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"Use MOSFET body didoes"

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Tim Williams
Guest

Sat Jan 07, 2017 7:56 pm   



"John Larkin" <jjlarkin_at_highlandtechnology.com> wrote in message
news:mrev6cp210lbdap42l093254cislbbqbjv_at_4ax.com...
Quote:
Well, the trivial solution would be supply inductance.
^^^^^

How would you snub the current spike?


You quoted it above!

As the Bible says:
If the bypass cap causes you trouble, rip it out!

Quote:

Snarkiness is not usually a successful design methodology.


But the designer is a human (for now), and human coping mechanisms are
therefore part of successful design methodologies.

Sarcasm. Alcohol. Sleep. All fantastically useful design tools! :-)

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

John Larkin
Guest

Sat Jan 07, 2017 11:08 pm   



On Sat, 7 Jan 2017 06:56:53 -0600, "Tim Williams"
<tiwill_at_seventransistorlabs.com> wrote:

Quote:
"John Larkin" <jjlarkin_at_highlandtechnology.com> wrote in message
news:mrev6cp210lbdap42l093254cislbbqbjv_at_4ax.com...
Well, the trivial solution would be supply inductance.
^^^^^

How would you snub the current spike?

You quoted it above!

As the Bible says:
If the bypass cap causes you trouble, rip it out!


Adding an inductor into the supply pin of the switcher would certainly
change things.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

Tim Williams
Guest

Sun Jan 08, 2017 8:48 pm   



"John Larkin" <jjlarkin_at_highlandtechnology.com> wrote in message
news:ci427cl9jelq0s06619ej6qbdlkih2bnj4_at_4ax.com...
Quote:
Adding an inductor into the supply pin of the switcher would certainly
change things.


It works fine if you have an external switch, but most integrated regulators
don't have separate VCC (logic supply) and switch drain (or PMOS source)
pins.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

John Larkin
Guest

Tue Jan 10, 2017 1:51 am   



On Sun, 8 Jan 2017 07:48:50 -0600, "Tim Williams"
<tiwill_at_seventransistorlabs.com> wrote:

Quote:
"John Larkin" <jjlarkin_at_highlandtechnology.com> wrote in message
news:ci427cl9jelq0s06619ej6qbdlkih2bnj4_at_4ax.com...
Adding an inductor into the supply pin of the switcher would certainly
change things.

It works fine if you have an external switch, but most integrated regulators
don't have separate VCC (logic supply) and switch drain (or PMOS source)
pins.

Tim


The charge stored in the substrate diode will be pretty much constant
(spontaneous recombination is slow) so Vcc would be dragged down hard.
Lots of nC.

Even with discrete fets, I'd sure prefer to eliminate the
step-recovery spike, rather than adding an inductor and suffering the
results.

That's sort of a new buck switcher topology, a synchronous half-bridge
with inductors at both the input and output.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

Tim Williams
Guest

Tue Jan 10, 2017 3:52 am   



"John Larkin" <jjlarkinxyxy_at_highlandtechnology.com> wrote in message
news:0km77cl3p7obj0v0khd8ncsri99ehqk7ko_at_4ax.com...
Quote:
The charge stored in the substrate diode will be pretty much constant
(spontaneous recombination is slow) so Vcc would be dragged down hard.
Lots of nC.


Yep. Recovery is slow, no matter how you cut it. MOSFETs necessarily make
bad junction diodes (it's a doping thing).

Quote:
Even with discrete fets, I'd sure prefer to eliminate the
step-recovery spike, rather than adding an inductor and suffering the
results.

That's sort of a new buck switcher topology, a synchronous half-bridge
with inductors at both the input and output.


The missing piece is adjustable dead time. If they provided this feature,
it could be trimmed to 0 +/- 15 ns, say, rather than the usual
overly-cautious and EMC-inducing 45 +/- 15 ns most chips have.

