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Posted on 17 July 2019

Pulse-Width Modulated Fan Controllers for Automotive Applications

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PWM controllers for fan motors can also offer many benefits

There are two areas inside the motor vehicle which require fans to facilitating cooling. These are the engine compartment, where primarily the engine cooling system needs to be cooled, and the passenger compartment for the HVAC system.

By Eric Hustedt, Jim Tompkins, and Peter Sommerfeld, Electronic Motion Systems

 

The fans for both of these applications are typically brushed DC motors, which need controllers to regulate their speed. This then allows different levels of ventilation which in turn allows control of cooling.

There are three “traditional” methods of controlling the speed of fans in a motor vehicle.
- Shunt resistors
- Series/parallel configuration
- Linear controllers

The shunt resistor method simply switches various resistors in series with the fan to slow the fan down, this is simply dissipating the extra power in the resistor, since that power has to come from the engine you can see this is a waste of fuel! The power wasted can easily exceed 100W for a 400W fan. As well as the inefficiencies, there is a limited speed selection range.

The series parallel configuration, while not being as inefficient as the shunt method, only gives two speeds, full power or under ¼ power due to the load characteristics of a fan motor. The Linear Controller uses a transistor as a variable resistor to be able to continuously adjust fan motor speed. This still has the disadvantage of dissipating power in the variable resistor and wasting fuel.

Pulse-Width Modulated Control

Pulse-Width Modulation (PWM) for motor control is a method of changing the effective voltage and therefore the current a motor is supplied with. This is done by turning a switch on and off at a high speed (e.g. around 20kHz) and varying the “on” time to attain the required motor speed. As long as the switching speed is fast enough (technically, much greater than the overall time constant of the motor), the load does not see the difference between the equivalent DC voltage and the PWM voltage (see Figure 1). PWM does have significant reductions in power loss in the switch, which increases efficiency of the motor controller. The reason for this is explained in the following paragraph.

PWM with 20 percent duty cycle and the corresponding PWM voltage and average voltage

Since in a PWM system the switch is either “on” and carrying current with most of the voltage dropping across the load and almost no voltage dropping across the switch, or “off” with full voltage dropping across the switch but no current flowing, it means there is never the condition for significant power dissipation where both voltage and current need to be present simultaneously. Figure 2 shows a Field- Effect Transistor (FET) being used to switch a load of 1 Ohm on and off. The resistance of the FET, the RDS, can be varied from very high resistance (off-state) to very low resistance (on-state). If the FET is turned off, the voltage will drop entirely across the FET, but because of no current flowing through FET and load, there will be no power dissipation. If the FET is turned on (RDSon typically mOhm) most of the voltage will drop across the load since its resistance is typically much higher than the RDSon of the FET. If the FET were to be turned on sufficiently to allow half of the voltage to drop across the load, the RDS would need to be the same value as the resistance of the load.

Power dissipation in a switch (FET) between off and on-state

This however would lead to a very high power dissipation as shown with the peak in Figure 2. To avoid this PWM control toggles between on-state and off state of the FET such that for a certain duty cycle (ratio between on time and toggle period) the desired average voltage drops across the load. This then is realized at greatly reduced levels of power dissipation.

Figure 3 shows the efficiencies of a PWM control compared with standard resistor control for a fan motor. The corresponding power losses are shown in Figure 4. The power dissipated will impact the fuel efficiency of a motor vehicle. This fuel can be saved by controlling fan motors through PWM.

Efficiencies of a PWM fan controller compared with a shunt resistor controller

Power losses of a PWM fan controller compared with a shunt resistor controller

Additional functionality offered by PWM control

PWM controllers for fan motors can also offer many other benefits by the ability of including certain functionality, e.g.:

- Efficient continuously variable speed control of the fan motor.
- Stall detection and protection in case of a locked rotor.
- Constant speed regulation.
- In-rush current limit or “soft start”.

Continuously variable speed control

Since the fan controller is not limited by either fixed resistors or series/parallel switching, it can run the fans at any desired speed. Although linear controllers can do this as well, PWM controllers can do this at almost no power dissipation. For engine cooling this means that the vehicle ECU can set the fan speed as required by the cooling system and can thus regulate the temperature more accurately improving engine performance. In HVAC applications it means that the speed is not limited by fixed steps, and the controller can change the speed slowly reducing noise impact.

Stall detection

A PWM control unit can have advanced stall detection algorithms built into the software. The unit can detect both a stalled and an over loaded motor at various speed settings. Should the fan be blocked or jammed for some reason this feature protects the fan against further damage or even fire, reduces the load on the vehicle electrical system and allows the fan to recover out of a temporary blockage like ice build up.

Speed regulation

In cabin fan applications it is desirable to prevent the fan motor speed changing with vehicle engine speed. This is caused by the variation of the battery voltage with changing alternator output. In traditional shunt resistor control a variation of the battery voltage causes a change in the fan speed in the cabin. This is most noticeable when coming to a stop (engine idle). PWM fan controllers for HVAC applications can detect the change in voltage and modify their output accordingly. This means at all but full power settings the fan speed does not change with battery voltage. This control is not possible at full speed because the fan is then directly connected to the battery and the controller therefore cannot hold the speed when the battery voltage drops.

EMS Engine Fan Controller Package

In-rush current limit

All motors when starting draw far more current than at their steady state running speed. This can be in excess of 3 times normal current draw! This puts an unecessary load on the vehicle electrical system, it can cause lights to dim momentarily, the alternator to over compensate etc. Large fluctuations in the load current on the alternator can also cause voltage spikes. This is not desirable for the vehicle electrical system! PWM Fan Controller can provide a soft start and a soft increase function, meaning that at any increase in speed, either from zero or a low speed, the controller actively prevents large currents being drawn from the battery.

Fan Controller packaging

The engine fan controller is located at the shroud of the front cooling module of the vehicle. It requires battery voltage and ground as supply inputs and a control input, which is used to set the speed it should control the fan motor to. This set-point is usually communicated to the fan controller by the engine management system. Its 2-pole output goes to the fan motor, which is also located at the shroud.

EMS HVAC Fan Controller Package

The engine fan controller housing has a motor and vehicle connector integrated. An insert-molded lead-frame forms the contact leads in the connector shrouds and connects to the PCB inside the housing. The PCB holds the microcontroller, the power stage along with all the peripheral circuitry. The PCB is heat-sunk by attaching it via thermal adhesive to an aluminium heatsink, which is over-molded into the housing. A lid seals the housing cavity and protects the circuitry. The HVAC fan controller is located in one of the air-ducts of the cabin cooling system. The input and output requirements are similar to the engine fan controller. The motor output and power and signal input connectors sit directly on the controller PCB, which holds the microcontroller, the power stage along with all the peripheral circuitry. The PCB is heat-sunk by attaching it via thermal adhesive to the aluminium heatsink, which is cooled by the air-flow in the HVAC duct. A plastic housing place on top of the PCB/heat-sink assembly protects the circuitry.

Conclusion

Fan controllers using pulse width modulation, greatly reduce the power loss and therefore increase fuel efficiency. This technology is now proven and validated for automotive application and has been used on production programs , both for engine fan cooling and HVAC fan cooling.

 

 

 

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