PCIM: BLDC motor chip has built-in algorithm

Allegro MicroSystems has announced a 3-phase sensor-less brush-less dc (BLDC) fan driver IC with sinusoidal outputs to minimise audible noise and vibration.

Allegro A5931

A5931 on left, and conventional equivalent on right

The chip, called A5931, includes output mosfets and a built-in control algorithm running on a state-machine – easing ASIL automotive qualification, claimed the firm, because there is no microcontroller whose software would have to be examined in detail.

Motor parameters are stored in on on-chip EEPROM.

Interactive (with on-screen sliders) Windows-based design software is available to establish appropriate motor parameters, or they can simply be typed in if already known.

Allegro A5931 BsideA5931 on left needs only a single-sided board – on the right is the conventional equivalent, according to Allegro

Start-up, in some ways the trickiest part of sensor-less operation, is through an align-and-hold process – treating it as a stepper motor – to generate enough back emf to determine rotor position.

Also for development, a built-in test mode allows many – millions if necessary, a spokesman told Electronics Weekly at PCIM – automated re-starts from stationary to gather start reliability data. One PC can control multiple development boards for start testing.

Originally developed for 25,000rpm server fans, speed control is 5% over temperature and typically 1% – necessary because in servers, said the spokesman, fans are sometimes operated in tandem where efficiency drops if the fan speeds are not matched.

A second version of the chip – A5932 – drives external n-channel mosfets for larger motors and operates in both sine and trapezoidal wave – the latter to increase torque.

 

 

 


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