As more industrial and consumer products shrink in size, the need to remove heat in an efficient, quiet way is becoming increasingly important. A PSoC produces an efficient, cost-effective means for accomplishing this.

Basics of fan operation

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Peripherals needed

This fan is required to operate from 600 rpm to 30,000 rpm, well within the capacity of a microcontroller assisted by analog and digital peripherals and an ideal application for a microcontroller-based PSoC. The following peripherals are required:

• A comparator to convert the Hall sensor’s differential analog outputs into a digital signal.

• An 8-bit PWM to control the magnitude of the coil current. To control the current’s direction, the microcontroller switches the PWM into the appropriate lower leg of the Field-Effect Transistor (FET) bridge. The PWM output frequency is set to 23.4 kHz (6 MHz/256), just above the human audio range.

• A 16-bit timer to measure the edges from the Hall sensor comparator. This timer will measure the time for all four phases of the motor because the four poles are not perfectly spaced. The timing for each pole is needed because driving the coil for the very last bit of each cycle does not produce any significant power while consuming a significant amount of current. To conserve power, the PWM is stopped some empirically determined amount of time before the end of the cycle. The PWM uses the previous known phase time to turn itself off early.

• An ADC to measure the shunt current and thermistor voltages.

• A 16-bit timer to measure the duty cycle to the incoming speed selection control signal. The timer is clocked at 24 MHz. The timer is set to measure the time for a falling edge, the next raising edge, and the next falling edge. The duty cycle is shown in the equation:

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Note that clock accuracy falls out of the equation. For example, a 24 MHz timer clocking a 25 kHz input has a period of 960 with accuracy much less than 0.2 percent.

The Cypress CY8C21323-24LFXI PSoC that controls this design comes in a 24-pin micro lead frame package, contains a microcontroller, and has the following peripherals:

• Four 8-bit digital blocks that can be configured as timers, counters, and PWMs and can be cascaded to make wider peripherals.

• Two analog blocks that can be used as single-ended programmable comparators. Combining one with a properly configured digital block produces a 10-bit ADC. Both of these blocks can be combined into a comparator with differential inputs.

States to share

At first glance, it appears that there are not enough peripherals to construct the required system elements. As Figure 2 shows, if the control is segmented into eight different states, many of the required elements can share the available system resources.

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Timing is broken down into two rotations, with each rotation having four phases. The PWM is needed for all eight phases, so it cannot be shared and thus requires a single digital block. Each phase starts with the transition on the Hall sensor, which comprises two analog blocks. At this point, all the FETs are turned off and the appropriate high-side FET is turned on. The PWM is connected to the appropriate low-side FET and turned on. The PWM uses the previously calculated speed to determine when to turn itself off.

From just before the end of Odd₃ to just after the start of Odd₀, two of the digital blocks are configured to make a 16-bit timer and measure the four even phase widths. This information is used to calculate the fan speed. Note that the speed is not measured for odd rotations.

In the middle of Odd₀, one analog block and one digital block are reconfigured to build an ADC and measure the shunt current. After finishing, the analog blocks are reconfigured to rebuild the Hall comparator. The same is done in Odd₁ to measure the thermistor voltage. In Odd₃, two digital blocks are reconfigured to measure the incoming PWM’s duty cycle. When finished, they are reconfigured to measure fan speed, and the cycle repeats.

With this configuration, only three of the digital blocks are used. The fourth is available for additional free features.

As demonstrated by this design example, a PSoC allows for reduced components and performance improvements in a brushless DC motor. This type of dynamic reconfiguration enables designers to share system resources as well as reduce system costs.

Dave Van Ess is a principal applications engineer and member of the technical staff at Cypress Semiconductor, based in San Jose, California. He has 30 years of experience designing microcontroller-based hardware, software, and analog systems and holds eight patents for medical systems, signal processing design, and PSoC digital block enhancements. Dave graduated with a BSEE from the University of California, Berkeley in 1977.

Cypress Semiconductor

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dwv@cypress.com

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