Flyback diode spec?

Discussion in 'The Projects Forum' started by madindehead, Jan 5, 2012.

  1. madindehead

    Thread Starter New Member

    Oct 20, 2010
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    Hey guys,

    Quick question for you. As part of a uni project, we are designing a circuit to drive an inductive load.

    What type of flyback diode would be suitable for a solenoid which is drawing 60 amps on a 12V supply? I don't know the actual inductance of the solenoid sadly (not been informed of it yet from rest of team).

    Not my strongest suit sadly, I am better at designing VLSI type circuits ;)

    Hope you can help thanks.
     
    Last edited: Jan 5, 2012
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Well for a short period of time, whatever current was flowing in the inductor will be flowing in the diode, until it trails off. So the diode should have a peak current rating of 60A, and the voltage shouldn't need to be rated much greater than 12V, since the diode itself prevents the normal high voltage spike.

    btw what unholy solenoid burns 720W?
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Being of a nervous disposition (having seen too many expensive little fireworks displays in my time), I would be more conservative.

    Why not go for a peak rating of at least 100A, to allow a bit of margin. You really don't want this thing to fail, because there is likely to be quite a big bang and some serious collateral damage if it does. I have visions of a large driver device going up in a puff of smoke.

    Similarly, there should be no reason to have the breakdown voltage close to 12V either, as in any case the "12V" figure may be subject to tolerances - is it from a lead/acid battery? Anyway, you should not have much trouble getting a device rated at 20V or more
     
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  4. madindehead

    Thread Starter New Member

    Oct 20, 2010
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    It's a solenoid to change gear in a sequential gear box ;)

    It has to shift in less than 200ms from the gear change being actuated and has to deliver 44 newtons. So it's only one for a small period of time, but needs to be small. Hence the big current.


    Yeah, we really do not want this thing to kill the power MOSFETs we are using, or fry the wires. The last people who did it used the wrong fuse size, and set the fire alarm off :p It's being driven from a LiPo battery, so again, safety is important.

    As for the voltage for the diode, is it the Repetitive Reverse Voltage I need to be close to 20V. It will need to be a pretty fast diode also.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Repetitive Reverse Voltage or Peak Inverse Voltage. This can be 20V or more, but not less.

    Note that the drop-out time of the solenoid may be extended (made more sluggish) by using a a simple flyback diode. Possible methods for dealing with this include a (very low resistance, high current rated!) series resistor, or a bunch of diodes in series to raise a bit of voltage. For lower current applications I might have suggested a Zener diode, but I doubt that you will easily find one rated for the peak current.

    Any attempt to weaken the flyback suppression is a bit of a calculated risk: if the mechanical performance will be OK with a plain diode it may be best left that way, but it may be better to check before you risk breaking a gearbox.
     
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  6. madindehead

    Thread Starter New Member

    Oct 20, 2010
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    Ok thanks :)
     
  7. madindehead

    Thread Starter New Member

    Oct 20, 2010
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    Sorry, one final question. You mentioned a current rating of 100A for the diode. Which current is this in the diode spec exactly?
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Repetitive peak current. Unless you were shifting every second, then a 100A continuous would be better suited ;)
    But still, if you can get anything beefier for a reasonable price, it will work as well and be even more reliable, and also it will likely have bigger size for nicer cooling.
     
    Last edited: Jan 6, 2012
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  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This is actually a little tricky, unless you know the inductance (which you have already mentioned you do not), and how often the coil will switch off.

    The highest current specified for a diode is generally called IFSM. This is the Max. Peak One Cycle Non-Repetitive rating, really a "once in a blue moon rating", half a cycle of 60Hz and not very often at all. I really would not recommend you using a diode at this rating for flyback.

    To be dead safe, you can use the IF(AV) rating, Max. Average Forward Current: This is normally quoted at a 50% duty cycle, so you can probably double it for the flyback (unless the coil current takes a lot more than about 10ms to decay - here a small fraction of an ohm in series could help.)

    A really economical design would involve getting detailed knowledge of the flyback current and careful studying of thermal derating charts so as to pick a device which just fits the bill, but you'd best not try that.

    You probably need to look at what your supplier can sell you at not too high a price - maybe $7? Don't bother with super-fast devices as they will cost maybe four times as much for no real benefit

    Here is an example device - not specially recommended but it looks about right.

    http://www.vishay.com/docs/93529/85hf.pdf

    Possibly somebody who does this stuff more often will advise you more economically, but with limited information I'd rather be careful.

    Edit: Repetitive peak sounds good if you can find it stated explicitly, and know for sure that your service requirements will line up with it. Otherwise an old coward like me would probably stick to a minimum of 50A IF(AV) rating.
     
    Last edited: Jan 6, 2012
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  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Turn off time is a function of the flyback diode voltage. For any inductor:

    V = L di/dt or

    dt = L di/V

    When di/dt is equal to a constant the shape of the current is a linear decay. As dt is inversely proportional to V a higher V means a lower dt, or a faster turn off.

    For a coil i is the on current, L is the inductance and V is the flyback diode voltage. Many relay companies use a zener diode in series with the rectifier diode to yield a higher V and thus a larger di for a faster turn off time. Those I have seen in production all used a 36V zener.

    If a suitable zener is not available then a resistor can be used. It is not as fast as a zener but is faster then nothing. Size it by ohms law where I is the coil current and V is the desired turn off voltage.

    If you use such a scheme keep in mind the voltage adds to the supply voltage at the drain of the switching fet.

    Finally, I can't see any reason the turn off time of the diode has any effect on this circuit.
     
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