4-Wire Pulse Width Modulation (PWM) Controlled Fans Specification

Fan Speed Control
14
3.3 Fan Speed Response to PWM Control Input Signal
The PWM input shall be delivered to the fan through the control signal on Pin 4 (see Section
2.1.4). Fan speed response to this signal shall be a continuous and monotonic function of the duty
cycle of the signal, from 100% to the minimum specified RPM. The fan RPM (as a percentage of
maximum RPM) should match the PWM duty cycle within ±10%. If no control signal is present
the fan shall operate at maximum RPM. See Figure 3.
Figure 3 Fan Speed Response to PWM Control Input Signal
Speed vs. PWM Duty cycle
0
10
20
30
40
50
60
70
80
90
100
0 102030405060708090100
PWM Duty Cycle
Min.
RPM
Min.
PWM
Undetermined
% of full speed
Max.
PWM
Max.
RPM
Minimum specified RPM must be 30% or less
(This example shows a minimum RPM of 20%)
3.4 Operation below Minimum RPM
For all duty cycles less than the minimum duty cycle, the RPM shall not be greater than the
minimum RPM. The following graphs and definitions show three recommended solutions to
handle PWM duty cycles that are less than the minimum operational RPM, as a percentage of
maximum.
In a Type A implementation the fan will run at minimum RPM for all PWM duty cycle values less
than minimum duty cycle. The minimum fan speed is controlled by design and can not be
overridden by the external fan speed controller. See Figure 4.