Jaguars vs Victors

[dsirovica]

Correct he Jag 40A limit is implemented in Firmware:
http://www.ti.com/lit/ug/spmu130c/spmu130c.pdf

Its a 1mOhm 4W resistor via an opAmp into the ADC3 on the Jag processor.

Do we have source code for the Jag firmware ? If so we could relax the constraints they have

Though this might violate FIRST rules?

Also in the same document you'll see that the Jag has 4 empy slots for 4 more MOSFETS which would make it like a Victor - looks like a Jag IS a Victor with 25% missing MOSFETs and a procesor.

Does anyone have a similarly detailed spec sheet on Victors to conclude a definitive comparison of the two?

There is charm in pursuing everything to its atomic structure - did someone already say that ?

Dean

[Ether]

Originally Posted by dsirovica

Do we have source code for the Jag firmware ?

The source code is freely available on the TI site. It's like an Easter Egg hunt to find it though.

If so we could relax the constraints they have

Not permitted.

looks like a Jag IS a Victor with 25% missing MOSFETs

I doubt the Jag and Vic are using the same FETs.

Does anyone have a similarly detailed spec sheet on Victors to conclude a definitive comparison of the two?

This question has been asked repeatedly here on CD. The answer always comes back no. Too bad, it would be very interesting to see.

[slijin]

Originally Posted by Ether

This question has been asked repeatedly here on CD. The answer always comes back no. Too bad, it would be very interesting to see.

I actually have a couple of dead 884s in the lab that I was going to open to see if I could identify points of failure. If possible (and if I remember), I'll ID the MOSFETs in there.

[Ether]

Originally Posted by slijin

I actually have a couple of dead 884s in the lab that I was going to open to see if I could identify points of failure. If possible (and if I remember), I'll ID the MOSFETs in there.

I think Al S. has already posted the Vic FET PN somewhere. I was talking about a complete schematic (such as is available for the Jag) so the discussions could be better informed when we are comparing the Jag and Vic.

[nitneylion452]

We use Jags for everything. Sure, we've burned a couple out, but that was namely due to carelessness in drilling and sending swarf (gotta agree, it is really fun to say) all over the swarfing place.

As for the Jaguar's over-current protection, it is a software feature. It is designed to pump out 40A happily as long as you'd like it to (so long as you have it properly ventilated) and is designed to cut out after 2 seconds of 60A. For more information, you can see the Jaguar FAQ.

[dsirovica]

A full schematic would be ideal. But the FET PN would go a long way as that is a major component. If you look at the Jag's power path, it has a 1mOhm resistor and two legs of the Hbridge in series with the motor.

The Jag FET spec says:
MOSFET N-CH, TO-220 40V/60V 80A

However it lists 3 FET PNs
IRFB3206PBF: 60V 120A 2.4/3 mOhm (typ/max)
FDP038AN06A0: 60V 80A 3.8 mOhm
FDP050AN06A0: 60V 80A 5 mOhm

We then have an effective series resistance of between 3.4 and 6 mOhm between the battery and the motor, and a max I of between 160 and 240A (I guess no worries on max I).

The real limitation will come from power dissipation on the worst case 5mOhm FETs.
Eg: for 40A, each FET carries 20A.
P=I^2*R =2W
Thermal resistance from Junction to Ambient (no fans open space)= 62 C/W
Ambient T=25C
Therefore operating T= 149 C
The max Junction T is 175 C.

At 50A, Operating T = 219 C --- sizzzle!

The fan will help, but the math for that is below my age level. And clearly we've seen much higher current levels through Jags and they've survived.
(maybe Jags rarely use the 3rd PN?)

at 50A 1st PN 2.4mOhms: Op T = 118 C --- What a difference!

In the case of adding a third FET in parallel (like the Victor has), and keeping with the same FETs:
The current per FET is reduced by 33%, and the power dissipation by 56%.

This shows that Victors can handle far greater max power than Jags - unless they have very inferior FETs...

[dsirovica]

Thanks Joe,

From the Jag FAQ:
"... 40 A of continuous current to a heavily loaded motor. However, it is capable of providing much higher currents, but for shorter periods of time. Jaguar provides 60 A for up to two seconds and provides 100 A for approximately 0.2 s. "

that precisely matches the results we saw.

[Ether]

Black Jag

FDP8441 FET 40V 80A 2.7 mOhm each x2 in parallel

60amps thru Jag = 30amps thru each 2.7 mOhm FET = 30^2*2.7e-3 = 2.4 watts per FET

[Gdeaver]

Don't forget switching losses from the fet turning on and off. The fet is in the linear region during each cycle for a small period of time. Fets do not turn of instantly. Also, the black jags are switching both h legs on and off constantly.
The Victors are low side switchers. Black jag fets always see some current. Jags do not have diodes or TVS devices to protect the intrinsic diode of the fet from inductive kick back when they turn off. The decision to remove 4 fets from the jag and not re-space them may have messed with the cooling. I like the linearity of the jags and the robustness and size of the victors. Our speed controllers do not have on die temperature protection and no heat sinking for a thermal reservoir. We don't have the perfect controller but our robots keep on competing well. May be some day we will have the perfect controller until then take take your pick they will work most of the time.

