Jaguars vs Victors

[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. Perhaps the FETs have changed since then.

I re-downloaded the Kit CD for RDK-BDC24* from the TI site before I posted the above. It still had the RevB schematic in it.

TI could make this information easier to find on their website. I didn't think to download a User's Manual to find detailed technical information like a schematic.


* from the TI website, quote:

Description

This board-specific kit CD contains:

Complete documentation, including Quickstart and User's Manual
Complete source code and schematics
StellarisWare® software including peripheral driver library and example source code

[Al Skierkiewicz]

Dean,
So lurkers are not confused, the power per device is not a max power spec on the FET but how much power is generated within the device during constant current due to the internal resistance. 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. The fan in the Victor seems adequate, it is closely spaced with the devices and the airflow is constantly pushing air away from the tabs of the FETs. Everyone should keep in mind that although each leg has devices in parallel, there are two legs in series with the motor during operation. This series resistance also serves to limit that max current available to the motor. Under normal conditions, the devices do not achieve the indicated temperatures in either controller. If they did we would have had numerous reports of thermal issues over the years.

[techhelpbb]

There is a bit of a catch 22 with the higher on-state resistance.

These robots are being driven by humans.
Human sees robot go too slow...tells robot to go faster.

Higher on-state resistance means that robot spends more time operating closer to 100% duty cycle (not at 100% duty cycle obviously, but more time with the MOSFETs on) than in a situation with lower on-state resistance.
The more time you spend on, the more surges from the motor you dissipate through the MOSFET's junctions instead of the body diodes (never mind the normal currents).

[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!

[dsirovica]

Trying to answer the few Qs above and then report on my crash-course on MOSFET-101:

Yes the Power (W) we address in this thread refers to the thermal power dissipated/generated inside a MOSFET as a result of driving a CIM motor. The absolute max power rating of these MOSFETs is much higher but that assumes ideal heat-sinks which we do not have in Jags nor Vics.

The most significant parameter we focussed on so far was the R DS(ON) resistance of a fully conducting MOSFET. Hence it is critical to now the Part Numer (PN) of the MOSFET in your speed controller. As usual, different generations (even batches) of Jags & Vics will have different PNs. One hopes the variations in the critical parameters will not be great - but life is not always such.

For the Jags we have discovered 4 PNs so far:
IRFB3206PBF 3mOhm
FDP038AN06A0 3.8mOhm
FDP050AN06A0 5mOhm
FDP8441 2.7mOhm

and for the Victors:
IRL3103 16mOhm
FDB6035AL 12.5mOhm

The R DS(ON) in mOhms is listed above as that is the critical parameter (so far). Since Power dissipation is P=I^2*R, one can see how important this number is.

As was pointed out, I may have mixed the use of RDS max and RDS typ, but as you will see shortly we have a bigger problem.


MOSFET-101:

A pretty good paper is "A Power MOSFET Tutorial":
http://www.microsemi.com/en/sites/de...es/APT0403.pdf

Using this and a few other (harder to read links below) papers on the web, I will summarise the pertinent points here:
http://www.eetimes.com/design/power-...tching-MOSFETs
http://www.btipnow.com/library/white...Parameters.pdf


Power losses (Pl) in any component operating in the switch-mode can be divided in three groups:
a) Conduction losses (Pc)
b) Switching losses (Psw)
c) Blocking (leakage) losses (Pb), normally being neglected as this is in the micro-Amp region.


a) Conduction Losses:
This is primarily our friend R DS(ON).
This value is highly affected by 2 other parameters:
(1) The Gate Voltage - the higher the gate the lower RDS. Luckily for us the Jag uses a MOSFET driver chip and one assumes drives it at near Battery voltage - I am guessing that the Victor has a similar driver. At 10V on the Gate we are down to the spec RDS.
(2) The temperature. The higher the junction empo the higher RDS. All MSOFET datasheets have that graph. The Y axis is typically a Normalized RDS, and X-axis T (C). One can see that from ambient 25C (spec RDS) to 150C RDS can almost double! So keep it cool


b) Switching Losses:
This is where it gets really messy...
There's 2 losses, Capacitance, and Crossover:
(b1) Capacitance loss:
This is due to charging and discharging the "capacitors" within a MOSFET. However with our 12V battery and 15KHz this is down to 1-2mW and also I think this energy is dissipated outside of the MOSFET so we can ignore it as far as heating goes.
(b2) Crossover loss:
This is more interesting. Essentially it is due to the "slow" swichover from full-ON to full-OFF and back. See Figures 14 and 15 in the first link above, and look at the bottom shaded "switching energy" graph. Now for some reason the MOSFETS we are using do not have a spec for this Eon, Eoff data. It is also apparent from the same graph that the Turn-On Rise Time, and Turn-Off Fall Time is not the time during which most of the energy is released! I find it hard to believe that the industry hasn't yet standardised on the representation of this parameter? We must have mentors that work at MOSFET manufacturing companies - how do we get to them?...

