On Jul 10, 10:08=A0pm, Tim Wescott <t...@seemywebsite.com> wrote: Okay, I jumped the gun--I thought you were switching power to the H- bridge.> I'm working on a motor control > board, and I done messed up. =A0The circuit I have has a PMOS transistor > that is supposed to block current from the motor when it is in generator > mode. =A0In a moment of supreme blondness (extra-supreme, if you figure > that I was, at the same time, counseling a customer not to forget that > power FETs have intrinsic diodes), I laid the thing onto the schematic > with the source toward the + supply and the drain toward the "load", > neglecting the intrinsic diode. > > So, can I just turn it around, as I show here? =A0There will be four mode=s> of operation:> (1), the board will be on, the H-bridge will be unpowered, > and the PMOS will not be activated.Power is applied to the H-bridge via the intrinsic diode. Works if the H-bridge transistors are guaranteed 'off.'> (2), the board will be on, the PMOS activated, and the motor will be driv=ing a load. Fine.>(3), the board will be > on, the motor will be driven by the external "load" and will be > generating, and the PMOS will be off, to prevent "back feeding" the power > supply.Can that produce open circuit voltages >Vgs(br)? If yes, you need a Vgs clamp.> (4), the board (and PMOS) will be off but the motor will be > driven, acting as a generator.See above.> So, in "normal" operation the current will always be going through the > PMOS "backward" -- the only time that the source of the PMOS will be > higher than the drain will (hopefully) be when the PMOS is turned off and > blocking current. > > Is this gonna work? =A0Is it even remotely normal? > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 pmos > .---|>|--. > =A0supply=A0o--+-+ ^ +--+-----o------------. > =A0 =A0 =A0 =A0 =A0 =A0D | | | S =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0| > =A0 =A0 =A0 =A0 =A0 =A0 =A0=3D=3D=3D=3D=3D =A0 =A0 =A0 =A0| =A0 =A0 =A0 ==A0 =A0 =A0| =A0H-bridge> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 .--=o---.> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 .-. =A0 =A0 =A0 =A0| =A0==A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 | | =A0 =A0 =A0 =A0| =A0==A0 =A0o----.> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 | | =A0 =A0 =A0 =A0| =A0==A0 =A0| =A0 =A0O> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 '-' =A0 =A0 =A0 =A0| =A0==A0 =A0| =A0 / \> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 | ==A0 =A0 =A0| =A0| =A0 | motor> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 | ==A0 =A0 =A0| =A0 \ /> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0'--------o =A0 =A0 =A0 =A0 | =A0 =A0 ==A0| =A0 =A0O> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 | ==A0 =A0 =A0o----'> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 | ==A0 =A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0.-. =A0 =A0 =A0 =A0'--=-o--'> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| | =A0 =A0 =A0 =A0 ==A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| | =A0 =A0 =A0 =A0 ==A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0'-' =A0 =A0 =A0 =A0 ==A0 =A0|> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0==A0 |> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0==A0=3D=3D=3D> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0==A0GND> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |/ > =A0 =A0 =A0 =A0on o-------------| > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=3D=3D=3D > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0GND > (created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)-- Cheers, James Arthur
MOSFET "Backwards" Current / Motor anti-generate
Started by ●July 10, 2012
Reply by ●July 11, 20122012-07-11
Reply by ●July 11, 20122012-07-11
Tim Wescott <tim@seemywebsite.com> wrote:>In 4-quadrant switching amplifiers, it's quite common to run current >through a MOSFET "backwards" when it is on -- i.e., to run an NMOS with >the current traveling from source to drain, instead of drain to source. > >Yet all the data sheets only show 1st-quadrant conduction, with current >(for an NMOS) running from drain to source. > >Which boils down to my real problem: I'm working on a motor control >board, and I done messed up. The circuit I have has a PMOS transistor >that is supposed to block current from the motor when it is in generator >mode. In a moment of supreme blondness (extra-supreme, if you figure >that I was, at the same time, counseling a customer not to forget that >power FETs have intrinsic diodes), I laid the thing onto the schematic >with the source toward the + supply and the drain toward the "load", >neglecting the intrinsic diode.Simply use 2 PMOS mosfets in series drain-to-drain. Just don't tie the gates together; they'll need seperate levels. Also make sure not to exceed Vgs limits. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
