> 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.brown@clear.net.nz

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