Component Selection of Series Resonant Converter

How do we size the inductor and capacitor of series resonant full brigde converter? For a specific resonant frequency, there are lots of Capacitor-Inductor combinations with the same resonant frequency.

Also, how to compute for current sizing of inductor? Can we directly use the average value of the current after the pre-regulator?

How about voltage sizing of the capacitor? Is the average output voltage of the pre-regulator enough?

Thanks.

Ripple in High Power Series Resonant Converter Pre-regulator

When is the optimum ripple in a series resonant converter pre-regulator? It's input voltage would be 120V 60Hz and if drawing 17A, I need 28mF of filter capacitance to have less than 5V of ripple. The capacitor cost more than $50. Are my values normal? Too conservative? Or too loose?

You might want to sketch the intended topology for clarity.

Regarding AC input and ripple, what kind of rectifier are you using? I would expect a power factor corrected active circuit according to recent power quality standards, respectively you get 100 Hz power pulsation in the primary DC bus. You can either try to filter the respective voltage ripple in the DC bus, or suppress by a regulated converter in the power conversion path.

Regarding AC input and ripple, what kind of rectifier are you using?

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Bridge rectifier.

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You might want to sketch the intended topology for clarity.

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I will send when I get home. I'm accessing through phone right now.

FootTea said:

there are lots of Capacitor-Inductor combinations with the same resonant frequency.

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As a general rule, select L & C values which have proper reactive impedance so that they carry your specified Amperes at your specified frequency.

For large A, choose large C, small L.
For small A, choose small C, large L.

As a general rule, select L & C values which have proper reactive impedance so that they carry your specified Amperes at your specified frequency.

For large A, choose large C, small L.
For small A, choose small C, large L.

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Yes. It just feels weird because the capacitor was too expensive so I though there might be other ways implementing it.

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FvM said:

You might want to sketch the intended topology for clarity.

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Below is the sketch of the topology.

I also searched about the PFC boost converter you're talking about. I was what I have in my mind when I was dealing with the filter capacitor issue. I just didn't know its name.

Power Factor Corrector Boost Converter Selection for Series Resonant Converter

Attached is a circuit diagram of ICE3PCS01G PFC IC of Infineon. A boost converter changes it voltage gain by changing the duty cycle. What I understand on its datasheet is that output is controlled by the voltage divider resistors in VBTHL_EN and in VSENSE. Am I on the right track?

Also, if this is switched on the KHz level, does that mean I get to use a low-capacitance filter capacitor for the same ripple on the DC bus?

P.S. How do I merge this to my old post?

BradtheRad said:

As a general rule, select L & C values which have proper reactive impedance so that they carry your specified Amperes at your specified frequency.

For large A, choose large C, small L.
For small A, choose small C, large L.

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I misread your reply. I thought it has something to do with ripple. Anyway, what is the concept behind that rule? What is the drawback of using a large inductance for high currents? How about using large capacitance for small currents?

FootTea said:

Below is the sketch of the topology.

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The LLC resonant converter is not just a matter of selecting a cap and an inductor. I suggest you to go back and reread the literature where you saw this topology and review its basic design equations...

I'm not sure if the circuit is intended as basic series resonant LC converter (T1 magnetizing inductance >> Ls) or LLC converter (T1 magnetizing inductance artificially reduced by an air gap). What do you want to achieve when choosing a resonant converter topology?

CataM said:

The LLC resonant converter is not just a matter of selecting a cap and an inductor. I suggest you to go back and reread the literature where you saw this topology and review its basic design equations...

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Actually I haven't seen selection of components for SRC.

From what I understood, the H-Bridge produces a square wave. We will then have a series RLC circuit. This resistance can be modeled as an equivalent resistance with the same power only just referred to the primary. The square wave can be modeled using its first fundamental. We can then compute the transfer function.


I only saw design procedure for SPRC.

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FvM said:

I'm not sure if the circuit is intended as basic series resonant LC converter (T1 magnetizing inductance >> Ls) or LLC converter (T1 magnetizing inductance artificially reduced by an air gap). What do you want to achieve when choosing a resonant converter topology?

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I just want to build a 30mA 35KV supply. The PVM400 schematic has lots of missing data. It's a half bridge topology with a snubber in parrallel with the primary winding of the transformer( I hope someone can explain this too.. ). I want it to be controlable so I chose series resonant when I saw its gain equation according to frequency. As of now, I don't need much of the complexity of the optimized SPRC design calculation I saw. Any other way I can accomplish this? Other topology says this is not feasible. Even in wikipedia, only these types of converters are listed on this power range.

FootTea said:

What is the drawback of using a large inductance for high currents?

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High L cannot pass high current, unless frequency is low. Apply the formula for inductive impedance: 2 * Pi * f * L

How about using large capacitance for small currents?

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Large C can pass small currents okay, however too large C is overkill and inefficient design.

If C contributes to resonant action, then large C tends to create low resonant frequency.

Consider two limits of resonant circuit impedance:

Lowest possible impedance is with Ls=0, series inductance formed exclusively by the transformer leakage inductance.

Upper impedance limit is practically set by resonance voltage, you don't wont more than several 10 percent of bridge output voltage for Ls/Cs resonance. Capacitor voltage rating and inductor form factor (~ I²L) are the constraints.

In case of the intended high voltage output, design considerations for the transformer and output rectifier are probably enforcing all other design parameters, e.g. useful switching frequency.