low output voltage high efficiecny AC/DC converter

[ku637]

Hi,
I need to build an AC/DC converter
Input 85-265
Output 1 = +18V/0.35A
Output 2 = +3.3V/2.5A

overall Efficiency needs to be close to 85%

While examining the flyback topology.It seems that for lower output voltages the rectifier diode loses comes into play and efficiency suffers

Is there any method to achieve the above stated voltages and efficiency.

IS there is any synchronous rectifier topology for flyback & if it exists can some one tell me how it will work.How the gate pulse is generated in the secondary to make it synchronous.?

Thanks for any help

 

[ZekeR]

Here's a flyback controller with a synchronous secondary gate drive, you should check it out. I think it can do what you need.

 

[ku637]

But it seems like a DC/DC converter ..how to use with AC to DC converter application..just putting some zener or something like that will do??

 

[BradtheRad]

It seems that for lower output voltages the rectifier diode loses comes into play and efficiency suffers

Yes, an ordinary diode (.6 V fwd threshold) dissipates several watts during each 'On' portion of the cycle, since you are drawing several amps during each pulse.

If you use a Schottky (low fwd threshold) diode then you gain some efficiency. Nevertheless if your output V is 3.3, the diode alone wastes 10 percent of the power.

IS there is any synchronous rectifier topology for flyback & if it exists can some one tell me how it will work.How the gate pulse is generated in the secondary to make it synchronous.?

This article might help.

**broken link removed**

 

[ZekeR]

But it seems like a DC/DC converter ..how to use with AC to DC converter application..just putting some zener or something like that will do??

To turn AC to DC, you need to use a bridge rectifier. The major components in the design (input capacitors, flyback transformer, etc) will need to be chosen specifically for your purpose. I'm assuming you don't care too much about PFC; if you do, then there's even more work to do.

I recommend you study some more about offline conversion. Here is a non-synchronous offline converter that you can look at for reference (it has built-in PFC, so don't be too surprised if the way some things are done is a little different).

 

[ku637]

To turn AC to DC, you need to use a bridge rectifier. The major components in the design (input capacitors, flyback transformer, etc) will need to be chosen specifically for your purpose. I'm assuming you don't care too much about PFC; if you do, then there's even more work to do.

Thank you for the reply.. I understand that for AC-DC conversion we need to have a bridge rectifier..but then u get a say 110*1.414=155.54DC or 230*1.414=325.22DC..This will be fed to to flyback transformer.. hoping that we select the transformer and the MOSFET of appropriate rating as u suggested..The Controller should start up from this high voltage DC right..so is that being done with some zener to bring down the DC to the input range of the controller??I checked the LT3825 but its not showing any AC/DC type application...may be isnt it designed for that purpose or so that was my doubt..may be im quite wrong here..plz correct me if im wrong
by the way I dont bother much about PFC right now..all i care is efficiency and low cost.

 

[mtwieg]

The LT3825 should be able to work as an offline converter, but yes you'll need a bit of extra circuitry, since it can't run directly off of more than 18VDC. Like many offline converters, the LT3825 has a built in shunt regulator (effectively a zener), so you can bias it's VCC with a resistor from the high voltage DC bus. That is actually sufficient in itself for powering the IC, but it's pretty inefficient, especially for offline applications. So they recommend using the feedback winding to provide the power to the IC after startup, which is much more efficient. The shunt regulator is still necessary for startup though.

Transformer design is going to be your greatest challenge, since you need specific turns ratios for four windings. The power-handling windings will need to be low ESR for good efficiency, and the secondary windings should be extra low ESR if you want decent cross regulation.

 

[goldsmith]

Dear ku637
Hi
You can use a simple single ended forward converter . because if efficiency is important for you , you can use fly back . and it's cause is , that your out put can't be without load . and of course with full load it's efficiency is lesser than forward converter . you can use a simple UC3845 as your driver .
Best Wishes
Goldsmith