Re: Buck converter, controller riding on the switch node

On Jan 17, 7:30 am, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:
MooseFET wrote:
On Jan 16, 6:06 am, Klaus Kragelund <klausk...@xxxxxxxxxxx> wrote:
Hi

Well, a typical buck converter with current mode control has a PWM
controller referenced to output ground, a current sense transformer to
bring down the current sense signal and a pulse transformer to level
shift the gate signal.

Another approach is the one from Fairchild with an integrated solution
riding on the switch node:

http://www.electronicsdesign.dk/tmp/AN2544_ViperInBuckMode3.pdf

To be independant on single source parts, I would like to do the same
with the standard PWM controllers like the SG3524, UC1843 etc as shown
in the scematics below:

http://www.electronicsdesign.dk/tmp/HighSideBuck.pdf

I haven't build it yet, but do any of you have experiences with this?
Ofcourse I need to make a seperate plane connected to the SW (switch)
node below the entire PWM controller circuit to combat leakage
currents by the parasitic capacitances/dV/dt.

Anything more to look out for?

I have done this same thing with the entire switcher controller flying
up and down.  The feedback used and op-amp with careful low pass
filtering to get the output voltage level shifted on to the domain
with the huge AC on everything.

Part of the trick is to make your planes look like this:

------------- Top switching parts
------------- Inner 1 interconnect
------------- Flying plane
------------- Nothing in this area
------------- Nothing in this area
------------- Bottom system ground

Around the outside of the area, you can stitch the ground from its
normal layer down to the bottom side in many places.  This basically
puts the switching stuff in a box.

There needs to be something along the lines of system ground above the
flying plane. Either interleaved and well connected ground islands or a
shielded enclosure. It's next to impossible to pass EMC otherwise. BTDT
(or, rather, clients called me with similar designs they had and asked
what to do about their EMC problems).

The shielded enclosure is from what I've seen about the only way to
go. If you put ground on the upper surface of the PCB, it raises the
capacitance too much. The more capacitance you have the bigger the
current pulses are at the edges. These current pulses look for a way
to get out and usually seem to find one.


You want to soften up the switching edges a little.  Any microwaves
created by the circuit will keep looking until they find a way to mess
you up.  One problem is with the input transistors of the op-amp.

Nibbles away at the efficiency. In the end all this needs to be priced
out. If you must go to 6-layer and shields the savings from not having
transformers might evaporate.


The example stack up I gave was from real life. I had other things on
the PCB that drove the layer count. It wasn't just for the flying
plane that I needed to go to that many layers.

BTW: I had a shield over the power section and one over the parts
that did RF and the whole thing was in a metal housing. The
connectors were metal backshelled an the two cables that went in and
out were both shielded.
.