UCC28951: UCC 28950/ 28951 : Is it viable to use full bridge topology (without center tapped transformer) on the secondary side utilizing E and F PWMs?

Sagar Modi

Part Number: UCC28951

Tool/software:

Project Specs:
Vo: 24VDC
Vin: 550V
P=500W
I am using UCC28951 (PSFB topology) IC for DC-DC isolated power supply project. I have a full bridge synchronous rectifier on the low voltage (secondary) side instead of a half bridge synchronous rectifier.
On the secondary side, PWM signals for E and F (SR FETS) coming from the UCC28951 ic are fed to diagonal switches of the bridge through a half-bridge gate driver ic (2EDL8124G3CXTMA1). Recently, I looked at the forum and found that PWM signals for E and F both could be high simultaneously during the free-wheeling duration.
https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1400335/ucc28950-q1-centre-tapped-or-full-bridge-synchronous-rectifier?tisearch=e2e-sitesearch&keymatch=UCC28951%252525252525252520full-bridge#
In our project, under the same scenario the gate drive ic would not pass PWM signals to the switches when both the signals (E and F) are high simultaneously and hence it will avoid shoot through. At the same time the anti-parallel diodes would be conducted during the free wheel time.
Do you see any issue in our design?

I have been following the TI design https://www.ti.com/tool/PMP7246. It uses UCC 28950 (PSFB) and in its schematic it is clear that E and F PWMs go to the secondary (low) side of the bridge. I have replicated the design. What is the issue here and why are you not recommending to use E and F Pwm signals to drive the secondary bridge?

Thank you.

11 months ago

0 Sagar Modi 11 months ago

Prodigy 20 points

https://www.ti.com/lit/df/tidrbx3/tidrbx3.pdf?ts=1727369952604

0 Mike O' 11 months ago in reply to Sagar Modi

TI__Guru* 86266 points

Hello,

Your inquiry has been received and is under review.

Regards,

0 Sagar Modi 11 months ago in reply to Mike O'

Prodigy 20 points

In the above image, the effective duty cycle of E and F is identical to the generated by the A,B,C and D switches.

0 Mike O' 11 months ago in reply to Sagar Modi

TI__Guru* 86266 points

Hello,

Not sure what is meant by this last figure that you markup, could you explain?

Regards,

0 Sagar Modi 11 months ago in reply to Mike O'

Prodigy 20 points

I took the above image from the datasheet of the UCC28951 and edited. The effective duty cycle from the A, B, C and D PWMs are shown in red on 4th plot from the top. You can ignore the PWMs I drew on top of PWM C and D signals.

Now, in order to transfer energy from primary to secondary, the secondary (low voltage) H bridge switches must switch be ON during the effective duty cycle (PWM). I guess that is why you recommended to utilize the A and D PWMs on secondary side instead of E and F PWMs.

My point is that if the gate driver ic present on secondary side that gives low output when the E and F signals both are high then we could use E and F PWM signals to drive the secondary low voltage side H bridge and that is what exactly done in the https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1400335/ucc28950-q1-centre-tapped-or-full-bridge-synchronous-rectifier?tisearch=e2e-sitesearch&keymatch=UCC28951%252525252525252520full-bridge#.

If you focus on E and F PWMS in the attached image, then I highlighted the output of gate driver ic which is identical to the effective duty cycle PWM shown in the 4th plot. That assures secondary side bridge is conducting in the right pattern to extract the energy out.

I am using FET switches with anti-parallel diodes on secondary (low voltage) side full bridge. So the anti-parallel diodes will help to circuit current during the free wheel time.

Thanks.

0 Mike O' 11 months ago in reply to Sagar Modi

TI__Guru* 86266 points

Hello,

Thanks for letting me know. It looks like you have this one figured out.

Regards,

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UCC28951: UCC 28950/ 28951 : Is it viable to use full bridge topology (without center tapped transformer) on the secondary side utilizing E and F PWMs?