Thread: Anyone try a Forward Converter?

Originally posted by Probedude_2000:
<STRONG>Presslab,
7.5V for microseconds? I understand that there are instantaneous high peak current loads but I can't see that your power supply would need to work down to 7.5V when it only lasts for microseconds.

Have you monitored the output during this brief dip and seen that it is affected? Seems that if it was you could stiffen the caps on the output to carry you through the dip.

Lastly at 125Khz and a 60W load, what is your peak primary current? Seems like it would be way more than 30A. Have you monitored the current sense resistor and verified that you are not saturating the core under load? (esp at 7.5V)

How hot is your core getting? (even when idling without air flow for say 10mins) The biggest problem I had with a similar design was finding a core that could handle the peak current ratings without saturating or being so inefficient that it would get scorching hot without much of a load connected.</STRONG>

The 7.5V dip is measured at the power supply itself. This causes the switching to stop for a brief moment, which trips the power good circuit on the secondary side. This resets the motherboard. I could fudge the power good circuit, but ideally it would be able to power through the dip.

I'm not sure of the peak current, I don't have any current mode control on it, hence no sense resistor. I haven't blown any transistors yet. Oh and by the way, there are two primary windings switched simultaneously, sharing the load between the two, as well as two transistors. They need a fan on them, but with a fan at ~80W they are about 120deg F. (Finger test, FWIW)

The core does not get perceptibly warm to the touch, even under high load. If the power supply is 'on' but with no load I think the idle current is around 150mA, not bad at all, although I haven't actually measured it, just read it off my test supply with an analog meter.

I've noticed some weird oscillation in the MHz at some duty cycles, I think it is my catch diode on the primary side, I'm going to try another and see if I can get rid of it.

Was the design you tried a Forward Converter or a Flyback? I could see core saturation with a Flyback especially if the core is not gapped.

I attempted a Flyback Converter a few months ago, and procedeed to blow most of the components at one point or another. Hence the Forward Converter topology I have now!

Presslab

It was a current mode flyback design. I was designing for a bit over 100W output (12V 5A, 5V 8A, -12V 100mA), using the SG3825A, so with inefficiencies it was being designed as a 130W'ish supply.

I was using large toroids as the core material at first and even disassembled a few E-cores from PC power supplies to get some gapped cores. Still the core inefficiencies resulted in the cores getting much too hot under mild loads due to the high dI/dt and the resultant high peak magnetic flux change through the core. Core saturation wasn't a problem until I tried to go to higher operating frequencies (this is why I stuck with 25Khz).

It was a nice small board however (about 3x3x2"). I'm thinking of pulling it off the shelf and redoing it with a push pull primary. That will help greatly with core losses/lower my peak primary current.

Originally posted by Raas:
<STRONG>Yeah.. me too..

Finnally a way to decent power..

Well, I'm going to take a psu apart, just to see if I can follow what they are talking about..

This just sound too interesting...

Hell.. I even want to make a website if I weren't going on a holiday in a couple of days (another 1,5 days to work)

Quick Question PressLab:
You said you did some testing.. and said that you had 60w of power with an 7v input.
Does this only concern the 12v line ? or is it the total power on all the voltages ? Just curious... Would be great to see that you could pump out 12v @ 5A with just an 7v input..

Keep up the good work mate !!!

Greetz</STRONG>

Most of the power was on the 3.3V and 5V lines. I had two 1 ohm resistors loading those lines, while running the PC and hard drive. So not a whole lot on the 12V line (just the hard drive). The 12V is not used for too much, I've hooked 3 drives up at once to test it, and they worked fine. I need a different load resistor to really test it all correctly, but it's good enough for me.

Presslab

Okay everyone,

Here are some pictures of the PSU. Don't make fun, it's a prototype!! I also have a drawing of the transformer winding stuff. I will work on the schematic tomorrow, so wait a day or two for that.

Presslab

DC-DC PSU Pictures