Phase shift full bridge SMPS not suitable for high output voltage?

[Flyback]

Hello,
Please confirm that the Phase Shift Full Bridge (PSFB) SMPS is not suitable for 7KW Public Hybrid Electric Vehicle battery chargers?

Spec of PHEV charger
Vin = 90-265VAC (single phase)
Vout = 250V-400VDC (7KW)

The high output voltage of 400V, coupled with the much higher output diode stress that occurs with a PSFB means that the PSFB is not suitable.
The following article confirms the unsuitability of the PSFB….

“High Efficiency DC-DC Converter for EV
Battery Charger Using Hybrid Resonant and
PWM Technique”
By Hongmei Wan…………

https://scholar.lib.vt.edu/theses/available/etd-05072012-141855/unrestricted/Wan_HM_T_2012.pdf

This document confirms my postulate….also, Mr Wan proposes that for this application, the way forward is a “hybrid” converter, which comprises both an LLC converter and a PSFB converter, together with a non-dissipative diode snubber network.

Mr Wan proposes this, because as in his report, the PSFB by itself is not suitable for this application.
The alternative to Mr Wan’s complex hybrid converter is just simply to parallel multiple hard-switched Full Bridge converters….which seems like the sensible way forward in truth, do you agree with all this?

..................................................................................................................
The absolute *golden* question concerning Phase shift full bridge (PSFB) converters, is that if the dead time and leakage inductance is improperly sized such that zero voltage switching does not occur, then are they more or less efficient than a plain, hard-switched Full Bridge? No-one addresses this point in any document anywhere. I suspect that the PSFB converter is less efficient than the plain full bridge if not “tuned” properly.

 

[Tony Stewart]

PSFB do have higher stress factors and limited ZVS load range.

For quick chargers minimum load may not impose a problem if the bulk of charging is in CC mode then CV as load reduces. It would have to have major cost benefits to accept the disadvantages.

 

[Flyback]

Also, please note the attached simulation, which shows a plain full bridge SMPS compared with a non-ZVS phase shift full bridge (PSFB) SMPS.
The non-ZVS PSFB has switching power dissipation peaks which are 5 times higher than the plain full bridge SMPS.....this appears to confirm that the PSFB, when in non-ZVS behaviour, has far higher switching loss than the plain full bridge.

This is a big downer for the PSFB compared to the plain full bridge. The app notes and PhD research papers didn't bring this important point forward.

 

[Tony Stewart]

THe journal article you reference points out these problems at low and no load.
2.2.5 Disadvantages of Phase-shifted Full Bridge Converter

Did you ensure the following?
In order to achieve ZVS turn-on the energy stored in the leakage and series inductance has to be larger than the energy stored in the output capacitances.

Then it goes on to talk about the problems with insufficient load.

During light loads, very little energy is stored in the primary side inductance L lk (can
be sum of the transformer leakage inductance and external inductance in series with the
primary of the transformer). This causes the lagging-leg to turn on under a hard switching
condition which can be seen from Fig.2.10 (c) and Loss of ZVS means extremely high
switching losses at high switching frequencies and very high electromagnetic interference
(EMI) due to the high di/dt of the discharge current. Loss of ZVS can also cause a very
noisy control circuit, which leads to shoot-through and loss of the semiconductor switches.
As the load increases the energy stored in inductor L lk increases. This energy is used to
charge the output capacitance of the devices that are turning off, and to discharge the
output capacitance of the complementary device, thus forward biasing the freewheeling
diode. Full output capacitive discharge is necessary to cause the lagging-leg to start
turning-on under the ZVS condition. The turn-off under the ZVS condition closely mimics
that of resistive turn-off. A switch with low output capacitance helps to achieve ZVS
process at light load, improving the efficiency. Hard switching occurs when the output
capacitance of the switch requires more energy than is available in the inductance L lk to
fully charge and discharge the switches. And the energy stored available to turn on
lagging-leg the diode is very small so its conduction time is very small even at full load.
The energy stored in L lk increases by a square law as load current increases. Therefore, the
stored energy in L lk increases rapidly at loads higher than the minimum load required for
ZVS. The large amount of available charging energy causes the switch drain-source
voltage to rise and fall at a linear rate.

 

[Flyback]

Thanks Tony Stewart,

Did you ensure the following?
In order to achieve ZVS turn-on the energy stored in the leakage and series inductance has to be larger than the energy stored in the output capacitances.

You're hitting the nail on the head, but what I am wondering is if we can deliberately operate the PSFB (phase shift full bridge) without ZVS and still have less switching loss than the Full Bridge SMPS?
The reason we want to operate it without ZVS is precisely because we don't want a high Llk value.....because if we do then we end up with high snubber losses on the secondary side, and also higher voltage stress on the output diodes which is already bad enough because our Vout for PHEV application is 250V-400V.

I mean, I know we are going to have "some" Llk anyway, and we are wondering if this will pre-discharge enough of the fet capacitance to result in reduced switching loss compared to the full bridge.....or......will the act of being in non-ZVS in a PSFB mean that we have more switching losses than a full bridge smps.?
The simulation above certainly suggests that a non-ZVS PSFB has more switching loss than the "equivalent" full bridge smps.

As the paper discusses (#1), PSFB (alone) is not so good for PHEV charger applications, and the author states that he needs to use a hybrid converter composed of a PSFB integrated with an LLC, also integrated with a non-dissipative snubber network......well....we certainly haven't the time to do a hybrid like that, so we are wondering, can we do a PSFB, and if the output diode voltage stress ends up being so high that we have to have non-ZVS, then will the non-ZVS PSFB have more or less switching losses than the plain full bridge SMPS?................As discussed, the simulation above certainly suggests that the non-ZVS PSFB has more switching losses than the full bridge SMPS.

But is this true, or is it a figment of the simulater in some kind of error mode?