Assembled my VESC and need help with motor detection

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joaolino
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Re: Assembled my VESC and need help with motor detection

Postby joaolino » 18 Aug 2017, 20:22


rew
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Re: Assembled my VESC and need help with motor detection

Postby rew » 20 Aug 2017, 09:54

Working from memory here...

It doesn't hurt to have the boost capacitors a bit bigger than the minimum. You need a bigger boost capacitor if your mosfet is large and has a big gate capacitance. According to TI the 7530 mosfets are actually just beyond the limit of what the DRV can drive. So having a bit bigger capacitors is not unreasonable. That said, having bigger capacitors there is not really a problem. Or rather, unless you go overboard, it is never a problem.

So... when the phase wire is low, there is a diode charging the boost cap to about 12V. Then when the gate needs to be driven high, there is some charge-sharing between the gate capacitance and the boost capacitor. The voltage on the boost capacitor drops a little while the gate is turned on. Now, in this case we can calculate the end voltage of the boost capacitor. We start out with THE capacitor formula: i = C dV/dt . We rewrite that to dv = i . dt / C and plug in Q= i dt: dV = Q/C . The datasheet for the IRFP7530 mentions about 230 nC for the total gate charge (I haven't looked it up today). So for a 100nF capacitor we get: dV = 230nC/100nF = 2.3V. Now the capacitor is charged from 12V through a diode, so it should be charged to about 11.3V when we begin, giving us 9.0V at the end.

Worst case, the whole system is running at only 9V, the LDO trying to make 12V might be putting out only 7V, a diode drop and a charge sharing later your gate is at 4V. That's in the perfect range for blowing up mosfets.

Increasing the capacitor to 220uF improves the margins by 1.3V, sounds like a plan. :-)

joaolino
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Joined: 31 Jul 2017, 21:03
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Re: Assembled my VESC and need help with motor detection

Postby joaolino » 23 Aug 2017, 20:00


rew
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Joined: 25 Mar 2016, 12:29
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Re: Assembled my VESC and need help with motor detection

Postby rew » 24 Aug 2017, 07:35

Yes, your "linear' assumption is very wrong.

The "min" means that under worst-case conditions the device may start consuming (significant) current at that voltage. If you use the device, say 1 in 1000 controllers that you sell will drain the battery down to 50.3V through the TVS when fully charged. That sounds acceptable to me.

But the curve is far from linear. At some point there is a quick rise in the current. Below that it is almost zero, above that it quicly goes up. That max clamping voltage occurs again under worst-case conditions: 391A with "bad luck" in the draw of the production at the worst possible temperature etc etc. But the max breakdown voltage is 55.3V. So at that point it starts consuming significant (> 5mA!) current. The current quickly rises, but MIGHT not reach 391A before the voltage becomes 77V. I would say that this is likely to help.

You can often mount a smaller capacitor on larger pads if you can find one that satisfies the requirements. Or you can take a larger capacitor. Or as you decided: one rated for a higher voltage. Whatever you like.

My vesc works just fine on 12S. But it doesn't use an DRV. And I use a >60V diode. Make sure the diodes are rated for > 60V.

joaolino
Posts: 10
Joined: 31 Jul 2017, 21:03
Location: Lisboa
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Re: Assembled my VESC and need help with motor detection

Postby joaolino » 26 Aug 2017, 09:04



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