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'[EE] how much you can adjust output V of a switch '
2005\12\21@195034 by rosoftwarecontrol

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part 1 398 bytes content-type:text/plain; (decoded quoted-printable)

I need to control the output V of flyback/froward
switch power supply. But it seems very hard, due to that isolation transformer, which has fixed tune
ratio.

I test several switched power supply, after I decreased out put V, it shows very low loading
capability.
Any idea and success about this? Do switched power supplies only have fix output V?


part 2 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2005\12\22@170623 by Richard Prosser

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On 22/12/05, microsoftwarecontrol <spam_OUTmicrosoftwarecontrolTakeThisOuTspamyahoo.ca> wrote:
> I need to control the output V of flyback/froward
> switch power supply. But it seems very hard, due
> to that isolation transformer, which has fixed tune
> ratio.
>
> I test several switched power supply, after I
> decreased out put V, it shows very low loading
> capability.
>
> Any idea and success about this? Do switched power
> supplies only have fix output V?
>
>
Most definately not.
Usually the output voltage is controlled by changing the PWM ratio of
the drive to the output devices. Other control methods exist though,
such as changing the frequency or adjusting the phases to two
different drivers.

The turns ratio on the transformer is normally set by the control
requirements at either full load or no load - or maximum or minimum
input voltage.

RP
RP

2005\12\23@014158 by Steve Smith

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The little wart supplied by microchip for the pigstart+ will work as a 12v
battery charger it's a little 30-40 w flyback I just changed the voltage
feedback r. wound it up from 9 to 13.7  an it works ok.

Turning the voltage down may be a problem for the aux supply for the drive
chip it may go below its minimum operational voltage and go discontinuous
most of these chips have a very low current consumption until a voltage is
reached to enable resistive startup from line voltage. look up the data for
a 3842 this is a standard drive chip for a flyback found everywhere


Rgds Steve

{Original Message removed}

2005\12\23@024204 by Vasile Surducan

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On 12/22/05, microsoftwarecontrol <.....microsoftwarecontrolKILLspamspam@spam@yahoo.ca> wrote:
> I need to control the output V of flyback/froward
> switch power supply. But it seems very hard, due
> to that isolation transformer, which has fixed tune
> ratio.
>
> I test several switched power supply, after I
> decreased out put V, it shows very low loading
> capability.
>
> Any idea and success about this? Do switched power
> supplies only have fix output V?

No. Take a look how works the old UC3842-3845 .
The output duty cycle can be modified below 50% or above 50%.
The output loading capability it's a matter of design.

Vasile

2005\12\23@031302 by Peter

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On Wed, 21 Dec 2005, microsoftwarecontrol wrote:

{Quote hidden}

Yes and no. The output voltage is a function of the duty cycle in the
primary for a given transformer. But the power is limited by the maximum
primary current in the transformer. So to reduce the secondary voltage
more while maintaining some of the power output capability, you could
'cheat' by reducing the primary voltage. This is a hack.

good luck,
Peter

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