Re: Buck converter, controller riding on the switch node

Klaus Kragelund wrote:
On 17 Jan., 18:27, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
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.


The initial idea of mine was expanded from the use of the NCP101X
switchers, which are integrated switcher referenced to the switch
node. I did do a 5W design with almost the same PCB stackup as
MooseFet described, but the enclosure was not shielded and we had no
problems passing the EMC. Thats why I would try it for a bit large
power rating (35W). I think the area of the plane will be almost the
same and the rise/fall times also, so I would expect to see around the
same CM noise as the 5W design.


Congratulations, running something like a NCP1014 with flying ground is not that easy. AFAIR those operate under 150kHz and thus under the radar screen with the fundamental but the harmonics can easily get you. Increased power can increase the conducted readings because it always finds its way out the mains side via capacitances but if you had 15dB margins there you'd probably be ok.


(as far as I remember we used a full ground plane and cutouts in the
inner layers and routing only on the flying layer surrounded by a
small plane)



--
Regards, Joerg

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