Power Supply with transformer 300V

Hello, i have i power supply of Siemens in one olde machine, and i have to repair it, but i don't know how.

the essencial, the 1 step is transform 220 VAC to 315VDC and then a MOSFET will pulse the 300VDC at 50KHz to a transformer that out 5V at secudary, the problem is that tranformer giver out 8V and don't work..

Anyone can tell me if i change the frequency,. the output volt will decrease?

Thanks.

Reply to
Dotinho
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It sounds as though you are talking about a switch mode power supply here. Although the switching frequency has some effect on the final output due to the fact that the transformer is designed to run at a particular frequency, the basic control for this type of supply is achieved by altering the mark space ratio of the drive to the MOSFET.

However, I'm not convinced that this is actually what you are talking about. Are you saying that the supply has a *DC* output of 8v instead of a correct

5v ? If so, then it is impossible to offer valid advice on what the problem might be, without knowing the *actual* regulation scheme that the supply uses. For instance, all voltage setting and regulation might be achieved by a control loop that operates on the mark space ratio of the MOSFET drive, in a scheme known as pulse width modulation, or PWM. On the other hand, there might be a basic fixed drive ratio that results in some arbitrary voltage a few volts above the required 5v - possibly the 8v that you are seeing. This might then be followed by either a linear 5v regulator, or even in some cases, a further switching regulator. Still further, the supply could use a combination of any or all of these schemes.

If you are not familiar with the principles of switchers, you might struggle to get to the bottom of any problem that there might be. These items are notorious for being very fickle. Also, very importantly, I would warn against working on the supply without it being at least connected via a proper bench isolation transformer. These supplies are VERY dangerous to the point of being potentially lethal, if you do not understand fully their principles, and the places to avoid touching.

Arfa

Reply to
Arfa Daily

Another point is that if you're testing it without a proper load, outputs could be higher or lower than spec'd.

There is general info on SMPS repair at the sites below.

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Reply to
Sam Goldwasser

Thanks Sam. A valid point I forgot to mention.

Arfa

Reply to
Arfa Daily

Its okay sam, but the PWM have some protections, and if output voltage was more than 6.5V it disable every thing, with or without load..

the tranformer are malfunction, and i can't get another becouse don't produce anymore, this power supply is very old, and i have to repair it, decrease the output tension, and i dont know how..

there is no regulation, just exist a small feedbak for output protecion, if voltage was superior to 6.5V it disable everything. thats all..

The shematic ir very simple, it rectifier 220VAC from nets, to 315VDV wih 4 diods and 2 big capacitors, than the tranformer is conected in serie with MOSFET, pulsing that 315VDC to tranformer, and the there is

3 output, one at 5VDC with 18A and another 24V with 0.5A...

The problem is the tranformer, the mosfet is directly connected to PWM with one wire, there is no regulation!!

How can i reduce the power changing the shema??

thanks

Reply to
Dotinho

I would suggest that you are missing something somewhere. OK, so the MOSFET is connected to the PWM with one wire, but I would be 99% sure that this is where your trouble lies. Something somewhere is generating that PWM, and it is likely doing it with the wrong mark space ratio. I would be very surprised if a switcher is producing a 5v rail specced to 18 amps, without some kind of basic feedback. Usually, a 5v rail in older kit, with that sort of current capability, is supplying a bunch of 'straight' or 'LS' 74 series logic. That stuff is critical of its supply voltage to around +/- 0.2v.

I can't actually think of a failure mode for the transformer, which would cause it to produce excess output. In fact, SMPS transformers are incredibly reliable across the board. Since they've become really popular over the last

25 - 30 years, I can only think of one item of equipment that had the transformer fail regularly, and that was an early DB sat receiver. In that case, the primary used to fail with shorted turns, which led to catastrophic failure of the MOSFET and input fuse.

How is the PWM generated in the first place, and are you absolutely certain that there is not an optoisolator hiding somewhere ? How does the overvoltage shutdown circuit work ? Are you sure that it does not also serve as the basic control loop, and only has the shutdown function as a last resort if control is lost ?

Arfa

Reply to
Arfa Daily

Arfa Daily escreveu:

ya ya, i know that. perhaps i should send the shmatic? is that okay?

dotinho

Reply to
Dotinho

No problem. Send it.

Arfa

Reply to
Arfa Daily

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