Mosfet and Inregrated Mosfet/Drivers, how to protect from inductive loads? - Page 1
Forum section: electronics / projects

author: jnz
author_rank: Frequent Contributor
timestamp: September 23, 2015, 07:02:57 pm
content: If I had a 22Amp @ 18VDC load, and wanted to drive an inductive motor.... As I understand it, the issue is that when the motor shuts off, it's still spinning and creating an inverted load back at my mosfet. Since the body diode will allow this to conduct, it'll go through my fet even if it's shut off, and the rdsOn might not be as low as it would be when on, the issue is then heat at my mosfet and I can't do anything except wait for it to stop. Am I getting that mostly correct?Attached a picture from an integrated driver that has a solution for this.Is A or B more effective? A is using a Zener it seems, B is a TVS, maybe. Is there another solution that might be better?The optional mosfet is odd to me, it's put there to "limit current at the diode" above it, but it's gate not being hooked to anything looks... wrong.What TVS or Zen diode values and mosfet value makes sense? How am I supposed to calculate what an unknown inductive load could be? What is this got hooked to something like a battery charger where it's not reverse polarity, but a higher output than the voltage source? I'm not sure how to specify these parts.Anything else I might want to consider to protect the mosfet/high side switch?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2015, 09:45:28 pm
content: The bEMF due to motion does not reverse direction. It's in the same direction as the source.The equivalent circuit of a motor looks something like:(Terminals) - Leakage inductance - DCR - bEMF (voltage source)bEMF, in turn, is a dependent source, something like a R || C for a motor with no load. R representing windage losses and C being rotor inertia.The bEMF due to leakage works just like any inductor, and will reverse direction (since it comes from V = L * dI/dt), and will only ever deliver as much current as the transistor drew through it.It's very convenient to wire up a motor in a standard buck converter circuit. The clamp diode takes care of inductance, and also tends to brake the motor if it should not turn in reverse. Standard switching supply design practice applies -- use a current mode controller (this protects the switch, and also sets the torque limit), use filtering for EMI and efficiency if needed; keep the switch-diode-(supply bypass cap) loop short.Tim

author: jnz
author_rank: Frequent Contributor
timestamp: September 24, 2015, 01:53:45 am
content: Ok, I was talking about two things. Motor stopping (bEMF) and someone hooking up a battery charger or some other higher voltage than source.As to the rest of your post... I know SOME of those words... I'll re-read a few times to try and better process.

author: singapol
author_rank: Frequent Contributor
timestamp: September 24, 2015, 03:21:43 am
content: Quote from: jnz on September 24, 2015, 01:53:45 amRead up on surge protection, it's not only turn offs you should worry about but turn ons too in this case of a battery which can source a tremendous amount of current. The high side switch and the mosfets are the primary components that need surge and operational protection. Although there are zener for highside switch and body diode for mosfet you still need external diode for it apart from a gate resistor. A snubber (resistor capacitor) is also required for the mosfet.It is most likely you will have to consider the pros and cons of using TVS, MOVs and NTCs. TVS are fast acting while MOVs have better surge capacity depending on physical size while NTCs take on the transients directly.You have to also consider if snubbers affect the switching time of the circuit depending on the values used.All need testing in actual circuit.

author: calexanian
author_rank: Super Contributor
timestamp: September 24, 2015, 03:29:11 am
content: In so much as the protection of the mosfets you will want to have at least a TVS across motor windings. MOV's may not be fast enough to catch a fast spike. In regard to the back EMF and motor deceleration and such, its a vastly more complicated subject.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 24, 2015, 06:13:22 am
content: Done right, a brushed DC motor (I'm assuming here...) controller is a two-switch buck converter, synchronous (two FETs) or not (one FET and one diode), and won't necessarily need much "extra protection". TVS in motor leads? Never seen one.You need the second switch (a freewheeling diode is automatic!) to continue circulating energy stored in the motorwindingsin the motor. When you do this, there won't be huge spikes. Only switch recovery times, deadtimes and layout parasitics cause some, but nothing inherent to motor control - read about buck converters, inductor being the motor, output capacitors removed and output shorted. Then you'll be able to use the much more vast knowledge there.It's wrong approach to think about inductive spikes and then try to deal with it. Motor controller is practically a DC/DC converter, and the correct way to think is to provide a current path for inductor current, all the time, i.e., never let the inductor (motor in this case) loose if there is current flowing. Hence the freewheeling/catch/flyback/whatever diode - many names. Diode continues circulating current (stored energy) in the motor just the same way it was circulating there before the main FET switched off.The voltage generation (back-EMF - happens with a permanent magnet motor as it turns) is a separate thing, and you can see that the voltage generated is always lower than the DC bus voltage you use to turn the motor - unless someone physically turns the motor to higher RPM than your controller voltage is capable of. The direction of the voltage also isn't reversed.Regenerative braking happens by working as a boost converter - i.e., shorting the back-EMF generated voltage through the motor winding inductance using the second switch (cannot be a diode if you want to do this), then releasing the short. Works exactly like a boost converter, in a synchronous bi-directional buck converter configuration.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 24, 2015, 12:28:55 pm
content: Quote from: jnz on September 24, 2015, 01:53:45 amAh, backflow is natural because the MOSFET body diode will carry it. If this is a possibility, then you need to make sure the current is controlled or limited, or prevented (using a series blocking diode, if you don't mind the loss of efficiency).Tim

author: jnz
author_rank: Frequent Contributor
timestamp: September 24, 2015, 04:17:26 pm
content: Quote from: T3sl4co1l on September 24, 2015, 12:28:55 pmThat's the thing. I very much do.The attachment I posted is supposed to limit current coming back from inductive loads, or so I thought it said. I'm not sure what the optional part of that image is doing I guess. I see how putting another mosfet inline with the output would do it, same as the series resistor idea, right? But that should be a lot less lose of efficiency. Issue is I need just as beefy a mosfet as my primary. Which then negates the reason for going with an integrated solution over discreet and then of course added cost.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 25, 2015, 12:05:14 am
content: The schematic isn't of much help, as none of the components are labeled and the two boxes are utter mysteries...If you want an "open circuit" state, you can use a synchronous rectifier (half bridge), which will be low loss while operating, and can be tristated for a no-current state as well (unless the motor overspins or reverses, or some idiot connects a charger to the wrong side, in which case the body diodes still do their job of course).If you need a still better "open" state, use a back-to-back MOSFET and isolated driver as an SSR. Put a MOV across the output terminals, so the MOSFETs don't get punished if a very high voltage is applied (say some even better idiot plugs the motor output terminals into the AC line?).Tim