In many products, running the fan at low or medium speed is sufficient to dissipate heat, while allowing the highest speed mode to be reserved for worst-case scenarios. The circuit described in this article uses linear voltage control to reduce fan speed and thus noise by running the fan at a DC voltage lower than the manufacturer's full rated voltage.
SMBus Temperature Sensor IC
Commercially available SMBus temperature sensor ICs include a sensor that measures the ambient temperature around the IC and a device that supports one or more external sensors (i.e., some inexpensive transistors connected to diodes ).
The SMBu communication interface provides an easy connection to the system microcontroller, while the temperature sensor's measurement parameters can be configured through writable registers.
Figure 1: The relationship between temperature and fan speed set by the control circuit in this paper.
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Many SMBus temperature sensors have one or two outputs that become active (typically go low) when the temperature exceeds a certain limit (programmed into the IC registers). Designers can expect typical sensor accuracy of 1 to 3°C and resolution as fine as 1/8°C.
Common drive voltages for most DC brushless fans are +5V and +12Vdc. Fans running at full speed can produce objectionable noise, so it is important to reduce the fan speed as much as possible. When running DC fans at reduced voltage and as the fan ages, the fan starting voltage can become a limiting factor, as bearing wear causes the required starting voltage to increase.
The actual operating voltage range of fans varies widely. A fan rated at +5V by one manufacturer might start with 2Vdc, while another fan of the same size
Figure 2: Two transistors implement a 5V fan drive.
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Intelligent Control
1. Two transistors realize 5V fan drive:
The circuit shown in Figure 2 is useful for fan products powered by +3.3V and +5V. When the temperature is below the two limit settings, the open-drain outputs Out1 and Out2 are set high, causing R1 and R2 to pull the gates of P-channel FETs Q1 and Q2 high, turning them off. When the temperature exceeds limit 1 in Figure 1, Out1 goes low, turning on Q1 and applying approximately 3V to the fan through Schottky diode D1. When Out2 goes low, Q2 turns on and applies 5V to the fan. D2 ensures that the 5V supply does not act back on the 3.3V supply through Q1.
The circuit is highly power efficient because the transistor base does not consume current, it acts as a switch, connecting the fan directly to the power rail. Selecting a P-channel FET with Ron < 0.75Ω @ Vgs = 3V keeps the voltage drop and power consumption low. Low power consumption allows the fan to use a small SOT-23 device to achieve a rated current of 400mA @ 5V.
2. Single transistor to achieve 5V fan drive
Figure 3: A single transistor implements a 5V fan drive.
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The circuit shown in Figure 3 uses a PNP transistor to control the three speeds of the fan: off, medium, and high. When the temperature is below the two limit settings, both Out1 and Out2 go high. No current flows through the base of Q1, so it turns off and the fan voltage is 0V.
When the temperature exceeds limit 1, Out1 is driven low and the resistor divider R1/R2 sets the voltage at the base of Q1 to 1.8 V. Since the base voltage is Vbe, the emitter voltage will be 0.7 V higher than Vbe, making the fan voltage 2.5 V (50% of the full-scale voltage).
When Out2 goes low, it pulls the base of Q1 down to ground level, and the base current is limited by the maximum absorption capacity of the IC output, typically 6-8mA@Vol=0.4V. Due to the limited base current, the gain of Q1 should be greater than 100 to ensure minimum voltage drop and strong transistor drive capability. The voltage drop between the output device and Q1 limits the maximum fan voltage to 4.1V (82% of the full-scale voltage).
3. Single transistor to achieve 12V fan drive
The circuit shown in Figure 4 is slightly different from the single transistor circuit and can drive the fan at low, medium, and high speeds. This arrangement allows a 12V fan to be controlled by an IC with a maximum output voltage of 5V.
Figure 4: A single transistor implements a 12V fan drive.
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When both IC outputs are high, low speed is set by resistors R1 and R3. The R1/R3 voltage divider sets the Q1 base voltage to 5.0V, providing about 6.3V (52% of full-scale voltage) to the fan. When Out1 goes low, medium speed is achieved, with R2 sinking current to set the base voltage to 2.5V and the fan voltage to 8.8V (73% of full-scale voltage). When Out2 goes low, the high speed voltage reaches 11.1V (92% of full-scale voltage).
Conclusion
The three speeds of the fan can be controlled using an SMBus temperature sensor, providing high system design flexibility and low cost. The fan speed can be set for quiet operation by using the lower two fan speeds for operation in normal and above average power consumption situations. The highest speed can be dedicated to operation in extreme temperature conditions, when cooling takes precedence over quiet operation.
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