Supply inductance resolves the question of "how much current is drawn in
shoot-through?". It's simply V = L * dI/dt, as with anything else done with
inductors! No need for burning up transistors, it's just reactive energy.
The energy can be "stirred" back into the supply, or burned in a resistor
because it's not much.

A buck converter, operated in shoot-through, can have ZVS switching under
all load conditions. Good for speed!

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

John Larkin
Guest

Tue Jan 10, 2017 8:30 am   



On Wed, 04 Jan 2017 20:29:43 -0500, krw_at_notreal.com wrote:

Quote:
On Wed, 04 Jan 2017 10:47:56 -0800, John Larkin
jjlarkinxyxy_at_highlandtechnology.com> wrote:

On Wed, 4 Jan 2017 12:48:08 -0500, bitrex
bitrex_at_de.lete.earthlink.net> wrote:

On 01/04/2017 12:10 PM, John Larkin wrote:
On Wed, 4 Jan 2017 11:58:44 -0500, bitrex
bitrex_at_de.lete.earthlink.net> wrote:

On 01/04/2017 11:52 AM, John Larkin wrote:
On 4 Jan 2017 08:14:07 -0800, Winfield Hill <hill_at_rowland.harvard.edu
wrote:

bill.sloman_at_ieee.org wrote...
John Larkin wrote:
Tim Wescott wrote:

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.

Some mosfet body diodes make nice step-recovery diodes.

So they can spray really high-frequency noise around the circuit?
Just what you want when you have to meet EMI standards.

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.


This is a really classic SRD waveform:

https://dl.dropboxusercontent.com/u/53724080/Parts/LM3102_SwitcherRise.JPG

This switcher is basically unusable. A paralleled external schottky
only helped some.


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.


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.

How about a Schottky in parallel with the low-side FET/Diode?


We tried that, and it helped some, but not enough.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

Piotr Wyderski
Guest

Wed Jan 18, 2017 3:31 pm   



Tim Wescott wrote:

> A FET with a body diode that has slow recovery can become smoke pretty quickly.

Depends on the switching frequency. If it is low, say 50Hz, the diodes
can be extremely useful. For example, in my synchronous rectifier
they trigger the rectification half-cycle. Here are some excerpts
from my article (a cover story in one of Polish hobby magazines):

https://s30.postimg.org/7rmju7xq9/IMG_0991.jpg
https://s29.postimg.org/cfdxfmb7r/schemat.png

It makes use not only of the parasitic diodes, but also of the
parasitic gate capacitance, which greatly simplifies the circuit.
The small toroids on the MOSFET legs are the place where the
magic happens. For small currents the device is purely Graetz,
for, say, >1A it is purely synchronous and in the transition zone
it continuously morphs between these two states. It is very cool,
for <50A it requires no additional heatsinking (the PCB is made of IMS),
didn't test it above that value.

Best regards, Piotr

Tim Williams
Guest

Thu Jan 19, 2017 7:11 am   



Oh, and FYI, I once built a 400kHz industrial inverter (~5kW) that used the
body diodes just fine. And that was with the old high voltage VDMOS, the
kind with huge capacitance, terrible Rds(on) and ~800ns recovery time!

Obviously(?), this was only even remotely possible because it was a resonant
ZVS application. The body diodes recovered acceptably by the time the
channel was handling normal (drain-positive) current.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com


"Piotr Wyderski" <no_at_mail.com> wrote in message
news:o5n95q$ouf$1_at_node1.news.atman.pl...
Quote:
Tim Wescott wrote:

A FET with a body diode that has slow recovery can become smoke pretty
quickly.

