smps with a fixed duty cycle?

The classic linear power supply (transformer, bridge rectifier, cap) relies on AC at a fixed frequency of 60 Hz (50 Hz outside the USA?), so that's basically a 50% duty cycle, right?

And if higher frequency means less iron is required for the transformer...

Would a simple SMPS, operating at several hundred kHz, with a fixed 50% duty cycle still outperform the linear power supply?

Thanks,

Michael

Reply to
mrdarrett
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... or would it just be a miniature space heater? :)

Michael

Reply to
mrdarrett

I don't know what duty cycle means in that context. Most such supplies have rectifier conduction spikes that are fairly narrow.

Yes. Airplane power systems are 400 Hz to save weight.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

You are trying to compair two totally different things. The linear supply could care less about the AC frequency in one sense. As long as the transformer is rated for the frequency and the capacitors are sized for the frequency , the pulsating DC has nothing to do with the regulation.

For the linears the higher the frequency within limits the less iron can be used and the smaller the filter capacitors.

Reply to
Ralph Mowery

The point I was trying to make is, no feedback was used to make an attempt to modify the primary waveform shape in any way for the 50/60Hz transformers.

If someone were to design an SMPS with no feedback loop whatsoever (which normally changes the duty cycle), running at a constant 50% duty cycle, what would be the ramifications?

Thanks,

Michael

Reply to
mrdarrett

** Some linear PSUs have a triac in the primary circuit that allows the DC output voltage to be regulated.
** Such PSUs are in fact common, where regulation is not essential and the aim is to reduce the weight and cost of having a large iron transformer.

Generally known as a "square wave inverter" they can be found in many high powered audio amplifiers. The output DC still contains ripple at twice the supply frequency but requires little filtering as a rectified square wave is pure DC.

The switching is mostly done with IGBTs these days and can have outputs up to 10kW.

.... Phil

Reply to
Phil Allison

There's no 50% duty cycle. The positive half-wave and negative half-wave both contribute. There's a 'conduction angle' that can be calculated, and there are series-choke options that will change the conduction angle, if that matters to you. But that doesn't just depend on power supply design, it also depends on load.

Reply to
whit3rd

Depends on the type. if it's a forward converter it will behave senibly.

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Reply to
Jasen Betts

Cheap potted SIP DC/DC converters are usually unregulated forward converters running about 50%. With a fixed DC input, the output is usually stable enough to run opamps and logic and stuff.

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One of my products makes isolated 24 volts into +-10, with more regulators downstream.

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John Larkin         Highland Technology, Inc 
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John Larkin

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