Reply by ●July 11, 20122012-07-11
wrote in message=20 news:2d75bd2e-0149-440d-91e0-21262901ee2d@v9g2000vbc.googlegroups.com...> On Jul 11, 7:55 am, Tim Wescott <t...@seemywebsite.com> wrote:>> My question is, can I run a MOSFET "backwards", reliably, all day.> I say yes, I've used back to back fets for 230V AC, fast > enough to turn off if shortedThe back-to-back MOSFETs form a solid state switch that is equally good = on=20 AC or DC, in both directions, so it won't prevent the motor from = generating=20 current into the source if its voltage is higher. But you can add a = current=20 sensor to turn off the switch if the current changes polarity and tries = to=20 charge the source. I made a simple LTSpice circuit. But the trick is how = to=20 drive the gates of the two MOSFETs without using an isolated supply. It=20 might be possible with an NMOS and a PMOS, with the PMOS on the positive = side and the NMOS on the negative side. Yes, it's possible, and here it = is: http://www.enginuitysystems.com/pix/NMOS_PMOS_Switch.png http://www.enginuitysystems.com/pix/NMOS_PMOS_Switch.asc I used a +/- 15V square wave for the motor as generator, and I varied = the=20 supply voltage to show when the generator signal starts driving through = the=20 switch when it's on. Some of it also comes through when the source = voltage=20 is less than 3.5V, but that can probably be fixed. There's probably a = point=20 at which the generated current doesn't matter. The NMOS back-to-back circuit does not have any problem: http://www.enginuitysystems.com/pix/NMOS_Back_to_back.png http://www.enginuitysystems.com/pix/NMOS_Back_to_back.asc Paul=20
Reply by ●July 11, 20122012-07-11
On Wed, 11 Jul 2012 11:11:31 +0200, Fred Bartoli wrote:> Tim Wescott a écrit : >> In 4-quadrant switching amplifiers, it's quite common to run current >> through a MOSFET "backwards" when it is on -- i.e., to run an NMOS with >> the current traveling from source to drain, instead of drain to source. >> >> Yet all the data sheets only show 1st-quadrant conduction, with current >> (for an NMOS) running from drain to source. >> >> Which boils down to my real problem: I'm working on a motor control >> board, and I done messed up. The circuit I have has a PMOS transistor >> that is supposed to block current from the motor when it is in >> generator mode. In a moment of supreme blondness (extra-supreme, if >> you figure that I was, at the same time, counseling a customer not to >> forget that power FETs have intrinsic diodes), I laid the thing onto >> the schematic with the source toward the + supply and the drain toward >> the "load", neglecting the intrinsic diode. >> >> So, can I just turn it around, as I show here? There will be four >> modes of operation: (1), the board will be on, the H-bridge will be >> unpowered, and the PMOS will not be activated. (2), the board will be >> on, the PMOS activated, and the motor will be driving a load. (3), the >> board will be on, the motor will be driven by the external "load" and >> will be generating, and the PMOS will be off, to prevent "back feeding" >> the power supply. (4), the board (and PMOS) will be off but the motor >> will be driven, acting as a generator. >> >> So, in "normal" operation the current will always be going through the >> PMOS "backward" -- the only time that the source of the PMOS will be >> higher than the drain will (hopefully) be when the PMOS is turned off >> and blocking current. >> >> Is this gonna work? Is it even remotely normal? >> >> pmos >> >> supply o-----+^+--------o------------. >> ||| | | >> === | | H-bridge >> | | .--o---. >> | .-. | | >> | | | | o----. >> | | | | | O >> | '-' | | / \ >> | | | | | | motor | | >> | | \ / >> '--------o | | O >> | | o----' >> | | | >> .-. '---o--' >> | | | >> | | | >> '-' | >> | | >> | === >> | GND >> |/ >> on o-------------| >> |> >> | >> | >> | >> | >> === >> GND >> (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) >> >> >> > So you're after a "no drop" diode and don't want to switch the PSU rail? > > In that case it's perfectly OK. Your mosfet can be driven because the > source voltage is defined by the mosfet internal diode.OK. That's what I figured, but I wanted to check.> > Obviously, don't forget to switch the MOS "off" when the motor is in > source mode...Yup. And switch the thing on when the motor is being driven (unless I like toasty transistors). -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●July 11, 20122012-07-11
