Phase Shift Full Bridge SMPS is massively over-hyped?

[treez]

Do you agree that regarding the choice between LLC converter and Phase Shift Full Bridge (PSFB) converter, you would virtually always choose the LLC converter? You would only choose the PSFB if either input or output voltage were highly variable.
The reason the LLC is always preferred over the PSFB is because the LLC has far less modes of operation where destructive latch-up of the FETs can take place due to reverse recovery of the primary side FET’s intrinsic diodes. Do you agree?

After all, in an LLC , the mentioned reverse recovery problem only happens during startup, but can be avoided by a good soft start. So, LLC always better than PSFB(?)

The below articles (1 to 3) tell of the reverse recovery problem in the PSFB…..it cannot be avoided with the PSFB, so you just have to use special FETs with ultra low Qrr and hope you don’t fall victim to the problem.


1…“MOSFET body diode recovery mechanism in a phase-shifted ZVS full bridge DC/DC converter”
https://www.st.com/st-web-ui/static...ical/document/application_note/CD00171347.pdf

2…“FCS Fast Body Diode MOSFET for Phase-Shifted ZVS PWM Full Bridge DC/DC Converter”
https://www.fairchildsemi.com/application-notes/AN/AN-7536.pdf


3…Design of Phase Shifted Full-Bridge Converter with Current Doubler Rectifier
file:///C:/Users/andrew/Downloads/ZVS_Full-Bridge.pdf


Top of page 7 of the following shows how there are far less failure modes of the primary side FETs in an LLC converter than in a Phase shift full bridge converter.

“Primary Side MO S F E T Selection for LLC Topology”
https://www.infineon.com/dgdl/Infin...y.pdf?fileId=5546d46147a9c2e40147d3430e927e5d
Top of page 7:-
“”Design considerations of the power MOSFET in the primary side LLC resonant converter are relatively simple compared to a phase-shifted full-bridge converter, as it is not affected by leading or lagging waveforms with regards to the power MOSFET. With the exception of the capacitive load resulting from overload or short-circuit conditions, during startup only system reliability issue caused by incomplete body diode reverse recovery of the power MOSFET need to be considered””

…so the LLC converter only has to be protected from primary FET failure during startup, whereas the Phase shift full bridge converter has more modes of operation where it may undergo catastrophic destruction of its primary side fets due to reverse recovery/latch-up.

 

[treez]

The PSFB being discussed here appeared to have a spec of..
Vin = 400-600Vdc
Vout = 65vdc
Pout = 7.5kW
Fsw = 100khz

..Presumably the PSFB was chosen instead of the LLC converter because
1…The PSFB needs no series capacitance to carry the power current (and film caps are hard to heatsink)
2….The Vin had quite a range on it (400 to 600vdc)

The LLC is surely always preferred over the PSFB whenever…
1…Vin and Vout are fixed.
2….You’ve got room for the series capacitors (resonant capacitance)
3….The load is very steady with few large transients
(?)

 

[treez]

LLC vs Phase shift full bridge SMPS's

Would you agree, that generally speaking...
if Vin and Vout are fixed,
And load transient behaviour is limited (mostly steady load)
Transient response doesnt have to be very fast..
Then LLC is always chosen over Phase shift full bridge?

 

[D.A.(Tony)Stewart]

Re: LLC vs Phase shift full bridge SMPS's

I like Belgiium's recent million dollar win of the highest density inverter from 450dc to 240Vac 60Hz with centre neutral.

It was funded by Google and spec'd by the IEEE.

Just over 13 cu. inches for a 2kW inverter and acceptable Class B conducted , radiated and thermal emissions with a transient response to any load switch much less than a second within load regulation specs.

 

[asdf44]

I don't have much input at this time but this is a great thread. Thanks Treez for providing so much information, simulations etc. I love to see your posts and have downloaded many of your LT Spice sims.

I may have some input soon as we're looking to refresh a [as I currently understand it] late 90's phase shifted bidirectional inverter design. I'm not sure yet how much effort was put into the ZVS aspect of that design.

My current understanding is that the phase shifted FB lends itself quite well to use in an inverter and particularly a bidirectional one because zero crossings of either voltage or current are seamless (it uses synchronous switches on the secondary). But I need to gather some additional context before fully understanding the pros and cons of the different options.

I'm currently involved in a dual active bridge inverter design where the zero voltage crossings requires active switching of the final polarity bridge which adds distortion.