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High component count for long delay circuit (inrush resistor switch out)

Flyback

Well-Known Member
Thanks, yes, its necessary to keep to the right of that gain peak......the article is talking about being to the right of the "upper resonance frequency".......which is not generally a problem in LLC....i mean, ideal world, you would operate bang on the upper resonance frequency, but tolerances in caps etc...you'll end up a bit below it, or a bit above it........no problems........the article is saying you get reverse recovery when to the right (higher f) of the upper resonance frequency....which is a huge faux pas in the article.......its normal to end up operating above resonance.....quite usual.

If one operates an LLC resonant converter and experiences reverse recovery...then its really serious....the LLC could die in quick time...

Theres loads of LLC's out there that are on the knife edge...just waiting to go pop if the situation that causes rev rec happens.....eg overload, return from brownout, etc etc

The reverse recovery problem in LLC , and how to avoid it, is not stated enough in app notes about LLC.....its an absolute killer of the LLC ....infineon actually have an app note where they try to detect (in software), reverse recovery in LLC before it happens....and then avoid switching the fet on...........i have no idea if they succeeded with this.....but it would be an amazing feat if they suceeded.........The grunge way to avoid it is as in that thread just above.

But yes...the LLC and the PSFB both can suffer destructive reverse recovery......in fact, all "current sloshing" SMPS's can suffer it...........ie those SMPS's that slosh primary current about in order to turn the diode on just before the fet gets turned on (and so get ZVS)

Another "current slosher", is the Half bridge with no output inductor....just uses the leakage inductor....its often done with IGBTs to help thru the rev rec situation....and hope that the conditions that cause rev rec never happen.

I mean seriously, for that article to say that reverse recovery happens in normal operation of an LLC, is like saying its normal to do nuclear bomb testing in a big supermarket.
But i wouldnt complain about the article, as its very good in places, and i woudlnt want it taken down....its a harmless mistake because its so obviously wrong.
 
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danadak

Active Member
But i wouldnt complain about the article, as its very good in places, and i woudlnt want it taken down....its a harmless mistake because its so obviously wrong.

Flyback, I have been involved in doc mistakes in past, as a FAE, and so far raising
inaccuracies has been productive, the edits are done, end of story. Not in the least
human safety considerations could be impacted by this.

I will post a request to look at this, I think its worthwhile.

Regards, Dana.
 

danadak

Active Member
Thanks for that article...i am not sure about that doc...because on 6th page (marked pg 5), they say....
(regarding operation above the resonant frequency)....
QUOTE.....
**The rectifier diodes are not softly commutated and reverse recovery losses exist**

...this is completely wrong....and is a basic situation of the LLC converter.
And it is in fact, common to operate an LLC above f(res), because you are further from the capacitive region....and your RMS and peak output current is lower.....
This is what I got back from TI -

Hi Dana,


Here is the feedback of our product specialist,


When you operate above resonant frequency, secondary side rectifiers wont have zero current switching, so there will be reverse recovery.


Below and at the resonant frequency, secondary side rectifiers do have ZCS switching.


However, As move on left side of the gain plot curve (away from resonant frequency and towards left side of gain plot curve) of LLC, conduction losses will increase.


So ideal point would be to operate the converter at resonant frequency for rated power. So, the design should be made that way to achieve high efficiency.


Please refer the following screenshots for the current waveforms on both primary and secondary:


1652708364394.png


1652708426086.png


1652708472398.png


Regards, Dana.
 

Flyback

Well-Known Member
Thanks Danadak, but they are wrong....if you run the attached LTspice sim you can see.

Also attached is sec diode voltage when above resonance......no reverse recovery......when diode current goes off, the voltage across the diode was already zero...the reverse voltage did not build up across the sec diode until the diode current had gone to zero.......green is sec diode currnt...red is sec diode voltage..

Reverse recovery, AYK, is when a diode that is conducting current, suddenly has a reverse voltage put across it....and a large current spike flows through the diode in reverse direction...........as can be seen, this does not happen with LLC above resonance.
 

Attachments

  • HalfbridgeLLC_70khz_Lres 20uH.asc
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danadak

Active Member
Thanks Danadak, but they are wrong....if you run the attached LTspice sim you can see.

Also attached is sec diode voltage when above resonance......no reverse recovery......when diode current goes off, the voltage across the diode was already zero...the reverse voltage did not build up across the sec diode until the diode current had gone to zero.......green is sec diode currnt...red is sec diode voltage..

Reverse recovery, AYK, is when a diode that is conducting current, suddenly has a reverse voltage put across it....and a large current spike flows through the diode in reverse direction...........as can be seen, this does not happen with LLC above resonance.

Next reply -

Hi Dana,


According to our product specialist,


During above resonance, the current slew rate is very high (compared to other modes) just before current going to zero.


I believe this is what it causes the reverse recovery (reverse recovery is a function of current slew rate).


Regards,


Danilo Austria Jr.
Texas Instruments Customer Support
 

Flyback

Well-Known Member
Sorry but they are wrong again.........i can see what they in fact meant to say...............they in fact are talking about a situation which happens when the dual sec windings of a CT output LLC are not well coupled....but they make no reference to that whatsoever....what they have actually written is complete and utter nonsense.

It looks to me that some specialist, who maybe no longer works there, explained it to the writer, and the writer got it all mixed up.

Somebody else has read this, and it looks like they posted here...


The answerer to that thread is one of the foremost SMPS designers (and currently working) in the western world today.
 

sagor1

Active Member
Hm.... I would have used a simple adjustable 0-5 seconds timer relay on the input AC side, if there was a fixed AC voltage (either 120V or 240V), to bypass any shunt/NTC.
On a power bump, it resets immediately.
The issue with this approach is that it is not "universal" voltage, but for a fixed input voltage.
I'm sure one could create a 555 timer to do the same thing, and use either a 12V or 24V relay to switch the bypass contacts on the main AC feed. One would need a relay with a heavy duty contact rating, like a 841-S-2A-C1 relay which is rates 25A at 240V
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Hm.... I would have used a simple adjustable 0-5 seconds timer relay on the input AC side, if there was a fixed AC voltage (either 120V or 240V), to bypass any shunt/NTC.
On a power bump, it resets immediately.
The issue with this approach is that it is not "universal" voltage, but for a fixed input voltage.
I'm sure one could create a 555 timer to do the same thing, and use either a 12V or 24V relay to switch the bypass contacts on the main AC feed. One would need a relay with a heavy duty contact rating, like a 841-S-2A-C1 relay which is rates 25A at 240V
Historically it's usually been done with a TRIAC rather than a relay, and a simple delay circuit that turns it ON after a couple of seconds - which is why I was staggered by the stupendous number of spurious components used. Usually it's not as complicated as using a 555, but a 555 would still be fairly simple.

For a simple example:

 

sagor1

Active Member
Well, I use a simple step start on a 2kw amplifier, but with a transformer it is easy to put a relay on the primary and when the capacitor bank on the secondary charges up, the primary voltage rises enough to trigger the relay to bypass the current limiting resistor. It all happens within about 100ms or less.
 

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