit will certainly not be balanced.

So here is an example of why we should have been shown the driver circuit, because it is probably in the driver portion that the problem exists. If we could see that circuit then we could be much more aware of what the problem might be. Otherwise, if we are only guessing, we may not guess correctly.

And also we should have been given the intended voltages and currents for the output of the supply. We don't need the details, just the voltage and currents for the output will be enough.

 

Ok. Here is all the applicable circuitry. It's an extract from 31 pages of schematic. I don't have any specs to share although output voltages are shown. Don't know the currents. I'm lucky to have the schematics.

 

THANK YOU!!
This is certainly an uncommon arrangement of power supply and regulators. What I think I see is diodes D27 and D30 being short circuits of their associated negative voltage regulators. THAT would explain the burning of the 25 turn transformer windings. Also, the purpose of the resistors shunting the voltage regulator inputs is not obvious to me.

And also I do not understand the reason for positive bias being applied to the negative outputs of those same regulators. That is not commonly seen in power supply circuits.

So it certainly appears is that the transformer winding failure is caused by an apparent diode caused overload current.