Degauss Write Current Decay Control Circuit
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Solution Overview
Problem
Existing degaussing circuits in mass data storage devices are process-dependent and result in nonlinear decay of write current, affecting the performance of subsequent read and write operations due to residual magnetism in write heads.
Innovation Solution
A control circuit with a current mirror and programmable ramp voltage generator is used to control the decay of write current, ensuring a linear and process-independent decay to zero from any initial current value, independent of the number of enabled DAC transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a traditional degaussing circuit with MOS transistors and resistor discharge is used, then the write current can be reduced to zero, but the decay time varies due to process dependence and different initial current values
Solution Approach 1:
The patent changes the control parameter from voltage-based discharge (which is process-dependent) to current-based control through a current source. The current source provides a constant discharge current that is independent of process variations, ensuring consistent decay time. The degaussing circuit uses a current source I_bias that flows through a capacitor C, creating a linear voltage decay that translates to linear current decay in the write head, regardless of process variations in MOS transistor characteristics.
Solution Approach 2:
The patent prepares the degaussing circuit in advance by having a dedicated current source and capacitor ready to immediately begin the linear decay process when degaussing is initiated. The circuit architecture is pre-configured with the current source connected to the capacitor, which is normally charged during write operations. When degaussing starts, the pre-positioned current source immediately begins discharging the capacitor in a controlled linear manner, eliminating the need for complex real-time adjustment during the decay process.
2Adaptability or versatility
If the number of enabled DAC transistors varies, then different initial write current amplitudes are achieved, but the decay characteristics become unpredictable and nonlinear
Solution Approach 1:
The patent changes the control mechanism from voltage-based (which creates nonlinear decay when capacitance varies) to current-based control. The current source I_bias provides a constant discharge current that is independent of the number of enabled DAC transistors. This current charges or discharges the capacitor C at a constant rate, creating linear voltage decay regardless of the initial voltage level or the number of active transistors in the DAC, thereby ensuring linear current decay in the write head.
Solution Approach 2:
The patent introduces a capacitor C as an intermediary energy storage element between the current source and the write head circuit. This capacitor acts as a buffer that smooths out the discharge process, converting the constant current from the current source into a linear voltage decay that drives the write current. The capacitor decouples the initial current amplitude (determined by DAC transistor configuration) from the decay characteristics (determined by I_bias and C), making the decay linear and predictable regardless of the initial conditions.
3Reliability
If voltage discharge through a resistor is used to degauss the write head, then the current can reach zero, but the decay is uncontrollably nonlinear and process-dependent
Solution Approach 1:
The patent replaces the passive resistor-based voltage discharge mechanism with an active current source-based system. Instead of relying on Ohm's law (I = V/R) where current decay follows an exponential curve determined by RC time constants, the invention uses a controlled current source that provides a constant current I_bias. This active control mechanism substitutes the passive, process-dependent resistor discharge with an active, process-independent current control, enabling linear and predictable current decay.
Solution Approach 2:
The patent implements implicit feedback through the capacitor voltage. The current source I_bias charges or discharges the capacitor C at a constant rate, and the resulting voltage across the capacitor feeds back to control the write current amplitude. This feedback mechanism ensures that the write current follows a linear decay pattern, as the capacitor voltage linearly increases or decreases based on the constant current flow, providing automatic control without needing explicit measurement and adjustment.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution achieves a consistent and controlled decay of write current to zero, improving the reliability and performance of degaussing operations by eliminating process-dependent variations and nonlinear decay issues.
Implementation Method 1
A current mirror having a reference current path and a bias current path in which current in the reference current path is mirrored
Implementation Method 2
The programmable ramp voltage is applied to a voltage-to-current converter which is connected to the current diverting circuit
Data Source
AI summary
A control circuit to provide a control current to control an amplitude of a write current in a magnetic media drive. The control circuit has an output circuit for providing the control current with an amplitude dependent on a bias voltage. A bias current path provides the bias voltage to the output circuit, and a current diverting circuit is connected to divert current from the bias current path. A programmable ramp voltage generator operates in response to a degauss enable signal, and a voltage-to-current converter receives the programmable ramp voltage to control the current diverting circuit to divert current from the bias current path at a rate determined by the programmable ramp voltage. The bias voltage and the write current decay according to the programmable ramp voltage. The write current decay can be made linear and independent of a beginning write current amplitude.