Page 83 - CNC Robotics

CNC Robotics

APPUCATION NOTE
stability of the operating frequency discussed to the speed of the switching device and main-
above can occur. At this point the constant fre- tains a VF that limits the peak voltage within the
quency chopping technique becomes impractical allowable limits. When the diodes are not inte-
to implement and a chopping technique that uses grated they must be provided externally. The di-
constant off time frequency modulation like imple- odes should have switching characteristics that
mented in the PBL3717A, TEA3717. TEA3718, are the same or better than the switching time of
and L6219 is more useful. the output transistors. Usually diodes that have a
reverse recovery time of less than 150 ns are suf-
ficient when used with bipolar output devices like
Why Won 't the motor move the L298N, The 1N4001 series of devices,for ex-
Many first time users of chopping control drives ample, Is not a good selection because it is a
first find that the motor does not move when the slow diode.
circuit Is enabled . Simply put the motor is not gen- Although it occurs less frequently, excess current
erating sufficient torque to turn. Provided that the can also destroy the device. In most applications
motor is capable of producing the required torque the excess current is the result of short circuits in
at the set speed, the problem usually lies in the the load. If the application is pron to have shorted
current control circuit. As discussed in the pre- loads the designer may consider implementing
vious section the current sensing circuit can be some external short circuit protection (7].
fooled. In extreme cases the noise is so large that
the actual current through the motor is essentially Shoot through current, the current that flows from
zero and the motor is producing no torque. An- supply to ground due to the simLJtaneous conduc-
other symptom of this is that the current being tion of upper and lower transistors in the bridge
drawn from the power supply is very low. output. is anoth er concern. The design of the
L298N, l293 and L6202 all include circuitry spe-
cifica lly to prevent this phenomena. The user
should not mistake the reverse recovery current
Avoid Destroying the Driver of the diodes or the parasitic structures in the out-
Many users have first ask why the device failed in put stage as shoot through curren t.
the application. In almost every case the failure
was caused by electrical overstress to the device.
specifically voltages or currents that are outside
of the device rating s. Whenever a driver fails, a SELECTED REFERENCES
careful evaluation of the operating conditions in IllSax, Herbert., "Stepper Motor Driving" (AN235)
the application is in order. [2]"Constant Current Chopper Drive Ups Stepper-
The most common failure encountered is the re- Motor Performance" (AN468)
sult of voltage transients generated by the induc- [3]Hopkins, Thomas. "Unsing the L6506 for Cur-
tance in the motor. A correctly designed applica- rent Control of Stepping Motors" (AN469)
tion will keep the peak voltage on the power
supply, across the collector to emitter of the out- [4]"The l297 Steper Motor Controller" (AN470)
put devices and, for monolithic drivers, from one [5]Leenouts,Albert. The Art and practjce of Step
output to the other within the maximum rating of Motor Control. Ventura CA: Intertec Communica-
the device. A proper design includes power sup- tions Inc. (805) 658-09 33. 1987
ply filtering and clamp diodes and/o r snubber net- [6]Hopkins. Thomas. "Controlling Voltage Tran-
works on the output [6]. sisnts in Full Bridge Drivers" (AN280)
Selecting the correct clamp diodes for the appli- [7]Scrocchi G. and Fusaroli G. "Short Circuit Pro-
cation is essential. The proper diode is matc hed taction on L6203". (AN279)

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