20khz PWM @2kA, 24V

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Electric go-kart project is getting out of hands and we need more current...

Has anyone done something similar or has nice ideas on output stage?

Single to-220 mosfet can take about 70-80A continuously, so I guess that around 30pcs of to-220 mosfets parallei should do the job..

Switching 2kA in 500nS or so is a another thing, I am affraid that layout is "intresting". 4oz copper is thickest PCB I can get, probably good enough for mounting decoupling caps but needs some stiffening with copper bars to carry 2000A.

Plan was to parallei 2 mosfets and 2 schottkys per "module" and run separate 6mm2 wire from each module to motor(half a meter maybe)
This way dynamic current sharing between mosfets should be better

Previous pwm-controllers can take 800A cont. and 1kA shortly(half a sec and bonding wires waporize)
 
:whazzat: :whazzat: :whazzat:
2kA @ 24V = 48kW????? I hope you don't have a single electric motor....

for the controller: I think do you need a rigid copper bar (section at minumum 500 mm2) for a current of 2000A, so I think the controller must be fixed directly on the motor(s).

Modular controller is a great improvement, you can add as many modules do you need...
 
acid_k2 said:
:whazzat: :whazzat: :whazzat:
2kA @ 24V = 48kW????? I hope you don't have a single electric motor....

for the controller: I think do you need a rigid copper bar (section at minumum 500 mm2) for a current of 2000A, so I think the controller must be fixed directly on the motor(s).

Modular controller is a great improvement, you can add as many modules do you need...
Actually we have 2 motors and need 2 controllers
:D

2kA is only for very brief perioids on acceleration, 2-5 seconds max. I think motor and copper busbars can be rated for average current(around 300-500A I guess) but it wont help if mosfets are vaporising every time I hit pedal to the floor.

Higher voltage would make things a bit easier, but unless we get better motors and batts we have to live with 24v
 
Any chance to run the motors in series?

Then you'd have 48V @ 500 amps which may be more achievable.

The problem with low voltages is the I^2 * R losses. I remember trying to build an inverter for a MIG welder in my student days.

Are you just PWM'ing the battery direct to the motors? (no step up/down of voltage) ?

-Len.
 
len_scanlan said:
Any chance to run the motors in series?


Are you just PWM'ing the battery direct to the motors? (no step up/down of voltage) ?

-Len.

Series connection should be possible but we still need huge currents as plan was to use 2x2000A controllers, with series connection "only" one 2000A controller would be needed.

Yeap, it's direct pwm to motor, motor inductance takes care of smoothing the current. Previously we have used 1khz PWM frequency to ease on switching losses and current sharing problems but 20khz would result better low-power torque and get rid of that annoying 1khz whine.
 
IGBTs will have too much forward voltage drop, the dissipation will be huge. I think you're limited to N type Mosfets

Some options:

55V, 120A, Rds=0.0035ohm
http://www.st.com/stonline/products/literature/ds/13200/stp185n55f3.pdf

40V, 75A (180 peak), Rds=0.0032ohm
http://www.irf.com/product-info/datasheets/data/irl1404zpbf.pdf

40V, 80A, Rds=0.0027ohm
http://www.fairchildsemi.com/ds/FD/FDP8441.pdf

I didn't know Fets had come so far in terms of on resistance until I pulled down a few datasheets.

Even with the vanishingly small Rds(on) of these ones, you'll still need about 30 in parallel to make up the current rating. Mounting many TO-220 packages to a bus-bar will be challenging.

-Len.
 
Re: 30 mosfets...

djQUAN said:
have you considered using ISOTOP cased fets and buss bars?
ISOTOP fets cost an arm and leg :(
I can get maybe 15pcs to-220 transistors with a cost of one ISOTOP (construction time/cost would be another thing in commercial product, but not an issue here)

BWRX said:
Or IGBTs, especially since this is a low switching frequency and high current application.
IGBT would be obvious choice above 300V or so, but not so good with 24v. 30pcs 3mOhm mosfets in parallei makes 0.1mOhm total resistance, resulting 0.2v voltage drop at 2kA and 400W power loss. IGBT's conduction loss is more than 5 times that.

acid_k2 said:
There is a problem paralleling mosfets (or other devices): current unbalancing...

If you have time to spend, read this interesting document:
http://www.irf.com/technical-info/appnotes/para.pdf

Yes, current sharing is my biggest worry. Plan was to use symmetric layout for 15 modules (2x mosfet and 2x diode+4supply coupling caps) and connect these parallei with separate 50cm wires. Even then there is worries of mosfet parameter spread and problem of switching all 30 gates simultaneously. I did some spice sims with 10 parallei mosfets and truckload of parasitic inductances and other variables(mosfet Vt, rdson, diode Vf, coupling cap loop inductances, busbar resistances, gate drive variations and what else...)
 
to minimize complexity in hooking up all of those fets in parallel.... I think you could mount all the fets on the heatsink with no insulator and use the tab as the drain terminal. that would minimize thermal resistance from case to heatsink and it would make a really nice bussbar. :D

although there would be a need to insulate the heatsink from the chassis.
 
Btw, input capacitor ripple also some sort of challenge.
500A rms input ripple current is not that easy for electrolytics.
Published ripple current ratings for 1000uF 35V caps are something like 1-3A, so that would make something like 300pcs of 12x30mm caps. I was going to get away abusing these caps with 10 times their rated ripple current :hot: Forced fan cooling should ahelp a lot.


Reasoning behind farming small electrolytics is that bigger electrolytics(ie 100mF) have smaller surface area to dissipate ESR generated heat per capacitance/price and bigger thermal resistance between internals and casing. (again, screw-mount caps would be much more compatible with bus-bars)
I tought even adding cooling fins to caps :rolleyes:
 
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