Phase shift Full Bridge converter has hard switch-off of primary FETs

[treez]

Hello,
This document (below) about Phase Shift Full Bridge (PSFB) converters, on page 21, states that a plain Full Bridge SMPS has a hard-switched turn off of the primary FETs. –Which is true, but so too does the Phase Shift Full Bridge converter, so what is the author talking about?…

“High Efficiency DC-DC Converter for EV
Battery Charger Using Hybrid Resonant and
PWM Technique”
https://theses.lib.vt.edu/theses/available/etd-05072012-141855/unrestricted/Wan_HM_T_2012.pdf

(by the way, I am not speaking about the “modified” form of PSFB that has additional capacitors placed in parallel to the primary FETs)

 

[treez]

SMPS article is wrong about Half Bridge?

Hello,
I have just started reading an eminent power engineers document (from an eminent power engineering institution of world wide repute) on hybrid EV chargers, but then it states (on page 9), the following….

"High Efficiency DC-DC Converter for EV Battery Charger Using Hybrid Resonant and PWM Technique "
https://theses.lib.vt.edu/theses/available/etd-05072012-141855/unrestricted/Wan_HM_T_2012.pdf

The half-bridge topology is only compatible with voltage-mode control. The ½Vin voltage balance at the
midpoint between C1 and C2 is not maintained with current-mode control

..this is just wrong. How did this document get published? This is simple, basic knowledge on SMPS being misunderstood. I am now wondering if its worth actually reading the rest of the article at all?
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Hello,
The same article, on page 40 also states a totally incorrect statement about diode reverse recovery in LLC resonant converters operated above the resonant frequency….

Moreover, the secondary side diode turns off with large di/dt, which can cause large reverse recovery on the diodes. Furthermore, the high di/dt turn off of the diodes cause extra voltage stress on the diode, which makes the circuit less reliable.

Page 41 also re-iterates this incorrect statement…

Operation above the resonant frequency allows the circulating current to be minimized, but the rectifier diodes are not softly commutated such that the reverse recovery loss might be severe.

The attached (LTspice) simulation of an LLC operated well above its resonant frequency shows that there is no reverse recovery in the secondary diodes at all when operating above the resonance frequency.
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Hello, this thread is related to this, and is in response to this thread.....
https://www.edaboard.com/threads/342326/
......which I do not believe is “closed”….

…..Thanks, but sorry, post #81 of the following thread….
https://www.edaboard.com/threads/342663/
….states (with references) how the Phase Shift Full Bridge converter (PSFB) has far more potential failure modes and “gotcha’s” than the LLC converter.

With the PSFB in light_load/no_load, the situation of reverse recovery of the PSFB’s primary fets’ intrinsic diode and the subsequent distructive latch-up of these FETs is enough to put anybody off the PSFB for life?…surely?……in the PSFB, you simply cannot avoid this problem (severe reverse recovery) , you must run its dreaded gauntlet, -all you can do is use FETs with a low-as-possible Qrr and hope you don’t kill them.

Regarding short circuit of the LLC’s output, admittedly it needs handling…..but most LLC controllers actually limit the switching frequency (so it goes no lower than a certain chosen value) which goes a long way to surviving output_short_circuit. Also, LLC controllers such as ICE2HSO1G have an overcurrent pin which has multiple protection levels….initially it increases the operating frequency in response to slight overcurrent, but immediately latchs off if the current goes above a higher threshold.

So surely we can state that the PSFB is only for use when the input voltage ranges over a wide range. (because the LLC admittedly isn’t so good at this)
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Now I am really confused, the article of the top post, on page 41, states the following….

LLC seems a good candidate, but the variable frequency control makes it very complicated to handle all the conditions, and it is not easy to regulate the output current, which cannot meet the constant current charging requirement for EV battery charger.

..As such, please can anybody state why on earth an LLC converter is so challenging to use with a feedback loop closed on its output current? I have done LLC LED drivers before and it was no problem. Does anybody know if this “problem” of using an LLC converter to regulate its output current is actually really a problem at all?

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It's just that PS-FB is very straight forward to design and no hidden control issues for dynamic loads and short circuits, also less parts, current mode control makes FS-FB bullet proof, a LOT more testing is required for LLC at high powers as sudden shorts put a LOT of current through the main switches...!

…..Though Page 12 of the following document by Infineon points out the suite of “gotcha’s” that await the Phase shift full bridge designer….
“Design of Phase Shifted Full-Bridge Converter with Current Doubler Rectifier”
https://www.digikey.com/-/media/Ima...en-US&ts=be77e52b-3ca6-4cb3-b12d-1988f0050c3e

Quoting here from page 12 of the above document (concerning the problems of PSFB converters)..

Robust body diode with fast reverse recovery. Although that in normal operation the body diode current/charge are softly commuted, in some conditions such as startup, load transient, light load or low leakage inductance, body diode may have hard commutation, it may not have a channel conduction following its own conduction, or channel conduction might be too short and not enough to completely sweep out the reverse recovery charge, in such case, as the MOSFET turns off with a high dv/dt while there are still residual charge in the body diode region, the charge leaving the body diode P-region may bias the parasitic npn BJT, causing false turn on and destruction of the MOSFET.

….That tells that it’s the PSFB that can blow up following transients, I’ve never heard of a failure mode for an LLC that involves load transients within the actual load range of the LLC converter (ie not talking about overload transients or shorts). The PSFB doesn't sound "Bullet-Proof" to me.