Tim Wescott wrote:> > On Wed, 11 Jul 2012 00:49:21 -0400, P E Schoen wrote: > > > "Tim Wescott" wrote in message > > news:-fadnZXyVsu9bmHSnZ2dnUVZ_hudnZ2d@web-ster.com... > > > >> No, I didn't mean to have the source at the supply -- that's what I > >> have now, and it's failing at Job 1, which is to keep from backfeeding > >> the supply. > > > >> Having the thing fail to block the supply when the motor isn't > >> generating is just fine -- the H bridge is off until I turn it on, and > >> I intend to turn the PMOS on hard before the H bridge goes on. It's > >> preventing the motor from ramming power backwards through my circuit > >> that I'm trying to avoid. > > > >> If I put a diode in there it'll be the biggest power consumer on the > >> whole board, by a mile. So that's right out. > > > > How high is the BEMF when the motor is turned off? Is there a lot of > > energy stored in the motor due to an inertial load, or is it possible > > that the shaft could be driven at a higher speed to produce a higher > > generated voltage? There will normally be some generated spikes during > > the dead-time of the PWM switching of the H-bridge, which is usually > > dealt with by using snubbers, or even by adding a capacitor across the > > motor. > > There is a possibility that Bubba might grab the output shaft when the > thing is supposed to be off and give it a spin. > > Or that Bubba might turn the output shaft faster enough that the motor > will generate more than the supply. > > I need a method to turn the _supply_ to the H-bridge OFF at _any time_. > > > If you want to eliminate any possibility of reverse power flow, you can > > turn on both bottom (or both top) MOSFETs in the bridge, for braking > > mode, which will quickly stop the motor by dissipating the energy. As > > long as there is not a large inertial load or a chance of generation due > > to external drive of the motor shaft (as in an EV rolling downhill), the > > power should be of reasonable level. > > That won't work when the rest of the circuit is powered down. > > > Another method is to use an N-channel MOSFET and a high side gate > > driver. But that would not be an easy PCB patch. > > That won't work for the same reason that a P-channel won't -- because > power MOSFETs have intrinsic diodes that let the current run "backwards" > when the FET is off. > > To reiterate: > > My question is, can I run a MOSFET "backwards", reliably, all day. >Sure. That's the usual supply polarity reversal protection circuit, when you can't stand a diode drop: PFET with gate grounded, source to + supply, drain to the rest of the circuit. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 11, 20122012-07-11
On Wed, 11 Jul 2012 18:27:38 -0400, Phil Hobbs wrote:> Tim Wescott wrote: >> >> On Wed, 11 Jul 2012 00:49:21 -0400, P E Schoen wrote: >> >> > "Tim Wescott" wrote in message >> > news:-fadnZXyVsu9bmHSnZ2dnUVZ_hudnZ2d@web-ster.com... >> > >> >> No, I didn't mean to have the source at the supply -- that's what I >> >> have now, and it's failing at Job 1, which is to keep from >> >> backfeeding the supply. >> > >> >> Having the thing fail to block the supply when the motor isn't >> >> generating is just fine -- the H bridge is off until I turn it on, >> >> and I intend to turn the PMOS on hard before the H bridge goes on. >> >> It's preventing the motor from ramming power backwards through my >> >> circuit that I'm trying to avoid. >> > >> >> If I put a diode in there it'll be the biggest power consumer on the >> >> whole board, by a mile. So that's right out. >> > >> > How high is the BEMF when the motor is turned off? Is there a lot of >> > energy stored in the motor due to an inertial load, or is it possible >> > that the shaft could be driven at a higher speed to produce a higher >> > generated voltage? There will normally be some generated spikes >> > during the dead-time of the PWM switching of the H-bridge, which is >> > usually dealt with by using snubbers, or even by adding a capacitor >> > across the motor. >> >> There is a possibility that Bubba might grab the output shaft when the >> thing is supposed to be off and give it a spin. >> >> Or that Bubba might turn the output shaft faster enough that the motor >> will generate more than the supply. >> >> I need a method to turn the _supply_ to the H-bridge OFF at _any time_. >> >> > If you want to eliminate any possibility of reverse power flow, you >> > can turn on both bottom (or both top) MOSFETs in the bridge, for >> > braking mode, which will quickly stop the motor by dissipating the >> > energy. As long as there is not a large inertial load or a chance of >> > generation due to external drive of the motor shaft (as in an EV >> > rolling downhill), the power should be of reasonable level. >> >> That won't work when the rest of the circuit is powered down. >> >> > Another method is to use an N-channel MOSFET and a high side gate >> > driver. But that would not be an easy PCB patch. >> >> That won't work for the same reason that a P-channel won't -- because >> power MOSFETs have intrinsic diodes that let the current run >> "backwards" when the FET is off. >> >> To reiterate: >> >> My question is, can I run a MOSFET "backwards", reliably, all day. >> >> > > Sure. That's the usual supply polarity reversal protection circuit, > when you can't stand a diode drop: PFET with gate grounded, source to + > supply, drain to the rest of the circuit.Thanks Phil. I figured it was perfectly OK. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●July 11, 20122012-07-11
