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Forward converters without explicit reset windings

When I first learned of forward converters -- largely from Pressman's book -- they were drawn with "reset" windings and catch diodes to ensure that the magnetizing current had somewhere to go when the power siwtch turned off. This is shown in, e.g., Linear Tech's application note 19: page 16 of

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. However, looking at contemporary controller ICs for forward coverters, that reset winding is gone. See, e.g., the "typical application" for the LT1950 here:
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.

In thinking about this, it seems to me that when the power switch turns off, since current is flowing in the secondary anyway, there isn't some huge voltage that the power switch has to deal with (assuming leakage inductance is zero for the moment)... instead the magnetizing current just gets dumped into the secondary (almost like a flyback), with the core thereby seeing a reset voltage that's Vsec-Vout. Is this correct?

The application note referenced above states that, "Without this [reset winding and diode clamp] switch voltage would jump all the way to breakdown at the moment the switch is opened due to the magnetizing current flowing in the primary." That doesn't seem to follow, since producing such a voltage implies that the flux in the core would have to drop rapidly, and since the secondary is conducting there's no reason to expect such a precipitous drop.

Thoughts?

---Joel

Reply to
Joel Kolstad
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Most forward converters driven by a MOSFET have large enough capacity in the FET drain to ground and transformer primary to absorb the magnetizing energy before the drain voltage exceeds 2xVin which is where most reset windings clamp. If you examine the drain voltage starting at turn off you will see a step up to Vin then half a sine wave, who's peak is less than

2xVin and returns back to Vin before the next turn on. If it does not return before the next turn on, you then need the extra winding and catch diode. Designed many forward converters, never used that winding. Studying the toff rise time and haversine voltage yields much info on the parasitics of the MOSFET and transformer. Cheers, Harry
Reply to
Harry Dellamano

Hi Harry,

:-) I appreciate that definitive answer! I understand why now -- to keep the flux circulating the right way, the magnetization current needed in the secondary is opposite that which the secondary's rectifier diode will allow. (In other words, the diode needs to be oriented such that you've now built a flyback instead of a forward converter.)

Ah, there's the "secret." Too bad this doesn't appear on data sheets!

...or a snubber, I suppose.

That's quite helpful information, thanks again.

---Joel

Reply to
Joel Kolstad

I would ring a bell of caution on that one. Steady state your converter waveforms might look all pretty and seem wonderful. However during start-up/overload/current limiting or some other fault condition you may/will still end up with overvoltages on components and possible excess power dissipation.

Then there's manufacturing spread and the chance that some kind manufacturer will change the process and your mosfet will become an 'improved' device. Last, but not least, when you've left or even while your there someone will substitute a different device without knowing how you were relying on Cds and not bother to look at what the result is......

DNA

Reply to
Genome

Responding to myself here: Maxim has an application note on these so-called "resonant reset" forward converters here -->

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Reply to
Joel Kolstad

The main thing you should think about is that the Volt-seconds which the transformer gets when your MOSFET is on HAS TO be equal to the Volt-seconds when the MOSFET is off. How you control this depends on how much power your MOSFET can take. IT you just let it fly into the MOSFET drain, it''' avalanche until well, like I wrote, until V*s off equals V*s on. If you have a rest winding, you can control exactly what voltage the transformer gets when being reset.

Reply to
HVYMETAL

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