PSFB abnormal heating and current oscillations

[Albert.b]

Hello everyone,


I am designing a PSFB converter with a current doubler topology in the rectification stage, using the UCC28951PWR controller. The rest of the circuit follows the topology provided in the datasheet: the drivers and the controller are referenced to the secondary ground, and the full-bridge MOSFETs are driven through gate drive transformers. The input voltage range is 30–120 V, the output voltage is 14 V, and the total output power is 1 kW.

The issue I am facing is that the full-bridge MOSFETs are heating up excessively—reaching 90°C in just one minute of operation at a 3 A load.

(Lagging leg - Left-side / Leading leg - Right-side)

As far as I can tell, ZVS is being achieved in both legs for all the possible loads, which to he honest, it's a bit odd to me as ZVS is meant to happen at higher loads, so I may be missing something here.

I am attaching an image that shows the switching behavior on the lagging leg for a load of 3A, and amall shim inductor (0.22uH) in series with samll tx leackage inductance (0.044uH) :

• Yellow: Voltage at the intermediate node of the branch.
• Green: High-side MOSFET gate voltage.
• Blue: Low-side MOSFET gate voltage.

Until now, I have only been monitoring the MOSFET gate voltages, which do not reveal everything. If abnormal current is flowing through the body diode, it may be hidden from my observations. To gain better insight, I placed a shunt resistor in series with the high-side MOSFET of the leading leg to monitor the current flow for any irregularities.

The first set of measurements (see attached image) shows the following waveforms for the leading leg:

• Yellow: Current through the high-side MOSFET.
• Green: High-side MOSFET gate voltage.
• Orange: Voltage at the intermediate node of the branch.
• Blue: Low-side MOSFET gate voltage.

The concern arises when a large current oscillation appears in the MOSFET while its channel is disabled, which I suspect is causing the heating issue.

In another measurement, the waveforms observed are:

• Yellow: Voltage at the middle node of the leading leg.
• Green: Current through the low-side mosfet of the leading leg(A).
• Orange: Voltage at the middle node of the lagging leg.

Another observation is that the current oscillations through the MOSFET occur when any MOSFET in the bridge turns off and the node at the midpoint changes its voltage value. In a PSFB converter, when a MOSFET turns off, the voltage at the midpoint shifts polarity (from 0 to Vin or vice versa) because the parasitic capacitances in the branch of the MOSFET are charged and discharged by the current maintained briefly by the transformer’s inductance. This rapid voltage change might be causing the oscillations, as the inductance in the current path resonates with other elements.

If that’s the case, I’m unsure what changes I could make to the schematic to suppress these oscillations.

When comparing the experimental waveforms with a simulation of the same circuit under identical conditions, it was observed that, aside from the noise present in the real implementation, the current waveforms through the MOSFETs match closely:

The theoretical circuit does not exhibit ringing, but it does appear in the physical implementation. Therefore, the issue could stem from the PCB design or the way the components are arranged. Additionally, the current paths in each switching cycle can be described as follows:

• The current path during one half-cycle, which forms an almost closed loop (Red).
• The current path during the other half-cycle, which extends further, forming approximately one and a half loops (Blue).

But this is just an observation as I am not sure.
Iam attaching the schematics and the images in case the quality is too low.

Any help would be very much appreciated, thank you

 

[kd502]

Did you use scope probe ground clip? Ringing everywhere look like scope probe grounding problem.
Ground connection must be as short as possible:

Gate voltage rise and fall times look quite long. Zoom in the slope of the gate voltage and drain voltage try to estimate the switching times, calculate the switching losses.

 

[Albert.b]

Hello,


This "noise" or ringing appears regardless of which probe I use — even with the current probe. I believe it's real and likely the cause of my heating issue.


Regarding the rise and fall times, perhaps you were referring to the second image, where they do look a bit slow. I’ve tested many gate resistor values, even removing them entirely, until the transitions were fast enough. Yet, the heating problem persisted. Also, if I'm achieving ZVS, then the rise time shouldn’t matter that much, right?


Here are some of the questions I have related to this heating issue:

• Am I using the correct topology? I've been using a current doubler rectification topology because I assumed it was compatible with the UCC28951PWR controller, even though this topology is not shown in the datasheet.
• Am I truly achieving ZVS (as it seems in the second image), or am I missing something? To me, it doesn't seem normal to achieve ZVS with loads as light as 3A. So maybe I’m misinterpreting the oscilloscope waveforms.

This problem feels quite fundamental — more like a conceptual error rather than just a matter of component selection.

 

[kd502]

This "noise" or ringing appears regardless of which probe I use — even with the current probe.

Current waveforms usually have more ringing. When I see ringing on gate signal I suspect probe ground I'm not talking about which probe you used but how you connected ground of that probe.
Did you use spring shown in my picture ?

I believe it's real and likely the cause of my heating issue.

Ringing usually have small energy. Exception is when high and low side switch conducts. In that case ringing is not the cause.
Show us the waveform with current falling slope stretched to whole screen.

Am I truly achieving ZVS (as it seems in the second image), or am I missing something? To me, it doesn't seem normal to achieve ZVS with loads as light as 3A. So maybe I’m misinterpreting the oscilloscope waveforms.

Looks like ZVS. Current flows through the inductive load. Nothing unexpected.

This problem feels quite fundamental

Fundamental question. What are the switching loses? Transistors without heatsink can dissipate only 1W. Do the math. Is it possible or not?

Also, if I'm achieving ZVS, then the rise time shouldn’t matter that much, right?

Rise time doesn't matter but fall time matter. You have hard switching there.
What is the peak current before turn-off?

 

[mtwieg]

My first guess would also be that most of the observed ringing is not "real" and is the result of how the signals are probed... but the fact that you're observing such heating indicates a real issue.

This "noise" or ringing appears regardless of which probe I use — even with the current probe.

Please describe what probes you're using in detail.
For example, you show waveforms of high side Vgs, is this measured with a differential probe or by subtracting two measurements?
Also you mentioned above measuring current by measuring voltage across a shunt resistor, but here you mention a "current probe". The distinction between the two is very important. Also need to know which side of the FET the current is measured on.
These details affect whether we assume the ringing is "real" or not.

In practice, ringing like this usually originates from the rectifiers on the secondary side. Are you using any snubbers on the secondary? Are you using synchronous rectifiers?

 

[Easy peasy]

lack of quality film caps ( or MLCC suitably rated ) across the fets on the H bridge - top D to bottom S

this is a classic newbie thing

 

[Easy peasy]

@Albert.b just to recap - the reason for the overheating mosfets is the lack of heat-sinking.

not helped by how far apart the fets are - with no local decoupling.