AN2154 datasheet(9/16 Pages) STMICROELECTRONICS

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AN2154

The PWM generator block of ST7MC motor control peripheral for SVPWM

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There are several possible ways to distribute time t0 (associated to one of the two null

sectors) inside the PWM period.

The first possibility is shown in Figure 6. It utilizes ST7MC Motor control peripheral 12-bit

PWM generator in edge-aligned mode.

Please note that the edge alignment implies the presence of the passage between the null

state 000 and the null state 111 and, then, the need of three contemporary switches

commutation.

Literature demonstrates that, in order to reduce the number of commutation and switching

losses, it is preferable to utilize a states sequence where successive states are also

adjacent. This means that passing from one state to the successive one should occur with

only one switch commutation.

This implies that the null state 111 must be always situated after one of the states 110, 101,

011, and null state 000 must be always situated after one of the states 100, 001, 010.

Figure 7 shows a typical switching pattern generated by ST7MC Motor control peripheral 12

bit PWM generator in "center-aligned" mode. It can be observed that the above rules are

respected.

Please be aware that, to be coherent with what has been stated in the previous paragraph,

the PWM period should be reported as T0, while in Figure 7 it has been indicated as 1/fs. It

is convenient, in fact, from this point on, to indicate with T0 only a half of the PWM period. It

is easy to understand that Equation 10 - 12 are still valid in the case that ta, tb are taken as

shown in Figure 7.

Please observe that time t0, associated with the null states, has been divided into two parts:

t0‘ is relative to the permanency time of state 111 and t0‘‘ is relative to the permanency time

of state 000. Since Equation 12 does not give any information about the values of these two

times, their choice could be arbitrary. Nevertheless, literature demonstrates that the best

total harmonic distortion on line to neutral voltage and on motor current is achieved when:

Equation 13

In this case the modulation is called symmetrical SVPWM.

Figure 6.

Edge-aligned SVPWM patterns

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