Modular Approaches to Brushless DC Motor Drives

-May 20, 2010

Brushless DC motors require power transistors that switch coils on or off. You can use discrete power transistors in a circuit of your own design or use a reference design from microcontroller, motor, and transistor suppliers. As an alternate, two new devices can simplify the drive electronics and provide ancillary circuits that limit current, detect over- and under-voltage conditions, short circuit, and so on.

First, the DEC Module 24/2 from Maxon Motor will drive a brushless DC motor (BLDC) with up to 48 watts of power from a 8- to 24-V power source. The BLDC motor must use Hall-effect sensors to detect the rotor's position. The small module, which measures 24.2 x 20.4 mm (0.95 x 0.8 inches) includes a full-bridge power-drive circuit for each of the three motor windings and inputs for three Hall-effect sensors.
 


An on-board microcontroller lets developers set motor direction, limit current, and choose the motor type they plan to use. You can find more information about this module on the Maxon Motor Web site at: shop.maxonmotor.com/ishop/download/article/367661.xml. Click on the "Downloads" tab to go to the 18-page data sheet (Operating Instructions) for the module.

An analog input sets the motor speed. For an input voltage between 0 and 0.1 volt, the motor operates at its minimum speed. Between 0.1 and 5.0 volts, the motor speed tracks the voltage. Equations in the data sheet for the DEC Module 24/2 let you calculate either the speed for a particular voltage or the voltage needed to drive a motor at a specific speed. The analog input for speed control makes this approach suitable for applications when engineers don't want to invest time in motor-control designs and need to concentrate more on the overall system needs. If your MCU has a DAC output, you're all set. But you can use an MCU's PWM output and a low-pass filter to produce a control voltage, too.

If you want to try this modular approach, you can order a DEC Eva-Board evaluation board, part number 370652 from Maxon. Check the company's main Web site for a local sales office: http://www.maxonmotor.com. Digi-Key lists the price for an SA303-IHZ chip at $US 23.95, 1 piece, (598-1784-ND), and the price for an Eva-Board at $US 164.79 (598-1787-ND).

Second, Cirrus Logic has a BLDC motor-drive IC, the SA303-IHZ, in its Apex Precision Power product line. You will find a data sheet at: www.cirrus.com/en/pubs/proDatasheet/SA303U_A.pdf. The 64-pin quad flat-pack device supplies control logic for internal high- and low-side MOSFET switches that control the three coils in a BLDC motor. Because you can control individual MOSFETs, you could use this device to control any high-power load such as a relay or solenoid--not just a motor--within the current and voltage limits for the SA303. 

Internal fault logic detects a short-circuit or a temperature fault and two independent outputs signal these conditions to your microcontroller (MCU). A separate output indicates a current-limit condition and three analog outputs let an MCU measure the current that passes through each high-side P-channel FET. The fault outputs do not disable the SA303, so the MCU must respond to the faults and decide what to do.

The MCU delivers the pulse-width modulator (PWM) outputs to the SA303 chip to turn on or off the P- and N-channel FETs. Of course you do not want to turn on the high-side and now-side FET for an output simultaneously, which would cause a short circuit or "shoot through" condition through the FETs. That condition should not happen, but a glitch in your PWM firmware might cause it to occur. Thus, the SA303's protection circuit will turn off both FETs if this condition occurs. The data sheet does not indicate whether this short-circuit condition will trip the IC's SC output.

The SA303 operates with a 10-60-V supply and a 3A continuous rating. If you need higher currents, look at the SA306-IHZ and 306A-FHZ devices in the Apex family.

How do you drive BLDC motors? Do you use a module or design your own circuits?

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