초록 ▼

A circuit is arranged to pulse-width modulate the duty cycle of a power supply signal that is used to control the rotational speed of a fan in a temperature-controlled system. The control system includes an edge detector for monitoring a tachometer signal that is provided by the fan. The tachometer

A circuit is arranged to pulse-width modulate the duty cycle of a power supply signal that is used to control the rotational speed of a fan in a temperature-controlled system. The control system includes an edge detector for monitoring a tachometer signal that is provided by the fan. The tachometer signal is monitored at a rate (e.g., one Hertz) that is substantially lower than the rotational speed of the fan. The control system also includes a timer for recording time intervals between selected edges of a monitored tachometer signal. The fan speed is determined from the recorded time intervals. The control system extends the duty cycle of the pulse-width modulated signal to ensure that the fan power remains stable over the period in which the tachometer signal is to be monitored. Maintaining fan power ensures the validity of the tachometer signal during the period that the fan power is maintained.

대표청구항 ▼

1. A circuit for measuring a rotational speed of a fan, comprising:a fan having a pulse-width modulated (PWM) power terminal that is used to control the speed of the fan;a switch that is coupled to the PWM power terminal; wherein the switch is configured to alternately apply and disconnect power fro

1. A circuit for measuring a rotational speed of a fan, comprising:a fan having a pulse-width modulated (PWM) power terminal that is used to control the speed of the fan;a switch that is coupled to the PWM power terminal; wherein the switch is configured to alternately apply and disconnect power from the PWM power terminal in response to a PWM signal;a tachometer that is configured to produce a tachometer signal that is proportional to the speed of the fan, wherein the tachometer is configured to produce the tachometer signal using power from the PWM power terminal;a PWM controller that is configured to produce the PWM signal, wherein the PWM controller is further configured to extend the active period of the duty ratio of the PWM signal for a time that is sufficient for a valid measurement of the speed of the fan to be made from the tachometer signal, whereby the speed of the fan is not substantially affected by the extension of the active period of the PWM signal; andand an edge detector timer that is configured to measure the time interval between a first edge of a first type of a first pulse of the tachometer signal and the next following edge of the first pulse, wherein the next following edge of the first pulse has a type that is opposite to the first type of the first edge of the first pulse, wherein the measured time interval is used to determine the speed of the fan. 2. The circuit of claim 1, wherein the PWM power terminal is coupled to a positive terminal of a power supply. 3. The circuit of claim 1, wherein the PWM power terminal is coupled to a ground terminal of a power supply. 4. The circuit of claim 1, wherein the PWM power terminal is coupled to a negative terminal of a power supply. 5. The circuit of claim 1, further comprising a temperature sensor for measuring a device that is cooled by the fan. 6. The circuit of claim 5, wherein the controller is further configured to adjust the duty ratio of the PWM signal in response a temperature measurement from the temperature sensor. 7. The circuit of claim 1, wherein the controller is further configured to adjust the duty ratio of the PWM signal in response to the measurement of the speed of the fan. 8. The circuit of claim 1, wherein the controller is further configured to periodically measure the speed of the fan wherein the rate of making the measurement is substantially slower than the speed of the fan. 9. The circuit of claim 1, wherein the controller is further configured to periodically measure the speed of the fan wherein the rate of making the measurement is at least eight times slower than the speed of the fan. 10. The circuit of claim 1, wherein the PWM controller is configured to extend the active period of the duty ratio of the PWM signal for a time that is sufficient for a half-pulse measurement to be made, wherein the active period is shorter than 50 milliseconds. 11. The circuit of claim 1, wherein the PWM controller is configured to extend the active period of the duty ratio of the PWM signal for a time that is sufficient for a two-pulse measurement to be made, wherein the active period is shorter than 50 milliseconds. 