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4-Wire Pulse Width Modulation (PWM) Controlled Fans

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Introduction
Overview

This specification defines the intended operation of a fan that implements the Pulse Width
Modulation (PWM) control signal on the 4-wire fan interface. The introduction of 4 wire PWM
controlled fans is a means to reduce the overall system acoustics. The expectation is a 4 wire
PWM controlled fan when properly implemented will be significantly quieter than a similar 3
wire fan.

Current

Steady state operation fan current draw (with 12.6 V applied, with fan operating in the free stream
condition) shall not exceed 1.5 A.
Fan current spike during start-up operation (with 12.6 V applied, with fan operating in the free
stream condition) shall be allowed to exceed 1.0 A, up to 2.2A maximum for a duration of no
greater than 1.0 sec.

Tachometer Output Signal

Fan shall provide tachometer output signal with the following characteristics:
• Two pulses per revolution
• Open-collector or open-drain type output
• Motherboard will have a pull up to 12V, maximum 12.6V

PWM Control Input Signal

The following requirements are measured at the PWM (control) pin of the fan cable connector see
Figure 7 and Table 1:
PWM Frequency: Target frequency 25 kHz, acceptable operational range 21 kHz to 28 kHz
Maximum voltage for logic low: VIL = 0.8 V
Absolute maximum current sourced: Imax = 5 mA (short circuit current)
Absolute maximum voltage level: VMax = 5.25 V (open circuit voltage)
This signal must be pulled up to a maximum of 5.25V within the fan.

Pulse width modulation (PWM), or pulse duration modulation (PDM), is a modulation technique used to encode a message in a pulsed signal. Although this modulation technique can be used to encode information for transmission, its main use is to allow the control of the power supplied to the electrical devices, especially inertial charges such as motors. In addition, PWM is one of the two main algorithms used in photovoltaic solar battery chargers, the other is tracking the maximum power point.

The average value of the voltage (and current) supplied to the load is controlled by turning the switch between the power supply and the on / off load at a rapid speed. The longer the switch compared to the off periods, the greater the total power supplied to the load.

The PWM switching frequency must be much greater than that which would affect the load (the device using the power), ie the resulting waveform perceived by the load must be as smooth as possible. The speed (or frequency) at which the power supply should change can vary considerably depending on the load and the application, for example

Switching must be done several times a minute on an electric stove; 120 Hz in a lamp dimmer; between a few kilohertz (kHz), and tens of kHz for an engine drive; and well in the tens or hundreds of kHz in audio amplifiers and computer power supplies.

The term duty cycle describes the proportion of time 'on' to the regular interval or 'period' of time; a low duty cycle corresponds to low power because the power is off most of the time. The duty cycle is expressed as a percentage, 100% is fully activated.

The main advantage of PWM is that the loss of power in the switching devices is very low. When a switch is off there is virtually no current, and when it is turned on and the power is being transferred to the load, there is almost no voltage drop across the switch. The loss of power, the product of voltage and current, is therefore close to zero in both cases. PWM also works well with digital controls, which, due to their on / off nature, can easily set the required duty cycle. PWM has also been used in certain communication systems where its duty cycle has been used to transmit information through a communications channel.

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