Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
1998-12-31
2001-03-27
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C327S408000, C327S530000, C327S546000
Reexamination Certificate
active
06208194
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates, generally, to a MOSFET control circuit with controlled channel voltage drop and, more particularly, to an improved circuit which provides reverse bias blocking through the use of a diode, but which utilizes a voltage-controlled MOSFET channel in lieu of the blocking diode for forward voltage conduction.
2. Background Art and Technical Problems
Control circuits are often required to deliver a pre-determined threshold voltage to an electrical load, even when the input voltage dips below the threshold voltage required to drive the load. In the case of power supply circuits, for example, it is quite common to augment the power supply with a bank of capacitors configured to discharge their stored energy to the load during the transient periods in which the input voltage source dips below the require threshold voltage, thereby insuring that adequate voltage is supplied to the load at all times. This is particularly important in applications involving health and safety, for example in the avionics industry to maintain power supply to avionics computers, flight control systems, and other on board aircraft electrical loads.
Presently known control circuitry for supplying a threshold voltage to a load notwithstanding transient reductions in the input voltage below the load threshold typically involve the use of a power converter circuit. In particular, a power supply includes a diode through which power is applied to the load, with a bank of capacitors disposed in parallel with the load. Thus, when the input power supply through the diode exceeds the threshold voltage to be applied to the load, the capacitors remain charged and the load “sees” an applied voltage which is at least as great as the threshold level required by the load. When the input voltage (supplied by a battery, generator, or the like) dips below the threshold level, such that the output voltage exceeds the input voltage, the diode blocks current flow in the reverse bias direction, and the capacitors release their energy to the load to maintain the voltage supplied to the load at or above the threshold level until the input source again reaches the load threshold. This technique for maintaining threshold voltage to the load, however, is disadvantageous in several respects.
In particular, the reverse blocking diodes employed in presently known power supplies typically exhibit a voltage drop on the order of 0.5 to 0.8 volts. At high current values, a substantial amount of power is dissipated by the diode, such that the circuit liberates a significant amount of thermal energy. In order to dissipate the heat in a controlled manner, heat sinks and associated hardware are often employed. Moreover, high operating temperatures contribute to diode failure in many applications.
Control circuits are thus needed which effectively isolate the load from the source during transient low voltage conditions at the source, but which reduce power losses and high operating temperatures associated with presently known systems.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a MOSFET control circuit which overcomes many of the shortcomings of the prior art. In accordance with a preferred embodiment of the present invention, a MOSFET control circuit employs a metal oxide semiconductor—field effect transistor (MOSFET) for supplying voltage to a load when the MOSFET is in the forward biased condition (i.e., when an input voltage level exceeds the threshold voltage level by a pre-determined amount). In accordance with a further aspect of the present invention, the MOSFET control circuit is configured to turn off the MOSFET when the input voltage level dips below that pre-determined level required to maintain the MOSFET in the forward biased condition. When the MOSFET is turned off, the diode internal to the MOSFET device prevents current flow in the reverse bias direction (i.e., the internal diode prevents current flow from the output to the input), effectively isolating the load from the input voltage source, thereby allowing the capacitors to discharge their energy to the load to maintain a voltage supply to the load which is at or above the output load threshold voltage level. Thus, although a diode is still employed to prevent current flow in the reverse direction, the diode need not fuction to supply the current to the load when the MOSFET is in the forward biased direction. Rather, current flows through the MOSFET channel (as opposed to flowing through the diode) when the MOSFET is in the forward biased condition.
In accordance with a further aspect of the present invention, conducting current through the MOSFET channel substantially mitigates power losses vis a vis prior art systems inasmuch as the voltage drop across the MOSFET channel can be significantly less than the 0.7 volts typically associated with a diode. This results in more power efficient conversion and reduces the high temperatures associated with prior art schemes in which current flows through the diode to the load. In accordance with a further aspect of the present invention, the use of a MOSFET for conducting current eliminates the need for expensive and cumbersome heat sinks.
REFERENCES:
patent: Re. 36179 (1999-04-01), Shimoda
patent: 5475332 (1995-12-01), Ishimoto
patent: 5744878 (1998-04-01), Wachter et al.
patent: 5886561 (1999-03-01), Eitan et al.
patent: 5959494 (1999-09-01), Fotouhi
Dinh Paul
Honeywell International , Inc.
Wells Kenneth B.
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