12. A circuit for measuring a rotational speed of a fan, comprising:means for alternately applying power to and removing power from the fan in response to a pulse-width modulated (PWM) signal;means for producing a tachometer signal that is proportional to the speed of the fan, wherein the means for producing the tachometer signal is configured to produce the tachometer signal by using the PWM power of the fan;means for extending the duty ratio of the PWM signal such that power remains applied to the fan for a time that is sufficient for a valid measurement of the speed of the fan to be made from the tachometer signal; andand means for measuring the time interval between a first edge of a first type and the next following edge having a type that is opposite to the first type of a pulse of the tachometer signal, wherein the measured ti me interval is used to determine the speed of the fan. 13. The circuit of claim 12, wherein the tachometer signal comprises pulses that are proportional to the speed of the fan. 14. The circuit of claim 13, wherein the PWM signal is extended such that at least one valid pulse is produced in the tachometer signal. 15. The circuit of claim 13, wherein the PWM signal is extended for a time that is sufficient for a half-pulse measurement to be made, wherein the active period is shorter than 50 milliseconds. 16. The circuit of claim 12, further comprising means for controlling the speed of the fan in response to a valid measurement of the speed of the fan. 17. The circuit of claim 12, further comprising means for controlling the speed of the fan in response to a temperature measurement from a device that is cooled by the fan. 18. A method for measuring a rotational speed of a fan, comprising:alternately applying power to and removing power from the fan in response to a pulse-width modulated (PWM) signal;producing a tachometer signal that is proportional to the speed of the fan, wherein the tachometer signal is produced using the PWM power of the fan;extending the duty ratio of the PWM signal such that power remains applied to the fan for a time that is sufficient for a valid measurement of the speed of the fan to be made from the tachometer signal; andand measuring the time interval between a first edge of a first type and the next following edge having a type that is opposite to the first type of a pulse of the tachometer signal, wherein the measured time interval is used to determine the speed of the fan. 19. The method of claim 18, wherein the tachometer signal is produced such that the tachometer signal comprises pulses that are proportional to the speed of the fan. 20. The method of claim 19, wherein the PWM signal is extended such that at least one valid pulse is produced in the tachometer signal. 21. The method of claim 19, wherein the PWM signal is extended for a time that is sufficient for a half-pulse measurement to be made, wherein the active period is shorter than 50 milliseconds. 22. The method of claim 18, further comprising controlling the speed of the fan in response to a valid measurement of the speed of the fan. 23. The method of claim 18, further comprising controlling the speed of the fan in response to a temperature measurement from a device that is cooled by the fan. 24. The method of claim 18, further comprising periodically measuring the speed of the fan wherein the speed of the fan is measured at a rate that is substantially slower than the speed of the fan. 25. A circuit for measuring a rotational speed of a fan, comprising:a fan having a pulse-width modulated (PWM) power terminal that is used to control the speed of the fan;a switch that is coupled to the PWM power terminal; wherein the switch is configured to alternately apply and disconnect power from the PWM power terminal in response to a PWM signal;a tachometer that is configured to produce a tachometer signal that is proportional to the speed of the fan, wherein the tachometer is configured to produce the tachometer signal using power from the PWM power terminal;a PWM controller that is configured to produce the PWM signal, wherein the PWM controller is further configured to extend the active period of the duty ratio of the PWM signal for a time that is sufficient for a valid measurement of the speed of the fan to be made from the tachometer signal, whereby the speed of the fan is not substantially affected by the extension of the active period of the PWM signal; andand an edge detector timer that is configured to selectively measure the time interval between a first edge having a first type of a first pulse of the tachometer signal and a next following edge of a successive pulse, wherein the next following edge of the successive pulse has a type that is the same as the first type of the first pulse of the tachometer signal. 26. The circuit of claim 25, wherein the edge detector timer that is further configured to selectively measure the time interval between the first edge having a first type and a third edge of a third pulse that follows the next following edge of the second pulse, wherein the third edge has a type that is the same as the first type of the first pulse of the tachometer signal.