Fan speed control circuit

1. A fan speed control circuit to control the rotation speed of at least one fan, each of the at least one fan corresponding to one load and being to dissipate heat of the corresponding load, an inductance being connected between a processing chip and one load, each of the at least one processing chip converting the inputted voltage to the PWM signal containing an unique duty cycle and outputting the converted PWM signal, each of the at least one inductance converting the PWM signal outputted by the corresponding processing chip to the direct current voltage and outputting the direct current voltage to the corresponding load, the duty cycle of the PWM signal outputted by each processing chip increasing as the corresponding load consuming more electrical power, the fan speed control circuit comprising:

at least one counter corresponding to the at least one processing chip, each of the at least one counter counting the number of PWM signals outputted by the corresponding processing chip; and

a control chip connected between each of the at least one processing chip and the corresponding inductance, and being connected to the at least one counter; the control chip being configured to store a relationship table recording a plurality of duty cycle intervals and a plurality of rotational speeds of the fan; each of the duty cycle intervals corresponding to one of the rotational speeds of the fan; and the control chip being further configured to control each of the at least one counter to add one upon obtaining one pulse outputting by the corresponding processing chip, obtain a preset number of PWM signals outputted by each of the at least one processing chip when the counting of the counter reaches the preset number, determine each average value according to the duty cycle of the obtained preset number of PWM signals outputted by each of the at least one processing chip, determine which duty cycle interval each average value is in, according to the relationship table, determine each rotational speed of the fan corresponding to the determined duty cycle interval according to the relationship table, and control each fan to rotate according to the determined corresponding rotational speed of the fan.

2. The fan speed control circuit as described in claim 1 , wherein the control chip sums all the obtained duty cycles of preset number of PWM signals outputted by each of the at least one processing chip, to determine each total duty, and divides each total duty by the corresponding number of the obtained PWM signals, to determine the each average value.

3. The fan speed control circuit as described in claim 1 , wherein the control chip:

stores an initial duty cycle of the PWM signal, when the load consuming the least amount of electrical power, the PWM signal containing the initial duty cycle;

subtracts the initial duty cycle of the PWM signal from each duty cycle of the preset number of PWM signals outputted by each of the at least one processing chip to determine a preset number of comparison values; and

sums each preset number of comparison values to determine each total comparison value, and divides each total comparison value by the corresponding number of the obtained PWM signals to determines each average value.

4. The fan speed control circuit as described in claim 1 , wherein the control chip:

stores an initial duty cycle of the PWM signal, when the load consuming the least amount of electrical power, the PWM signal containing the initial duty cycle; and

sums each total obtained duty cycles of preset number of PWM signals outputted by each of the at least one processing chip to determine each total duty cycle, divides each total duty cycle to the corresponding number of the obtained pulses by determine each average duty cycle, and subtracts the initial duty cycle of the PWM signal from the determined each average duty cycle to determine each average value.

5. The fan speed control circuit as described in claim 1 , wherein the control chip controls the counter to reset the counting of the counter to zero when the counting of the counter reaches the preset number.

6. The fan speed control circuit as described in claim 1 , further comprising a thermal sensor to detect temperature of the environment of the loads, the control chip further storing a standard temperature and an offset table, the offset table recording a plurality of temperature differences and a plurality of rotational speed offsets of the fan, each temperature difference corresponding to one rotational speed offset, wherein the control chip:

obtains the temperature detected by the thermal sensor, and subtracts the stored standard temperature from the obtained temperature to determine a temperature difference;

determines a rotational speed offset of the fan corresponding to the determined temperature difference; and

adds each rotational speed of the fan corresponding to the each determined duty cycle interval with the determined rotational speed offset to determine each total and actual rotational speed of the fan, and further controls each fan to rotate with the corresponding determined total and actual rotational speed of the fan.

7. The fan speed control circuit as described in claim 6 , wherein when the temperature difference is negative, the rotational speed offset of the fan is negative, and when the temperature difference is positive, the rotational speed offset is positive.