Are resonant converters bi-directional?

Thread Starter

SiCEngineer

Joined May 22, 2019
147
A bit of a general question here! In resonant converters, the power is typically transferred from the source to the load. However, is it possible to transfer power bi-directionally such as within a dual-active-bridge?

In some applications where the load current can transition from full load to zero load, and no energy is transferred to the secondary side through the transformer - where does the resonant energy held within the inductors and capacitors go? Does it just gradually dissipate, or is it held in them until the load is connected again? Is there a way to transfer this resonant tank energy back to the source, or is this just defeating the purpose of a resonant converter?

At the moment, I have a buck regulated resonant half bridge converter. However when the load current transitions from full load to zero load, the current in the resonant tank goes to zero and no power is transferred to the secondary side. But what I don't understand is what happens to the energy stored within the tank at this instant? Could the buck be replaced with something else which is bi-directional to recover the energy back to the source? The current through the buck converter also (for some reason?) drops to zero amps, and the voltage on the capacitor rapdily decays. I could place a very large cap here to hold the charge but I would like to avoid that.

Topology attached.
 

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WendellB

Joined Feb 14, 2020
12
Dear Sir;

This is the second time I have attempted to reply to someone on this site. I find it difficult to do so for lack of operating instructions. Last time I tried I apparently sent two lists of my technical videos and was accused of spamming and locked out for two months. Anyway I will try just once more.

To answer your first question, yes the LCC topology can work bi-directionally. It has been done by the Chinese and I found this technical paper entitled: "Operation Characteristics of the Bidirectional LCC Resonant DC-DC Converter". I am sure you can find it on the web.

You asked where the stored energy in the resonant components goes when you operate at light of no load. It is returned as current to the capacitor in parallel with the DC source voltage to the power supply, a capacitor usually referred to as the
reservoir capacitance. The voltage simply raises a little and less current is pulled from the DC source.
I have been designing the LCC topology since 1990 and recomment you look at two of my videos, and I will type what you must
enter when connected to YouTube.

"LCC RESONANT CONVERTER DESIGN IN 2016" and
"TRANSFORMER DESIGN, CAPACITANCE REFERRED TO THE PRIMARY WINDING"

I noticed in your schematic that there is a capacitor across the primary winding with a resistor in series with it
I should mention that that capacitance is often referred to as the CEP or Effective capacitance seen across the power transformers primary winding, as you have showed it. However that is not a real physical capacitor. It is the capacitance seen looking into the primary circuit wise and is really just the capacitance of the transformer which is created by the capacitive stored energy in the insulation spaces between the transformer windings and some from spacial secondary capacity transferred to the primary winding via the turns ratios squared.

It is important to never place a physical capacitor across the primary of an LCC converter for it must appear between the leakage inductance referred to the primary and the magnetizing inductance of the primary winding. Else you will have two resonant circuits which will raise havoc with the pulsing waveforms seen both in a simulation and in real life.

I am not sure if I can do this but I can help far more to help if we can communicate using Skype and I you can view my computer screen. My email is (email address removed by moderator -sorry, forum policy). I am a retired senior electrical engineer living in Wise VA.

I use computer simulation and modeling very heavily and can show you what can be done and share those experiences with you if you wish. I am about to offer my experience in mentoring to All About Circuits and explore the possibilities too.
Hope I was somewhat helpful.

Best regards

Wendell
 
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Thread Starter

SiCEngineer

Joined May 22, 2019
147
Hello Wendell,

Thank you for your response. At the moment the capacitor is supposed to represent the parasitic capcatiance - should it be on the seondary windings, instead? Would that work better for simulation? The resistor was just to avoid PLECS simulator giving errors about dependent voltage sources being connected in parallel.

I will email you in my spare time at work tomorrow, I am from the UK but hopefully we can find some time to talk on Skype.

Speak to you soon and hope you remain on the site!

