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Old 2007.10.28, 02:51 AM   #1
LBRC
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New kid on the block Fairchild FDS8858CZ

Mentioned these in a MA-010 FET thread but decided they rate their own thread after spending a day playing with them.

On paper they’re very nice with a -7.3 amp continuous drain current for the P-channel and a total P+N on-state resistance of 54.5mΩ at 4.5V, for comparison the Vishay 4562 lists a -6.2 amp continuous drain current and a total P+N on-state resistance of 58mΩ at 4.5V.

Data Sheet Link: http://www.fairchildsemi.com/ds/FD/FDS8858CZ.pdf

Max rated continuous drain current at 25° C:
N-channel 8.6 amps
P-channel -7.3 amps

On-State Resistance:
N-Channel
• Max rDS(on) = 20mΩ at VGS = 4.5V, ID = 7.3A
P-Channel
• Max rDS(on) = 34.5mΩ at VGS = -4.5V, ID = -5.6A

Of course what the manufacturer lists on the data sheet and how they actually hold up in a Mini-Z is not always the same thing but so far for me they seem to be living up to the specks. First I put a 1x1 set in an old MR-01 and after tentatively playing around with different motors I got brave enough to pop in an Atomic T2 with a 6 then eventually 7 tooth pinion to run some wall bashing 50 lap sets on a 2xWide-L RCP track. Ran out of courage at the 7 tooth pinion though, even with a listed 7.3 amps I thought anything more would be asking for a miracle, besides a T2 in a MR-01 on a small track is bit more than I can handle. The question is how long will they last as a 1x1 with a motor as hot as the T2? My guess is that even though it has now gone through 4 NiMH battery sets and nearly 400 laps it’s just plain lucky to be alive.

After the MR-01 I put a 2x2 in a MA-010 with one of the blue MZWS BB practice motors (I believe it’s a 47 turn); noticeable power increase over the 3010’s with the same motor and batteries, very snappy throttle response but still quite linear, just the way I like it, and with an estimated/rated 14.6 amps of current handling ability even in an AWD the 2x2 should be able to handle just about any motor I'd want.


The Big Question:
Do I like them more than the 4562’s? Don’t know yet only time will tell, maybe. What I can say without a doubt is that I like the price, 82 cents each or $54.60 for 100 from Digi-Key, and if your looking for FETs the 8858’s are definitely worth considering.
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Old 2007.10.28, 10:04 AM   #2
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Quote:
Originally Posted by LBRC
Mentioned these in a MA-010 FET thread but decided they rate their own thread after spending a day playing with them.

On paper they’re very nice with a -7.3 amp continuous drain current for the P-channel and a total P+N on-state resistance of 54.5mΩ at 4.5V, for comparison the Vishay 4562 lists a -6.2 amp continuous drain current and a total P+N on-state resistance of 58mΩ at 4.5V.

Data Sheet Link: http://www.fairchildsemi.com/ds/FD/FDS8858CZ.pdf

Max rated continuous drain current at 25° C:
N-channel 8.6 amps
P-channel -7.3 amps

On-State Resistance:
N-Channel
• Max rDS(on) = 20mΩ at VGS = 4.5V, ID = 7.3A
P-Channel
• Max rDS(on) = 34.5mΩ at VGS = -4.5V, ID = -5.6A

Of course what the manufacturer lists on the data sheet and how they actually hold up in a Mini-Z is not always the same thing but so far for me they seem to be living up to the specks. First I put a 1x1 set in an old MR-01 and after tentatively playing around with different motors I got brave enough to pop in an Atomic T2 with a 6 then eventually 7 tooth pinion to run some wall bashing 50 lap sets on a 2xWide-L RCP track. Ran out of courage at the 7 tooth pinion though, even with a listed 7.3 amps I thought anything more would be asking for a miracle, besides a T2 in a MR-01 on a small track is bit more than I can handle. The question is how long will they last as a 1x1 with a motor as hot as the T2? My guess is that even though it has now gone through 4 NiMH battery sets and nearly 400 laps it’s just plain lucky to be alive.

After the MR-01 I put a 2x2 in a MA-010 with one of the blue MZWS BB practice motors (I believe it’s a 47 turn); noticeable power increase over the 3010’s with the same motor and batteries, very snappy throttle response but still quite linear, just the way I like it, and with an estimated/rated 14.6 amps of current handling ability even in an AWD the 2x2 should be able to handle just about any motor I'd want.


The Big Question:
Do I like them more than the 4562’s? Don’t know yet only time will tell, maybe. What I can say without a doubt is that I like the price, 82 cents each or $54.60 for 100 from Digi-Key, and if your looking for FETs the 8858’s are definitely worth considering.
Great write up!

