MCP8026 datasheet(32/64 Pages) MICROCHIP

32 / 64 page

MCP8025/6

DS20005339B-page 32

 2016 Microchip Technology Inc.

Ramp Mode

At the end of the Lock mode, Ramp mode is entered. In

Ramp mode, the microcontroller steps through the

commutation state machine, increasing linearly, until a

minimum speed is reached. Ramp mode is an

open-loop commutation. No knowledge of the rotor

position is used.

Run Mode

At the end of Ramp mode, Run mode is entered. In Run

mode, the Back EMF sensor is enabled and

commutation is now under the control of the phase lock

loop. Motor speed can be regulated by an outer speed

control loop.

4.3.3.3

PWM Speed Control

The inner commutation loop is a phase-lock loop,

which locks to the rotor’s position. This inner loop does

not attempt to modify the position of the rotor, but

modifies the commutation times to match whatever

position the rotor has. The outer speed loop changes

the rotor velocity and the inner commutation loop locks

to the rotor’s position to commutate the phase at the

correct times.

The outer speed loop pulse-width modulates the motor

drive inverter to produce the desired wave shape and

voltage at the motor. The inductance of the motor then

integrates this pulse-width modulation (PWM) pattern

to produce the desired average current, thus controlling

the desired torque and speed of the motor. For a

trapezoidal

BLDC

motor

drive

with

six-step

commutation, the PWM is used to generate the

average voltage to produce the desired motor current

and motor speed.

There are two basic methods to pulse-width modulate

the inverter switches. The first method returns the

reactive energy in the motor inductance to the source

by reversing the voltage on the motor winding during

the current decay period. This method is referred to as

fast decay or chop-chop. The second method

circulates the reactive current in the motor with minimal

voltage applied to the inductance. This method is

referred to as slow decay or chop-coast.

The preferred control method employs a chop-chop

PWM for any situation where the motor is being

accelerated, either positively or negatively. For

improved efficiency, chop-coast PWM is employed

during steady-state conditions. The chop-chop speed

loop is implemented by hysteretic control, fixed off-time

control or average current mode control of the motor

current. This makes for a very robust controller, as the

motor current is always in instantaneous control.

The motor speed presented to the chop-chop loop is

reduced by approximately 9%. A fixed-frequency PWM

that only modulates the high-side switches implements

the chop-coast loop. The chop-coast loop is presented

with the full motor speed, so, if it is able to control the

speed, the chop-chop loop will never be satisfied and

will remain saturated. The chop-chop remains able to

assume full control if the motor torque is exceeded,

either through a load change or a change in speed that

produces acceleration torque. The chop-coast loop will

remain saturated, with the chop-chop loop in full

control, during start-up and acceleration to full speed.

The bandwidth of the chop-coast loop is set to be

slower than the chop-chop loop so that any transients

will be handled by the chop-chop loop and the

chop-coast loop will only be active in steady-state

operation.

TABLE 4-3:

COMMUTATION STATE MACHINE

State

Outputs

BEMF

Phase

HSA

HSB

HSC

LSA

LSB

LSC

CE = 0

OFF

N/A

BOOTSTRAP

OFF

N/A

LOCK

OFF

N/A

OFF

Phase B

OFF

Phase A

OFF

Phase C

OFF

Phase B

OFF

Phase A

OFF

Phase C

MCP8026 Datasheet(PDF) 32 Page - Microchip Technology

MCP8026 Datasheet(HTML) 32 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Datasheet Download

Link URL