SiC
 

TeeKay6

Joined Apr 20, 2019
572
Dear Sir;

This is the second time I have attempted to reply to someone on this site. I find it difficult to do so for lack of operating instructions. Last time I tried I apparently sent two lists of my technical videos and was accused of spamming and locked out for two months. Anyway I will try just once more.

To answer your first question, yes the LCC topology can work bi-directionally. It has been done by the Chinese and I found this technical paper entitled: "Operation Characteristics of the Bidirectional LCC Resonant DC-DC Converter". I am sure you can find it on the web.

You asked where the stored energy in the resonant components goes when you operate at light of no load. It is returned as current to the capacitor in parallel with the DC source voltage to the power supply, a capacitor usually referred to as the
reservoir capacitance. The voltage simply raises a little and less current is pulled from the DC source.
I have been designing the LCC topology since 1990 and recomment you look at two of my videos, and I will type what you must
enter when connected to YouTube.

"LCC RESONANT CONVERTER DESIGN IN 2016" and
"TRANSFORMER DESIGN, CAPACITANCE REFERRED TO THE PRIMARY WINDING"

I noticed in your schematic that there is a capacitor across the primary winding with a resistor in series with it
I should mention that that capacitance is often referred to as the CEP or Effective capacitance seen across the power transformers primary winding, as you have showed it. However that is not a real physical capacitor. It is the capacitance seen looking into the primary circuit wise and is really just the capacitance of the transformer which is created by the capacitive stored energy in the insulation spaces between the transformer windings and some from spacial secondary capacity transferred to the primary winding via the turns ratios squared.

It is important to never place a physical capacitor across the primary of an LCC converter for it must appear between the leakage inductance referred to the primary and the magnetizing inductance of the primary winding. Else you will have two resonant circuits which will raise havoc with the pulsing waveforms seen both in a simulation and in real life.

I am not sure if I can do this but I can help far more to help if we can communicate using Skype and I you can view my computer screen. My email is (email address removed by moderator -sorry, forum policy). I am a retired senior electrical engineer living in Wise VA.

I use computer simulation and modeling very heavily and can show you what can be done and share those experiences with you if you wish. I am about to offer my experience in mentoring to All About Circuits and explore the possibilities too.
Hope I was somewhat helpful.

Best regards

Wendell
If you have not already done so, take a look here:
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With regard to communicating with @SiCEngineer, I don't think you can initiate the conversation but I think that SiCEngineer has enough points that he could "start a conversation" with you. Might be worth a try. I will caution that "conversations" via AAC are not (apparently) intended for extensive use to avoid the forum display.
 

WendellB

Joined Feb 14, 2020
12
Hello Wendell,

Thank you for your response. At the moment the capacitor is supposed to represent the parasitic capcatiance - should it be on the seondary windings, instead? Would that work better for simulation? The resistor was just to avoid PLECS simulator giving errors about dependent voltage sources being connected in parallel.

I will email you in my spare time at work tomorrow, I am from the UK but hopefully we can find some time to talk on Skype.

Speak to you soon and hope you remain on the site!

SiC
Hi SiC

A little more on the capacitance shown across the primary of an LCC topology.
When designing the LCC I always calculate and work with the CEP primary capacity and at first literally connect it across the primary winding of the power transformer, mostly for convenience. But when I do, the coupling coefficients for the transformer ARE ALWAY A VALUE OF ONE, this is possible using circuit simulation but in the real world, the coupling koefficients are never one but some value above zero and lesss than one. So using a value of 1 there is no 2nd resonant affect that I spoke above in my first response to you. If the Koupling coefficient is less than one the you must place the capacitance on a secondary winding.

The actual capacitance of a real physical transformer is the static or so called parallel plate capacitance seen between the winding layers of the transformer which is then multiplied by a factor which in turn is a function of the ac voltage across each pair of windings. And it is called a multiplication factor. The video I referenced about transformer capacity explains how it is derived and used in some detail. Hope this answers your question more clearly.

Best regards
Wendell
 
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