Snubber advice pls!

[Mosaic]

Hi all:

I have a simple flywheel snubber Schottky across a 12V Lead Acid batt I am experimenting with. Pulsing from a 36V capacitor bank.
The battery's DC-R is around 25mΩ and AC-R is 15mΩ.
The battery is accepting 4A avg and holding at about 16V.

My concern is that the flywheel schottky snubber seems to be permitting up to 15+V kickback for several µS as measured across the terminals of the flywheel schottky.
Why is this?

This is the 50SQ80 spec sheet.
https://www.farnell.com/datasheets/3251.pdf
The V/µS of the voltage kickback is 500V/µS, well below that of the Schottky @ 10KV/µS

See the scope waveforms annotated below. The blue trace is the current flow to the battery as measured by a CT.



EDIT:
BTW I am investigating using the inductive kickback to depolarize the cells to improve the pulse charge acceptance. You can see that the positive going pulse width is significantly reduced with this method for a fixed avg current acceptance!

 

[crutschow]

Without a schematic I can't answer your question.

 

[Mosaic]

crutschow.
Here it is:

 

[crutschow]

That spike may be due to lead inductance.
Where are you measuring the voltage relative to the diode and the battery?
How long are the connections?

 

[Mosaic]

Actually, as i am measuring this AT the flywheel diode terminals their should be very little 'spike. No lead inductance to speak of (2 cm total).

 

[crutschow]

Actually, as i am measuring this AT the flywheel diode terminals their should be very little 'spike. No lead inductance to speak of (2 cm total).

Then I don't understand where the spike is coming from either.
Is that spike a problem?

 

[Mosaic]

Ok, things are not as bad as they seemed at first...
1) The -ve going spike must exceed the battery voltage (16V) before the diode conducts. That seems accurate. Then there is a 8V spike over the batt V with a bit of decayed ringing (while the current [blue trace] continues to collapse] which must include the Vf of the diode. Perhaps we're looking 7V of 'extra' voltage, some of which will accrue to parasitic inductance and lead resistances. Guessing at perhaps 2 or 3V due to that ( the pulse current flow can be 250A + thru the diode based on a 0.1Ω current sense test I did earlier). That leaves 4V unexplained. I am wondering if Vf change at high pulse currents can account for that. I see no spec on that in the spec sheet.

The spike induced failure event.
The unsnubbed >50V spike was progressively killing an avalanche rated NFET (60 Vds, 4Vth at ±20Vgs spec.) that handles battery discharge loading for battery internal resistance tests. I have since added an 18V Zener Vgs clamp to that NFET. I am guessing it's possible with all the fast edge, ringing high currents circulating that parasitic inductance & capacitance in the assembly could cause some Vgs hammering. I noted that the (40Vce) NPN 2n3904 (efollower push pull) in the NFET gate drive (10Ω gate resistor) started to 'leak' 1.7V thru its collector- emitter after the NFET failure (Vds short). That implies the gate voltage spiked (over 40V) somehow.

Also interesting is another identical NFET used as a drain - source battery polarity protector in the same discharge circuit was fine. Its gate drive is fed by a 18V amplified zener regulator as it is directly exposed to the kickback pulses otherwise.

UPDATE:
The installed snubber appears to be doing the job so far. Max bounce (parasitic drain gate capacitance) on the FET gate is a bit under its Vth. The actual snubbed kickback excursion at the FET terminals is 24V (8V spike + BattV) when all the parasitics are in play.

Comment:
The snubber design is a bit unusual in that it had to accommodate a possible reverse polarity connection on the battery. Thus a stock freewheel diode could be destroyed in that event. The diode is now in series with a low ESR 1000uF,50V DC blocking cap. In parallel with the cap is a 0.4Ω , 2A PTC. The PTC discharges the cap during operation and acts as an E-fuse in the event of batt. polarity reversal, thereby protecting the pulse diode. It works, I tried reversing the batt!

Compared to using a bank of 4 or 5, bipolar 1.5KE series TVS units with 1 ohm limiting resistors, this solution snubs a lot better and makes a lot less heat!

 

[MrAl]

Hello,

It is unclear what you are doing here. You're talking about an inductive circuit but you show only a voltage source, battery, and diode. Diodes start to *conduct* very very fast, even notoriously 'slow' diodes like 1N4004...they just dont recover as quickly. This means you are not showing what is really there.