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SMPS: I'm Topologically Challenged

Hi,

I'm in need of a pointer to a couple of representative circuits so that I can get my brain around the particular configuration used in the SMPS I'm working on. It is a Boschert OEM type (XL131-3604E, HP

0950-1879) used in an HP54502 DSO. Before I accidentally blew the switching transistor, it would only work in bursts causing the several output series regulators to deliver less than perfect DC with negative-going saw tooth waveforms on top. The unit has worked in the past but, before this happened, the mother board would now and then reset perhaps due to dirty DC.

The switching circuit consists of a single TIPL755A power transistor, two transformers, a CNY21N opto-isolator (fed from the +5V line), a

2N2647 unijunction transistor and one each of PN2222A and TIP41C transistors as well as the usual collection of diodes, resistors and capacitors. My difficulty is that this topology is unfamiliar to me and I cannot easily see how a single unijunction oscillator with the help of two other transistors could form a PWM. I would have expected at least a flip-flop but this circuit is all discrete.

Currently I'm taking the easy way out and am checking everything, as far as I can, and replacing all the semiconductors on the input side of the PSU; the multiple output regulator circuits being a nightmare, I'm hoping the fault isn't in there. I've tried hard to find the schematic for this unit but don't now ever expect to do so; the HP 'so-called' service manual does not contain any useful information.

BTW, I've read Sam's repair FAQ and 'googled' away for ages.

TIA - Joe G3LLV

Reply to
Joe McElvenney
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When I was faced with a confusing discrete build SMPS a year ago, I found a book in the public library, old enough stock to be helpful. Had "switch mode" or "switched mode" power supplies in the title. IIRC it was that major USA publisher of electonic titles, the name escapes me at the moment.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

On Sat, 13 Dec 2008 14:34:43 +0000, Joe McElvenney put finger to keyboard and composed:

See ON Semiconductor's SWITCHMODE Power Supply Reference Manual:

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It discusses various SMPS topologies including buck, boost, buck-boost, push-pull, half-bridge, full-bridge, half-forward.

It includes many circuit examples.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.
Reply to
Franc Zabkar

On Sat, 13 Dec 2008 14:34:43 +0000, Joe McElvenney put finger to keyboard and composed:

The service manual for this TV chassis has an in-depth discussion of how its SMPS works:

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The SMPS consists of discrete transistors and an optocoupler for feedback. Regulation is achieved by monitoring the current in the primary winding and turning off the chopper when the current rises to a level determined by the feedback voltage from the optocoupler.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.
Reply to
Franc Zabkar

IMHO the most common failures are some of the electrolytics at the front end, so if you have an ESR meter check these first. Then any Schottky diodes. Then the opto-isolator.

--
*In some places, C:\\ is the root of all directories *

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

Hi,

To those who replied to my 'cri de coeur' - many thanks.

Franc - for the pointers to the book and the TV schematic which appears to use a complementary pair to simulate a unijunction transistor and so should be of great help.

"Diverse Devices" - for a very informative web site and the public library suggestion.

Dave - for your comments on the main electrolytics.

All have been taken on board and will be made use of. I am currently awaiting a couple of spare TIPL755A transistors from the States and expect to be on the job in the New Year.

Happy Christmas - Joe

Reply to
Joe McElvenney

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