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Application Note

4.2

Efficient Implementation of the SV-PWM

Table 4-2. seems to show that the duty cycles have different expressions in each sector. A thor- ough study of these expressions show that since sin(x) = sin(π x) , all these duty cycles can be

written in a unified way as

dk =

2Vsm E3

sin(θ ′′)

and

d k +1

=

2Vsm E3

sin(θ )

, with

θ ′′ =

π 3

θ

and

θ ′ = θ (k 1)

π

3

. Since these expressions no longer depend on the sector number, they can be

denoted

as

da

and

db

. Since θ is always between 0 and

π 3

,

computing

da

and

db

requires

a

sine table for angles inside this interval only. This greatly reduces the amount of memory required to store this sine table.

The AT90PWM3 provides the 3 power stage controllers (PSC) to generate the switching wave- forms computed from the Space Vector algorythms.

The counters will count from zero to a value corresponding to one half of the switching period (as shown on the lower side of Fig. 4), and then count down to zero. The values that must be stored in the three compare registers are given in Table 4-3.

Figure 4-4.

Inverter switch waveforms and corresponding compare register values

5

7546A–AVR–12/05

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