Re: Buck converter, controller riding on the switch node


Joerg wrote:

MooseFET wrote:
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. ...

Yep, same experience here. Last month I had to convince a client to do
the same on such a switcher. That didn't exactly create outbursts of joy
with the materials guys because it's a custom part. However, there
just isn't really any other low cost way.

... 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.


" ... and usually seem to find one" :-)

Well said. EMI is like water or synthetic oil. You seal over here and
then ... phssss ... it starts to ooze out over there.


More like fat. No matter how much they push, it bulges somewhere
else. Its funny to see someone with a big roll of fat over or under
what they think makes them look thinner. You need to watch out in case
it spits, because it can kill someone from the recoil! ;-)


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