Post by thread:[EE] how much you can adjust output V of a switch

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)

On 22/12/05, microsoftwarecontrol <spam_OUTmicrosoftwarecontrolTakeThisOuTyahoo.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

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}

On 12/22/05, microsoftwarecontrol <.....microsoftwarecontrolKILLspam@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

On Wed, 21 Dec 2005, microsoftwarecontrol wrote: {Quote hidden}> 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? 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