TPS2378EVM-602: The flyback structure in User's guide

Aaron Chen (FAE)

Intellectual 1310 points

Part Number: TPS2378EVM-602
Other Parts Discussed in Thread: TPS23731EVM-095, TPS2378

Hi Experts,

When i was reading the user guide of TPS2378EVM-602.

I found that the mosfet is not at output trace.

what is the difference between them.

Normally, the mosfet of the flyback is at output trace, am i making sense?

over 3 years ago

+1 Michael Pahl over 3 years ago

TI__Mastermind 21320 points

Hello,

Yes I think you are making sense. You are saying normally the drain of the diode is connected to the output inductor (L2 in this schematic). Connecting it to the inductor is called putting the FET on the high side, and how it is currently pictured is considered putting the FET on the low side.

We have learned over time that this is not a good idea. The reason is that most MOSFETs are built with three pins for source, one pin for gate, and then 4 pins and a pad for the drain. Therefore, the drain is where the thermal dissipation happens. To assist in thermal dissipation, we suggest using a large copper plane.

the issue with Low side FETs is that the drain is now a high dV/dt plane because it switches between the left side of the transformer and PGND. So making this plane large will create bad EMI.
Therefore, we now create a conflict -- we need large plane for thermals, but we need a small plane for EMI.

If we put the FET on the high side (like TPS23731EVM-095), then the drain is connected to the inductor and the dV/dt is not large, so the EMI contribution is minimized.

Note the TPS2378 is an older part, so we were (are) still learning.

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

0 Aaron Chen (FAE) over 3 years ago in reply to Michael Pahl

TI__Intellectual 1310 points

Hi Michael,

To recap, putting the FET on the low side, is not a good idea, because of the thermal, once it put at the low side, it needs large copper plane. Also, a large plane with high dV/dt creates the worse EMI performance. right?

Does it have any advantage of putting FET at low side?

0 Aaron Chen (FAE) over 3 years ago in reply to Aaron Chen (FAE)

TI__Intellectual 1310 points

Does it mean that the mosfet of new flyback device all put at high side?

0 Michael Pahl over 3 years ago in reply to Aaron Chen (FAE)

TI__Mastermind 21320 points

Hey Aaron,

Yes! The large plane is needed on the drain, but on the low side it creates a big EMI noise.

Therefore, all new designs for PoE do not have this. We have tried to learn from our mistakes :)

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

0 Aaron Chen (FAE) over 3 years ago in reply to Michael Pahl

TI__Intellectual 1310 points

Hi Michael,

I see.
Does it have any advantage of putting FET at low side?

BR,
Aaron Chen

0 Michael Pahl over 3 years ago in reply to Aaron Chen (FAE)

TI__Mastermind 21320 points

Not that I know of -- maybe there is, but for us we now always put them on the high side. So the advantages are not worth the disadvantages to us.

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

0 Aaron Chen (FAE) over 3 years ago in reply to Michael Pahl

TI__Intellectual 1310 points

I see, then, why TI designed it at low side for old solution?
Was that a common structure from textbook?
or Did there any old paper or literature discuss about this?

0 Michael Pahl over 3 years ago in reply to Aaron Chen (FAE)

TI__Mastermind 21320 points

Hey Aaron,

To be transparent, these designs were made before my time at TI. I can check with the designers to ask about their design choices.

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

0 Aaron Chen (FAE) over 3 years ago in reply to Michael Pahl

TI__Intellectual 1310 points

Hi Michael,

I really really really appreciate your help.
I know it will take time, if it's possible, maybe we could have more discussion by mail, also loop the designer.
Looking forward to it.

0 Michael Pahl over 3 years ago in reply to Aaron Chen (FAE)

TI__Mastermind 21320 points

Hey Aaron I checked with the designer, and there is no real reason they did it on the low side. It has just been done this way, it was done this way, and then since then we learn the best way :)

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

0 Aaron Chen (FAE) over 3 years ago in reply to Michael Pahl

TI__Intellectual 1310 points

Hi Michael,

Btw, is the red square named Synchronous Rectifier?

0 Aaron Chen (FAE) over 3 years ago in reply to Aaron Chen (FAE)

TI__Intellectual 1310 points

I founded this cool material for discussing the pros and cons.
www.ti.com/.../slua861.pdf

0 Aaron Chen (FAE) over 3 years ago in reply to Aaron Chen (FAE)

TI__Intellectual 1310 points

I will study it, thank you for the support.

0 Michael Pahl over 3 years ago in reply to Aaron Chen (FAE)

TI__Mastermind 21320 points

Hey Aaron,

Yes, the red square makes it a synchronous rectifier. Whatever is on the secondary determines the topology.

Sounds good, thank you for the material! i'll review it too.

Thanks.

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

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TPS2378EVM-602: The flyback structure in User's guide