Anyhow, I did some back of the envelop and assuming similar energy curves as APT50M75B2LL for our Jags E switching is 0.5W. For the Vics that would be negligible due to the slower PWM cycle.

Therefore, if we use the following:
Jag MOSFET FDP8441
Vic MOSFET FDB6035AL
Motor Current 40A and 60A
Operating at red-hot 150 C junction T
PWM duty cycle 99%

we get the following power dissipation at the ON MOSFETs (40/60A):
Jag = 1.8/3.4W
Vic = 3.2/7.3W

And (no fan) T operating:
Jag = 137/238C
Vic = 227/479C

Jag wins hands down!

Something doesn't compute - as the Vics should all be burnt !
1. The fan probbaly has a very strong heat dissipation effect.
2. The Jags could be driven much harder than the Vics if only we dissabled the software cut-out mechanism
3. Lets open up a few Jags & Vics and confirm the PNs (maybe we can replace the PNs on the Vics with the better ones - they are only a couple of $ each -just kidding - its probably against the rules!


Here is the math for those who want to tinker:
P= I^2*R
R= RDStyp * the Temp factor

T=P*RJA+Tamb

Where:
I is the current in the ON MOSFET in Amps. Motor Current divided by 2 for Jag and by 3 for Victor.

R is from the MOSFET datasheet. Use RDS(ON) typical. Temp factor from the datasheet table.
Jag FDP8441 RDStyp=2.1mOhm, Temp factor (150C)=1.55
Vic FDB6035AL RDStyp=11mOhm, T factor (150C)=1.65

RJA is the Thermal Resistance Junction to Ambient from the datasheet:
Jag FDP8441 RJA=62C/W
Vic FDB6035AL RJA=62.5C/W

Tamb is 25C

[dsirovica]

Eric wrote his post while I was researching mine:
Hey Eric how did you guess the W numbers so accurately ?

Yup its all in the fan! If it gets us 10C/W then the even the VICs at 60A come down to <100C! We should do a temp measurment on those.

The 15KW avalanche energy was a total red herring. Just ignore it we are not avalanching in FIRST


Basicaly to place a simplified answer to the title of this thread:

The VICs win in terms of being able to get more power out of them.
The Jags are overly protective and their power could be increased significantly without harming the system - especially for a FIRST 2.5 minute competition event.

Dean

[Mark McLeod]

A current Vex Victor 884 uses an Infineon 2N03L13
Now I seem to remember that change being introduced circa 2007.

http://www.infineon.com/dgdl/SPP_B_4...12b4271adf3b95

Had a travel meeting tonight and got a chance to check one I just purchased.

[Al Skierkiewicz]

Guys,
You are scaring me and half the population as well. While you are taking the approach of worst case analysis, the common use for these devices is much less than predicted. A good FRC design will generally not choose to run at ~60 amps (the max power power output of the CIM). While short bursts into this range are common, they are not continuous. At 60 amps, the CIM would be dissipating over 300 watts and we know that few teams are burning up their motors or their controllers. So I believe that most teams are running in the recommended normal operating area of 30 amps or less. At this current the temperature numbers seem more in line with experience in the field for both devices using fans.

[dsirovica]

Al: Thank You for bring us back to earth!

It is by no means necessary to be able to follow this debate in order to be a great FIRST participant.

One of the genius ideas behind FIRST is that it enables kids of different backgrounds and skills to actively participate by leveling the playing field. The design of the KOP, the game rules, strategies, community effort, etc. The body of knowledge behind the scenes and the way it is organized to serve a noble cause is just phenomenal.

As a new mentor, and one of the little cogs in the FIRST ecosystem, I was dismayed when one morning after another 1AM night of frantic redesign a few days before bag-day, the robot showed up with all its Jags replaced by Vics! The reasoning was – they kept cutting out, we always have problems with them, we've always switched back to Vics and we carry a bag of replacement Vics as they die!

Well needless to say, in the professional world, this is not a satisfactory assessment of the situation and resulting decision...

BTW, on a more humorous note, that same night when the kids where left home alone :-) our robot almost went for a midnight skinny dip – one wheel was over the edge of the pool! Maybe it was a Jag that prevented that fiasco!

I see the pursuit of this thread as a way to close one of a gazillion of open issues that many struggle with and forever debate on these fora. I believe that a little extra effort to push for a definitive understanding and resulting recommendation is going to save this community a huge effort. We must though ensure that the mechanism exists to capture, save, and make this knowledge readily available when next time someone asks Jag vs. Vic? Can you just imagine what the total effort spent on this is ? Our team has spent many person-days on this just this season. It has a huge spill-over effect too. Our mechanical design called for a JAG-based PID shooter control, so all that work is down the drain too!

