One of the problems you run into with a MOSFET is drain to gate capacitance. So as the gate is discharging (going high), the drain is going low, and the capacitance between gate and drain slows the discharge of the gate by providing some charging current.
That is probably part of why a 20:1 change in resistance only resulted in an 11:1 change in Off switching time.
So it would be reasonable to suggest, other things being equal, that if we are doing PWM, to reduce the frequency, so that these "edge cases" happen less often? Obviously this problem goes away if we are switching infrequently, like turning garden lights on and off.
There is more explanation in the tutorial than I had above, plus a low-side driver (to sink current). If you spot any mistakes please let me know.
@polymorph - I scaled R2 back to 1k, it seemed we were getting diminishing returns on low-value resistors, and that would keep the current drain on Q1 down. The switching time is now about 7.5 µS.
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:
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.
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.
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?
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.
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 somewhereSIOV metal oxide varistors