Fan Control Circuit Design With PWM Driving

DC fan control and DC motor control are a lot alike and if you think about how a fan works. A speed control circuit for a fan operates under a simple type of driving modulates of DC power delivered to the fan. The same technique is used for motor control, where the average power delivered to a motor is used to control the rotational speed. This is done with PWM driving, which is essentially the same technique used in switching regulators to control power delivery.

DC fan control circuits with speed control will use PWM driving to control average power delivery as described above. Depending on the required current delivery, control circuits for large and small fans could be designed from discrete components or from an integrated fan controller. Building these circuits or systems involve selection of a few simple elements:

• An oscillator and comparator with adjustable hysteresis

• A simple temperature sensor

• A FET that modulates power delivery

• Stable DC power source

With these ingredients, you can also design a basic fan control circuit with or without sensing and feedback.

DC Fan Control Circuit Topology

DC fan control circuits generally pick from the list of items mentioned above; a block diagram showing the general topology is shown below. The controller in this block diagram can use an integrated controller, where the drive and control circuit are packaged in a single IC, or discrete components and a gate driver.
 

In this topology, the ability to deliver power with a signal begins with an oscillator. The output from an oscillator becomes a PWM waveform with the required duty cycle. That signal then modulates a FET or group of FETs such that some desired average power is delivered to the fan. That average power delivered to the fan then sets the fan’s speed.

Here, an important point needs to be made about the different kinds of DC fan control circuits, as these different circuits could have different capabilities. DC fan control circuits but have some of the following features:

• A feedback loop that is used to sense and regulate the fan at a fixed speed

• Ability to sweep or adjust through a range of speeds

• The fan could have 2 wires, 3 wires, or 4 wires

If you want to build a fan control fan control circuit, you must know what type it will connect to. Controllers can use either type of fan, while discreet controllers have to be specifically designed for either 2-wire, 3-wire, or 4-wire fans. These two types of speed control circuits are found in the table below.

Integrated Controller Circuit

An integrated controller circuit uses a specific ASIC with power delivery and speed control functions built into it. Typically, you do not need to provide an external oscillator or PWM signal, and oftentimes you do not need an external FET because it will be built into the fan or controller. Fan control or speed control ASICs are also designed to work with specific temperature sensors, the most common being a simple thermistor. An example with a 4-wire fan in a block diagram is shown below.

This particular fan controller uses four wires: two for power, one for PWM signal delivery, and one for speed sensing (the tachometer or locked rotor wire). Power delivery to the fan is not monitored directly, instead the speed of the fan is monitored. Alternative usage with this type of controller would be 3-wire usage, where the fan must be modulated with an external FET. Another possibility is to use a 4-wire fan as a 3-wire fan, where the tachometer signal is left unmonitored.

The current being drawn through the fan may be monitored internally if the ground return from the fan has a connection to the controller, or the power could be monitored with a current sense resistor in series with the fan’s power line. The fan turn-on and turn-off thresholds can also be adjustable based on the value of the thermistor and any external configuration resistors or capacitors.

Discrete Fan Controllers

Discrete components would need to be used when high power is required in the fan. Larger fans that require more power might use an H-bridge for power delivery, which would require phase-shifted PWM signals to modulate the group of FETs. Therefore, you would need a gate driver that is specifically designed for an H-bridge.

In a circuit with discrete components, how do you maintain desired speed and how do you trigger the fan to come on at certain temperatures? This could involve some additional logic and a requirement to monitor the fans average power. For a 3-wire or 4-wire fan, you could monitor the tach output, but note that the tachometer signal only outputs while the fan is on. This means the fan needs to be periodically run at full DC in order to get an actual tachometer reading.

Simple Fan Control Circuit Simulation

To simulate a fan control circuit, you will generally be simulating the control loop and the PWM generator in the time domain. The FET arrays will also need to be simulated to ensure they fully open when modulated and thus deliver the required power to the fan. Make sure your CAD system has these capabilities if you need to evaluate your fan control circuit before PCB layout.

Whenever you need to design and evaluate fan control circuits with gate drive functions, make sure you simulate your designs with the complete set of tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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