Jaguars vs Victors

[EricVanWyk]

Originally Posted by dsirovica

The IRL3103 has 16mOhms, and FDB6035AL 12.5mOhms.
For a 40A motor curent through a 3-FET Vic, this is 2.8W and 2.2W respectively and an operating temp of 201 and 163 C.

Compare that to a two FET FDP8441 Jag: 1.08W and 92 C.

According to this the Jag has superior power.

Reality says different...

These figures don't necessarily say that the Jaguar has superior power, it says that it has superior efficiency at DC. For a given motor current, Jaguars will produce less heat.

At 60 Amps, I predict that a Victor will produce ~7W heat per FET (21W per leg), and a Jaguar will produce ~3.5W per FET (7W per leg). At 40 Amps, I predict 2.5W/ FET for Victor and 1.5 for Jaguar. I included conduction and gate loss for both, but only included switching loss for the Jaguar as I was unable to predict it for the Victor.

I think my numbers should be within 30-50% of reality. I'll wait about 15 minutes after posting to see how thoroughly Ether tears my math apart

Originally Posted by dsirovica

Also my temp calcs assume no fan so that may increase power dissipation significantly - anyone care to analyse forced air vs ambient power dissipation for these geometries?

Unfortunately, I left all my thermal simulators behind when I switched hats to software.

You can sort of eye-ball it by looking at heat sink spec sheets and backing out a bit. For example, the cheapest TO220 heat sink I can find is Aavid's 5073. If I'm reading things correctly, it is 25C/W in stagnant and 4C/W in a tornado. If I use the back of a napkin and wave my hands enough, I might be able to convince myself that we could hope for a 3x to 6x case to ambient impedence reduction from the fans... ish?

At the very least, it is enough of a reduction that we aren't above the 175C junction temperature limit.

I ran my calculations assuming 10C/W case to ambient because it is a round number within the range. I don't know what the real number is, I'd bet higher.

Originally Posted by dsirovica

3. I am worried by the "Single Pulse Avalanche Energy (Note 1) 947 mJ"
of the FDP8441. If that means you turn 1Joule into heat on every swichover - we are trullly fried!

If that was the case, each Jaguar would be generating 15kilowatts of heat from this mechanism alone. This is several times the entire robots power budget. This is a rating for how much energy it can handle when the voltage across the FET exceeds the breakdown threshold.

This paper does a much better job of describing avalanche events than I can: http://www.irf.com/technical-info/appnotes/an-1005.pdf

Originally Posted by Ether

I've heard the Vics do not do this, and here's my speculation why they do not: the PWM period of the Vics is so long that even for relatively high duty cycles (say 50%), the motor inductive current would decay before the end of the OFF portion of the cycle, and, for higher motor speeds the motor back EMF would take over and start shoving current in the other direction, effectively reversing the torque. This would reduce the average torque output of the motor. Anyone care to comment?

I agree with your statement. The Vic chop rate is well below the electrical time constants of the motors.

Originally Posted by Al Skierkiewicz

Dean,
Please see that the ON resistance of the IRL3103 goes down as the Gate to source voltage goes up. The 12mohm is the more accurate figure.

Yep. The 16mOhm number is for driving the gate with 4.5V. When the junction is at 25C, 12mOhm is the expected figure. When the junction is warm, expect 30-50% more resistance per figure 4. For a Victor pushing 60 Amps, I predicted that the junction temperature would be ~104C and ~17mOhms per FET. This was with a very optimistic thermal impedence estimate, so the actual resistance is likely to be higher.

Originally Posted by Matt Krass

Also, swarf. Swarfing swarf. (Really, try to say it out loud with a straight face)

Matt

Swarf!