Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2000-08-21
2001-05-29
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S050000
Reexamination Certificate
active
06239991
ABSTRACT:
FIELD OF THE INVENTION
The present application relates generally to an electronic control circuit. More specifically, it relates to an electronic control circuit used to provide continued operation of a pulse width modulation driven fan during a PWM input signal failure.
BACKGROUND OF THE INVENTION
Electronic equipment often uses cooling fans to maintain system temperature at proper levels. The speeds of fans can be controlled as a function of the equipment temperature or loading conditions. It is essential that fans operate reliably because the malfunction or breakdown of a fan could be destructive to electronic equipment that relies on the fan to prevent it from overheating.
One known method to control fan speed is by applying a pulse width modulation (PWM) to the input of the fan motor. With PWM systems, the duty-cycle, or on time, of the input voltage pulses can be varied to control the fan speed. One type of prior art circuit that uses PWM to control fan speed is shown in FIG.
1
. Referring to
FIG. 1
, a signal from a PWM source is applied to a common-emitter type circuit that includes a resistive network R
1
, R
3
and R
4
and a transistor Q
1
. The output of this circuit is an amplified and inverted reproduction of the PWM input signal. When the PWM signal turns on transistor Q
1
, the fan motor is not driven because the transistor has a low resistance and most of the current flowing from the positive voltage source flows through pull-up resistor R
5
, through the transistor Q
1
and goes to ground. On the other hand, when the PWM input signal is low enough that transistor Q
1
is turned off, the current from the positive voltage source flows through resistor R
5
and then through the output to drive the fan motor. One drawback associated with this type of prior art control circuit is that if the PWM input malfunctions and produces a steady high or above ground d.c. signal sufficient to turn transistor Q
1
on, the circuit will produce a low or no voltage output signal that will, in turn, terminate operation of the fan. Accordingly, this type of control circuit does not reliably compensate for PWM signal failures, and can lead to fan shut down, overheating and possible damage to electronic equipment.
What is desired, therefore, is a PWM driven electronic control circuit for a fan or motor that reproduces the PWM signal during normal operation and that will compensate for a malfunction or complete failure of the PWM input signal and ensure the fan's continued operation.
SUMMARY OF THE INVENTION
The present invention is directed to a control circuit and method for controlling an electric motor driven by a voltage source, such as a fan motor, and for driving the fan motor at an approximately full-duty cycle, or other predetermined speed in response to a failure or malfunctioning of the control circuit. The control circuit comprises a pulse width modulation (PWM) source having an output; a switch having a control and being coupled between a voltage source and the motor; and a capacitor having an input coupled to the PWM output, and an output coupled to the switch control. During a substantially direct voltage level at the PWM output indicative of a failure or malfunction of the PWM source, the capacitor maintains at its output a voltage level sufficient to cause the switch control to connect the voltage source to an input of the electric motor and drive the electric motor at a full duty-cycle. A diode of the control circuit is coupled between the PWM output and the switch control for providing direct current restoration at the output of the capacitor such that when the voltage level of the PWM output is time varying, an input voltage at the motor input substantially corresponds to the voltage level of the PWM output.
Accordingly, one advantage of the electronic control circuit and method of the present invention is that during normal operation the PWM input signal is reproduced for the fan or motor, and in the event of a failure or malfunction of the PWM input signal, the fan or motor is driven at a full or substantially full duty cycle. As a result, the control circuit and method of the present invention ensure continued operation of the fan in the event of a PWM input source failure or malfunction to thereby protect sensitive electrical equipment.
REFERENCES:
patent: 4409933 (1983-10-01), Inoue
patent: 4841207 (1989-06-01), Cheyne
patent: 4965504 (1990-10-01), Ueda et al.
patent: 5457766 (1995-10-01), Ko
patent: 6037732 (2000-03-01), Alfano et al.
patent: 6040668 (2000-03-01), Huynh et al.
Ajro Metali
Harlan George H.
Cummings & Lockwood
Nidec America Corporation
Sterrett Jeffrey
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