Comments requested on MOSFET high-side driver

[Coding Badly]

How is the MOSFET temperature with the new resistors?

[dlloyd]

I use MOVs when switching transformers and solenoids and believe you could get much greater speed improvement with your MOSFET high-side motor driver. The speed improvement comes from clamping higher than the 0.7V of a diode. I haven't used MOVs across a motor, but I wonder what speed improvements could be had with an MOV rated for 16V DC such as this:
http://www.digikey.com/product-detail/en/B72210S1140K551/495-6514-1-ND/4931646

Also check this post:
http://forum.arduino.cc/index.php?topic=294268.msg2064128#msg2064128

[polymorph]

Good trade-off.

Yes, if the application is fine with a slower PWM, that saves switching losses and the other losses associated with it like driver current losses.

The 1N4001 is not a fast switching diode. It is a garden variety line frequency rectifier, but it switches On fast enough to absorb most inductive transients. What dlloyd is referring to is more of an issue with the speed of magnetic field collapse in a coil. This is an issue with relays as a low clamp voltage can cause contact burning as it slows release. In that case, adding a resistor in series with the clamping diode can speed that up.

For a motor, it is going to generate a voltage as it continues to turn, but it will be in the same polarity as the voltage applied and not spike up any higher. Well, except a motor is also made of inductors, so I'd still keep the diode.

For your MOSFET power calculations, I'd say 2^2 rather than 4. It is clearer, then, that the number is 2A squared.

I try, as a general rule, to keep device temperatures under 75C when possible. 150C may be a maximum, but by mentioning, you may be unintentionally giving people the impression that it doesn't really have any drawbacks.

[nickgammon]

How is the MOSFET temperature with the new resistors?

Well, it still got up to around 32 °C, which I am not that impressed with, but I think my test conditions are not helping. With a fixed current from the lab supply it is dropping the voltage to around 3.6V which then means I think it is not properly switching on.

I think I'll retry with a lower current, thus allowing the voltage to rise.

For your MOSFET power calculations, I'd say 2^2 rather than 4. It is clearer, then, that the number is 2A squared.

Fixed.

[nickgammon]

Ach, make that a higher current, allowing the voltage to rise. But then the higher current heats the thing up more.

It's tricky setting up good test conditions.

[dlloyd]

Missed the fact that the traces were generated using a 2 ohm resistive load ... would be different with inductance included.

[nickgammon]

Ah, I thought that was a general recommendation. I haven't used MOVs before, would you just put it across the load like the diode? And what specs would apply in this case?

[dlloyd]

Yes, directly across the load is best. I've used them successfully for many applications - mainly transformers in AC circuits. Since a transformer and motor are closely related ... hence my suggestion. For sizing, the first spec. I look at is the working voltage. In a 120VAC circuit, I use MOVs rated for 150VAC continuous, the clamping voltage is higher. Then I look at the surge current rating and energy rating.

If the 12V supply is regulated, then any MOV with a continuous DC voltage a few volts higher than this would be OK. They're even simpler to use than a diode because polarity doesn't matter.

[nickgammon]

So as a working recommendation, the one you mentioned (B72210S1140K551) with:

Varistor Voltage (Min) 19.8V
Varistor Voltage (Typ) 22V
Varistor Voltage (Max) 24.2V
Current - Surge 500A

... would probably be OK for a 12V motor?

But what does this mean?

Maximum DC Volts 16VDC

Is that basically saying that it is basically open circuit up to 16V but then conducts (up to 500A) starting at 19.8V?

[dlloyd]

Yes, that's how I understand it ... it works like back to back 16V zener diodes with a bit of series resistance. There are some detailed graphs somewhere SIOV metal oxide varistors

[larryd]

back to back zener diodes

See Transorbs:
http://en.wikipedia.org/wiki/Transient-voltage-suppression_diode

http://en.wikipedia.org/wiki/Transient_voltage_suppressor

[JoeN]

Aren't the tradeoffs being discussed exactly why many manufacturers offer MOSFET gate driver ICs?  Maybe something to add at some point is how performance characteristics change when using one of those, though choosing one that is representative of driver ICs as a whole might be difficult.

[zoomkat]

Would a lower value for R2 help here? Any suggested values?

Have you tried using a pot to vary the resistance to see if you can find a "sweet spot" for the resistance?

Also have you tried reducing the value of R1 some?

[polymorph]

This might be a bit confusing now, zoomkat, as he's modified the schematic to reflect changes made. Originally, both R1 and R2 were 10k.

[nickgammon]

See Transorbs:
http://en.wikipedia.org/wiki/Transient-voltage-suppression_diode

http://en.wikipedia.org/wiki/Transient_voltage_suppressor

Well, the interesting thing to come out of this discussion has been the transient suppression. Like many people I have read all over the place to "put a diode over an inductive load" but now there are lots of suggestions about types of suitable diodes, MOVs and now Transient-voltage-suppression diodes.

http://en.wikipedia.org/wiki/Flyback_diode
http://en.wikipedia.org/wiki/Snubber
http://zone.ni.com/reference/en-XX/help/375472A-01/switch/inductive_load/
http://electronics.stackexchange.com/questions/31014/where-should-i-put-the-kickback-diode-in-a-transistor-switch

From the above page:

On a side note: 1N4001 is a bit slow for this application. I usually see 1N4148

I initially had a 1N4148 however:

I would also use a 1N4001 diode, that one is just a signal diode.

So it seems that agreement on the diode type is hard to reach.



Other pages mention how useful the diode is without specifying how to choose one:

http://www.sealevel.com/support/article/AA-00470/0/How-to-Switch-Highly-Inductive-Loads-Using-Digital-I-O.html