Bidirectional LLC...

[Piotr Wyderski]

Hi,

Every converter with synchronous rectification should be capable of
bidirectional operation. But there seem not to be any practical
realisations of bidirectional LLC, except for some vague conference
papers. And even then it is not a pure LLC, but a CLLC or split-rail
LLC. Why?

Best regards, Piotr

 

[legg]

On Tue, 22 Sep 2020 08:22:46 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

Hi,

Every converter with synchronous rectification should be capable of
bidirectional operation. But there seem not to be any practical
realisations of bidirectional LLC, except for some vague conference
papers. And even then it is not a pure LLC, but a CLLC or split-rail
LLC. Why?

Best regards, Piotr

Suggest you try it. Just because you can draw a topology on
paper and give it a name, doesn\'t mean it has a practical
application. Not all of the relevent Cs and Ls operating
in an \'LLC\' converter appear in its nomenclature. Not all
operating states that are possible for the topology are
of practical use at any specific time..

Bidirectional tri-ports, multiresonant or otherwise, isolated
or otherwise, have been used in UPS systems since the 80s,
with varying success. All have pros and cons, depending upon
the voltage ratios, range of regulation and direction of power
flow.

Suggest you examine the vague conference papers more closely.

The problem is efficient, versatile bidirectional power flow
- not \'why can\'t I do it?\'.

RL

 

[Piotr Wyderski]

legg wrote:

> Suggest you try it.

Seems to work OK in Spice. In the reverse direction it is a push-pull
pumping a resonant tank. The half-bridge then becomes a synchronous
voltage doubler or even can be entirely deactivated if the transformer
ratio is sufficiently high. For 30:1 the parasitic MOSFET diodes do the
job just fine with an amp or two.

The waveforms look fine, the converter reacts to frequency changes as
expected. So if it looks OK, so why there are no practical
implementations widely available? In the best case it can be described
as a curiosity.

Theory says it\'s OK, Spice says it is OK, practice says verboten. Why?

Just because you can draw a topology on
paper and give it a name, doesn\'t mean it has a practical
application.

A high-efficiency converter of such a simple structure should have
plenty of practical applications. A DAB requires twice as many switches,
a bidirectional PSFB is about the same and can be considered a DAB
driven in a fancy way.
> Suggest you examine the vague conference papers more closely.

Sure, but there are not many of them, which raises my concerns even
without reading them. And hence comes the question.

The problem is efficient, versatile bidirectional power flow
- not \'why can\'t I do it?\'.

The efficiency should be expected to be high in at least one direction,
while in the other it is at least satisfactory. Very good approach if
power flows mostly in one direction.

Best regards, Piotr

 

[Bill Sloman]

On Tuesday, September 22, 2020 at 4:22:54 PM UTC+10, Piotr Wyderski wrote:

Hi,

Every converter with synchronous rectification should be capable of
bidirectional operation. But there seem not to be any practical
realisations of bidirectional LLC, except for some vague conference
papers. And even then it is not a pure LLC, but a CLLC or split-rail
LLC. Why?

There may not be all that many practical applications so far..

Domestic roof-top solar power operating into a home battery might be one. There isn\'t that much of it around, but it is certainly becoming more popular in Australia.

The grid won\'t pay you anything like as much for the power you feed into the grid as they charge you for power they sell you from it, so it makes sense to store the power you generate during the day and run your house on it overnight (while it lasts).

For large scale solar farms you want convert the low voltage direct current from the arrays into high voltage AC as close as possible to the panels - you can use much thinner and cheaper wire to connect the arrays together and back to the grid, and the power is only going to go one way.

With roof-top solar the wires aren\'t long enough for this to be much of an issue.

If domestic roof-top solar with battery becomes a mass-market item, you may see high volume production of devices aimed at this market.

Electric cars and trucks with regenerative braking might be another such market. That might be closer to maturity.

--
Bill Sloman, Sydney

 

[legg]

On Tue, 22 Sep 2020 15:08:03 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

legg wrote:

Suggest you try it.

Seems to work OK in Spice. In the reverse direction it is a push-pull
pumping a resonant tank. The half-bridge then becomes a synchronous
voltage doubler or even can be entirely deactivated if the transformer
ratio is sufficiently high. For 30:1 the parasitic MOSFET diodes do the
job just fine with an amp or two.

The waveforms look fine, the converter reacts to frequency changes as
expected. So if it looks OK, so why there are no practical
implementations widely available? In the best case it can be described
as a curiosity.

Theory says it\'s OK, Spice says it is OK, practice says verboten. Why?

Suggest you breadboard your spice circuit. If it works as required
under the range of input and output conditions needed, then you may
have a viable application.

