SMPS Forward Active Reset

gearheadgene · 1st May 2011, 12:06 PM

1) With forward design, multiple output, does it really matter which winding is used for regulation? I think that as long as the PWM regulates on one output, then all the windings will have the correct voltage (not like flyback, for example)

2) I need a bias winding for the PWM. I would prefer to make it flyback flavor for simplicity. With the rest of the transformer in forward mode, plus the active reset, I'm afraid there won't be enough energy in core to supply the bias supply. If so, I'll just go ahead and make the winding forward mode as well.

regards

gene

sawreyrw · 1st May 2011, 10:36 PM

I would need to think about this a little more to be sure, but I don't think the forward converter regulation issue is much different from a flyback. So I would regulate the highest power output, but be sure none of the others go out of spec. Yes, I would make the bias winding a forward winding, but it will cost you another inductor.

gearheadgene · 2nd May 2011, 11:32 PM

I will design with population option so that I can choose any of the outputs for regulation. At least the flexibility will let me recover if it turns out wrong.

Seems to me that flyback and forward are a lot different. On forward, no matter which output is chosen for reference all the other outputs are set by turns ratio. Since no power is stored in core, no problems of one winding starving another, right?

sawreyrw · 3rd May 2011, 02:48 AM

A forward converter and a flyback converter are a lot different, but the secondary voltages of the forward converter are related by the turns ratios. Neglecting the diode drops and assuming the output inductor currents never go to zero the average output voltages of the forward converter are Vave=D*Vsec.

hesener · 3rd May 2011, 08:49 AM

The output voltages are related by the turns ratios, but there is the so-called issue of "cross-regulation", meaning they will not all regulate as well as the one output voltage that is regulated, reason being the different magnetic couplings, winding resistance, different rectifiers etc etc.

As mentioned by sawreyrw, regulating the highest power output is a good idea, but its still necessary to check that the other voltages do not go too high. you may use post-regulation (e.g. a linear reg) or take of a turn or so to keep these voltages in spec.

gearheadgene · 3rd May 2011, 05:36 PM

One reason I bring this up, is the high-power winding may not always be active. This supply is going to +/-40 (amp), +/-7 (pre-regulated analog supply) and +5.0 (logic). It is conceivable that the 40V portion may be inactive at times. Maybe the answer is that the 40V rails need a minimum load in order to keep the regulation - so place some active or passive load to keep things tidy.

hesener · 4th May 2011, 07:17 AM

Then I would definitely use post regulation for logic and analog - once those voltages go above their limits due to no load on the 40V, it will be time to say goodbye..... ,-(

1st May 2011, 12:06 PM	#1
gearheadgene diyAudio Member Join Date: Jul 2005 Location: CT	SMPS Forward Active Reset 1) With forward design, multiple output, does it really matter which winding is used for regulation? I think that as long as the PWM regulates on one output, then all the windings will have the correct voltage (not like flyback, for example) 2) I need a bias winding for the PWM. I would prefer to make it flyback flavor for simplicity. With the rest of the transformer in forward mode, plus the active reset, I'm afraid there won't be enough energy in core to supply the bias supply. If so, I'll just go ahead and make the winding forward mode as well. regards gene

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1st May 2011, 10:36 PM	#2
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	I would need to think about this a little more to be sure, but I don't think the forward converter regulation issue is much different from a flyback. So I would regulate the highest power output, but be sure none of the others go out of spec. Yes, I would make the bias winding a forward winding, but it will cost you another inductor.

2nd May 2011, 11:32 PM	#3
gearheadgene diyAudio Member Join Date: Jul 2005 Location: CT	I will design with population option so that I can choose any of the outputs for regulation. At least the flexibility will let me recover if it turns out wrong. Seems to me that flyback and forward are a lot different. On forward, no matter which output is chosen for reference all the other outputs are set by turns ratio. Since no power is stored in core, no problems of one winding starving another, right?

3rd May 2011, 02:48 AM	#4
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	A forward converter and a flyback converter are a lot different, but the secondary voltages of the forward converter are related by the turns ratios. Neglecting the diode drops and assuming the output inductor currents never go to zero the average output voltages of the forward converter are Vave=D*Vsec.

3rd May 2011, 08:49 AM	#5
hesener diyAudio Member Join Date: Mar 2005 Location: Munich, Germany	The output voltages are related by the turns ratios, but there is the so-called issue of "cross-regulation", meaning they will not all regulate as well as the one output voltage that is regulated, reason being the different magnetic couplings, winding resistance, different rectifiers etc etc. As mentioned by sawreyrw, regulating the highest power output is a good idea, but its still necessary to check that the other voltages do not go too high. you may use post-regulation (e.g. a linear reg) or take of a turn or so to keep these voltages in spec.

3rd May 2011, 05:36 PM	#6
gearheadgene diyAudio Member Join Date: Jul 2005 Location: CT	One reason I bring this up, is the high-power winding may not always be active. This supply is going to +/-40 (amp), +/-7 (pre-regulated analog supply) and +5.0 (logic). It is conceivable that the 40V portion may be inactive at times. Maybe the answer is that the 40V rails need a minimum load in order to keep the regulation - so place some active or passive load to keep things tidy.

4th May 2011, 07:17 AM	#7
hesener diyAudio Member Join Date: Mar 2005 Location: Munich, Germany	Then I would definitely use post regulation for logic and analog - once those voltages go above their limits due to no load on the 40V, it will be time to say goodbye..... ,-(

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