Author Topic: EEVblog #260 - Tracking Pre-Regulator Simulation in LTspice  (Read 17976 times)

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Offline mightyzen

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Re: EEVblog #260 - Tracking Pre-Regulator Simulation in LTspice
« Reply #25 on: March 24, 2012, 10:23:24 am »
@amspire thanks for your example!

If the numbers just do not work, then a single inductor solution will not work and you will have to go to a transformer. With a transformer, you could reduce the peak current to more like 4A to 8A for these specs.

Could you elaborate a bit on the transformer?  That was the only part that was not clear to me.
 

Online ejeffrey

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Re: EEVblog #260 - Tracking Pre-Regulator Simulation in LTspice
« Reply #26 on: March 24, 2012, 11:41:14 am »
Could you elaborate a bit on the transformer?  That was the only part that was not clear to me.

A transformer coupled SMPS lets you take advantage of the turns ratio so that you can operate more near 50/50 duty cycle.  This keeps your peak currents to a more manageable level.  You can do this as a flyback converter which uses the transformer as the energy storage (as a direct replacement for an inductor) or a forward converter where the transformer is used in the usual fashion, and there is a separate inductor for energy storage.
 

Offline amspire

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Re: EEVblog #260 - Tracking Pre-Regulator Simulation in LTspice
« Reply #27 on: March 24, 2012, 11:52:30 am »
A boost converter with a transformer instead of an inductor is basically a flyback converter. Since you can choose your ratios, you could pick a 2.8:25 ratio. The transformer primary is connected exactly the same way as the inductor was, but the discharge cycle now happens from the secondary winding.

This would mean that at maximum voltage, the duty cycle is 50%.  So if you have a 50% duty cycle on the secondary, then for a 2A average output, you would need 4 A peak in the secondary or more.

What I forgot to explain properly is that with the transformer ratio, it would mean 36A peak in the transformer primary or more. It was 20A or more when the primary had a 90% charging duty cycle. It makes sense that it goes up to about 36A with a 50% charging duty cycle.

So the transformer can be designed to give you much less ripple current in the secondary circuit at the expense of more ripple and a higher peak current in the primary. Sometimes it is worth the effort if the quality of the output, or the ESR peak current rating of the secondary components is the prime concern. Transformers are always more expensive then inductors and tend to be much harder to design.

Richard.
« Last Edit: March 24, 2012, 11:55:34 am by amspire »
 

Offline mightyzen

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Re: EEVblog #260 - Tracking Pre-Regulator Simulation in LTspice
« Reply #28 on: March 24, 2012, 04:58:41 pm »
Clear, thanks, the diagram helped.
 


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