Depends on the switching frequency. If it is low, say 50Hz, the diodes
can be extremely useful. For example, in my synchronous rectifier
they trigger the rectification half-cycle. Here are some excerpts
from my article (a cover story in one of Polish hobby magazines):

https://s30.postimg.org/7rmju7xq9/IMG_0991.jpg
https://s29.postimg.org/cfdxfmb7r/schemat.png

It makes use not only of the parasitic diodes, but also of the
parasitic gate capacitance, which greatly simplifies the circuit.
The small toroids on the MOSFET legs are the place where the
magic happens. For small currents the device is purely Graetz,
for, say, >1A it is purely synchronous and in the transition zone
it continuously morphs between these two states. It is very cool,
for <50A it requires no additional heatsinking (the PCB is made of IMS),
didn't test it above that value.

Best regards, Piotr


Tim Williams
Guest

Thu Jan 19, 2017 7:20 am   



"Piotr Wyderski" <no_at_mail.com> wrote in message
news:o5n95q$ouf$1_at_node1.news.atman.pl...
Quote:
Depends on the switching frequency. If it is low, say 50Hz, the diodes
can be extremely useful. For example, in my synchronous rectifier
they trigger the rectification half-cycle. Here are some excerpts
from my article (a cover story in one of Polish hobby magazines):

https://s30.postimg.org/7rmju7xq9/IMG_0991.jpg
https://s29.postimg.org/cfdxfmb7r/schemat.png

It makes use not only of the parasitic diodes, but also of the
parasitic gate capacitance, which greatly simplifies the circuit.
The small toroids on the MOSFET legs are the place where the
magic happens. For small currents the device is purely Graetz,
for, say, >1A it is purely synchronous and in the transition zone
it continuously morphs between these two states. It is very cool,
for <50A it requires no additional heatsinking (the PCB is made of IMS),
didn't test it above that value.


Hmm, cute!

Not entirely sure of the behavior, but it certainly looks to have
interesting feedback behavior. The IR21531S seems to be supplying a refresh
clock, as it were, but that would imply it's constantly figeting, which
would need a lot of inductance somewhere to absorb those events (perhaps the
power transformer and TVS is enough?).

The feedback connection reminds me of the old BJT inverter circuits, that
used load current to drive base current: the switching transistors were
driven at constant-hFE, the ratio being defined by turns ratio between
emitter and base windings. The controller actually forced the normally-on
inverter 'off', by shorting out the drive transformer!

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

John Larkin
Guest

Thu Jan 19, 2017 8:30 am   



On Tue, 3 Jan 2017 12:59:02 -0500, bitrex
<bitrex_at_de.lete.earthlink.net> wrote:

Quote:


Not to change the subject (never!) but enhancement phemts can be used
as radical diodes.

https://dl.dropboxusercontent.com/u/53724080/Parts/Fets/Ephemt_Diode.JPG

It's a very fast, 1 amp, 3 pF diode.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

Piotr Wyderski
Guest

Fri Jan 20, 2017 5:59 am   



Tim Williams wrote:

Quote:
Not entirely sure of the behavior, but it certainly looks to have
interesting feedback behavior. The IR21531S seems to be supplying a
refresh clock, as it were, but that would imply it's constantly
figeting, which would need a lot of inductance somewhere to absorb those
events (perhaps the power transformer and TVS is enough?).


I'm not sure if I understand you correctly, but no, the output
current is very clean, as there is no high-current HF switching.
If you neglect the S1-S4 "transformers", the IR21531/MOSFET gate
driver part it is just a quadrupled full bridge forward converter,
producing ~10V_at_5mA DC and operating at ~300kHz. The not shown
filtering capacitance is the parasitic MOSFET gate capacicance,
in this particular case in order of 20nF each. At this frequency
it is more than enough to provide stable 10V gate voltage, which,
under the above assumption, means that the oscillator IC is working
hard to keep all the four MOSFETs constantly on.

