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Browse Prior Art Database

Slew Rate Limiting to Prevent Flyback Current

IP.com Disclosure Number: IPCOM000108950D
Original Publication Date: 1992-Jul-01
Included in the Prior Art Database: 2005-Mar-23
Document File: 3 page(s) / 69K

Publishing Venue

IBM

Related People

Anderson, TB: AUTHOR [+4]

Abstract

In a current driver used to drive a voice coil motor in a direct access storage device (DASD), flyback current due to the inductive load can be driven back into the supply causing poor voltage regulation. With a digital control system, a combination of current step size limiting with microcode and a switchable analog filter can be used to to prevent flyback current.

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Slew Rate Limiting to Prevent Flyback Current

       In a current driver used to drive a voice coil motor in
a direct access storage device (DASD), flyback current due to the
inductive load can be driven back into the supply causing poor
voltage regulation.  With a digital control system, a combination of
current step size limiting with microcode and a switchable analog
filter can be used to to prevent flyback current.

      Current drivers for voice coil motors usually include diodes to
limit the voltage across the coil when the current direction is
reversed. These diodes are either discrete diodes as used with
bipolar drivers or the intrinsic diodes of a power MOSFET.  With
current flowing through the coil in one direction, if the polarity of
the driver is reversed, current keeps flowing in the same direction
through the coil but, instead of drawing current out of the supply,
current is sourced back to the supply through the protection diodes.
An example of this occurs when a voice coil actuator switches from
accelerate to decelerate during a seek.

      To prevent the flyback current through the protection diodes,
it is necessary to bring the coil current to zero before reversing
the polarity of the driver output stage.  This can be done in a
digital control system by including microcode to limit the maximum
rate of change of input to the current driver to something less than
what would produce flyback current.  An example of code to do this is
shown i...