"Tim Wescott" <tim@seemywebsite.com> wrote in message news:lNmdnUk_QZ51iGDSnZ2dnUVZ_u0AAAAA@web-ster.com...> Right. That's my situation. And I'm asking about the FET in the back to > back pair that has the current running "backwards" (because the top FETs > in my half-bridges are the 'forward' FETs. > > How kosher is it to do this? What unexpected problem am I going to have?Absolutely fine! I've never seen a MOSFET whose body diode wasn't rated for the same current as the channel. As long as the gate is "on", the channel will conduct equally well in both directions. If the channel's resistance is sufficiently low so as to keep voltage drop under Vf, the diode will never even see injected charge -- which isn't a concern for "on/off" duty, but will keep "recovery time" negligible, if for some reason you needed it to reverse quickly. The only time it's not acceptable is when Rds(on) is insufficient to keep the diode off, and the charge injection through the body diode disrupts other junctions -- this can only occur in monolithic devices, where the injected charge turns on a parasitic VDD-to-VSS SCR structure. The gain of this SCR is intentionally very low, so that it can't be triggered at normal levels (supposedly up to 100mA peak, on input pins, for 74HC logic), which isn't to say it's impossible. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Reply by ●July 12, 20122012-07-12
Tim Wescott wrote:> In 4-quadrant switching amplifiers, it's quite common to run current > through a MOSFET "backwards" when it is on -- i.e., to run an NMOS with > the current traveling from source to drain, instead of drain to source. > > Yet all the data sheets only show 1st-quadrant conduction, with current > (for an NMOS) running from drain to source. > > Which boils down to my real problem: I'm working on a motor control > board, and I done messed up. The circuit I have has a PMOS transistor > that is supposed to block current from the motor when it is in generator > mode. In a moment of supreme blondness (extra-supreme, if you figure > that I was, at the same time, counseling a customer not to forget that > power FETs have intrinsic diodes), I laid the thing onto the schematic > with the source toward the + supply and the drain toward the "load", > neglecting the intrinsic diode. > > So, can I just turn it around, as I show here? There will be four modes > of operation: (1), the board will be on, the H-bridge will be unpowered, > and the PMOS will not be activated. (2), the board will be on, the PMOS > activated, and the motor will be driving a load. (3), the board will be > on, the motor will be driven by the external "load" and will be > generating, and the PMOS will be off, to prevent "back feeding" the power > supply. (4), the board (and PMOS) will be off but the motor will be > driven, acting as a generator. > > So, in "normal" operation the current will always be going through the > PMOS "backward" -- the only time that the source of the PMOS will be > higher than the drain will (hopefully) be when the PMOS is turned off and > blocking current. > > Is this gonna work? Is it even remotely normal? > > pmos > > supply o-----+^+--------o------------. > ||| | | > === | | H-bridge > | | .--o---. > | .-. | | > | | | | o----. > | | | | | O > | '-' | | / \ > | | | | | | motor > | | | | \ / > '--------o | | O > | | o----' > | | | > .-. '---o--' > | | | > | | | > '-' | > | | > | === > | GND > |/ > on o-------------| > |> > | > | > | > | > === > GND > (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) > >If you don't want regen power at all, then put a series diode after the PMOS switch. For that matter, you don't even need the PMOS switch if you don't ever need regen energy. One could even put a clamping R circuit (DBr) around that diode as part of a power supply over voltage and switch in the DBr when needed to drain it off. have a good day. Jamie
Reply by ●July 13, 20122012-07-13
On Wed, 11 Jul 2012 00:55:47 -0500, Tim Wescott <tim@seemywebsite.com> wrote:>On Wed, 11 Jul 2012 00:49:21 -0400, P E Schoen wrote: > >> "Tim Wescott" wrote in message >> news:-fadnZXyVsu9bmHSnZ2dnUVZ_hudnZ2d@web-ster.com... >>=20 >>> No, I didn't mean to have the source at the supply -- that's what I >>> have now, and it's failing at Job 1, which is to keep from =backfeeding>>> the supply. >>=20 >>> Having the thing fail to block the supply when the motor isn't >>> generating is just fine -- the H bridge is off until I turn it on, =and>>> I intend to turn the PMOS on hard before the H bridge goes on. It's >>> preventing the motor from ramming power backwards through my circuit >>> that I'm trying to avoid. >>=20 >>> If I put a diode in