Back to the topic for a moment:
Thanks Mark for the latest Vic PN 2N03L13
It is very similar to the other one I used in the math FDB6035AL
The parameter of interest is RDS the New PN is
10.3mOhms(25C), 15.5mOhms(150C) so just a little better – not too significant.

BTW: in my posted equation for the MOSFET Power dissipation I forgot to show the Pswitching contribution, so the formula should be
P= I^2*R + Pswitching which is 0.5W for our Jags and 0W for Vics

The numbers presented are still valid.


Al is also right – as others have said – we are investigating an area of operation that is near, and beyond the “red-zone”. Proper design would call for staying well clear of the edge of the normal operating zone (kids take note!). Jag designers know that, so they designed the cut-outs to ensure the system stays clear of that zone. We all know that once a system is deployed it will be pushed -accidentally, or on purpose, to its extreme. So the Jags just cut out, while the Vics burn!

It is like the 0-11 setting in the Spinal Tap amplifier :-) users want, and will always push to go to 11!
So I do think the Jag should allow for an “11” option and a disclaimer User Beware :-)

Seriously, IMHO, the Jag can relax its protection a bit and still be quite safe. Especcially I think there could be two modes (1) for regular long steady-state operation, and (2) for short bursty operation like a FIRST competition. Yes yes I can hear you say users of (1) will use the (2) setting to solve a mechanical design problem... c'est la vie!

[Al Skierkiewicz]

Dean,
If I were to hazard a guess, I would think that the trip characteristics are more to protect the onboard power supplies over the FETs. At the currents and durations listed, I would guess that the voltage drop to the three terminal regulators is likely to cause them to drop out if the trip points were to be relaxed.

[techhelpbb]

Originally Posted by Al Skierkiewicz

Dean,
If I were to hazard a guess, I would think that the trip characteristics are more to protect the onboard power supplies over the FETs. At the currents and durations listed, I would guess that the voltage drop to the three terminal regulators is likely to cause them to drop out if the trip points were to be relaxed.

It's actually quite easy to brownout the Jaguars already with the sorts of systems common to this competition. As I have suggested elsewhere it's a shame that the Jaguars don't have a separate place to insert power for their logic (something common to older educational robot designs), a lower drop out regulator or more energy storage (larger capacitor, battery, etc).

Of course the bigger problem with the brownout as it is, is that if you're not in the default modes you have no non-volatile place to store things if you do brownout. If that changed perhaps it would be less an issue.

[Joe Johnson]

Can we all agree that the FIRST control system, while each change seemed to make sense at the time, had morphed into something that has cross over the line if sanity?

I am quite serious.

FIRST should really think things through from start to finish.

I know that it is unlikely that the CRIO is going anywhere soon, but that doesn't mean that we have to keep everything else.

If I were king, I think I'd look hard at a PDBoard that was 3-4" thick, essentially nothing but relays, FET and fins, hermetically sealed with a brain to do the switching, a big fat battery connection for current going in, a bunch of smaller (Non Wago please) connections where the current goes out to the motors.

I think it makes a ton of sense.

Joe J.

[EricVanWyk]

Originally Posted by Joe Johnson

Can we all agree that the FIRST control system, while each change seemed to make sense at the time, had morphed into something that has cross over the line if sanity?

I am quite serious.

FIRST should really think things through from start to finish.

I know that it is unlikely that the CRIO is going anywhere soon, but that doesn't mean that we have to keep everything else.

If I were king, I think I'd look hard at a PDBoard that was 3-4" think, essentially nothing but relays, FET and fins, hermetically sealed with a brain to do the switching, a big fat battery connection for current going in, a bunch of smaller (Non Wago please) connections where the current goes out to the motors.

I think it makes a ton of sense.

Joe J.

Yes. A thousand times yes. Sometime over a beer I'll have to show you the (rejected) Power Interface. I don't know when I'll get the chance to resurrect it, but I swear it will happen one of these years.

[techhelpbb]

I don't know about major overhauls (there are already unforeseen consequences from that sort of thing in the past 14 years I've been involved in U.S. FIRST).

Plus, many teams have invested a small fortune in parts.

I am quite comfortable with the Victors, and I think that they have historically functioned quite well.

That being said, rolling everything into a single module to add to the cRIO carries with it the disadvantage that the students don't get to really glue it all together.

Additionally, this module idea with everything in it is...sort of the like the Jaguars with a single CAN bus...a single point of failure. I should hope that great care would be taken to insure parts would be available quickly if things go badly and the smoke comes out.

I think the Jaguars just need really 2 things:

1. A good comprehensive, easy to grasp, document set designed to help design with them within this environment.

2. A little design work so that the next revision better addresses a few of the things I often see vex people.

(On a side note I very much favor modular design. It leaves room for much innovation and it allows competition between part sources that...when supervised properly...helps keep the quality high and the price low.)

[Al Skierkiewicz]

Joe,
We really need to get together and talk. Are you up for a challenge? I have an assignment you might be interested in.
Al