Hello all!
I am not confident with my understanding of DC inductors and flyback.
What would be the operational and control differences, from the common PWM brushed DC motor power using a FET or BJT with a shunting flyback diode, versus using a high breakdown-voltage device (IGBT?) and no flyback diode needed to protect the device?
Is this a viable control option, and what considerations would there be in operation?
Thanks!
If you use an avalanche rated FET the "flyback diode" is internal. If you use a BJT you will need the diode. Unless you are getting into power a IGBT would not be necessary. The difference in the control you might see is with the FET you get better performance because of the lower voltage drop across the device. The reason for the flyback is when power is applied to an inductor it stores the charge, when the connection is opened the inductor reverses polarity and discarages its energy as fast as it can. The voltage will rise until limited by an external device.
Thanks for your reply, and I followed that fine. My original design for this 10A 24V motor used an N_MOSFET and power diode, but both the shunted potential heat in a small enclosure and the additional large parts invited a re-think, and so...
gilshultz:
The reason for the flyback is …
Understood, and so my question is relative to diode versus no diode. Assuming the control device (FET, BJT, IGBT, etc) is rated for breakdown voltage higher than the flyback voltage and has no body diode, and assuming noise is not a limiting factor; I will re-phrase the questions as:
• Will the control or performance of the motor be improved using a diode versus not using a diode?
Single switching element or H-bridge motor driver.
In a fet H-bridge the 'other' fet takes care of flyback, through it's body diode.
Leo..
Single. Although it wouldn't seem to make a difference to the question?
w427:
• Will the control or performance of the motor be improved using a diode versus not using a diode?
gilshultz:
If you use an avalanche rated FET the "flyback diode" is internal.
New one on me, please elaborate.
w427:
Hello all!I am not confident with my understanding of DC inductors and flyback.
What would be the operational and control differences, from the common PWM brushed DC motor power using a FET or BJT with a shunting flyback diode, versus using a high breakdown-voltage device (IGBT?) and no flyback diode needed to protect the device?
Is this a viable control option, and what considerations would there be in operation?
Thanks!
Use a freewheel diode and you won't shorten the lifetime of the motor by high voltage on the winding insulation,
won't be generating impulsive electrical noise to everything in the vicinity (and interfering with
wireless comms), and you can use cheap stock low-on-resistance MOSFETs (high voltage parts are much
higher on-resistance or exotic and expensive - IGBTs are slower to switch and less efficient for PWM).
Basically by snubbing the high voltage you prevent many problems that you'd otherwise have to solve
in expensive ways (like needing screened cables, higher voltage parts, higher rated wiring in the motor...)
But this is all moot since for motor control you use an H-bridge of MOSFETs which already contain the
4 freewheel diodes as part of their structure for free.
Thanks for the input, and that generally covers my understanding thus far. Assuming then that there are only benefits to using a diode, and no (zero) benefits to omitting the diode for performance or control, I will use one.
As the fan setup I am controlling is unidirectional, my circuit only employs one MOSFET and one flyback diode. Again assuming there are no benefits to using a more expensive and complex H-bridge for this purpose, I should stick with my simple circuit. Alternate theories welcome. 