Electricity: motive power systems – Automatic and/or with time-delay means – Speed or rate-of-movement
Reexamination Certificate
2000-06-29
2001-07-17
Ro, Bentsu (Department: 3746)
Electricity: motive power systems
Automatic and/or with time-delay means
Speed or rate-of-movement
C318S464000, C388S912000, C324S166000
Reexamination Certificate
active
06262549
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to cooling fans. More particularly, the present invention relates to a fan speed pulse filter for a pulse width modulation (PWM) fan.
2. Description of Related Art
Fan cooling is typically used to cool electric system, such as microprocessors and power suppliers, when the power consumption of the system exceeds its natural cooling capacities. However, fan failure can result in overheating, and may cause permanent damage to the system. Therefore, it is desirable to be able to detect a slow fan speed or fan failure and initiate an automatic shutdown to avoid thermal runaway. By equipping a Hall-effect switch or by sensing a fan motor current, many fan systems or fans include a tachometer output, which outputs pulses per revolution of the fan. The tachometer output of the fan is connected to a CPU or input-output (I/O) ports for a fan speed monitor and control.
FIG. 1
illustrates the tachometer output pulses when the fan is operating at full speed. In general, the speed of the fan is measured in revolution per minute (RPM); speed is determined by measuring the period (Tp) of the tachometer pulses. The RPM is defined by RPM=60/(Np*Tp), wherein Np is the pole number of the fan.
Many fan controllers use PWM control, with fan speed determined by the duty cycle of the PWM signal. The PWM mode provides highly efficient fan control and prevents the fan from locking during a start-up period. However, generating the tachometer pulses has the following difficulties.
The tachometer only operates when power is applied to the fan.
FIG. 2
shows the output pulses of the tachometer. As shown in
FIG. 2
, Tp
1
is the period of the tachometer pulse, and an incorrect pulse period Tp
2
is produced when the PWM signal is off, causing an inaccurate RPM for the fan. However, the tachometer only operates correctly when proper power is applied. When the applied power is incorrect, an inaccurate RPM is output; as a result, the fan is driven by a slow slew of the PWM signal. Namely, noise pulses
20
occur at the rising time Tr or the falling time Tf of the PWM signal, as shown in FIG.
3
.
In addition, the tachometer may output phantom pulses when the power is rapidly applied to the fan.
FIG. 5
shows the phantom pulses
32
of the tachometer pulse, in which the fan is driven by a fast slew PWM signal.
Noise, the phantom pulse, or an incorrect period of the tachometer pulse introduces an incorrect RPM for the fan. It is necessary to provide a fan speed pulse filter to filter the noise and the phantom pulses and generate a non-interrupted pulse output.
SUMMARY OF THE INVENTION
The invention provides a fan speed pulse filter for a PWM fan. The filter comprises a comparator and a latch circuit. The PWM switch connects to the power source to generate a PWM signal. The fan unit, including a tachometer pulse (tacho-pulse) output, is controlled by the PWM signal to determine the fan speed. The comparator receives the PWM signal, wherein the comparator generates a reset signal by comparing the PWM signal with a reference voltage. The latch circuit, coupled to the comparator and the fan unit, is used to receive the reset signal and the tacho-pulse and to output a pole signal (filtered tacho-pulse containing no phantom pulses).
In addition, fan speed pulse filter for a PWM fan further comprises a synchronizer, an up-counter, a Q-buffer, a down-counter and an oscillator. The synchronizer is coupled to the output of the latch circuit to generate an output signal without interrupt when the PWM signal is off. The up-counter couples to the synchronizer. The Q-buffer is coupled to the up-counter and the synchronizer. The down-counter is coupled to the buffer and the synchronizer. Further, the oscillator is coupled to the synchronizer for providing system clock signals to the synchronizer, the up-counter, the buffer and the down counter.
The up-counter, the Q-buffer, the down-counter and the oscillator mentioned above can be integrated into the synchronizer as a whole.
Advantageously, the fan speed pulse filter for a PWM fan comprises two major parts: one is used for eliminating phantom pulses, and the other is used for outputting a continuous pulse output.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 4110676 (1978-08-01), Edick et al.
patent: 4138728 (1979-02-01), Tung
patent: 5727928 (1998-03-01), Brown
patent: 5883510 (1999-12-01), Torre et al.
patent: 6008603 (1999-12-01), Jones et al.
patent: 6135718 (2000-10-01), Yang
Hsueh Cheng-chi
Lin Jenn-yu G.
Yang Ta-yung
Huang Jiawei
J. C. Patents
Ro Bentsu
System General Corp.
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