there it'll be the biggest power consumer on the >>> whole board, by a mile. So that's right out. >>=20 >> How high is the BEMF when the motor is turned off? Is there a lot of >> energy stored in the motor due to an inertial load, or is it possible >> that the shaft could be driven at a higher speed to produce a higher >> generated voltage? There will normally be some generated spikes during >> the dead-time of the PWM switching of the H-bridge, which is usually >> dealt with by using snubbers, or even by adding a capacitor across the >> motor. > >There is a possibility that Bubba might grab the output shaft when the=20 >thing is supposed to be off and give it a spin. > >Or that Bubba might turn the output shaft faster enough that the motor=20 >will generate more than the supply. > >I need a method to turn the _supply_ to the H-bridge OFF at _any time_. > >> If you want to eliminate any possibility of reverse power flow, you =can>> turn on both bottom (or both top) MOSFETs in the bridge, for braking >> mode, which will quickly stop the motor by dissipating the energy. As >> long as there is not a large inertial load or a chance of generation =due>> to external drive of the motor shaft (as in an EV rolling downhill), =the>> power should be of reasonable level. > >That won't work when the rest of the circuit is powered down. > >> Another method is to use an N-channel MOSFET and a high side gate >> driver. But that would not be an easy PCB patch. > >That won't work for the same reason that a P-channel won't -- because=20 >power MOSFETs have intrinsic diodes that let the current run "backwards"==20>when the FET is off. > >To reiterate: > >My question is, can I run a MOSFET "backwards", reliably, all day.IIRC back to back power fets is one way of making DC SSRs. ?-)
Reply by ●July 15, 20122012-07-15
On Wed, 11 Jul 2012 18:27:38 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Tim Wescott wrote: >> >> On Wed, 11 Jul 2012 00:49:21 -0400, P E Schoen wrote: >> >> > "Tim Wescott" wrote in message >> > news:-fadnZXyVsu9bmHSnZ2dnUVZ_hudnZ2d@web-ster.com... >> > >> >> No, I didn't mean to have the source at the supply -- that's what I >> >> have now, and it's failing at Job 1, which is to keep from backfeeding >> >> the supply. >> > >> >> Having the thing fail to block the supply when the motor isn't >> >> generating is just fine -- the H bridge is off until I turn it on, and >> >> I intend to turn the PMOS on hard before the H bridge goes on. It's >> >> preventing the motor from ramming power backwards through my circuit >> >> that I'm trying to avoid. >> > >> >> If I put a diode in there it'll be the biggest power consumer on the >> >> whole board, by a mile. So that's right out. >> > >> > How high is the BEMF when the motor is turned off? Is there a lot of >> > energy stored in the motor due to an inertial load, or is it possible >> > that the shaft could be driven at a higher speed to produce a higher >> > generated voltage? There will normally be some generated spikes during >> > the dead-time of the PWM switching of the H-bridge, which is usually >> > dealt with by using snubbers, or even by adding a capacitor across the >> > motor. >> >> There is a possibility that Bubba might grab the output shaft when the >> thing is supposed to be off and give it a spin. >> >> Or that Bubba might turn the output shaft faster enough that the motor >> will generate more than the supply. >> >> I need a method to turn the _supply_ to the H-bridge OFF at _any time_. >> >> > If you want to eliminate any possibility of reverse power flow, you can >> > turn on both bottom (or both top) MOSFETs in the bridge, for braking >> > mode, which will quickly stop the motor by dissipating the energy. As >> > long as there is not a large inertial load or a chance of generation due >> > to external drive of the motor shaft (as in an EV rolling downhill), the >> > power should be of reasonable level. >> >> That won't work when the rest of the circuit is powered down. >> >> > Another method is to use an N-channel MOSFET and a high side gate >> > driver. But that would not be an easy PCB patch. >> >> That won't work for the same reason that a P-channel won't -- because >> power MOSFETs have intrinsic diodes that let the current run "backwards" >> when the FET is off. >> >> To reiterate: >> >> My question is, can I run a MOSFET "backwards", reliably, all day. >> > > >Sure. That's the usual supply polarity reversal protection circuit, >when you can't stand a diode drop: PFET with gate grounded, source to + >supply, drain to the rest of the circuit. > >Cheers > >Phil HobbsI use N-channels, "bass-ackwards", to prevent the body diode from conducting when doing charge-pump gimmicks to generate a negative supply. (Integrated MOSFET's and most discrete's are symmetric in their behavior except for the body diode.) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