The trick is based on my discovery that about one ampere-turn is
perfectly enough to drive this particular ferrite into saturation.
In essence, the beast is magamp-controlled, with the high-current
MOSFET source pin having dual role, i.e. it is also the single-turn
control winding of a saturable reactor. :-)

If the body diode current is high enough (i.e. ~800mA+), it saturates
the S1/S2 or S3/S4 toroid pair and the reactance of the other winding
disappears, effectively connecting Tr2 or Tr3 to the oscillator and
thus producing gate voltages on the appropriate MOSFETs, which shorts
the conducting body diodes and reduces heat dissipation. Then it is
self-sustaining. If the current decreases below the saturation
threshold, the magamp cores reset and turn off the MOSFETs even more.
And since the magamps are not voltage, but current-controlled, the
entire device doesn't care whether there are filtering caps or not,
exactly as a diode bridge does. In order not to inject HF into the
output/mains, there are two oppositely connected magamp pairs instead of
just one, i.e. 4 small control toroidal cores in total.

If you perform a bit more detailed analysis, you'll discover that this
mode of control is inherently stable. In practice I wasn't able to
devise a crazy enough, yet realistic scenario to derail the control
circuit, but to be at the safe side, there is the T5/T6 protection
AND gate with the transistors having much lower threshold voltage
than the main switches. If somehow both halves were to conduct
simultaneously, the oscillator would be stopped, resetting the
synchronous rectification control system.

Best regards, Piotr

Tim Williams
Guest

Fri Jan 20, 2017 8:30 am   



"Piotr Wyderski" <no_at_mail.com> wrote in message
news:o5rgck$3kd$1_at_node1.news.atman.pl...
Quote:
The trick is based on my discovery that about one ampere-turn is
perfectly enough to drive this particular ferrite into saturation.
In essence, the beast is magamp-controlled, with the high-current
MOSFET source pin having dual role, i.e. it is also the single-turn
control winding of a saturable reactor. Smile
snip


Ah, gotcha.

Cool stuff :)

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

John S
Guest

Sun Jan 22, 2017 8:34 pm   



On 1/18/2017 10:08 PM, John Larkin wrote:
Quote:
On Tue, 3 Jan 2017 12:59:02 -0500, bitrex
bitrex_at_de.lete.earthlink.net> wrote:

Something seems wrong here but I can't place exactly what it is:

http://www.edn.com/design/power-management/4443225/Body-diodes-simplify-UPS---energy-recovery-circuits

Not to change the subject (never!) but enhancement phemts can be used
as radical diodes.

https://dl.dropboxusercontent.com/u/53724080/Parts/Fets/Ephemt_Diode.JPG

It's a very fast, 1 amp, 3 pF diode.



The data sheet says Vgs 7V absolute max. Did you really use 10V?

John Larkin
Guest

Mon Jan 23, 2017 12:29 am   



On Sun, 22 Jan 2017 07:34:44 -0600, John S <Sophi.2_at_invalid.org>
wrote:

Quote:
On 1/18/2017 10:08 PM, John Larkin wrote:
On Tue, 3 Jan 2017 12:59:02 -0500, bitrex
bitrex_at_de.lete.earthlink.net> wrote:

Something seems wrong here but I can't place exactly what it is:

http://www.edn.com/design/power-management/4443225/Body-diodes-simplify-UPS---energy-recovery-circuits

Not to change the subject (never!) but enhancement phemts can be used
as radical diodes.

https://dl.dropboxusercontent.com/u/53724080/Parts/Fets/Ephemt_Diode.JPG

It's a very fast, 1 amp, 3 pF diode.



The data sheet says Vgs 7V absolute max. Did you really use 10V?


Yes. That's all my function generator would do. Here's the DC drain
curve:

https://dl.dropboxusercontent.com/u/53724080/Parts/Fets/ATF_Leakage.JPG

They would probably die somewhere between 25 and 30 volts. Maybe
negative gate voltage would reduce drain current; I should have tried
that.

The voltage ratings of RF parts are tricky. The makers may assume an
RF peak swing of twice the DC drain voltage, or at least the numbers
look that way. I usually test RF parts, fets and transistors and
diodes, to destruction and then guardband that.

I haven't tested the two diode configs to destruction; they may be
different.




--

John Larkin Highland Technology, Inc

lunatic fringe electronics

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