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Old 19th Jul 2018, 10:26 am   #1
Ted Kendall
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Default Obsolete Sunpower switchmode PSU

This may seem an odd place to put a post with this title, but said standalone power supply is used with the hardware control panels for my Sadie DAW. I haven't used the panels for a while, but wanted to get them going again, so I hitched them and the supply up and switched on. No output from the supply, and after a few seconds a "crack" and a funny smell.

Fortunately, I have another of these supplies, but it is of the same vintage and I'm loth to power it up without understanding why the first one failed. I need two, anyway, and Sadie's replacements are £100 a pop. I could build a decent linear for a quarter of that, but would rather get these going if I can.

Now I don't like working on switchmode supplies - life's short enough as it is, so I'm treating this like a 'scope EHT - cold checks, probability and substitution. They remind me of what my wife calls "small, yappy-type dogs" - undersized, temperamental and liable to bite at the slightest provocation, or none.

Initial inspection shows that the input fuse has popped and the current sensing resitor has got hot and failed o/c. The undamaged supply tells me this is .33 ohms. The main cap still measures close to the claimed 100uF with 0.5 ohm ESR. Nevertheless, I'm intending to start by changing the caps and replacing the sensing resistor. Question is, could a duff cap reasonably account for this failure, or should I look elsewhere as well?

Sunpower deny all knowledge of the thing and can't supply information, incidentally.
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Old 19th Jul 2018, 10:56 am   #2
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Default Re: Obsolete Sunpower switchmode PSU

If the sensing element has gone, odds are that the switching element is also toast. Once the switching element has been destroyed it's easy for HT to also destroy the low voltage primary side control circuitry - unless the drive is via an isolating transformer.
A dried up cap can be the cause of destruction like this if it causes the switching element PWM drive signal to have spikes or noise on it.

Having said that, controller IC's such as the UC384x have a lockout that will prevent drive if the ancillary voltage is low or unstable.
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Old 19th Jul 2018, 11:19 am   #3
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Default Re: Obsolete Sunpower switchmode PSU

This sounds like a failure pattern I've seen before, especially in VCR power supplies. If the current sense resistor has failed, as Chris says, it's almost certain that the main switching transistor has gone short-circuit and is likely to have taken the controller IC or other semiconductors with it. A classic cause of this is the startup components: there's usually a high-value resistor or two, plus an electrolytic capacitor, which provide a low-voltage power supply to the controller IC. If the capacitor dries up, as they are wont to do, the startup supply gets noisy and uncertain, the controller loses its marbles or the drive to the switching transistor's base or gate gets weak, and things blow up. This capacitor can get missed because it's not one of the 'headline' items that get all the attention.

It should be possible to point out the likely culprits from a photograph of the PCB in the other power supply. You're looking for one or two resistors of value >100k, and one or more capacitors on the primary side, probably quite small - 100uF/25V, that sort of thing.

The main (big, high voltage) input capacitors are probably OK, since they're quite large and the ripple current they handle is small. However, it's worth replacing them while you're in there unless you have complete confidence in them. It's the capacitors on the secondary side that really suffer from ripple current related heating.

I don't know this particular unit but another tip is that if there's a standby power supply this is often in worse condition than the main one - it's likely to have run at its rated load for thousands of hours even when the equipment was nominally 'off'. I have seen this in PC power supplies as well as consumer/prosumer electronics.

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Old 19th Jul 2018, 12:50 pm   #4
Ted Kendall
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Default Re: Obsolete Sunpower switchmode PSU

Thanks, gents - that all sounds quite logcal. There is indeed a liittle electrolytic in the line of fire which I shall drag out and test - it's going to be replaced anyway, but comprehension is half the point of this.

