improving cross regulation with synchronous rectification

Post-regulating off a winding of the main isolaton transformer, depending on topology, ends up having to deal with the voltage stress, lower duty cycle and leakage energy associated with same.

It can be done, but there's seldom a profit generated by the approach, and it usually means discrete components in series with rectifiers that are there already - doubling losses. I've only used it where a single extra output was required as a late addition, at a time when discrete switching regulators were rare and overpriced.

Integrated controllers have a much easier time dealing with pre-regulated DC inputs,where the second converter losses need not be as large as when operating at the stress levels of the actual pre-regulator.

CS5101 from ON was one of the early parts developed specifically for the job in single-ended forward converters. I used a simple 3842, synchronzed by the input waveform being regulated.

RL

Forgot to mention that there are other methods of reducing cross-regulation. These involve degenerating the main regulation loop, or introducing a negative impedance in series with the main regulation loop, to counteract main load effects (only) on main duty cycle.

As this is only one regulator, improving the performance of all auxiliaries, it can pay off to develop loss-free methods. All that's left to contend with in auxiliaries, is load regulation.

RL

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The idea is to use synchronous rectification on both the main and aux outputs. this increases efficiency considerably, especially for the low 1.8v. (1.8v is for a dual 400mhz DDS.)

The regulation comes about when the output needs to be reduced by controlling the MOSFET that is used as a synchronous rectifier:- a) not turning the MOSFET on but wich still leaves the body diode in series. b) allowing the MOSFET to let current flow back from the output into the transformer, before the main switch is turned on ofc, wich doesnt lose energy. the later helps cross regulation even when it is only used on the main output.

If you look at the MAX1652, MAX1653, MAX1654, MAX1655, they show you the principle involved.

Yes there are a number of integrated switching ICs wich just need an inductor and capacitor, the good ones like lm2594 are quite good but a bit expensive, the cheap hysteretic ones I dont like much, they are inclined to screech at light loads.

Actually ive seen some dual output fully integrated switcher ICs with synchronous rectification such as

Might be worth reducing my input voltage range.

Colin =^.^=

These parts are straight DC-DC regulators. They may employ synchronous rectification, but they have nothing to do with the reduction of cross regulation effects seen in an auxiliary rectifier output. If you look at the data sheet for the CS51001, you will see the switches in series wth the normal rectifiers, using the output incyctor that would normally be required in a single-ended forward topology. The same principle allows it's (very lossy) use in series with flyback rectifiers. (see St Micro AN511) The use of active synchronous rectifiers in place of standard rectifiers has no inherent advantage in reducing cross-regulation effects, except at light loads where output inductors of standard rectifiers can become discontinuous.

I fear that I've misunderstood your original posting. If I'd known you were talking about the uses of DC-DC post-regulators, I would not have responded. DC-DC post regulators don't give a damn what kind of input regulation they are correcting, or where it comes from. they are affected by crossregulation as much as any other load circuit is.

It's only when post-regulators are not used, or when headroom is critical (ie to reduce linear losses), that the reduction of cross regulation effects becomes important in multi-output converters.

RL