Kris
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Old 2007.10.28, 10:17 AM   #3
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need help with fet specs. how will i go about comparing these new fets of yours against the irf 7389? i have no clue where to look -,-

data sheet

http://www.irf.com/product-info/data...ta/irf7389.pdf

thanks
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Old 2007.10.28, 07:16 PM   #4
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Quote:
Originally Posted by dohc
need help with fet specs. how will i go about comparing these new fets of yours against the irf 7389? i have no clue where to look -,-

data sheet

http://www.irf.com/product-info/data...ta/irf7389.pdf

thanks
Not sure it can be done in a 10,000 words or less?!!

With most of the FETs we use comparing the total on-state resistance for both the P and N channels and the mosfets max continuous current drain will tell you 98% of what you need to know.

On-State Resistance or “rDS(on)” will give you an idea of how efficient the mosfet is, the lower the value the more battery power reaches the motor. Putting mosfets in parallel reduces the resistance by half so that a 2x2 will have half the resistance of a 1x1, the practical result being that a car with a 2x2 will be faster than the same car with a 1x1 of the same mosfets.

Maximum Continuous Current will give you an idea of how much current the motor can draw before you see and smell smoke, so the larger the value the “hotter” the motor you can use. Not important if you are only running stock class motors but very important if you wish to run low turn hot motors.

For the motor to go in one direction the car needs to use both a P and an N channel.
• So for total resistance you need to add the P and N channel On-State Resistance.
• And for current handling ability the weakest link sets the limit so only the P-channel matters since being the lowest it will burn out first.

Typically you would compare the 8858’s 20 milliohm N-channel resistance at 4.5 volts and its 34.5 milliohm P-channel resistance at 4.5 volts, totaling a nice and low 54.5 milliohms to the 7389’s 46 milliohm N-channel resistance at 4.5 volts and 98 milliohm P-channel resistance for a total of 144 milliohms.

Compare total P+N-channel resistance at 4.5 volts:
• 8858 = 54.5mΩ for a 1x1 and 27.25 mΩ for a 2x2
• 7389 = 144mΩ for a 1x1 and 72mΩ for a 2x2

And P-channel continuous drain current:
• 8858 = -7.3A for a 1x1 and -14.6A for a 2x2
• 7389 = -5.3A for a 1x1 and -10.6A for a 2x2

So usually that would be all you’d need to know, as you can see a 2x2 7389 would still have more rated resistance than a 1x1 of the 8858’s while only handling 3.3 more amps, and even worse at 3x3 the 7389 would start to look ridiculous only handling 1.3 more amps than a 2x2 of the 8858s while still having noticeably more resistance...

However things are not quite so simple with the IRF 7389 it takes much more to see what is actually going, yes at first glance the on-state resistance will seem high and the current handling good but not great when in fact because of power dissipation and switching it’s actual performance in a Mini-Z is quite a bit better than that type of quick uneducated glance at a data sheet would lead you to suspect. Cold as seen above it can’t compare to the 8858 or a 4562 but in some cases where they are hot in the middle of a race and consequently less efficient due to the increased resistance a 7389 is able to stay cooler with consequently lower resistance values.

The other problem is with stacking, data sheets rarely give a good indication about what will happen when you run a particular mosfet in parallel without modifying the driver circuitry too, yet alone what happens when you plop one right down on top of the other, something the manufacturer never intended. I am/was a 7389 fan for 2x3 and 2x4 stacks preferring them to the 4562’s especially in a 2x4+ stacks, however the more mosfets in a stack the greater the chance something will go wrong and with the right mosfet there is rarely a true need for a 2x3 or 2x4 stack.

For racing things become even more complicated since the “fastest car” and best lap times rarely have anything to do with each other. Things like how the mosfet and motor combo responds to the throttle movement are just as, if not more important, than resistance, current handling ability, speed, and power. Unfortunately that’s nearly impossible to find on a data sheet it’s a mater of trial and error to see what works for you.
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Old 2007.10.28, 09:29 PM   #5
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thanks a lot for the information. i learn new things everyday ;]
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Old 2007.10.28, 09:45 PM   #6
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LBRC that was the most infomative info about FETs I've ever heard. Good on ya! Also why I stickied it

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Old 2007.10.31, 03:42 PM   #7
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Quoted by LBRC "and with an estimated/rated 14.6 amps of current handling ability even in an AWD the 2x2 should be able to handle just about any motor I'd want."

I happen to disagree with the seemingly wide spread opinion that stacking FET's increases the current capacity. I beleive that stacked FET's are still only good up to the current capacity of a single FET, but the load is placed across both FET's and therefore the FET can run cooler. I don't think the motors we use pull more than 2 to 3 amps, anyway. We do get the benefit of reducing the resistance with stacking. To increase the current capacity the FET's would have to be wired in series.
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Old 2007.11.01, 12:43 PM   #8
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Have been trying to figure out how to be nice and break it to you gently but it’s sort of like trying to explain when someone posts with conviction that the world is flat.

