Fan speed detection in the presence of PWM speed control

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;
• and
• and 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 time interval is used to determine the speed of the fan.

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;
• and
• and 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.

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;
• and
• and 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.