AN414

Application description

Doc ID 1876 Rev 2

7/14

Figure 5.

PWM timer operation (Operation of the Autoreload PWM timer: The CPU

controls the period with the Reload register, and the duty cycle with the

Compare register.)

3.3

Voltage compensation

Using such feature on a Permanent magnet DC motor, the speed is kept almost constant by

keeping the motor voltage constant even if the rectified mains voltage is strongly swinging

across the filter capacitor. So a closed loop speed control is not necessary and the speed

sensor is saved.

A compensation feature is implemented in order to:

●

Obtain a motor speed insensitive to mains voltage variations (static performance),

●

Reduce acoustic motor noise and dissipation by reducing the motor current ripple

(dynamic performance).

The average motor voltage Vmot must be kept constant. The duty cycle d is modulated as a

function of the direct voltage Ud around a reference point given by:

●

User request duty cycle (d0),

●

Nominal direct voltage (Udnom).

Vmot = d x Ud

Vmot = constant = d0 x Udnom

To achieve this function, Ud is measured using the A/D converter of the ST6265C and

quantized on 32 levels, and d0 is quantized on 16 levels. A duty cycle correction is picked up

from a look-up table versus Ud and d0. Then the correction is added to d0 and the sum d is

loaded in the PWM timer register.

The table could also contain values chosen specifically for the application.

In order to reduce motor current ripple, the control loop must be fast, so as to minimize the

time between voltage reading and duty cycle updating.

TIMER

PWM OUTPUT

255

COMPARE

t

REGISTER

000

SWITCHING PERIOD

DUTY CYCLE

MS19693V1