If I have driven the system right, there are pictures here showing the board, the fuse and the fried resistor. The big !00uF/400V checks out OK for uF and ESR - about 0.5 ohm - but will be replaced anyway. Noxt step is to check the switching transistor, then...I think the control chip is a Texas, so I'm hopeful of finding a replacement if needs be.
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Old 19th Jul 2018, 1:03 pm   #5
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Default Re: Obsolete Sunpower switchmode PSU

I test a lot of equipment fitted with these and similar power supplies. There is no way you will get any service information from them. However you will find most of the components are standard but don't be tempted to buy cheap control chips from eBay or the like. Source them from a reputable supplier like Farnell. Good luck.
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Old 19th Jul 2018, 2:01 pm   #6
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Default Re: Obsolete Sunpower switchmode PSU

I agree that getting a circuit diagram is most unlikely. As it appears to use an off the shelf controller chip, examining the data for that - and especially any application circuits, will give you a fairly good steer on what's what.
In this instance, I wouldn't advocate "live working", Just change the brown crispy bits, electrolytic caps and controller IC. Verify everything else else with a meter, get your prayer mat out and apply power
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Old 19th Jul 2018, 2:08 pm   #7
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Default Re: Obsolete Sunpower switchmode PSU

Hi,
if I was working on a switched mode supply now I would use a lamp limiter before applying power for the first time. Would a 15w lamp be small enough to protect things ?
I agree with the comments about cold testing everything and not working live !
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Old 19th Jul 2018, 2:49 pm   #8
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Default Re: Obsolete Sunpower switchmode PSU

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Originally Posted by dragonser View Post
if I was working on a switched mode supply now I would use a lamp limiter before applying power for the first time. Would a 15w lamp be small enough to protect things ?
This isn't always a good idea with switched mode power supplies. The reduced mains voltage through a lamp limiter could cause all sorts of trouble at startup, and could even damage the power supply. It's very unlikely that the power supply was designed to be run via such a high resistance.

To give a couple of examples, one power supply I've just worked on has an auto 120V/240V switching circuit. Starting up via a lamp limiter could conceivably trigger it in to thinking it's working on 120V, then once things are up and running, the lamp cools, its resistance decreases and the power supply sees 240V. In its 120V mode that's not good news. Who knows whether it can handle that situation? Maybe it can, but this is not a good time to find out!

More commonly, the startup of a switched-mode power supply is always a delicate time. If the mains is coming via a high resistance, it's possible that the power supply could get stuck trying to start itself repeatedly, or even end up in a state where the main switching transistor isn't being driven properly because of the reduced supply voltage. Partly-switching-on a power transistor or MOSFET is the quickest way to kill it.

I'd advise checking for obvious short circuits, then hitting it with full mains.

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Old 19th Jul 2018, 2:56 pm   #9
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Default Re: Obsolete Sunpower switchmode PSU

I used to do a lot of SMPS repair and my approach, after checking all the obvious stuff like rectifiers and line shorts used to be to hang the scope probe (unearthed scope) on the collector of the main switching transistor and bring the power up slowly on a variac. Normally you will see some switching by the time you are up to about 40 or fifty volts. You can then begin to interpret what you see.

This method has been fairly successful but you must be aware of the dangers of using a possibly live scope, don`t do it if you are not sure you know what you are up to.
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Old 19th Jul 2018, 3:58 pm   #10
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Default Re: Obsolete Sunpower switchmode PSU

Quote:
Originally Posted by barrymagrec View Post
I used to do a lot of SMPS repair and my approach, after checking all the obvious stuff like rectifiers and line shorts used to be to hang the scope probe (unearthed scope) on the collector of the main switching transistor and bring the power up slowly on a variac. Normally you will see some switching by the time you are up to about 40 or fifty volts. You can then begin to interpret what you see.

This method has been fairly successful but you must be aware of the dangers of using a possibly live scope, don`t do it if you are not sure you know what you are up to.

Indeed !

My approach is to leave the 'scope earthed and feed the unit under test via an isolating transformer. I have an all in one variac / isolating transformer with voltage and current meters on the front for this type of work. It also has the merit of increasing line impedance, though obviously not as much as a lamp limiter. I always fault find at 110V whenever possible.
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Old 19th Jul 2018, 4:24 pm   #11
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Default Re: Obsolete Sunpower switchmode PSU

Read and appreciated, all. I'm going the cold test, replace crispies and stand well back method. Get your titters out...
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Old 19th Jul 2018, 6:59 pm   #12
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Default Re: Obsolete Sunpower switchmode PSU

Looks like Chris is on the money here.

