MOSFET Switch Design.

[LocoYokoPoco]

This is for all you electronics savvy people.

I'm in the process of designing my own "active braking" MOSFET switch and I'm a bit curious after seeing a few schematics on the web.

Namely this one:
http://fireworksairsoft.webs.com/BETA%20SCHM.JPG

OR my clarified version:

http://i.imgur.com/1bBGN.png
(Sorry, that +V on the trigger side should be ground instead)

I have a few questions about this design.

1) Why use a TVS Diode? I really don't see the point of it, especially where it is placed. Correct me if I'm wrong, but all that diode does is clip the voltage to 24V going back to the switch? Pointless, no?
EDIT: Hmm I could see a use for this diode after all... it's probably there to prevent a voltage spike across the gate and source that exceeds the VGS threshold. Can someone confirm that?

2) I know that MOSFETs have a built in "diode" across the drain and source, but they often have slow recovery times. Therefore, wouldn't it be a good idea to place a Shottkey diode across the MOSFETs anyways?

3) Wouldn't a flyback diode across the motor be a good idea? Why is it absent in that schematic?

Any input would be appreciated.

[XZIVR]

Hmm, regarding the flyback diode, doesn't the mosfet brake by just shorting the positive and negative wires on the motor? That's how it works in rc cars anyway. I would think then that a flyback diode would be redundant in this case, but maybe I dont properly understand its function?

[LocoYokoPoco]

Originally Posted by XZIVR

Hmm, regarding the flyback diode, doesn't the mosfet brake by just shorting the positive and negative wires on the motor? That's how it works in rc cars anyway. I would think then that a flyback diode would be redundant in this case, but maybe I dont properly understand its function?

Yes, the top MOSFET would short out the motor to brake it, but I think having a flyback diode across the motor / MOSFET drain and source would still be a good idea. The flyback diode prevents the inductive voltage spike from damaging the MOSFETs by clamping the voltage to the Vf (forward bias voltage) of the diode. Though as you said, I'm not sure if it is necessary in this case. BUT, without it, the braking mosfet would be taking in some big voltage spikes, no?

Can someone clarify?

[XZIVR]

Well, I use a couple of triggermasters and dont recall seeing that on their schematic. So far I havent cooked them. I believe I once tried this though, using a big rectifier diode and performance wise there was no precievable difference.

[LocoYokoPoco]

Originally Posted by XZIVR

Well, I use a couple of triggermasters and dont recall seeing that on their schematic. So far I havent cooked them. I believe I once tried this though, using a big rectifier diode and performance wise there was no precievable difference.

Yea, there shouldn't be any performance difference. However, with a flyback diode, it should improved reliability.

Speaking about Triggermaster, I might make an ARM Micro controller based programmable motor driver with on the fly FPS and RPS measurements as well as round counter. Maybe after I finish with school though.

EDIT: After doing some more research, the TVS is actually completely useless, unless you're switching the MOSFETs via a micro controller or a really crappy switch. All the TVS does is prevent inductive voltage spike from reaching the switch.

[LocoYokoPoco]

Successfully bread boarded/prototyped my MOSFET design:

http://www.youtube.com/watch?v=KfGpn...9U0NOHL-SmGpa5

Now let's see if I can fit all of this inside a V2 gearbox (doubtful).

[Wetmelon]

i think the TVS' just in there "just in case". As it's named... it suppresses transient voltages. We're talking probably up to 400V. I would be really surprised if you needed it in that schematic, but it's never a bad idea to add one

And P.S. technically it's not a flyback diode. A flyback diode is for draining reverse current induced by an inductor's magnetic field collapsing. TVS's are for limiting voltage to avoid.... overvoltage :P

[ruinz00]

I ran my TM M14 this year with the original design. I really didn't see a big difference with and without it. I did have to jump up to a 9.6v from a 8.4v battery.

[LocoYokoPoco]

Originally Posted by Wetmelon

i think the TVS' just in there "just in case". As it's named... it suppresses transient voltages. We're talking probably up to 400V. I would be really surprised if you needed it in that schematic, but it's never a bad idea to add one

And P.S. technically it's not a flyback diode. A flyback diode is for draining reverse current induced by an inductor's magnetic field collapsing. TVS's are for limiting voltage to avoid.... overvoltage :P

Again, the TVS appears to be there to protect whatever is behind the driving circuit. So in most cases, it would be your switch/trigger contacts. However, the trigger contacts are more than able to handle this voltage. It is only when you interface to the driver circuit with maybe TTL, CMOS or a micro controller, you want to have the TVS. Even then, I would use a Zener diode instead of a TVS.

I didn't say the TVS was a flyback diode .
And technically speaking, the flyback diode clamps voltage spikes due to the transient nature of an inductor. This is because when current to an inductor goes from a large amount to suddenly stopping, it causes a big dI/dt, which produces a large voltage.

[johnpcunningham]

The TVS is bidirectional to protect against voltage transients going to the GATE of the MOSFET. It protects against positive and negative transients, where as a diode would only work in one direction based on where it is placed.

My experience shows fly back diodes going from the drain to the source are necessary since the EMF of the motor will produce a very high and negative voltages at the drain of the N-channel FET. When the voltage goes negative at the Drain, it forward biases the internal parasitic diode in the MOSFET and it will generally blow.

For these guns, something in the 1Amp range will work. Cathode goes to the Drain, Anode goes to the Source.

[KenTsui]

Would removing the diodes from the above design affect the function of the mosfet?