This is the "partial fullbridge" topology (2 mosfets and 2 diodes) from the previous post roughly prototyped.
And the overall setup. The test solenoid is a Takaha CH12840062. This in stock form is a total slug and definitely not the greatest choice of blaster bolt drive noid, incidentally.
I just used random crap I had around - some IRLR7843s for the switching devices, 1N5819 diodes,
and for the highside drive I used a discrete bootstrap driver as in
traditional ESCs. A tiny bit of code on a mega328 to drive the power
stage, take a switch input and spit out a drive waveform (in this case generic 50ms on/50ms off).
Anyway, so this is what the voltage across the winding is doing with the regenerative fast decay case:
(10ms/div, 5V/div)
Exactly what we expected - it flies up to the DC bus level (mostly charged 4 cell pack) for the 50ms on-time, then flies down and is caught to negative (DC bus + 2 schottky diode drops).
Note that the total current decay time is not really the 30.4ms I had the cursors on. That is the duration of the negative voltage transient on the winding - but current is only really flowing when the voltage is below negative DC bus i.e. during the flattish, ~10ms long period right after switch-off. The exponential rampy part at the end happens after the diodes have mostly stopped conducting entirely and the voltage decays back down (up) through parasitics. The small but existent step up at ~30ms after switch-off is an artifact of the power stage switching to slow decay mode by the low-side fet being turned on at 30ms after switch-off (random value I picked). With a bootstrap high side driver in this arrangement the low side must be on at some point whenever otherwise unimportant (like this) during the off-time to present a path for the high side bootstrap capacitance to charge.
Now what happens if we run that again, but in SLOW decay mode - exactly equivalent to the typical circuit with a flyback diode across the winding? (20ms/div this time as I had to back off to see all of the decay time!)
Here I did that by keeping the low side fet permanently on, so we're
doing high-side switching and the flyback diode is the one coming from
ground up to the switch node. I of course did that because the high side driver is bootstrap and we can't keep the high side on as the cap will slowly discharge through parasitics.
What we're expecting here is DC bus level for the on-time and then one diode drop below ground (small, because this is a schottky) whilever winding current is still flowing through the flyback diode. And that's exactly what we get.
Note how long that negative period lasts! Here's that a bit closer (500mV/div):
That weird low frequency ringing in the setup that went down to -2V on the switch node wants our attention, but here the ~120ms of actual decay time is the shocking matter...
And yes, the solenoid immediately felt PROFOUNDLY more sluggish to release during these shots.
Now what happens far as trying to cycle quickly?
I have had this noid cycling (unloaded) very reliably at 10Hz with the 50/50ms timing using the regenerative fast decay setup:
No problem. Now, this is the same noid, the same timing parameters, the same otherwise hardware, the same battery powering the setup at the same voltage (immediately swapped between modes)... the only variable here is the decay mode which is now slow (aka, standard flyback diode configuration).
Okay; so I would say there might indeed be an issue here!
Now, this noid is a 6.2 ohm coil that is probably more turns and more inductance than typical sub-3 ohm blaster bolt solenoids and is also a bit lacking on return spring hence is making a more dramatic example of this than usual, but clearly something major is afoot here with this decay mode thing and the notion that a simple flyback diode is a hindrance to fast-cycling solenoid actuators. This will be very interesting to apply this topology to more known-quantities like the FTW Hyperdrive and quantify the timing margins gained over the flyback diode approach.
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