Stacking the mosfets puts two or more in parallel, and even without any formal electronic education you should be able to see how with two working side by side the current handling ability of the circuit is doubled since you then have two mosfets sharing the same load instead of just one. As for the “series” statement mosfets are not batteries that’s the sort of nonsensical statement that shows you lack an understanding of basic electronic fundamentals to the point what a mosfet is and how it works.

What I suggest you do is to pick up an Atomic T2 or 33 turn PN Racing Anima motor to install in a Mini-Z for a few hard laps around a track, after the smoke clears try it with a stacked set of mosfets.

Now you could make a good argument by saying that power handling, as in watts, and heat dissipation is the real problem, while max continuous current on the data sheet is just a quick easy indication of how the proposed set of FETs will do, the goal being to get as much power through the mosfets and to the motor as possible while driving without overheating and destroying the mosfets in the process and reducing rds(on) by placing the mosfets in parallel is the key to the power equasion. It’s true that we don’t run high amperage continues current through the mosfets, heck I doubt the circuit board pathways could take it, it’s the constant spiking and stall current that kills our mosfets, but technicalities aside the fact remains that putting the mosfets in parallel by stacking allows us to double the current handling ability of the ESC on the circuit board.
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Last edited by LBRC; 2007.11.01 at 01:22 PM.
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Old 2007.11.01, 04:45 PM   #9
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Well said LBRC...

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Old 2007.11.02, 08:51 AM   #10
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The more I learn the more I know I don't know!!

This is what makes the forums interesting
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Old 2007.11.02, 10:06 AM   #11
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Quote:
Originally Posted by LBRC
Have been trying to figure out how to be nice and break it to you gently but it’s sort of like trying to explain when someone posts with conviction that the world is flat.

Stacking the mosfets puts two or more in parallel, and even without any formal electronic education you should be able to see how with two working side by side the current handling ability of the circuit is doubled since you then have two mosfets sharing the same load instead of just one. As for the “series” statement mosfets are not batteries that’s the sort of nonsensical statement that shows you lack an understanding of basic electronic fundamentals to the point what a mosfet is and how it works.

What I suggest you do is to pick up an Atomic T2 or 33 turn PN Racing Anima motor to install in a Mini-Z for a few hard laps around a track, after the smoke clears try it with a stacked set of mosfets.

Now you could make a good argument by saying that power handling, as in watts, and heat dissipation is the real problem, while max continuous current on the data sheet is just a quick easy indication of how the proposed set of FETs will do, the goal being to get as much power through the mosfets and to the motor as possible while driving without overheating and destroying the mosfets in the process and reducing rds(on) by placing the mosfets in parallel is the key to the power equasion. It’s true that we don’t run high amperage continues current through the mosfets, heck I doubt the circuit board pathways could take it, it’s the constant spiking and stall current that kills our mosfets, but technicalities aside the fact remains that putting the mosfets in parallel by stacking allows us to double the current handling ability of the ESC on the circuit board.
Well Now, I stand corrected, but man you are brutal!
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Old 2007.11.08, 11:36 AM   #12
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good stuff!!!
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Old 2007.11.10, 02:38 PM   #13
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Seems now there are a few more very viable FETs out there besides the venerable 4562. Very interesting.
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Old 2007.11.15, 01:35 AM   #14
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Any thoughts re: input capacitance? I've been rethinking my stance on linearity vs. driveability.
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Old 2007.12.10, 11:17 PM   #15
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I was able to try some of these on an F1. I received the car with an kyosho X-speed and 3004 fets. This thing was a dog I thought that it might just be a problem with the car. So, I changed to a NIP Speedy 05, but it still was ridiculously slow off the line and speed was really low. I began to think this car had some type of board problem, but I decided to change the fets to see if it made a difference



I went straight to a 2x2 stack since I did not want to mess around with the board too much fearing I might lift a pad. Anyways, the fet job went well. The car now had much better punch that was controllable and decent speed for a car that weighs the same as an MA-010. I later stole my bros HFAY motor and it felt really nice in the car with more punch and speed.

I have a fetted 2x2 4562 MR-02 and I know its not a direct comparison, but it would be hard to figure what each car had. I'd definitely recommend these for the price alone. I will possibly get a chance to run a decent mod motor soon, but with the fets rating, I don't think its hot enough to really see what they can take.

I've heard a 2x2 stack of 4562 fets can take an anima in an mr-02, so these should possibly take something with lower turns but for most of us, anything lower would probably be useless for circuit racing
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