AN2267

Outline to various approaches

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Outline to various approaches

A BLDC motor driven in a conventional 6-step method greatly resembles a brushed DC

motor. Hence, one may choose to regulate the average DC link current. But this actually

results in constant power operation for the motor because at constant DC link voltage, if the

average link current is regulated at a certain value, it effectively regulates the power at that

point for any variation in motor load, and the average load current / motor torque varies

inversely with speed depending on the load. Any effort to compensate the average DC link

current data with the duty cycle to obtain average phase current will be impaired by a filter

time constant, rendering this option ineffective.

Since the DC link current does not reveal winding currents during PWM “off time”, one may

choose to monitor all 3 winding currents and build a regulator. But this requires two current

sensors to monitor any two phase currents, while the third phase current can be

reconstructed from these two. However, the cost of these sensors makes this option

expensive.

A third option would then be to regulate the peak current per PWM period. Though it is

inexpensive and easy to implement, it is not exactly linear. During PWM on time, at lower

duty cycles, when both speed and BEMF are small, the phase current rises much faster

than at higher duty cycles when the speed and BEMF are large. The same peak currents

per PWM period represent different average currents at different duty cycles. An intuitive

geometric approach will reveal this as shown in Figure 1. A typical variation in average

current vs duty cycle at a given peak current reference is shown in Figure 2.

Figure 1.

Peak current regulation at different duty cycles with BEMF load

Figure 2.

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dutycycle

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