AN857 データシートの表示（PDF） - Microchip Technology

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AN857

Brushless DC Motor Control Made Easy

Microchip Technology 

AN857

The numbers at the top of Figure 2 correspond to the

current phases shown in Figure 1. It is apparent from

Figure 2 that the three sensor outputs overlap in such

a way as to create six unique three-bit codes corre-

sponding to each of the drive phases. The numbers

shown around the peripheral of the motor diagram in

Figure 1 represent the sensor position code. The north

pole of the rotor points to the code that is output at that

rotor position. The numbers are the sensor logic levels

where the Most Significant bit is sensor C and the Least

Significant bit is sensor A.

Each drive phase consists of one motor terminal driven

high, one motor terminal driven low, and one motor ter-

minal left floating. A simplified drive circuit is shown in

Figure 3. Individual drive controls for the high and low

drivers permit high drive, low drive, and floating drive at

each motor terminal. One precaution that must be

taken with this type of driver circuit is that both high side

and low side drivers must never be activated at the

same time. Pull-up and pull-down resistors must be

placed at the driver inputs to ensure that the drivers are

off immediately after a microcontroller Reset, when the

microcontroller outputs are configured as high-imped-

ance inputs.

Another precaution against both drivers being active at

the same time is called dead-time control. When an

output transitions from the high drive state to the low

drive state, the proper amount of time for the high side

driver to turn off must be allowed to elapse before the

low side driver is activated. Drivers take more time to

turn off than to turn on, so extra time must be allowed

to elapse so that both drivers are not conducting at the

same time. Notice in Figure 3 that the high drive period

and low drive period of each output, is separated by a

floating drive phase period. This dead time is inherent

to the three-phase BLDC drive scenario, so special tim-

ing for dead-time control is not necessary. The BLDC

FIGURE 3:

THREE PHASE BRIDGE

+VM

commutation sequence will never switch the high-side

device and the low-side device in a phase, at the same

time.

At this point we are ready to start building the motor

commutation control code. Commutation consists of

linking the input sensor state with the corresponding

drive state. This is best accomplished with a state table

and a table offset pointer. The sensor inputs will form

the table offset pointer, and the list of possible output

drive codes will form the state table. Code development

will be performed with a PIC16F877 in an ICD. PORTC

has arbitrarily been assigned as the motor drive port

and PORTE as the sensor input port. PORTC was

chosen as the driver port because the ICD demo board

also has LED indicators on that port so we can watch

the slow speed commutation drive signals without any

external test equipment.

Each driver requires two pins, one for high drive and

one for low drive, so six pins of PORTC will be used to

control the six motor drive MOSFETS. Each sensor

requires one pin, so three pins of PORTE will be used

to read the current state of the motor’s three-output

sensor. The sensor state will be linked to the drive state

by using the sensor input code as a binary offset to the

drive table index. The sensor states and motor drive

states from Figure 2 are tabulated in Table 1.

+VM

+VM

A High

control

A Low

control

B High

control

To A

B Low

control

-VM

C High

control

To B

C Low

control

-VM

To C

-VM

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DS00857B-page 3

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