Parallel versus Series Resonance Converter Topology - Pros/cons

[Klaus Kragelund]

Hi

I'm working on a resonance converter and have read a little bit on the
subject thus far. Most recommend using a series resonance topology, but
I cannot seem to find the excact reason why the parallel resonance
converter is disregarded.

The block diagram is a follows:

www.microdesign.dk\tmp\parallel_resonance_conv.jpg

In this parallel resonance converter the OSC is adjusted to exactly the
resonance frequency. Offcourse this will let the voltage envelope
continue to rise on the LC circuit, but that is taken care of by the
optocoupler which will shut down the driver when the voltage is to high
on the output

So in essence the converter is operating burst like on the resonance
point

In this converter the current is cycled between the L and C in the LC
circuit, so there is little current in the driver. On the contrary the
series converter has large current in the drive.

But what is the drawbacks to an parallel LC circuit like this versus a
standard series LC resonance converter? Something that I'm missing?

Thanks

Klaus

 

[Tim Williams]

But what is the drawbacks to an parallel LC circuit like this versus a
standard series LC resonance converter? Something that I'm missing?

Transistors are most efficient switching quick and solid. A capacitor is
like a train hitting a brick wall. Series resonant has an inductor in
series, so the switching harmonics are handled softly. Peak currents are
low and the world sleeps soundly.

If you must have a parallel resonant tank, you can feed it with a series
inductor (check my website for a related project), but the behavior is a bit
more complicated (showing both series and parallel resonant behavior).

Tim

 

[Frithiof Andreas Jensen]

Hi

I'm working on a resonance converter and have read a little bit on the
subject thus far. Most recommend using a series resonance topology, but
I cannot seem to find the excact reason why the parallel resonance
converter is disregarded.

There isn't any - it depends on what one needs the converter to do. In
general, parallel resonant converters tend to be current-fed i.e. large
inductor on the input, so they regulate slower. The Mapham Inverter is the
classic version. Other kinds work on dumping energy into the parallel tank
and regulate by changing the dump frequency - Philips did a "Parallel
Resonant Power Supply?" PRPS application note.

If your reason for wanting a resonant converter is getting a clean
sinusoidal output voltage then the large parallel capacitor on the output
helps a good deal. Often those converters are running at a *fixed* frequency
and the output voltage is controlled by adjusting the Inductor part of the
L/C resonator (For large setups, such as an UPS, the "inductor" is a
full-bridge feeding a constant current DC/DC converter - the current sets
the "Inductance"). Cancelling part of the flux by transformer action can
work too.