Need help with a UC3843 based SMPS repair

[SCR]

B_force, excellent guess, it is indeed a guitar amp.

I removed all conponents from the transformer output and checked again but the symptoms did not change. Good chance the transformer is gone, will do some more tests.

Fallback plan, are there small commercial power supplies available with symmetric outputs? (+-48V @2-4A, +-16V @1A)  There is not enough space to just install four small Meanwells.

[SCR]

P.s. What confuses me somewhat right now is that the output voltage (the setup with the 10 ohm shunt) is exactly like in the specs, but that should not be the case if some of the primary windings got shorted out since this would change the winding ratio, hmm...

[bktemp]

Did you try to replace D525?
To make the diode get hot, it needs to dissipate a lot of power. This can happen in 2 ways for a diode:
- due to a high forward current: Because of the low voltage drop you need a couple of amps to make the diode get hot)
- due to a higher than normal reverse current: Because the reverse voltage can be high, a rather small current can generate a significant amount of power.

Can a high forward current flow in this circuit? Probably not, because all of the current flowing thru D525 must have been stored in the transformer, and forward SMPS transformers have a high inductance to avoid any high magnetization currents. If the core gets satured because of a too low switching frequency or a too high duty cycle the current can get quite high, but the stored energy is still rather low. Maybe you can try to increase the switching frequency and look if the diode gets less warm.

My guess is still a faulty D525: I had diodes that tested fine at 5V but had a leakage current of several mA at 100V and even more at their rated 400V. From the switching waveform it looks like something is shorting the winding where D525 is connected to for the first part of the on-cycle. Maybe D525 has a too high turn-off time therefore generating a lot of heat (high recovery lossed because of some fault).

[bktemp]

There is no current limit after a short initial delay, because Q515 disables the current sense signal.
So any excessive overcurrent will destroy the mosfets.

Oh god. I'd ignored that bit since why would anyone do that?
Ha ha, actually strictly according to the schematic, there is NEVER any current feedback. What's that dot doing there?

I noticed that, but it makes no sense, therefore it is most likely just an error in the drawing. Also some parts of the cicruit are connected to GND, other ones to -165V but there is no connection between GND and -165V.

But it is still a bad design: They could have added an overload protection easily by putting a thyristor on the current sense resistor, shorting UC3843's power supply when the peak current gets too high.

[SCR]

Did you try to replace D525?

Not yet, i need to order some STTH812DI and MUR8100EG first, the ones i have at hand are not fast enough. Should have some results on tuesday.
I only did a low voltage test on D525, so a defect sounds quite likely given that the winding ratio seems to be still ok - thanks!

[SCR]

Tried both diodes but the symptoms did not change unfortunately. D525 and the MOSFETS still get hot (about 58C). I've fed 20V DC after the rectifier and the consumption while idle is around 190mA. Maybe the recovery time of STTH812DI and MUR8100EG is too short (currently at 50-75ns) as you mentioned before, i'll see if i can find something slower.

Could the overheating take place because the field/input at 4-6 is faulty?
E.g. the origin of the problem is at 4-6 and we just see some symptoms at 2-7?

[bktemp]

You could try to increase the frequency by adjusting R516: When the resistance gets lower, both the frequency increases and the duty cylce decreases, reducing the peak current. If it does not reduce the current, you could also try removing all secondary side diodes.

If one of the windings is shorted partially, the peak current will be much higher. If you have a LCR meter you can try to measure the inductance, but you probably have to desolder the transformer to get correct measurements. The inductance is probably in the range of 500uH to a couple of mH. It can be higher, but it should not be lower.

[SCR]

You could try to increase the frequency by adjusting R516: When the resistance gets lower, both the frequency increases and the duty cylce decreases, reducing the peak current. If it does not reduce the current, you could also try removing all secondary side diodes.

The power supply came set at 142 KHz, the maximum i could set up with R516 is 145 KHz, unfortunately.

If one of the windings is shorted partially, the peak current will be much higher. If you have a LCR meter you can try to measure the inductance, but you probably have to desolder the transformer to get correct measurements. The inductance is probably in the range of 500uH to a couple of mH. It can be higher, but it should not be lower.

Transformer was removed and tested after i checked the MOSFETS in the beginning, i only tested the transformer at 1KHz though:

4-6: 2 mH/660 Ohm
1-7: 10uH/0.2 Ohm
2-7: 1.97mH/418 Ohm
10-9 and 10-11: 40uH/0.8 Ohm
15-14 and 15-16:  9,9uH/0.2 Ohm

2-7 and 4-6 are slightly different but probably should be identical (1:1 windings), that might imply a small winding short in the beginning or end of 2-7. Still not a good explanation about the power uptake in idle when applying 20V. Will have a look at the MOSFET output and winding outputs with the scope tomorrow.

BTW, i've removed all secondary diodes before the D525 tests to make sure there is no influence from the secondary side.

[b_force]

Did you actually replaced the UC3843 already?

All looks pretty fine to me.

[bktemp]

The transformer looks fine. If you have removed all secondary side diodes, there is not much else left. You could disconnect R532 and D523 and C524 (should be ok while testing using low voltages).
If it still draws a high current, you could look at the current waveform (R509).

