12v=19v 4,7A dcdc

[N-Channel]

Well, I don't know what to say. It works beautifully. If I remove the feedback, it spikes to over 24V when minimally loaded. Respectfully, I refer you to App notes on Linear's LT3782, 2-F Step-up Converter driver chip. (Not the chip I used. I used the SG3525, though the principle is the same)

Not wanting to argue here, but this is what I have experienced.

[megajocke]

Interesting, I'll have to look at that. I'm not doubting the performance of your circuit either. Even if it isn't/wasn't a flyback it still is able to do what you want it to do. Your boards look very nice! Am I correct that it is wound like a push-pull transformer primary would be?

I'd expect the opposite MOSFET to clamp the output - but the leakage inductance lessens this of course. How much load can it take above 24V?

[megajocke]

I must add:

I am sorry if I did offend you in some way, didn't mean to. I should probably have worded my posts a bit differently. I tend to forget some of the important differences between English and Swedish when writing...

Regards,
Joakim

[N-Channel]

No big deal. I know the "lost-in-translation" thing can be a PITA (Pain In The A$$) sometimes.

As for the windings, yes, you are correct in understanding that it is wound like the primary side of a transformer. And, I must say that I chose my words poorly: The other half of the MBR2545CT doesn't clamp the spike, the other MOSFETs source does. The spike is limited to 19.5V by the feedback section of the '3525. As for loads above 24V, I don't know- I've never tried it. Good idea to do sometime.

Another Application note to look at is the Unitrode division of Texas Instruments, UCC3806. This is the BiCMOS version of the 1846/2846/3846 family of current-mode PWM chips. Last diagram shows the "autoformer" topology being substituted for the true transformer where isolation is not necessary.

[N-Channel]

Originally posted by luka
Hi Steve

Nice work as always, PCB looks like DIY, without silk and solder mask, I always like plated holes, makes life more easy. Is that heatsink big enough? Will you take one more pic when finnished? Maybe take output ripple at full output? Anyway look great and I hope it works that way to

Thanks, Luka. Tried first unit last night, promptly blew a 10A Fast-blow fuse when I plugged it in. Probably a bad solder-job. I have crappy Radio Shack soldering irons- I guess ya' get what ya' pay for. Just borrowed one of the technicians' Weller soldering stations that goes up to 830 Fahrenheit or 443 Centigrade. This should work much better. Will report tomorrow on results.

If I can translate Fluke readouts to screen, I will post waveforms.

[megajocke]

Is it this appnote?

http://focus.ti.com/lit/an/slua150/slua150.pdf

If you look at the schematic of the autoformer topology it does have an output inductor like a regular forward converter. When switches are off diode cathode node is at Vin. When a switch is on the node will sit at 2*Vin.

If you remove the inductor from this topology it will be like those SMPS often used for car audio - no output inductor but still some regulation is possible as long as one doesn't try to go too low down in voltage thanks to the leakage inductance. I'm pretty sure you could get better performance by substituting the inductor you have with a ferrite core transformer as I'm pretty sure most energy is delivered directly during the forward cycle right now and very little by flyback action. Adding an output inductor after output rectifier on top of that might give even better performance. (But one more component)

[luka]

I guess ya' get what ya' pay for

that has been right most of the times with me too

If I can translate Fluke readouts to screen, I will post waveforms.

Please do.

[N-Channel]

Too late to put inductor right after rectifier, board pattern is fixed. But, I have C-L-C output 2200uF-2uH-2200uF.

[Eva]

It's never too late when it comes to correct the wrong circuit topology...

I suspect there is not such a thing as a buck, boost or flyback converter with a single inductor, two windings having opposite polarities and two "push-pull" switches. It's probably relying on the small (air) leakage inductance for working, rather than on main (core) inductance as expected.

[lumanauw]

If you remove the inductor from this topology it will be like those SMPS often used for car audio - no output inductor but still some regulation is possible as long as one doesn't try to go too low down in voltage thanks to the leakage inductance. I'm pretty sure you could get better performance by substituting the inductor you have with a ferrite core transformer as I'm pretty sure most energy is delivered directly during the forward cycle right now and very little by flyback action. Adding an output inductor after output rectifier on top of that might give even better performance. (But one more component)

Talking about SMPS used in car audio, they usually put quite big capacitor bank for secondaries, without any inductor.
Is putting big cap bank (without L) in secondary has the same effect with LC (where C is small compared to the first case)?