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Brushed DC motor: Resistor in series with freewheeling diode

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langwadt:

--- Quote from: Nominal Animal on August 22, 2021, 01:05:35 am ---Right.  I've used MOSFETs as switches, and played with ideal diode models, so I should have realized.

I took the time to look at a few circuit diagrams of full step stepper drivers, and work the conventional current flow in each phase, to make sure I understand the situation. I really should have looked at the functional block diagram in the A4988 datasheet first.  The datasheet even mentions that at 1.5A current, the body diode voltage drop is max. 1.2V.

So, I definitely now think it's the lower voltage drop compared to the intrinsic diodes, and the reduced thermal stress, that explains why these stepsticks tend to fare better (fewer issues and breakdowns) with those extra Schottky diodes.  The datasheet mentions the package thermal resistance is 32°C/W on a four-layer PCB (these ICs are cooled via the bottom, with vias), with PD=4.0W at 20°C ambient temperature; they cannot really dissipate much power at all.

--- End quote ---

I exepect the chip to turn on the FETs so the instrinsic diodes are only during the transition, so the voltage will beRdson * I 

T3sl4co1l:
The winding inductance is often used as the filter choke in a buck converter; depending on how the controller deals with this, it could be that it's causing body diode conduction from time to time, in which case schottky diodes could help.  This can be prevented by working in forced continuous current mode (one transistor always on).

I haven't checked if these controllers typically use this mode or what.  I do know older stepper drivers for example, tend to use a single switch, and catch diode, to implement the buck control as a hysteresis current limiter (e.g. L297+L298) -- as a result, the winding is simply open circuit when it's not driven, so there's some advantage to be had with schottky.  (But also such old drivers as L298 have relatively huge voltage drops too, making schottkys a pretty obvious win. :P )

Tim

langwadt:

--- Quote from: T3sl4co1l on August 22, 2021, 05:16:53 am ---The winding inductance is often used as the filter choke in a buck converter; depending on how the controller deals with this, it could be that it's causing body diode conduction from time to time, in which case schottky diodes could help.  This can be prevented by working in forced continuous current mode (one transistor always on).

--- End quote ---

from the datasheet they either reverse the output of the hbridge or short the coil using the lower transistors, for either fast or slow decay



--- Quote from: T3sl4co1l on August 22, 2021, 05:16:53 am ---I haven't checked if these controllers typically use this mode or what.  I do know older stepper drivers for example, tend to use a single switch, and catch diode, to implement the buck control as a hysteresis current limiter (e.g. L297+L298) -- as a result, the winding is simply open circuit when it's not driven, so there's some advantage to be had with schottky.  (But also such old drivers as L298 have relatively huge voltage drops too, making schottkys a pretty obvious win. :P )

--- End quote ---

afaikt from the datasheet they are bipolars so no diodes, and in both cases even in forced conduction there will be some dead time where the diodes will conduct

Ernest1:
I have a  drill here that has a diode in parallel with motor through a switch on the trigger. When the motor trigger is not pressed the switch is closed with the diode. when the trigger is pressed the diode is disconnected and the motor is being driven by pwm. When you let go of the trigger, the switch is connected while the motor is running and the diode motor brakes.

The original diode was shorted (fx2000g) or equivalent (DST2045AX). I placed a much smaller 1n5404 instead because that's the biggest I have and it stops the motor instantly but I'm not sure about longevity. Would it help to add a big resistor (wattage) in series with it to reduce the stress on the diode while keeping some braking?

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