Quote:
Originally Posted by princeton
They are actually 200V zener diodes that act as flybacks when drain voltage goes less than source (ground) but also act as voltage limiters to protect my 300V mosfets.
No there are no input capacitors. I will probably place some input capacitors across the battery terminals. I didn't see a need for them taking up space in my controller. The only real purpose I can see they serve is to protect the battery from rapid discharge current and prolong its life. They are not absolutely necessary in all systems, in my opinion.
Yes, the mosfets are holding up the heavy gauge copper wire but I intend to put some insulated bracing to hold the copper wire to help prevent the leads from being broken off.
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There have been several responses about the diodes. I particularly like Paul's! Also note that diodes don't share current well unless they're thermally coupled - that is, one diode isn't allowed to get hotter than another (usually by placing them all near each other on the same heatsink). If one gets hotter than the rest, it hogs more current, gets even hotter, hogs even more current... thermal runaway ensues, and poof.
As for capacitors, they serve a couple of purposes. One, is that they help clamp the inductive voltage spikes from the motor. When the fet turns off, the voltage on the drain will rise due the motor inductance. When it rises above B+, the diode begins to conduct. At this point, the drain pin is essentially connected to B+ and the cap can help clamp the voltage spike.
Secondly, it smooths the supply current from the battery and the voltage spikes associated with it. Take the circuit as it is now without input caps. Let's say the controller is operating at 500A and maybe 50% duty cycle. When the mosfets turn on, 500A is drawn straight from the battery through the battery leads. When it's off, the current from the battery is zero (though the motor current keeps going due to the motor inductance). So, there will be this drastically changing current in the battery and it's wiring. (this is roughly what is called 'ripple current', btw).
The problem is that batteries don't like high current pulses - they'll likely heat internally and die early. Also, all wiring has parasitic inductance and that 500A won't stop instantly (see oprah) and'll cause (potentially destructive) ringing on the mosfets, and/or cause a ridiculous amount of EM noise (that long battery cable is like a big antenna!)
Adding capacitors will distribute the ripple current between the caps and the batteries, mitigate the parasitic inductance of the wiring, and bring a current source close to the switching section in order to minimize the parasitic inductance.
That's not to say the controller won't work as is, but it raises the question of how well it will work. If you find it doesn't meet the design goals, these are some possible options to look at when revising the design.