Hi everybody,
I'm currently repairing a Simax LV 250 PA power amplifier. Unfortunately I can't find any information about it on the internet, let alone a schematic...
Anyhow, the PSU is the culprit here and it looks rather straightforward. It is a pretty huge one, with the PCB measuring 15x42cm... The original fault was a shorted MOSFET and diode in the active PFC. I've replaced the parts and checked the whole PSU for other shorted semiconductors or defective resistors. Everything fine so far.
Now when I power up the supply with a variac, the supply voltage for the L4981 PFC controller slowly increases up to the point where it starts to operate, and then the PSU draws a short burst of 1,5A+, causing the supply voltage to drop a little, recharge and then repeat. While the large amount of current is drawn, the PSU makes a horrible squeaking noise. As far as I can tell it's the PFC inductor causing this. All this happens at a "mains" voltage of about 25VAC. I tried to increase it up to 100VAC, which then results in an even larger current draw and a louder squeaking noise.
Since I didn't find any semiconductors or resistors which might cause this, I suspect the PFC inductor itself. It has a little secondary winding (have a look at the attached datasheet schematic) which is fine and has no measurable short to the primary winding. The primary winding measures as a dead short, which is no wonder since it is heavy gauge. For some further testing I used a ring tester, pretty similar to this one: Anatek Blue Ring "Q" Meter Kit – Anatek Instruments. The primary winding gives me four rings, both in-circuit and out of circuit. This tells me that there must be some inductance left, but I'm not sure how much... One other thing I tried was this: while in circuit, I placed the ring tester between ground and either end of the inductor. From the rectifier side to ground was no measurable ringing, from the diode side to ground I got five rings, so a little more than the inductor itself. The PCB with the inductor removed doesn't cause any ringing at all, which makes me wonder whether the inductor might have an internal short somewhere?
The inductor is made by Block Trafo here in Germany, but it certainly is no off-the-shelf part 🙁 .I'm not even sure whether it has 0.9mH like the one in the datasheet application, so I probably won't be able to substitute it. Is there anything I might be overlooking which causes the inductor to squeak and make the PSU draw such huge amounts of current?
Any help is greatly appreciated 🙂
Regards,
Lasse
I'm currently repairing a Simax LV 250 PA power amplifier. Unfortunately I can't find any information about it on the internet, let alone a schematic...
Anyhow, the PSU is the culprit here and it looks rather straightforward. It is a pretty huge one, with the PCB measuring 15x42cm... The original fault was a shorted MOSFET and diode in the active PFC. I've replaced the parts and checked the whole PSU for other shorted semiconductors or defective resistors. Everything fine so far.
Now when I power up the supply with a variac, the supply voltage for the L4981 PFC controller slowly increases up to the point where it starts to operate, and then the PSU draws a short burst of 1,5A+, causing the supply voltage to drop a little, recharge and then repeat. While the large amount of current is drawn, the PSU makes a horrible squeaking noise. As far as I can tell it's the PFC inductor causing this. All this happens at a "mains" voltage of about 25VAC. I tried to increase it up to 100VAC, which then results in an even larger current draw and a louder squeaking noise.
Since I didn't find any semiconductors or resistors which might cause this, I suspect the PFC inductor itself. It has a little secondary winding (have a look at the attached datasheet schematic) which is fine and has no measurable short to the primary winding. The primary winding measures as a dead short, which is no wonder since it is heavy gauge. For some further testing I used a ring tester, pretty similar to this one: Anatek Blue Ring "Q" Meter Kit – Anatek Instruments. The primary winding gives me four rings, both in-circuit and out of circuit. This tells me that there must be some inductance left, but I'm not sure how much... One other thing I tried was this: while in circuit, I placed the ring tester between ground and either end of the inductor. From the rectifier side to ground was no measurable ringing, from the diode side to ground I got five rings, so a little more than the inductor itself. The PCB with the inductor removed doesn't cause any ringing at all, which makes me wonder whether the inductor might have an internal short somewhere?
The inductor is made by Block Trafo here in Germany, but it certainly is no off-the-shelf part 🙁 .I'm not even sure whether it has 0.9mH like the one in the datasheet application, so I probably won't be able to substitute it. Is there anything I might be overlooking which causes the inductor to squeak and make the PSU draw such huge amounts of current?
