Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1999-09-24
2002-09-03
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S067000
Reexamination Certificate
active
06445530
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of electrical control and signal processing devices, and more particularly, but not by way of limitation, to a method and apparatus for supplying controlled, bi-directional current to a device with minimal distortion at low currents.
BACKGROUND OF THE INVENTION
An H-bridge is a well known circuit construction that allows the application of bi-directional current to a load device, such as a motor that rotates in opposite directions or a coil that drives an audio speaker.
An H-bridge uses four switches, such as transistors, arranged in an “H” configuration. Each of the parallel “vertical portions” of the H-bridge is formed by serially connecting two of the switches; the load device is connected in series between the two vertical portions to form the “horizontal cross-member” of the H-bridge. For reference, this configuration is generally shown in prior art FIG.
3
. The top of the bridge is connected to a voltage source and the bottom of the bridge is usually connected to ground.
Current is passed through the load device in alternate directions by controlling the conductivity of opposite pairs of the switches. Generally, only one of the two switches in each of the vertical portions will be “on” at a time to prevent passing the drive current straight down the vertical portions, bypassing the load device and damaging the switches (since at maximum load conditions the drive current can have a substantial magnitude).
A classification system has generally been adopted by the electronics industry to describe the driving characteristics of various types of driver circuits. As will be generally recognized, a “Class B” device provides bi-directional current to a load device, but suffers from a small amount of distortion at low current levels as the current transitions from a first polarity to the opposite polarity.-ease
For example, an H-bridge that uses transistors, such as metal oxide semiconductor field effect transistors (MOSFETs), as the four switches and turns the non-conducting pair of transistors completely off, would generally behave as a Class B device, since a reversal in the polarity of the load current would require that the conducting pair of transistors be transitioned from a conducting to a non-conducting state and the nonconducting pair of transistors be transitioned from a nonconducting to a conducting state. Transistors (even fast MOSFETs) cannot change state instantaneously, and even in highly controlled manufacturing environments a population of discrete transistors will typically have transition characteristics that vary from part to part. Hence, this “transition delay” will result in noise in the applied current to the load device at very low levels of load current and at current polarity transitions.
A “Class AB” device, on the other hand, generally is described as a device that does not introduce such noise at low currents and at current polarity transitions. One common way to provide a Class AB H-bridge is to provide a small amount of quiescent current to the transistors sufficient to keep the nonconducting transistors “on” (at the lowest portions of the active region), but not sufficient to pass substantial amounts of current through the transistors. For example, an n-channel MOSFET conducts current from source to drain in relation to the magnitude of voltage impressed on a gate of the device. Thus, the gate voltage can be clamped to a low, non-zero value so that the device always at least conducts the quiescent current (and remains “operationally warm” as long as the circuit is active). A Class AB H-bridge can also be constructed using bi-polar transistors by supplying a small amount of current to the base of each transistor, but the power consumption (and heat dissipation) characteristics of such devices has led to a general migration toward the more efficient MOSFET technology for high performance applications.
While the passing of quiescent current through nonconducting transistors in an H-bridge has been successfully implemented, a pervasive problem faced by the industry is determining the appropriate voltage input necessary to obtain the desired current. Significant amounts of variation in gate-source voltage (V
GS
) versus source-drain current (I
D
) exist from part to part (and even over different temperature conditions), so an input voltage at the gate of one MOSFET in an H-bridge may be sufficient to establish the necessary quiescent conditions for that particular MOSFET, but insufficient for another MOSFET in the same H-bridge.
One prior art approach to resolving this condition is to fabricate all four MOSFETs of the H-bridge at the same time on the same semiconductor die in a lateral configuration. This will generally provide all of the MOSFETs with generally the same electrical response characteristics. Since semiconductor devices have response characteristics that are proportional to area., a fifth, proportionally smaller MOSFET can additionally be provided on the die. During operation of the H-bridge, circuitry can be used to adaptively determine a voltage sufficient to maintain the fifth MOSFET in the quiescent state and this voltage is then used on the remaining four MOSFETs in the H-bridge.
While operable, one significant drawback is the cost of such a circuit, since lateral MOSFET construction uses a relatively large amount of die area, and this can be significant especially in high current capacity MOSFETs. For this reason, semiconductor manufacturers are largely migrating away from lateral construction in favor of vertical construction, which is more efficient and generally provides more uniform conduction characteristics over a wider operational range. A vertical MOSFET generally comprises an array of very small transistor cells that are connected in parallel, with one laterally extending surface of the die (the “top”) metallized to form the source and the opposing side (the “bottom”) metallized to form the drain. The use of vertical MOSFETs, however, removes the ability to easily implement the “fifth” MOSFET approach, due to interconnection constraints.
There is a need, therefore, for an improved approach to providing an H-bridge driver circuit that exhibits Class AB characteristics that can be implemented in an inexpensive and efficient manner, without the drawbacks of the prior art. It is to such improvements that the present invention is directed.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and method for supplying load current to a load device.
In accordance with preferred embodiments, a disc drive comprises an actuator which supports a head adjacent a rotatable disc. A coil of the actuator is immersed in the magnetic field of a permanent magnet of a voice coil motor (VCM).
A servo circuit applies bi-directional current through the coil to rotate the actuator, and comprises four current sensing metal oxide semiconductor field effect transistors (SenseFETs) which are operably configured to form an H-bridge with the coil. Each transistor includes separately insulated source, drain, gate and sense terminals, with a source to drain conductivity determined in relation to a voltage applied to the gate terminal and a sense current from the sense terminal determined in relation to a magnitude of source to drain current. The transistors are preferably of vertical construction.
A driver circuit applies drive voltages to the gate terminals of alternating pairs of the transistors to pass a desired load current through the coil, with the drive voltages placing a selected pair of the transistors in a conductive state (to direct the load current through the coil) and the remaining pair of the transistors in a nonconductive state (to not conduct the load current passing through the coil).
A clamp circuit, connected to the gate terminals of at least one transistor from each of the pairs of the transistors, uses the sense current from the associated sense terminal in a closed-loop fashion to adaptively maintain the associated transistor in a quiescent state w
Crowe & Dunlevy
Hudspeth David
Seagate Technology LLC
Wong K.
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