Little cap, 47u/50v OK for value but off the clock on ESR. Chopper FET 2SK2545 short all ways up and board discoloured at leadouts. Probably enough to account for the blow-up, then.

So, replace big primary cap, 2SK2545, 470/25 output side caps (double diode on o/p checks out OK), controller chip, opto, small cap as above and cross fingers. Seem like a fair bet?
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Old 19th Jul 2018, 7:06 pm   #13
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Default Re: Obsolete Sunpower switchmode PSU

sounds good.
If the output side capacitors are decent quality they'll probably be ok, they wont blow the thing up on its own.
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Old 19th Jul 2018, 8:40 pm   #14
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Default Re: Obsolete Sunpower switchmode PSU

Minor comment, don't trust the opto, I always change them after a blow up.
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Old 19th Jul 2018, 8:45 pm   #15
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Default Re: Obsolete Sunpower switchmode PSU

Quote:
Originally Posted by Ted Kendall View Post
Looks like Chris is on the money here.

Little cap, 47u/50v OK for value but off the clock on ESR. Chopper FET 2SK2545 short all ways up and board discoloured at leadouts. Probably enough to account for the blow-up, then.

So, replace big primary cap, 2SK2545, 470/25 output side caps (double diode on o/p checks out OK), controller chip, opto, small cap as above and cross fingers. Seem like a fair bet?
Sounds good. Occasionally there are low value resistors (typically 4.7 Ohms or less) feeding the gate of the MOSFET, if fitted check they haven't blown open. A floating MOSFET gate is seriously bad news !
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Old 19th Jul 2018, 10:35 pm   #16
Ted Kendall
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Default Re: Obsolete Sunpower switchmode PSU

Quote:
Originally Posted by toshiba tony View Post
Minor comment, don't trust the opto, I always change them after a blow up.
Thanks, Tony - I agree and have ordered one of those too.
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Old 19th Jul 2018, 10:42 pm   #17
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Originally Posted by evingar View Post
Sounds good. Occasionally there are low value resistors (typically 4.7 Ohms or less) feeding the gate of the MOSFET, if fitted check they haven't blown open. A floating MOSFET gate is seriously bad news !
I think this would be the .33 ohm which has fried, but I'll be sure to check for any others around there...
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Old 20th Jul 2018, 1:15 am   #18
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Default Re: Obsolete Sunpower switchmode PSU

I would download a copy of the data sheet for the chip. It will give you quite a bit of the circuit diagram depending on the manufacturer. There is likely to be several chips with the same base number that will all work and have example circuits in there data sheets.
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Old 20th Jul 2018, 9:00 pm   #19
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Default Re: Obsolete Sunpower switchmode PSU

Progress - on a parallel track as yet, but progress nevertheless.

Most of the bits for the duff supply have arrived and been fitted, leaving only the chopper to go. Checks showed the ESRs on the big caps to be not too bad, but I changed them anyway for belt and braces since the thing's already popped.

However, bearing this in mind, I had a look at the other, recently untested supply. No burns, satisfactory ESRs on the bigger caps, ESR on the aforementioned small capacitor off the clock, as it was on the faulty supply. So, prayer mat and soldering iron out, change this cap, reassemble and switch on gingerly. Well, it seems to work. Respec', gentlemen!

Just one query on this example - it sings quietly to itself, and to be truthful I can't remember whether it used to or not. Of course, I've now got the twitch and am on the lookout for trouble! It could easily be HF magnetostriction, but I haven't otherwise used a beefy switchmode without a fan, which would easily drown this noise - it's rated at 4A total and cooling is passive through case vents.
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Old 20th Jul 2018, 9:16 pm   #20
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Default Re: Obsolete Sunpower switchmode PSU

It's not unusual for switchmode PSUs to sing, worse than line whistle; they sometimes drive me mad!
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