Power Supply with transformer 300V

[Dotinho]

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.

 

[Arfa Daily]

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.

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

 

[Sam Goldwasser]

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.

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.

http://www.repairfaq.org/sam/smpsfaq.htm

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Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: Anything sent to the email address in the message header above is
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subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[Arfa Daily]

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

Thanks Sam. A valid point I forgot to mention.

Arfa

 

[Dotinho]

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

 

[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

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

 

[Dotinho]

Arfa Daily escreveu:

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

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

dotinho

 

[Arfa Daily]

Arfa Daily escreveu:


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

dotinho

No problem. Send it.

Arfa