Any help is greatly appreciated 🙂
Regards,
Lasse
Attachments
Here's a quick shot of the primary side. I've drawn two rectangles. The bigger one is the PFC section, with the suspected faulty inductor in the upper left, and the smaller rectangle encircles (or ensquares? 
) the auxiliary psu. That is built around an 7815, is supplied by the secondary winding of the PFC inductor and powers both the PFC controller and the main PSU controller. It's fine when powered with a lab supply, but unfortunately the main PSU will not work without the PFC working correctly 🙁 .
) the auxiliary psu. That is built around an 7815, is supplied by the secondary winding of the PFC inductor and powers both the PFC controller and the main PSU controller. It's fine when powered with a lab supply, but unfortunately the main PSU will not work without the PFC working correctly 🙁 .Attachments
I would think that in most cases inductor wouldn't fail unless you would have gross overload ie. no fuse in system. Thinks to check would be everything in current path +IC itself
Yes, it's fine. Hooked up the scope across it and can see the spikes while the PSU draws current. Unfortunately I don't have a digital storage scope, else I would have shown you the scope plot.
I disconnected the drain from the rest of the circuit and connected an incandescent lamp instead. It does light up in the same "pumping" interval, but the high current draw and the squeaking are gone, so I assume the IC to be fine. The main PSU afterwards doesn't cause a huge current draw either, I've tested that without the PFC.
The inductor is the last thing to fail, unless the wire melts there is nothing that could possibly be damaged.
My primary suspect would be diode D1.
Double check it; if you already replaced it, check it again and confirm that you didn't inadvertently fit it backwards / reversed ...
My primary suspect would be diode D1.
Double check it; if you already replaced it, check it again and confirm that you didn't inadvertently fit it backwards / reversed ...
I've replaced it with an STTA type like in the datasheet, since that was what I had at hand, but the original was an Infineon CoolSiC. Guess I'll have to order such a thing then? Does that cause such a behaviour?
STTA should be fine; just check that it is still good and not reversed;
also inspect for solder shorts if you haven't done so yet;
my experience with such noises in boost / smps is that it indicates either a short or an open circuit condition;
by the way, boost converters require a minimum load to work properly - did you test it with appropriate load ? won't work without ...
2 Glühlampen 230v in series is what I use for testing (because of 300v + output of pfc)
also inspect for solder shorts if you haven't done so yet;
my experience with such noises in boost / smps is that it indicates either a short or an open circuit condition;
by the way, boost converters require a minimum load to work properly - did you test it with appropriate load ? won't work without ...
2 Glühlampen 230v in series is what I use for testing (because of 300v + output of pfc)
Checked that. Ordered a new SiC diode anyhow 'cause I needed some other parts from Farnell.
Didn't find anything short anywhere in that PSU. All the semiconductors measure fine, secondary side too.
The PFC doesn't even manage to get the main caps charged up, nor does the auxiliary PSU produce enough voltage to start up the main PSU. With this behavour the whole thing won't be able to start up on its own without an additional load, which doesn't sound plausible to me.
On the other hand it might well be that the damn thing needs someting north of 200VAC to start up properly, but then again... I've had dozens of TV sets with active PFC which all start up just fine well below 90VAC, without large current draw and certainly without any noise, so why should this be the exception? I didn't dare to feed it the whole 230VAC because I'm afraid that it might act like an IED 😱.
You tell me 😉
I do actually have a scope and know how to use it. I just don't know what else to look for. The MOSFET gets a clean drive signal at the Gate, looks like roughly 90% duty cycle in this case. It also manages to shunt the inductor down to GND. It just won't run continuously.
Anything you'd like me to try? I'll do my best to make some 'screenshots' from my Hameg HM 204 then...
That is good to know.
Have you look at current of inductor?
or replace inductor with your own for time being?
and 90% sounds all wrong if you don't have high load on output.
Have you look at current of inductor?
or replace inductor with your own for time being?
and 90% sounds all wrong if you don't have high load on output.
I've added two 25W lamps across the main capacitor, doesn't make a difference. Should I use a higher wattage instead?
