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'[EE]: Motor flyback diode theory?'
2001\05\01@172722 by Graham North

picon face
Hi,

I know that i need a flyback diode connected across the motor in my PWM
Motor Drive
Circuitry, but i am having a few problems with the theory.

I am writing my final year project report and am struggling for the theory
about the flyback
diode.

If someone knowledgable has a little time to spare, could they please give
me a few lines of
theory to get me started.

Thanks

Graham
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2001\05\01@192510 by Brent Brown

picon face
> Hi,
>
> I know that i need a flyback diode connected across the motor in my PWM
> Motor Drive
> Circuitry, but i am having a few problems with the theory.
>
> I am writing my final year project report and am struggling for the theory
> about the flyback
> diode.
>
> If someone knowledgable has a little time to spare, could they please give
> me a few lines of
> theory to get me started.
>
> Thanks
>
> Graham

Hi Graham,

Might be a bit long-winded, but I look at it this way. A DC motor
armature has some inductance. Inductance has the characteristic of
trying to maintain a current flowing, that is if you try to change the
current flowing in an inductor then the inductor produces a voltage
to oppose this change.

The energy required to do this is stored in the magnetic field of the
inductor. The worst thing (or should that be most interesting thing)
you can do to an inductor is try to interrupt a DC current flowing in
an inductor by opening the circuit. The magnetic field releases it's
stored energy to try and maintain the current flow, and as the circuit
resistance is now very high (open circuit) the voltage produced is
very high - check out Ohms law.

Important to remember: This voltage is called a "back emf" because
it is now in the reverse direction, but the current flow is in the SAME
direction. Check out the diagrams in your text books to make sure
this makes sense.

Now consider the motor armature, for this purpose, as just an
inductor. Flyback diode installed. In the "ON" phase of the PWM
cycle the inductor "charges up" it's magnetic field and current rises
following the typical charge curve. Flyback diode is reverse biased
and has no effect. Now when the "OFF" phase of the PWM cycle
comes along the inductor voltage is reversed, the instantaneous
current that was flowing previously MUST continue to flow, and it
does so through the now forward biased flyback diode. The current
decays according to the losses in this loop, namely the resistance of
the inductor and the voltage drop of the diode.

Without the diode:- the inductance dumps it's energy by producing
a large back emf that breaks something down somewhere and
dissipates the energy. Just like the sparks you see on the brushes/
commutator in a DC motor.

In a typical DC motor the inductance of the armature is much lower
than that of the field winding (if it has a field winding). With PWM
control the armature current is basically DC with what looks like a
small triangle wave superimposed. This is the charge/discharge of
the inductance during each PWM cycle.

The bakc emf diode must be rated at the same current as the
switching transistors. It needs to be fast, depends on the PWM
frequency, to minimise switching losses. As discussed on the PIClist
at other times, typical power diodes like 1N4007 devices are not
suitable. Shottky power diodes are good because they are fast and
have low voltage drop for lower power dissipation. To complement
high current MOSFETs suitable diodes are hard to find, but a cheap
option is to use another MOSFET just for it's inherent drain-source
diode. Many MOSFET data sheets characterise the performance of
this diode, need to check for specific part, but generally they are
fast and match the current ratings of the MOSFET itself.

Oops, did you say a few lines...

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  .....brent.brownKILLspamspam@spam@clear.net.nz

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2001\05\01@193047 by Alexandre Domingos F. Souza

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>have low voltage drop for lower power dissipation. To complement
>high current MOSFETs suitable diodes are hard to find, but a cheap
>option is to use another MOSFET just for it's inherent drain-source

       Or use old PC-Supply Schottky diodes, they are virtually free :o)

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2001\05\01@195730 by Brent Brown

picon face
> >have low voltage drop for lower power dissipation. To complement
> >high current MOSFETs suitable diodes are hard to find, but a cheap
> >option is to use another MOSFET just for it's inherent drain-source
>
>         Or use old PC-Supply Schottky diodes, they are virtually free :o)
>

Hey, good idea. What current rating would they be? 20A or so? My
experience is with 75A MOSFET's in TO-220 package, and they
worked out pretty cheap as far as 75A diodes go.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  EraseMEbrent.brownspam_OUTspamTakeThisOuTclear.net.nz

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2001\05\01@200524 by Alexandre Domingos F. Souza

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>>         Or use old PC-Supply Schottky diodes, they are virtually free :o)
>Hey, good idea. What current rating would they be? 20A or so? My
>experience is with 75A MOSFET's in TO-220 package, and they
>worked out pretty cheap as far as 75A diodes go.

       Around 30A in the 300W PSUs, enough for lot's of uses! Get the ones in the 5V rail

       BTW...I sent some messages today, and nothing of them around here...Internet is so slow today...

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2001\05\02@031425 by Roman Black

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face
Graham North wrote:
>
> Hi,
>
> I know that i need a flyback diode connected across the motor in my PWM
> Motor Drive
> Circuitry, but i am having a few problems with the theory.
>
> I am writing my final year project report and am struggling for the theory
> about the flyback
> diode.
>
> If someone knowledgable has a little time to spare, could they please give
> me a few lines of
> theory to get me started.
>
> Thanks
>
> Graham


Search the net for "Jones on steppers", it has
a lot of theory on flyback diodes and current
recirculation, although written for stepper
motors it has all the diode theory and pwm
current-limiting theory you will need.
:o)
-Roman

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2001\05\02@090552 by Olin Lathrop

face picon face
> I know that i need a flyback diode connected across the motor in my PWM
> Motor Drive
> Circuitry, but i am having a few problems with the theory.
>
> I am writing my final year project report and am struggling for the theory
> about the flyback
> diode.

This is definitely NOT how the physics works, but think of an inductor as
providing inertia for the current.  The higher the inductor, the higher the
inertia.  Trying to suddenly stop the current thru an inductor is like
trying to suddenly stop an inertial mass.  The mass will produce a large
spike in force (voltage).

Electronic switches like transistors are capable of shutting off current
very fast.  This can lead to very large voltage spikes.  Ten to fifty times
the supply voltage can be easily achieved without deliberately attempting to
do so.  These large voltage spikes can damage the electronics.  The flyback
diode provides a path for the "inertial" current from the inductor to go
without causing the voltage to go very high.

Go read about inductors and understand V = L * dI/dt.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, KILLspamolinKILLspamspamembedinc.com, http://www.embedinc.com

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2001\05\02@100312 by 1?Q?Alexandre_N._Guimar=E3es?=

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face
Hi,

> The bakc emf diode must be rated at the same current as the
> switching transistors. It needs to be fast, depends on the PWM


   Great explanation, Brent. One of the best I have seem on the subject.

   Could you clear another detail for me ? How does a snubber network
performs on this situation ?? Using 50 amps diodes is not very nice for big
loads. Could a snubber network be used as it would seem to me that it would
be cheaper at those high amps applications ?

Best regards,
Alexandre Guimaraes
RemoveMEalexgTakeThisOuTspamiis.com.br

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2001\05\03@230646 by Charlie Davis

picon face
Also reduces RF hash that can desensitize
an RC receiver. Shunt capacitance is also
used for the same reason.

Charlie

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