Bridgeless Totem Pole PFC.....not getting popular...why?

[Flyback]

Hello

The boss has asked me to evaluate a Bridgeless Totem Pole PFC (BTP-PFC) stage for our 1kW Offline Battery chargers.
However, recently I had to reverse engineer loads of Offtheshelf offline battery chargers to 3Kw , and none contained a BTP-PFC.
I am convinced there’s something of a trick question going on here, …its like the PFC’d resonant CUK converter that was supposed to do Vout regulation and PFC in a single stage, but it never took off.

What is the snag with the BTP-PFC?

For a start there’s the expensive GAN FETs that are needed. But what’s the gotcha?

(This by the way, wasn’t one of the questions asked to me about it, so I am not cheating.)

 

[ronsimpson]

a Bridgeless Totem Pole PFC

You could post a schematic or .PDF or some thing?
Found it!! Now I know what you are talking about.

 

[Flyback]

Thanks, here is a simulation of it.....or a very basic representation.
It has a greater common mode emi problem, which makes it less attractive compared to standard boost PFC.

 

[Flyback]

Thanks,

I am wondering why not one single Application Note on the entire internet actually mentions the very bad EMC problem of the BTP-PFC?……why this “cover-up” over the BTP-PFC?

The BTP-PFC brings with it a very bad common mode EMC problem due to the fact that the controller will inevitably be referenced to the DC Bus negative net. This net is by its nature, a very extensive net, going widely over the PCB (since it’s the reference point for much of the BTP-PFC circuitry) …the problem is, that in BTP-PFC, this net has a very high dv/dt with respect to earth ground….and this is the absolute recipe for bad common mode EMC issues. The attached (red) waveform shows the BTP-PFC problem (LTspice sim also attached)

With conventional Boost PFC, If you probe the Neutral input with respect to “DC Bus ground”, then the dv/dt with respect to the DC Bus ground isn’t so high. (as the green waveform shows)
However, with the BTP-PFC, the dv/dt between DC Bus ground and the Neutral is very high….and this means terrible common mode EMC problems. This situation is shown in the attached waveform diagram, which shows voltage between Neutral input and DC Bus ground for a BTP-PFC. (Red waveform)

And the key point is that DC Bus ground, is , by its very nature, a very extensive net, probably running over a great area of the PCB…so what we have , is this wide area of PCB copper which has a very high dv/dt with respect to Earth ground (since neutral is ultimately connected to earth ground). –This is the absolute perfect recipe for disastrous common mode EMC issues…the high dv/dt of a wide area of pcb copper with respect to earth ground.

The LTspice simulation attached actually shows the problem. I have never seen a real product in the market which actually uses BTP-PFC……and certainly on the entire internet you cannot find a single application note giving a conducted EMC scan of a BTP-PFC design.

I am beginning to wonder if this BTP-PFC is going to end up getting consigned to the scrap-heap, due to the impractically extensive EMC solutions that are needed to accommodate it?

In true cook-report fashion, i expose the BTP-PFC for the "dodgy geezer" that it is.

 

[Flyback]

Hello,
The attached is a BTP-PFC with pure analog control. I cannot see any reason why this would not work…can you?
Why are there no Pure Analog control solutions on the internet anywhere for BTP-PFC?
Why are all solutons using software based controllers?

 

[alec_t]

Why are all solutons using software based controllers?

I'd guess (a) fewer components, (b) easier to tweak/modify.