The screenshot shows the voltage across the current sense resistor RS (50mV/div, 2ms/div). Does that count as the current through the inductor? Unfortunately I can't tell you the value of that resistor, since it's simply a naked piece of metal with "L7" and "02J*" written on either side.
I'm unable to replace the inductor with anything because I don't have any cores at hand to wind my own. The only thing I might substitute would be a salvaged transformer from another SMPS, but then again I don't know anything about the turns ratio for the auxiliary winding...
The screenshot shows the voltage across the current sense resistor RS (50mV/div, 2ms/div). Does that count as the current through the inductor? Unfortunately I can't tell you the value of that resistor, since it's simply a naked piece of metal with "L7" and "02J*" written on either side.
I'm unable to replace the inductor with anything because I don't have any cores at hand to wind my own. The only thing I might substitute would be a salvaged transformer from another SMPS, but then again I don't know anything about the turns ratio for the auxiliary winding...
Attachments
The IC itself is fine, too. The new one doesn't make a difference.
When I supply the auxiliary power for both the PFC controller and the main PSU controller with a lab supply, the pulsing is gone and the squeaking noise from the inductor is already apparent with less than 10VAC at the input. The PFC controller still doesn't manage to get the main caps charged up (and still continuously draws huge amounts of current), but the main PSU is working fine. I can measure the symmetrical output voltage on the secondary side, albeit somewhat lower than with 400+VDC on the mains side.
I totally don't know what to do now. I tried to remove all the snubber caps and resistors and diodes one-by-one around the PFC circuit but to no avail. It seems like the MOSFET is consuming all those amps, because only with its Drain disconnected the high current draw is gone. When the IC is removed and the Gate shorted to Source instead, there's also no current draw, so the MOSFET shuts off fine and does not leak.
The only thing I can imagine is that the inductor has lost its inductivity somehow, doesn't "ring enough" and thus acts like a short circuit. Oscillator frequency is roughly 70kHz like it is supposed to be with 1n and 30k and visible at the Gate.
When I supply the auxiliary power for both the PFC controller and the main PSU controller with a lab supply, the pulsing is gone and the squeaking noise from the inductor is already apparent with less than 10VAC at the input. The PFC controller still doesn't manage to get the main caps charged up (and still continuously draws huge amounts of current), but the main PSU is working fine. I can measure the symmetrical output voltage on the secondary side, albeit somewhat lower than with 400+VDC on the mains side.
I totally don't know what to do now. I tried to remove all the snubber caps and resistors and diodes one-by-one around the PFC circuit but to no avail. It seems like the MOSFET is consuming all those amps, because only with its Drain disconnected the high current draw is gone. When the IC is removed and the Gate shorted to Source instead, there's also no current draw, so the MOSFET shuts off fine and does not leak.
The only thing I can imagine is that the inductor has lost its inductivity somehow, doesn't "ring enough" and thus acts like a short circuit. Oscillator frequency is roughly 70kHz like it is supposed to be with 1n and 30k and visible at the Gate.
It's alive!
Well, sort of...
R17 was the culprit for the strange behaviour. After replacing it and still supplying the auxiliary power to both of the controllers, the PSU is working again. PFC voltage goes up to 400V, no excessive current draw and no squeaking anymore. Secondaries are fine, too.
Unfortunately the auxiliary voltage is still missing 🙁. Guess I'll have to check some more high-value resistors...
Well, sort of...
R17 was the culprit for the strange behaviour. After replacing it and still supplying the auxiliary power to both of the controllers, the PSU is working again. PFC voltage goes up to 400V, no excessive current draw and no squeaking anymore. Secondaries are fine, too.
Unfortunately the auxiliary voltage is still missing 🙁. Guess I'll have to check some more high-value resistors...
Found the missing aux voltage. This time it actually was the inductor... 😱 One of the fine wires broke, right where it connects to the pin at the bobbin. Fixed it with a piece of wire. This must have happened when I put the inductor back in, because the winding was fine before.
I've only tested the whole PSU briefly yet. When it has to supply some amps on the secondary (variable current sink), the primary side still makes some audible noise, albeit much less than the squeaking before. I'll investigate that further.
I've only tested the whole PSU briefly yet. When it has to supply some amps on the secondary (variable current sink), the primary side still makes some audible noise, albeit much less than the squeaking before. I'll investigate that further.
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