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1. 

   Silntknight
   Moderator

   Hey, I have a motor (13.2V, 42A) and I have some MOSFETS that I am using to control it. I know that it will be using 42A almost continuously because the motor will be stalling nearly the entire time it is running. I can make this run in one direction easily using the MOSFETS, but I'd like to be able to reverse directions. I know this will lead to a MASSIVE voltage spike and other methods I've used in the past won't work to suppress it. I considered Zeners in anti-parallel across the MOSFET and a Schottky in anti-parallel across the motor, but because the direction of the current needs to reverse, they won't work. I haven't built RC snubber circuits before, but I am familiar with time's constant and the equations for such a circuit. Does anyone have any ideas about how I can suppress the spike no matter which direction it is in? I'd also like to not have to make an H-bridge because that would require a lot of MOSFETS (because I have two motors and I'm putting 3 MOSFETS in parallel for each motor currently to reduce heat dissipation).

   Also, I know this isn't quite the correct place to ask this, but I can't login to another (this type of question specific) forum.

   Thanks!

   Posted 4 years ago #
2. 

   zoofdxp
   Member

   Hi Silntknight, You may want to consider using a TransOrb, otherwise known as a TVS device, bidirectional in particular. This one, http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=5KP15CALFCT-ND
   has the appearance of a very hefty leaded diode, but acts basically like a high powered zener diode. It will not breakdown at your operating voltage of 13.2V because it has a standoff voltage of 15volts. Voltage breakdown begins at 16.7V, it can handle a repetative peak surge current of some 175 Amps, and like anything the voltage will grow a little during that peak current some 3 to 5 volts or so, maybe to 22 volts. This part is rated at 5000 Watts peak "spike" power. Make sure your components around the bridge can handle at least 25Volts for safety margin. As a final note...Place thing right across the motor leads. I'm assuming your using a 2-leaded DC motor, also I picked the reasonably biggest TVS I could find in a few moments, you may not need one that is quite this hefty, but its not that expensive either way. Keep in mind that their are reverse polarity body diodes in the mosfets that are part of the bridge that are able to direct the current into the power supply, they will share in absorbing back emf...the TVS diode will do its job during the peak transient.

   Posted 4 years ago #
3. 

   Silntknight
   Moderator

   Thanks! So this is basically a bi-directional diode? I think I understand its function, but not necessarily its operation. As I read your post, it appears to operate as a diode that is constantly reversed polarized, right? Could you draw a circuit for this? I realized that I do need an H bridge to make reverse motor operation possible, so I'm wondering if connecting it across the motor terminals would work for current on both directions.

   As far as clamping voltages go, 25V is fine. The motor should be able to handle it easily and the MOSFETS are rated for some 400V or so. Again, thanks! This is extraordinarily helpful.

   Posted 4 years ago #
4. 

   zoofdxp
   Member

   You got it right, it is a bi-directional Zener diode. You place it across the motor terminals. The H-Bridge provides 13.2 volts across the motor in both directions, the zener acts "invisible", like its not even there at this voltage (as you describe - constantly reversed polarized when the motor is in either direction). Only when spikes from back emf occur that raise the voltage high enough to "turn-on" the Zener voltage does it do its job. The spike is relatively short lived and the TVS can take the current surge without damage. If your Mosfets are 400V rated, I'm almost thinking you can get away with using no snubber or TVS, but I don't know the details of your motor or the current your pumping into to it to get a feel for how much energy we're dealing with, better safe than sorry! BTW, this type of TVS is commonly used in automotive circuits to deal with spikes from the alternator, the instrument clusters, sound system etc use them.

   Posted 4 years ago #
5. 

   zoofdxp
   Member

   While I'm thinking about it, depending on how large a motor you have, lets say...huge, an Resistor in series with a Capacitor snubber may be more appropriate. The RC combination takes on two states or two modes of operation very similarly as does the TVS, by proper selection of the RC values. Typically the resistor is low ohms, say 1 or 2 ohm and is reasonably hefty, like a large ceramic encased non-inductive wire-wound resistor. The spike voltage, due to the nature that it is actually spike, implies that the it is made up of high frequency signals. This high frequency energy shoots right through the capacitor, because that is what capacitor allow, right into the resistor. So during a spike the motor effectively has this low ohm resistor in parallel with it helping to dissipate and burn up the energy in the spike. During normal sustained DC, which has no frequency components to the signal, the capacitor doesn't conduct, the resistor gets no current, and the motor gets all of the DC from the H-Bridge and everybodies happy.

   Posted 4 years ago #
6. 

   Silntknight
   Moderator

   Thanks for all the info. You are right about the motor, they are, IMO, huge. I have two motors and each will be consuming 42A almost continuously, as each will be at stall for most of the time it is running. Basically, I'm running a good kW through the two motors for about a minute. Still, I like the idea of the TVS. I don't like RC circuits as much because they need to be calculated fairly precisely, and I'd rather not deal with that. That and for about $10 with shipping I can get a pair of TVSs that'll do the job. Again, thanks so much for the info!

   Posted 4 years ago #

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