Buck converter, controller riding on the switch node

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:

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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:

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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?

Thanks

Klaus

Reply to
Klaus Kragelund
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And bytheway, don't look to closely at components values, they were without thorough calculations :-)

Reply to
Klaus Kragelund

Not a bad idea but to get decent performance from this you'll need to change D25 for a controled switch (i.e. a mos) so that you really sample the output voltage and you might see some funny behavior too (think about what's happening when the ouput overshoots).

-- Thanks, Fred.

Reply to
Fred_Bartoli

x-no-archive:

I don't see how you addressed the issue that the Vcc and Vout are referenced to the real ground yet you need to reference them to Sw ground for the IC.

Mark

Reply to
makolber

Since VB and SW (the floating ground) would rapidly swing up and down with the switch action this can result in major EMC pain.

Maybe there is a way with the traditional concept to level shift with capacitors if the duty cycle remains in a "normal" range. Of course for the current sense that would almost have to be done differentially (or with a transformer).

--
Regards, Joerg

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Reply to
Joerg

When the gate is low and the MOSFET turned off, the diode D23 freewheels, so the SW voltage is negative one diode drop. So at that time the drive for the chip (VB) and the sense voltage (Out/24V) is transferred via diodes to the high side circuit. Actually, good thing you asked - Out and 24V should be the same node.

Regards

Klaus

Reply to
Klaus Kragelund

s: snipped-for-privacy@v18g2000pro.googlegroups.com...

nge

put

Good point, should sample a shortly after the gate is turned off. Thats actually how they do it in the integrated device for this function. Maybe the PWM should be a microcontroller (PSoc) to incorporate some intelligence into the PWM (softstart, hickup, sampling, reference etc).

Regards

Klaus

Reply to
Klaus Kragelund

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.

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.

Reply to
MooseFET

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).

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.

--
Regards, Joerg

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Reply to
Joerg

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.

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.

Reply to
MooseFET

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.

" ... 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.

[...]
--
Regards, Joerg

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Reply to
Joerg

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I always say that it is like light and conductors are like ideal mirrors. The light bounces around until it finds a hole.

RF beads (lossy inductors) help because they are black.

RF can even get out of a sealed box if the box is driven with an RF current. This isn't much like light or oil.

Reply to
MooseFET

[...]

Dang, some of mine are blue ;-)

Not if the box is a really good conductor and not very large compared to the wavelength. Same happens with oil. If you put a strain on the gear box it'll leak a little more :-)

--
Regards, Joerg

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Reply to
Joerg

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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.

(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

Klaus

Regards

Klaus

Reply to
Klaus Kragelund

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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Reply to
Michael A. Terrell

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Are you sure someone didn't sell you Real Fancy beads?

A lot depends on how much current is driven into the box. In one case it was a lot of amps at every multiple of 150KHz. It was a rack mounted power supply. I had to mount it with plastic hardware to keep the currents in the case from going through the rack.

Reply to
MooseFET

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.

--
Regards, Joerg

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Reply to
Joerg

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