Something seems wrong here but I can't place exactly what it is:
- posted
7 years ago
Something seems wrong here but I can't place exactly what it is:
It describes the way an APC UPS works. See video #504 in the eevblog.
(it is presented as a design idea but actually it is patented by another UPS company and probably licensed to APC)
Old hat. To be used with caution, because not all body diodes are created equal -- for some fast switching regulators you really want to parallel the FETs with Schottky diodes, as they recover quicker. A FET with a body diode that has slow recovery can become smoke pretty quickly.
-- Tim Wescott Control systems, embedded software and circuit design
Some mosfet body diodes make nice step-recovery diodes.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
So they can spray really high-frequency noise around the circuit?
Just what you want when you have to meet EMI standards.
-- Bill Sloman, Sydney
All MOSFETs do that, to one degree or another. Some have added soft-recovery diodes in parallel with the body diode, which helps, but designers of half-bridge stages, etc., are always struggling to prevent body-diode conduction, or limit its current*time, or slow switching, or snub any voltage spike from the sudden high dI/dt at snap off. The problem is worse at maximum switched current. And most off-line designs can't use added Schottky diodes, since high-voltage versions have too high an ON voltage.
-- Thanks, - Win
This is a really classic SRD waveform:
This switcher is basically unusable. A paralleled external schottky only helped some.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Didn't someone post a "design idea" here once where the cutoff ringing from the leakage L of a flyback was recovered into the output rail, somehow?
My pic was of a synchronous buck regulator, so there was no leakage inductance. The big-slam risetime may well have been scope+probe limited. The spike messed up opamps all over the board.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Would adding some R in series with the high-side bootstrap cap make a difference?
I doubt it. The big spike sure looks like step recovery in the substrate diode. The diode takes over from the low-side mosfet for about 20 ns, then when the high-side fet turns on, the diode fights it for a few ns, then snaps off.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
If it is step recovery then the fact that they don't bring the source connection on the high side switch out to a pin makes it seem like a SOL situation with respect to trying some other simple tricks to mitigate it.
I remember reading some paper where EMI-making ringing on the output node of a buck switcher of all types was canceled by sensing it thru a capacitor, amplifying it and passing it through the other winding of a common mode choke where the first winding was used as the energy-storage inductor.
Seems like a lot of work, though...
Yup. The fix is to use some other part.
IC designers need more critical design reviews, for things like this, and latchup, and other weird effects.
The current spikes must be huge. There is probably no reasonable way to fix this one.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
I remember years ago trying to use a Maxim part that was originally a Dallas Semi part, I think it was the DS1077 SPI-programmable clock oscillator.
A neat part, except for the fact that the example code in the datasheet didn't work, and the way they said the chip was addressed over SPI by a uP seemed to bear...no resemblance to reality.
"Designing" with it basically meant hunting through Wordpress blogs trying to find someone who'd managed to suffer through the process before.
One good maxim is
Never Buy Maxim.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
How about a Schottky in parallel with the low-side FET/Diode?
We tried that, but it only helped some, not enough to make the chip usable.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
I'd like to have seen your layout, too.
I suppose most users simply lived with the noise.
A common approach is to slow the switcher's dV/dt with an aggressive R+C snubber on the switched node. Yes, it can dramatically reduce the efficiency, but it quiets the noise. Did you try that? The other approach is to give up on synchronous converters.
-- Thanks, - Win
The spikes were making huge offsets in opamps all over the board.
That LM3102 was especially awful. We use other synchronous switchers without problems.
TPS54302 is a great little part. It switches fast but doesn't have a gigantic current spike.
It does spread-spectrum, too, which reduces official EMI.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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