Electrical Bias of Disk Assembly for Preventing Magneto-resistive Sensor Damage
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2005-Apr-04
Klaassen, KB: AUTHOR [+2]
A method is disclosed to prevent damage to magneto-resistive (MR) readback transducers in hard disk drives due to electrical breakdown or erosion of the sensors.
Electrical Bias of Disk Assembly for Preventing
A method is
disclosed to prevent damage to
magneto-resistive (MR) readback transducers in hard disk drives due
to electrical breakdown or erosion of the sensors.
medium in a hard disk drive is supported by a
conductive (Al) substrate and, in case of a thin-film disk, has a
conductive magnetic film. The thin overcoat may also be conductive.
If, in such a drive, a magneto-resistive (MR) readback transducer is
used (whose sensor element is exposed to the airbearing surface of
the slider) electrical breakdown, erosion or degradation of the
sensor element may occur if the MR sensor is not biased at the same
electrical potential as the disk substrate. Such damage can ensue if
electrical contact between the sensor (and its exposed connection
leads) is occurring due to, for instance, a conductive third body
wedged between the slider and the disk, a conductive asperity on the
disk surface, or a worn section of an otherwise non-conductive
overcoat. If the electrical potential difference between sensor and
disk is large enough, dielectric breakdown of the thin,
non-conductive overcoat can also cause sensor damage for close, near
contact, slider disk spacings. Depending on the resulting current
spike, the damage can be electro-erosion of a chunk out of the bottom
of the MR sensor. If, in this way, the bottom edge of the sensor is
eroded away, a recessed sensor results which gives a lower signal
amplitude. If the current spike is large enough the entire sensor
may be destroyed.
hard disk drives the disk assembly is held at
ground potential (i.e., the same potential as the ground return lead
of the system power supply). The MR sensor is connected to an input
amplifier which biases the sensor at 0 V center sensor potential.
This is made possible by giving the input amplifier (which is
contained in the Arm Electronics Module) a dual power supply. Such a
power supply delivers to the module at least two leads (usually
three: ground 0 V is also supplied), one carrying a positive power
supply voltage w.r.t. ground and one carrying a negative supply
voltage. This makes it possible in the input stage of the MR
amplifier to electronically maintain the center of the MR sensor to
interface for a low-end drive, however, only
contains 0 V, 5 V and 12 V. Now, the input stage of an MR
readback amplifier cannot be designed such that the center of the
sensor is at 0 V, since this would require one half of the sensor to
be biased below the lowest supply voltage (0 V). In addition, there
is not enough "head room" for the active devices (transistors) for
current-biasing the sensor and for reading out the sensor's signal
voltage. These devices would all be "bottoming out" against the 0 V
line without collector-base voltage head room.
To solve this problem, we propose the followi...