Schottky diode in SM PSU o/p?

Hi all,

I was given a 'dead' cheapo 20V Lithium drill battery, charger and wall wart to look at by a mate tonight.

Plug PSU (wart) into wall socket, clip 'remote' charger interface onto battery, switch socket on ... nothing.

I took the covers of the charger clip module and connected it onto the battery, measured 19 or so volts on the battery pins but nothing coming from the PSU.

However, after a few minutes I felt the PSU was slightly warm so assumed it was doing something?

I disconnected the PSU output leads from the PSU at the charger module and connected my bench PSU at 20 and with the current limited to the same 400mA as the PSU and the green 'Charged' LED came on constantly.

I rigged up a load (a couple of 12V 60W headlights in series) onto the battery and let it run for a while and then hooked it back up to the charger, charging LED now flashing, indicating it was charging.

DMM on the unloaded o/p of the PSU, nothing.

After leaving it disconnected for a while ... I cracked the PSU open with my toffee hammer and gave the board a look over, nothing obviously burned and no dry joints etc. I checked the bridge rec and a couple diode over and they seemed ok, but an axial series diode (SJ220 from memory) on the output that was driving a cap that went to the remote charger module seemed to be short?

I found the spec somewhere that suggested it was a 2A 1000V (?) device and I have a couple of questions please?

I know Schottky diodes have fast switching times (and a low forward voltage drop but not sure if that would be relevant here?) so is a Schottky required in what I think might be part of a 'charge pump' circuit because of the frequency of the output through the SMPSU and assuming I decoded the markings correctly, why would it need a 1000V device there?

Cheers, T i m

Reply to
T i m
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Use the Schottky diode. Sometimes a fast/soft recovery diode will work, bu t I've run into problems where the drive IC or transistor will run hot or f ail or the secondary voltage will be lower if the Schottky diode is subbed from original spec. Also, sometimes those diodes check very leaky even wh en they're not - depending on your meter and the polarity of the leads, ev en when not using the diode scale. You can put in a fast recovery to see i f the circuit works, but I wouldn't run it more than a few seconds.

Reply to
ohger1s

Output rectifiers of simple wide-range input flybacks will tend to see voltages stresses unrelated to the output voltage.

At the same time, HV diodes will be relatively inefficient, due to slow speed and high Vf.

If you can find real info on the part, it'll be easier to make a safe replacement. Seeing as the original part failed, it would be unwise to skimp on any of it's intended ratings.

RL

Reply to
legg

=============

** That cannot be a Schottky.

The highest voltage available is about 200V.

Most of your facts are WRONG !

..... Phil

Reply to
Phil Allison

Yeah, I will, I was just wondering why / if they were used in that role? ;-)

Ok.

I'll check it again but I believe I measured the resistance both ways and it looked short ... and the PSU wasn't working (but warming) and everything else I could check tested out ok.

Understood. I'll wait for the right things to arrive and keep my fingers crossed it was that. ;-)

Assuming the spec I looked up was correct, and it was a 2A device, and the PSU itself was rated as 400mA, I wouldn't have thought straight current would have taken it out so would there me a recommended 'safe' startup procedure that might put the least shock load on the diode?

eg, Should you connected charger clip to the battery first, then switch on at the wall (and likely what it says in the instructions) ... or switch on first (allowing the PSU to get the output cap charged up) and then connect it to the battery?

Wall wart / PSU lv Lead charging clip / LED status Battery.

Cheers, T i m

Reply to
T i m

Ah, ok.

Check.

Ah, it's an SR2200 JF (not SJ220, that was a result of my dyslexia ) that looks to be a

200V 2A Schottky Barrier Diode.

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Understood, I'll get the right thing ordered now.

Cheers T i m

Reply to
T i m

Oh?

The eBay listing suggested it was a 'Axial Lead Schottky Diode HER208 Ultra-fast Recovery Diode MIC 2A1000V DO-15' so maybe it was wrong / misleading?

Most of my facts were questions but as I have mentioned since and with a better / closer look, it's an SR2200 (not an SJ220) and that is indeed a 200V, 2A device so will order one now.

Cheers, T i m

Reply to
T i m

----------------

** The HER208 is a fast recovery silicon diode, not a Schottky.

... Phil

Reply to
Phil Allison

ms where the drive IC or transistor will run hot or fail or the secondary v oltage will be lower if the Schottky diode is subbed from original spec.

pending on your meter and the polarity of the leads, even when not using th e diode scale.

