UCC28950: Tools---Clarification in the Calculation of Primary MOSFET's Loss

Abu Bakar

Part Number: UCC28950

Hello,

We are currently working on designing a phase-shifted full bridge converter, utilizing the ucc28950 controller. In equation 59 (found on page 43 of the datasheet), the calculation for the loss in the primary MOSFETs is performed. This equation uses the root mean square (rms) current, which is the same as that used for the transformer windings.

However, there seems to be a potential oversight. Considering that each pair of MOSFETs conducts only for half a cycle, there arises a question: Shouldn't the calculation be adjusted to account for the duration during which the MOSFETs are actively conducting? Could you please provide clarification on this matter?

Regards.

over 1 year ago

0 Mike O' over 1 year ago

TI__Guru* 86106 points

Hello,

Is the equation you are referring too in the data sheet or application note?

Regards,

0 Abu Bakar over 1 year ago in reply to Mike O'

Prodigy 30 points

I'm referring page 43 in UCC28950 datasheet. please add one more question, in the next equation 60, it multiplies by 4 to calculate total losses in MOSFETs. Since only two MOSFETs (one in each leg) conduct at the same, the multiplication factor should be 2. What's TI thought on this?

https://www.ti.com/lit/ds/symlink/ucc28950.pdf?ts=1694613324335&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28950%253FkeyMatch%253DUCC28950%2526tisearch%253Dsearch-everything%2526usecase%253DGPN-ALT

Regards

0 Mike O' over 1 year ago in reply to Abu Bakar

TI__Guru* 86106 points

Hello,

I am reviewing your inquiry and will get back to you shortly.

Regards,

0 Mike O' over 1 year ago in reply to Mike O'

TI__Guru* 86106 points

Hello,

After reviewing the equation on page 43, I believe you are correct that total power loss will be multiplied by 2 instead of 4 for an accurate estimate of FET H Bridge Losses. Thanks for point that out and I will let the applications team now.

Regards,

0 Abu Bakar over 1 year ago in reply to Mike O'

Prodigy 30 points

Hello,

Thanks for the message. What about my earlier query regarding equation 59 on the same page, where it uses transformer RMS current to calculate the loss in MOSFETs??

Regards

0 Mike O' over 1 year ago in reply to Abu Bakar

TI__Guru* 86106 points

Hello,

I am evaluating your inquiry and will get back to you shortly.

Regards,

0 Mike O' over 1 year ago in reply to Mike O'

TI__Guru* 86106 points

Hello,

Equation 43 and 44 are used to calculate the transformer IRMS current in equation 45 which is the sum of the RMS currents squared.

It was derived from the below RMS currents that are in many power supply text books.

Based on estimating two of the FETs are always conducting in the HBridge RMS current through the FETs should be the same as the transformer.

So using the transformer RMS current to estimate FET losses is a good assumption.

Regards,

Power management

Power management forum

UCC28950: Tools---Clarification in the Calculation of Primary MOSFET's Loss