[SCR]

UC3843 was replaced in the beginning together with R521/Q513/Q522/R509/R508 and the PWM output looks fine. I'll focus on the waveforms around the transformer next and take a look at D523 and R532, as well as R509 if my current clamp manages measuring mA's.

Have replaced C524 against a WIMA MKS yesterday after D525 still got hot, but did no see any change afterwards.
I'm sure it's something very simple, i just haven't been able to discover it yet

[SCR]

A quick update, i put the scope on the MOSFET's gate and the drain (first image), as bktemp commented the wave does not look correct for a healthy forward transformer.

Next was the gate again and this time the transformers pin 2 which is the output of the second 330V winding for demagnetizing the core. I would have expected some kind of square wave but it looks like a DC voltage with spikes, maybe due to the phase cancellation with the main winding.

For this test, D524 and D525 have been exchanged against new components. As soon as i have found my differential probe i'll check transformer output 4-6 compared to 2-7 (the one in the second scope shot).

[bktemp]

No signal at pin 2 is strange, because your inductance measurement showed 1.97mH. So the winding can't be shorted or open.
It looks more like a shorted D525. That would explain all measurements (DC level at pin 2 and reduced inductance of the primary winding causing high current in the mosfets).
Can you verify D525 is not shorted?

[megajocke]

With that largeish 10 nF capacitor C524 connecting the demagnetization winding in that way to the drain node, we know that the windings must have 1:1 ratio.

This means the duty cycle can not be allowed to ever exceed 50 %. If it does, the transformer will saturate and a large current will flow through the FETs and D525. This matches the symptoms you observe and I think you also said the duty cycle was above 50 %.

Are you sure it's not supposed to be a UC3845 in there? Could it be that the schematic is wrong? Has someone been at it before you? (does soldering look non-factory made on the IC?)

[megajocke]

After a second look, it looks like they might be using an unusally large timing capacitor instead to provide the duty cycle limit as Ton earlier wrote...

Adding jumpers to JU505 would decrease the duty cycle (and frequency). But why would one design something this way? It's probably going to be unusually sensitive to temperature variations and differences between individual ICs, requiring adjustment at production or if the IC is ever replaced.

[rob77]

After a second look, it looks like they might be using an unusally large timing capacitor instead to provide the duty cycle limit as Ton earlier wrote...

Adding jumpers to JU505 would decrease the duty cycle (and frequency). But why would one design something this way? It's probably going to be unusually sensitive to temperature variations and differences between individual ICs, requiring adjustment at production or if the IC is ever replaced.

yes that whole design is literally stupid. 
a self-oscillating half-bridge driver driving 2 mosfets in push-pull would be a better solution for such a feedback-less SMPS ...

[SCR]

So, after doing some real work for a change back at trying to fix the SMPS

I played around a bit with the frequency/PWM setting of the UC3843 by adjusting R516 and discovered that i can get rid of the overshoots shown in my last posting by adjusting the frequency below 140 kHz.

At that frequency the overshoots vanished and the signal turned into a nice square wave (see scope image: yellow is the PWM output to gate, green is the MOSFET output, blue the secondary transformer output without load applied) the output current of the supply voltage also went down to 3 mA @20V (previously 190mA). Turning up the frequency creates the overshoot again, i am not sure why this happens though, looks like the transformer is resonating?



 

[bktemp]

It seems like the transformer was saturating, because the duty cycle was too high. Now it is close to 50%. Try going a bit lower to have more margin, because the slightest drift and it may go back to >50% and fail again.
That's a really bad design.
Maybe it could be a good idea to use a UC3845 instead of UC3843, because it is limited to 50% duty cycle (it blanks every other pulse, therefore the oscillator frequency needs to be doubled).

[SCR]

I think that the duty cycle was always at 50% or below, but the frequency seemed to matter. My previous tests at 43% duty and 147KHz yielded 190 mA @20V input (which would blow out the fuse when fed line voltage), whereas 49% at >140KHz results in 3 mA based on my testing today.

EDIT: The trimmer also changes the duty cycle above 140KHz, you were right. At around 50% and upwards the saturation starts and you can see the overshots appearing, also the primary input current rises up sharply.

[SCR]

The SMPS is up and running again, thanks for the support!

[MagicSmoker]

The SMPS is up and running again, thanks for the support!

Yeah, but for how long? This design is fatally flawed. I get the rationale for skipping voltage regulation in a guitar amp, but every other design choice made here either needlessly complicated the designer's life (e.g. - like choosing a controller IC with a 100% duty cycle limit in a transformer-isolated power supply and relying on grossly distorted timing component values to limit max duty cycle) or was done to intentionally defeat an intrinsic feature or function of the controller IC (e.g. - shorting out the current sense signal... to a current-mode controller IC).

I'm very curious about the model/brand of the guitar amp, and I suspect other participants in this thread are as well.

[SCR]

I've set the duty cycle to ~30-35% to be on the safe side which seems to work well even when applying some good load. These amps are known to be quite reliable, i think they have been sold since the early nineties. Good chance that the SMPS design has been changed in the meantime though.

http://www.gallien-krueger.com/mb-150s112-specs/

P.s. just noticed that the website shows a photo of the newer power supply, which features a toroidal transformer