[Ether]

Originally Posted by Gdeaver

Don't forget switching losses from the fet turning on and off. The fet is in the linear region during each cycle for a small period of time. Fets do not turn of instantly.

Yes. What would be your quantitative estimate of the overall effect (watts averaged over 67μs) of this switching (one rise and one fall every 67μs) for the FDP8441 when the current is 30 amps?

Also, the black jags are switching both h legs on and off constantly. Black jag fets always see some current. Jags do not have diodes or TVS devices to protect the intrinsic diode of the fet from inductive kick back when they turn off.

According to this post, for any given PWM command, in one leg of the H bridge the low-side FET pair is ON, and the high-side FET pair is OFF. In the other leg of the H bridge, the high and low FET pairs are alternately ON and OFF. Protection from inductive current is provided by shorting the motor through the low-side FETs during the OFF portion of the PWM cycle.

The Victors are low side switchers.

Not saying you're wrong, but what's your source for this info?

[dsirovica]

Ether, I assume you got the PN FDP8441 from an actual Jag as it is not one of the PNs in their spec sheet?

In any case it is very similar to the best performing one on their list.

Ahm, dating myself here, MOSFETs were not in my undergrad EE curriculum, hence my icon is the much loved 2N3055

Just looking at the datasheet for FDP8441 and the Jag Spec:

1. the FDP8441 max switching times are 77ns ON, 147ns OFF that is about 0.3% of the PWM cycle. So how much power can be relesed in such a short time into an inductive load? Further, the Watage and Temp calcs earlier assumed a 100% PWM duty cycle, anything less will double the FETs over which to dissipate the energy. Worst case would be 99.9% PWM duty cycle. I agree some additional wattage should be added but it may be unecessary?

2. What are all the data about Joules and Coulombs, I would love oto hear a scientific explanation on switching energy, anyone?

3. The Jag datasheet has a writup on how the MOSFET is used to sink motor current thus reducing power dissipation compared to a diode based solution. Maybe Victors don't do that so that would be a -ve for the Vic in this debate.

[Mark McLeod]

The Victor FETs used are IRL3103 (International Rectifier) or an equivalent like FDB6035AL (Fairchild Semiconductor. Depends on the year of manufacture, and I haven't looked that closely in a couple of years, so they might well be different now.

[Ether]

Originally Posted by dsirovica

Ether, I assume you got the PN FDP8441 from an actual Jag

I got it from the schematic in the RDK-BDC24 Rev B Hardware Design Package. The schematic shows a pair of FDP8441's for each of high+, high-, low+, and low- in the H bridge. Perhaps the FETs have changed since then.

the FDP8441 max switching times are 77ns ON, 147ns OFF that is about 0.3% of the PWM cycle.

The turn-on rise time is 24ns and the turn-off fall time is 17.9 ns. Which would be the correct numbers to use for this analysis?

The Jag datasheet has a writup on how the MOSFET is used to sink motor current thus reducing power dissipation compared to a diode based solution.

That's the alternating ON/OFF of the hi and lo side FET pairs mentioned in my previous post. If the source for that information is correct, then during the OFF portion of the PWM cycle, the motor is shorted through the low-side FETs, providing a low-resistance path for the inductive current in the motor to continue flowing (and producing torque). I assume that if the jumper is in the "coast" position, this behavior is modified for low duty cycles.

Maybe Victors don't do that so that would be a -ve for the Vic in this debate.

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?

[dsirovica]

Originally Posted by Mark McLeod

The Victor FETs used are IRL3103 (International Rectifier) or an equivalent like FDB6035AL

OK Houston we have a problem.

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...

Could it be that the Vics are simply driven red hot and we may be operating in out-of spec areas - we do see a lot of failed Vics?

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?


Now back to the switching power dissipation:

1. anything less than 100% duty cycle increases the number of FETS dissipating power. At 50% cycle we double the dissipation power.

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!

So we need help with (1) forced air power dissipation, and (2) switching energy loss in FETs.

I think we stumbled on a structural problem with FIRST: the kids are too young to know this level of detail, and the mentors are too old to know about this new MOSFET stuff, and object oriented programming - for me that meant you have a chair and desk object and a punch-card object ?

[slijin]

Originally Posted by Ether

I got it from the schematic in the RDK-BDC24 Rev B Hardware Design Package. The schematic shows a pair of FDP8441's for each of high+, high-, low+, and low- in the H bridge.

I thought Dean would've picked up on this (no offense!), so I didn't want to steal his thunder, but the current version is Rev C, and lists the following MOSFETs for the H bridge:

International Rectifier IRFB3206PBF
Fairchild FDP038AN06A0
Fairchild FDP050AN06A0

The BOM specs them all to be MOSFET N-CH, TO-220 40V/60V 80A.