Topologies do not always scale - their relationship to the quality
and cost of pracical components (with their limits and strays) often
restricts applications to a pretty narrow range of power, voltage,
current and frequency.

Just because you can draw a topology on
paper and give it a name, doesn\'t mean it has a practical
application.

A high-efficiency converter of such a simple structure should have
plenty of practical applications. A DAB requires twice as many switches,
a bidirectional PSFB is about the same and can be considered a DAB
driven in a fancy way.

Practical LLC converter circuits may appear simple. I suggest that
you try to design one from scratch for an application not previously
illustrated in a cookbook or app note. Obtaining magnetics or other
parts may be an issue.

Suggest you examine the vague conference papers more closely.

Sure, but there are not many of them, which raises my concerns even
without reading them. And hence comes the question.

The LLC designation is one of convenience, applied to specific
applications. This naming \'convention\' is one of convenience,
or intent, rather than accuracy or definition. It is probably the
last thing in mind (topological classification), when developing
practical circuitry. Whether you\'re performing a patent search,
or just googling, you will have to extend your terms of search
to find useful examples.

It\'s just a variety of series resonant converter - no shortages
of papers or references there. The third or fourth term make it
multi-resonant - for specific reasons. Is there a difference
between LLC and LCL, if the last two elements are parallel-resonant
anyways?

There are always more papers than practical and honest examples
of working applications. Always more chaff, than wheat. Try
searching for articles by the same authors, or tracking down
references to authors, companies or educational institutions that
they mention, if they smell right in the first place.

You might check some of Johannes Kolar\'s (large power) output,
Ashoka K.S.Bhat,or the research papers made available by Virginia
Tech (low power). Examples earlier than Y2K will involve
semiconductor and magnetic types and structures that you will
likely have no interest in exploring.

Contemporary semiconductor app notes only show up after they
have already achieved significant market penetration. Manufacturers
and customers don\'t care whether it\'s LLC, upside-down, or an
apple (well, possibly if it\'s an Apple . . .).

The problem is efficient, versatile bidirectional power flow
- not \'why can\'t I do it?\'.

The efficiency should be expected to be high in at least one direction,
while in the other it is at least satisfactory. Very good approach if
power flows mostly in one direction.

These are trade-offs that can make or break a bidirectional
power transfer application, just as they do battery or other
energy storage or transmission applications. They have to result
in advantages to justify their development or use.

RL

 

[Piotr Wyderski]

legg wrote:

> Suggest you breadboard your spice circuit.

It is better to ask before breadboarding. It can save a lot of time and
smoke. I tend to assume that I may be missing some detail that would
ruin this not very typical approach.

Practical LLC converter circuits may appear simple. I suggest that
you try to design one from scratch for an application not previously
illustrated in a cookbook or app note. Obtaining magnetics or other
parts may be an issue.

I have already done so several times, up to a kW. I use Octave for the
task. I prefer integrated magnetics, which implies custom parts. So I
would rather not discuss a unidirectional mid-power resonant converter
design, because I already can do that. I have no experience with
something really high power (30kW+), but I have no need for that either.

You might check some of Johannes Kolar\'s (large power) output,
Ashoka K.S.Bhat,or the research papers made available by Virginia
Tech (low power). Examples earlier than Y2K will involve
semiconductor and magnetic types and structures that you will
likely have no interest in exploring.

I think I have read most of the Kolar\'s papers, but his all efforts
related to bidirectional power transfer I know of seem to be
DAB-centric. I believe that DAB is too complex for my needs (even just
counting the parts: 8 switches instead of 4).

These are trade-offs that can make or break a bidirectional
power transfer application, just as they do battery or other
energy storage or transmission applications. They have to result
in advantages to justify their development or use.

Of course it always boils down to assumptions and design constraints.
Mine are very high power transfer efficiency (~97%) in one direction and
decent one (92%) in the other. The power level is about 1kW again.
A bang-bang charge control LLC should handle the typical transfer
direction pretty easily, but I am tempted to implement direction
reversal with exactly the same structure. The sims say it should be
doable, but google says that if I succeed, it will be a pretty unique
construction. Being unique is not always an asset, hence my concerns.

Best regards, Piotr

 

[Piotr Wyderski]

legg wrote:

Topologies do not always scale - their relationship to the quality
and cost of pracical components (with their limits and strays) often
restricts applications to a pretty narrow range of power, voltage,
current and frequency.

So I have finally found one example:

https://www.hpe.ee.ethz.ch/uploads/tx_ethpublications/06397282.pdf

From ETH Zurich, so can be trusted.

Best regards, Piotr