Quote:
Originally Posted by MrMin
Theme switch - quickly - Zippy batteries. I've kind of been obsessed about the fact that I've been reading zippies are tricky and not good etc...
I've been doing some basic calc....momentum.. 540 joules(thats 540 watts!) need to be dissipated over 1 second during braking.
The batt can handle about 1C, thats about 4-7 amps..maybe a little more. But we will see almost 100 amps...for 1 second..thats a long time in components lifetimes..
My idea is to cross 3 nice big zeners (with current limits)and 2 tvs diodes - I know its kind of overkill! on a tiny board which is between my batts and ESC (I have a loud maxpro buzzer there anyway - it reminds me when its time to put the batts to sleep). I've already done a spice circuit sim 3 times and it seems like it will work..
I'll give a list of components(I've got to source 3 x 5W(continuous) zeners and 2x1500W(burst) TVS diodes and a couple of current limit resistors..) and test the voltage etc... its all about "clamping".. When I've completed the "MrMin load dumper" or if you prefer "MrMin esc protector". BTW: Seems like caps will not help a lot, maybe with the ripple current and smoothing a bit.
BTW: A more elegant version would be active clamping/load dumping with IC's & power fets, but I'll see how well this works first...
What patrick said about the huge load is real. We got to keep the voltage below the 30v to protect the FETS.
Patrick, what do you think? I've done the spice sim and it works out..
This is part of this little investigation anyway...
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Lots of things that don't work look good in Spice....
Think about it this way -- A TVS is a large die zener diode. But until you have nearly the same die area in TVS as you do in FETs, then the TVS is just going to fail...
Remember that a FET can act as a zener diode -- they have what is called an "avalanche" rating. They can take a very large amount of energy -- but in an application like the MMM, it is usually not efficient to use TVS devices to protect the main FETs. In this type of application, the main FETs will still fail if you try to push everything too hard. You simply can't dissipate the energy in any kind of silicon device (within reason) -- you need to put it back into the battery...
I could spend the whole night discussing why -- but I probably won't convince you. I've spent 15 years designing motor controls, and still haven't seen a TVS be useful for anything other than protecting sensitive components like microprocessors. They simply can't dissipate enough energy to be useful in any other way -- and in fact typically can't take anywhere near as much excess energy as the main FETs themselves.
