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Separate Power board, tech spec controller


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1 minute ago, frenchie said:

Too bad that's 38 pages about everything but the Gotway. But I'll search it for data...

They are basically all the same board on every EUC 500W and above. No company his really invested any effort in control boards the last year or so.

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With a Wheatstone bridge I now have measured the resistance in the motor windings, thru each connection = blue-green, blue-yellow, green-yellow. Value was, as expected, the same on all connections i.e. two coils in series = 139 milli-ohm = 69.3 milli-ohm per coil.

The original MOSFET IRFB3207 is suposed to have maximum RDS(on) = 4.5 milli-ohm and therefore chews up some Power.

P = I2 * R

Total R = 69.3 + 69.3 + 4.5 = 143 milli-ohm.

The original MOSFET IRFB3207 is supposed to deliver 180A which thru the formula above leaves us 2247W per coil but, unfortunately also 146W in the component itself. This is of course "worst case" values. The truth is that the original MOSFET put in a TO-220 casing only can deliver 75A due to thin connectors. That is partly the reason Gotway doubled up by soldering a twin MOSFET in top of the original.

The value on Rjunction-case is maximum 0.45 C/W for the IRFB3207. If it has been properly mounted on a heat sink that is!! This gives us 0.45 * 146 = 65.6 degrees C in worst case. In reality we might have a current As a base addition to the heat needed to get rid of that's a little too much.

Kept in mind that the controller PWM-signal wont send out any different signal than before (open MOSFET is an open MOSFET, it will not send more Amps to the motor than the original setup, it just copes better with the current), the total current available is still set by the voltage/total resistance with a theoretical burst maximum at 67.2V/143milli-ohm = 469A (actually slightly higher since the max current should be calculated with the minimum total resistance, typical RDS(on) 3.6 milli-ohm = 472A). The average current is what is driving the wheel! IRFB3207 can cope with maximum burst current = 720A. The original design with only 6 MOSFET's seems to be correct. But the heat takes the better of them!! The heat sink is badly designed and much to thin where it counts. The average current might be as stated above by @lizardmech, a mere 20-30A.

 

However;

The MOSFET IRFP4368PBF has only 0.29 C/W and whats even better, RDS(on) = 1.85 milli-ohm! It also withstands a whopping 350A and as much as 195A in the connectors (TO-247AC is beefier than TO-220). Just to compare the worst case figure from the original MOSFET I use 180A again

P = I2 * R

Total R = 69.3 + 69.3 + 1.85 = 141 milli-ohm.

Pcoil is still 2247W

But the PMOSFET = 60W. And with a lower Rjunction-case at 0.29we only raise 17.4 degrees C, not 65.6!

 

Replacing to IRFP4368PBF will be easy to cool down, sturdier current-wise and absolutely not demanding twin-MOSFET. It'll be more than enough with 6 units. That way all the other problems Gotway has completely disregarded (or just failed to calculate) when the emergency soldered twin-MOSFET's, totally vanish. For instance, the biggest no-no is the 90-degree bend immediately right next to the casing, that's not OK! Also, the twin doesn't share the same heat sink profile. That gives you different temperature per individual MOSFET and therefore a large risk of switch oscillation. The MOSFET twins running forward thru "MotoB" also just happen to be in the middle heat-wise. This also builds up a much to high temperature in general.

Guess where I will mount my new MOSFET's ;0)

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You have to consider switching losses as well. Comparing the two infineon fets and the fairchild fet I'm using, the  IRFP4368PBF has low RDSon, however input capacitance is twice the original and rise/fall times are dramatically slower, it's also limited to 190A due to wire bonding in the package. The 12 fet board would suffer similar issues due to capacitance being doubled. In comparison the fairchild SMD fets have switching characteristics close to  IRFB3207 but the low 1mohm RDSon as well as a package rated for 300A.

The IRFP2368 will also spend more time in the miller plateau where resistance will be 3x higher. 

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On 1/11/2017 at 4:45 PM, frenchie said:

You are absolutely right. But at as low switching frequencies as this board use it shouldn't be that big issue?

 

What Fairchild SMD?

I think the issue is at lower currents the losses quickly add up. Even if it's just 2watts per fet you then have 12 watts heating the heatsink at all times, then when you do put a heavy load on it the fets are already at 60c+ and have a higher RDSon as a result.

These are the mosfets I have been using.

mouser.com/ProductDetail/Fairchild-Semiconductor/FDBL0150N80/

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  • 3 months later...

Funny, I've upvoted the first couple of posts back in Jan 7th, but have no recollection of ever reading this thread... Must have been drunk or something :P

But anyway, I'd be really interested in hearing how this turned out. Another thing I was wondering was that if the original gate-drivers don't pack enough "punch" to drive the higher gate charge-mosfets fast enough, do you think it's possible to buffer the gate-signals and use a separate card with it's own driver(s) and mosfets? What problems are likely to be faced, interference, too much delay between the original signal and the fets starting to conduct..?

Edited by esaj
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  • 7 months later...

Did this one die out, moved?

Not that I can follow 100% but interesting nevertheless and just send my ACM to shop today for inspection so..

 

Also thanks for the images, another image I see that have got no solder/splatter connecting the Green motor wire (C) to the 2mΩ just next to it like mine has, hard to accept when no other boards have it but not necessarily the reason for my sudden loss of balance either the whole board looks like a 5 year old played with fathers soldering gun and somehow manage not to get burned in the process and/or exposed?

The board though, ohh well. The pig fest reference in general is worthy, wish I had never seen that.

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