"Arfa Daily" writes:

"Dotinho" wrote in message
ps.com...
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.

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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