't run it more than a few seconds.

400ma is the maximum continuous duty design output, but during the first fe w cycles, it's essentially feeding a short circuit until the filter caps ch arge. Most modern smps circuits ramp up the drive in a controlled manner u sing a slow or soft start strategy to mitigate that. Still, sometimes diod es just short, and I've seen more random Schottky failures than other types .

I wouldn't worry about the start up procedure unless there are specific ins tructions to the contrary. Consumer electronics are generally designed to be plug and play regardless of the sequence.

Reply to
ohger1s

Understood.

This is the PSU OOI:

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The IC is a CR6338T.

Oh, ok.

Agreed, I was just thinking that this being a 'cheap and cheerful' example of this sort of thing, if they may have cut more corners than more expensive brands might, making them slightly less 'durable' under some circumstances ... how they handle the 'what if' situation?

But as you say, it could have just been one of those things and if it works it could be fine till it's worn out. ;-)

Cheers, T i m

Reply to
T i m

As I was looking back into it that's what I surmised Phil.

I've ordered the right thing now that should be here soon and I'll let the group know how I get on.

If it's not that there isn't much else outside the CR6338T. ;-)

Initially I was just going to look for a generic 20V, 400mA PSU but thought I'd open this one up just to see if there was anything obvious.

Cheers, T i m

Reply to
T i m

few cycles, it's essentially feeding a short circuit until the filter caps charge.

a slow or soft start strategy to mitigate that.

ailures than other types.

instructions to the contrary. Consumer electronics are generally designed to be plug and play regardless of the sequence.

Years ago when everything was discrete, we saw plenty of power supplies so badly designed that an excessive load or even a brown out would cause a cat astrophic failure (Sony). Today, an properly designed IC SMPS controller i s literally pennies each, and they have all that solved internally. Your I C does have a soft start internal to it and overcurrent protection, so the shorted Schottky diode did not seem to destroy the IC (check pin 5/6 to 4, should be no short on the internal mosfet).

Reply to
John-Del

;-(

Ok.

Phew.

5/6 to 4 test out like a pair of diodes but with a low resistance (either way) between 5 and 6?

Cheers, T i m

Reply to
T i m

so badly designed that an excessive load or even a brown out would cause a catastrophic failure (Sony).

, and they have all that solved internally.

, so the shorted Schottky diode did not seem to destroy the IC

5 and 6 should read short as they are the common drain pin and internally t ied together inside the IC. Don't use the diode scale when checking for le akage between those pins. Pins 5 and 6 are the drain, and pin 4 is the sou rce. Out of circuit, it should read virtually open between pins 5/6 to pin 4.
Reply to
John-Del

Just a quick update for the records ... I replaced the diode with the right / same one, taped the PSU together, stuck my DMM on 200V DV on the output and gingerly turned it on. There was a slight pause and then the DMM showed the voltage quickly (but not instantly) ramping up to 20 volts. ;-)

I switched it off, unplugged it and watched the output drop back down to 0V and then soldered the wires back onto the charger clip, screwed it back together and slid it onto the battery.

Plugged in, turned on, a second or so pause then the green 'Charging' LED started flashing ! ;-)

I'll leave it till it indicates it's fully charged then seal the PSU back up again [1] and should have a happy mate. ;-)

Thanks to all those who responded ... another load of stuff (charger, battery, drill) that has been saved from the waste ... for the sake of a few pence worth of diode (and my free time of course, which is the big issue for those not like us [2]).

Cheers, T i m

[1] I will leave him with the instruction that he's not to pass the thing onto anyone else. [2] Now to try and find out why our soup maker was working fine one minute and then completely dead the next. ;-(
Reply to
T i m

MOM?????

Reply to
John-Del

It tried to feed on your own soup?

Reply to
Sjouke Burry

At 90+ she still seems to be plodding along ... no, one of those electric jobbies where you just chuck the ingredients and some water in, press 'Chunky' or 'Smooth' and it beeps to tell you when it's done ...

Actually, since I mentioned it here I took it to bits, tested it in bits (carefully, another SMPSU) it seemed to come to life and it still seems alive now it's back together?

I couldn't see any dry / bad solder joints so will see how it goes and if it dies again, check all the crimps / plugs / sockets out.

(I bought a disability scooter that was my Dads, back of a mate with a fault and that turned out to be a connector within a multipole plug had pushed though and was not making contact).

Cheers, T i m

Reply to
T i m

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