Nope, Sir, it is not. I agree that inside the motor there is this rotating packet of coils. It is each coil of that packet that adds some inductance. So a motor has got some inductive behavior and, of course, some path for the current needs to get provided. With a simple one transistor stage on an inductive load (like for a solenoid or an unidirectional motor drive or one coil of a unipolar stepper or ...) you find always this inverse diode across the load. It is this diode that takes the inductive current when the magnetic energy comes back into the circuitry. With MOSFET H-bridge (!) motor drivers you are right, the body diodes need to take the current and dissipate some heat. With bipolar transistor bridges diodes are mandatory. Some higher power MOSFET designs also apply external Schottky-diodes to keep the power dissipation away from the body diodes. The L293D, being bipolar at its heart, already has the diodes integrated, exactly for this purpose.MasterOfGizmo hat geschrieben: ↑30 Apr 2021, 16:04All your colorful explanations miss the fact that the motor is an inductor. Switching its power supply between different voltage levels doesn't mean you are switching the motor simply on and off. Instead you are dealing with the residual energy stored in the coils of your motor.
There is some literature around (and also it is mentioned in the data sheets), that a shortage across the inductive load will also recirculate the current and it will do this with way less energy loss since the residual drop voltage drop is lower than that of a diode. And I agree to what is written there in the data sheet.
BUT:
- While a capacitor stores its energy by an electric field and thus a voltage, an inductivity stores its energy by a magnetic field and thus a current flow. With a capacitor the resistance between its terminals needs to be as high as possible, the lower this resistance, the more energy gets lost. With an inductivity it is quite the opposite and the resistance shall be as low as possible in the whole circuit to keep the current running.
- While the recirculation current is decaying it still generates some torque in case it is a motor (even with a stepper).
- With steppers you want a very long decay (lowest resistance losses) while the coil shall be powered. In case the next step is commutated this particular coil needs to get free of current as soon as possible and for this you want the shortest possible decay, highest resistance.
- There are special circuits that do not use a standard diode for recirculation but some zener with higher voltage. This indeed shortens the current decay way further. Well, this is used typically with unipolar drivers, not H-bridges.
- The DRV8833 is primarily intended as a stepper motor driver. Stepper motors designed for chopper-drivers usually have very low resistance and some inductivity. A chopper driver typically takes several kHz of frequency to controll the current draw through the stepper winding (hence the DRV8833 sense resistor capability).
- When the recirculation current died out you still have the EMF generated. Now, at this moment the motor will start to work as a generator, the current direction is opposite to its "motor state" and the electric energy produced comes from the movement.
And now consider the max. 75 us (exact value depends on motor data and circuitry choosen!) versus the 4 ms period when using SW generated PWM drive. Most of the "PWM off" time the motor is working as a generator and thus actively braking instead of idling until it receives the next kick of energy. Okay, the exact ratio also depends on the PWM ratio. To get a PWM H-bridge driver somewhat conserve some energy it has to be controlled by a PWM of several kHz at least. And then we have got lots of ohms due to the motor winding that just disspates away the electric energy as heat. Please take a look into switched regulator designs (namely the "buck" kind) to understand that resistance in the current storage path needs to be as low as possible if you want some noticeable efficiency.
There is one more thing to tell. Please consider the L293D internal setup applying the inverse diodes. When this kind of bridge is turned off, the motor current can idle through the diodes into the power supply. This is lots of voltage against the current must be driven and this gives a pretty fast decay. Please compare this with the respective pictures in the DRV8833 spec. Again, the DRV8833 is targeting primarily bipolar stepper applications and it could also serve DC motors. But you need to understand the differences.
Maybe you now understand that the "slow decay" mode is wrong when a brushed DC motors is PWMed for a certain reduced speed, the PWM commands "apply power / idle". But it could be used when you want to brake the thing to a stop - but please, here the PWM commands "short out / idle".
Oh, by the way: Some years ago I put an 8 Ohm speaker (which undoubtedly also has some winding and such an inductivity inside) as load resistor on a BC547 without any diodes. The circuitry you find across the web and in lots of (semi-)professional literature. No inverse diode and no way to "keep the current running". It is a real "short decay" mode exploited here. And still it works well when I use it today. Strange, isn't it?
Regards
H.A.R.R.Y.
Section 7.3.2, particularly Table 2, of the datasheet has all the info needed on PWMing the driver. I fear my knowledge doesn't go much further than that, Harry and MoG are obviously much more qualified for the details. I see that I'll have to revisit the question of PWM frequency and when to use fast and slow decay, too, when I'll switch to that driver, as planned.
I just tried another one and it works fine, so the first DRV8833 module died on me. I must have shorted some pin while breadboarding, I can't say.