Las relaciones SPA y Delito

The AC/DC converter topology shown in Figure 4.33 is composed of a three-phase diode rectifier with two identical series-connected capacitors and three bidirectional switches

Implementing Power Factor Correction in AC/DC Converters 125

D+ D–

D–

S

D+

FIGURE 4.34 Construction of a bidirectional switch.

(S_a, S_b, and S_c). The switches consist of four diodes and a MOSFET to form a bidirectional switch (see Figure 4.34).

These bidirectional switches are controlled by using HCC to ensure good supply current waveform, constant DC-link voltage, and accurate voltage balance between the two capac-itors. In Figure 4.33, the voltage sources vsa, v_sb, and vscdenote the three-phase AC system.

The waveforms and the current of phase a (isa) are shown in Figure 4.35.

For the circuit analysis (Figure 4.35), six topological stages are presented, corresponding to a half-cycle (0^◦ to 180^◦), which refer to the input voltage v_sa shown in Figure 4.35; for simplicity, only the components where current is present are pictured at each of those intervals.

In the interval between 0^◦and 30^◦(see Figure 4.36a and b), the polarities of the source voltages v_saand v_scare positive with that of v_sbnegative. When the bidirectional switch S_ais on, the source current i_saflows through S_a, and diodes D₅and D₆are on. The other diodes not shown in Figure 4.36a are off. When the bidirectional switch S_ais off, the current i_saflowing through the input inductor is continued through diode D₁and diodes D₅and D₆are still on.

The other diodes not shown in Figure 4.36b are off. The current commutation from S_ato D₁is at a certain moment determined by HCC. Diodes D₅and D₆offer a conventional rectifying wave. Switch S_aand diode D₁turn on exclusively, and offer the active compensation current.

In the interval between 30^◦and 60^◦(see Figure 4.36c and d), the polarities of the source voltages vsaand vscare positive with that of v_sbnegative. When the bidirectional switch Scis on, the source current iscflows through Sc, and diodes D₁and D₆are on. The other diodes not shown in Figure 4.36c are off. When the bidirectional switch S_cis off, the current i_scflowing through the input inductor continues through diode D₅, and diodes D₁and D₆are still on.

v_sa v_sb v_sc

v

0

wt i_sa

FIGURE 4.35 Waveforms of source voltages and current of phase a, i_sa.

v_sc v_sc is off, the current isaflowing through the input inductor is continued through the diode D₁, diodes D₅and D₆are still on. (c) 30^◦–60^◦; Scis on; (d) 30^◦–60^◦; Scis off, the current iscflowing through the input inductor is continued through the diode D₅, diodes D₁and D₆are still on. (e) 60^◦–90^◦; Scis on; (f) 60^◦–90^◦; Scis off, the current isc flowing through the input inductor is continued through the diode D₂, diodes D1 and D6 are still on. (g) 90^◦–120^◦; S_bis on; (h) 90^◦–120^◦; S_bis off, the current i_sbflowing through the input inductor is continued through the diode D₆, diodes D₁and D₂are still on. (i) 120^◦–150^◦; S_bis on; (j) 90^◦–120^◦; S_bis off, the current i_sbflowing through the input inductor is continued through the diode D₃, diodes D₁and D₂are still on. (k) 150^◦–180^◦; Sais on; (l) 150^◦–180^◦; Sais off, the current isaflowing through the input inductor is continued through the diode D₁, diodes D₃and D₂are still on.

Implementing Power Factor Correction in AC/DC Converters 127

The other diodes not shown in Figure 4.36d are off. The current commutation from S_cto D₅ is at a certain moment determined by HCC. Diodes D₁and D₆offer a conventional rectifying wave. Switch S_cand diode D₅turn on exclusively, and offer the active compensation current.

In the interval between 60^◦ and 90^◦(see Figure 4.36e and f), the polarity of the source voltage v_sais positive with those of v_sband v_scnegative. When the bidirectional switch S_cis on, the source current i_scflows through S_c, and diodes D₁and D₆are on. The other diodes not shown in Figure 4.36e are off. When the bidirectional switch Scis off, the current iscflowing through the input inductor is continued through diode D₂, and diodes D₁and D₆are still on.

The other diodes not shown in Figure 4.36f are off. The current commutation from S_cto D₂is at a certain moment determined by HCC. Diodes D₁and D₆offer a conventional rectifying wave. Switch S_cand diode D₂turn on exclusively, and offer the active compensation current.

In the interval between 90^◦and 120^◦(see Figure 4.36g and h), the polarity of the source voltage v_sais positive with those of v_sband v_scnegative. When the bidirectional switch S_bis on, the source current i_bflows through S_band diodes D₁and D₂are on. The other diodes not shown in Figure 4.36g are off. When the bidirectional switch S_bis off, the current i_bflowing through the input inductor continues through diode D₆, and diodes D₁and D₂are still on.

The other diodes not shown in Figure 4.36h are off. The current commutation from S_bto D₆ is at a certain moment determined by HCC. Diodes D₁and D₂offer a conventional rectifying wave. Switch S_band diode D₆turn on exclusively, and offer the active compensation current.

In the interval between 120^◦and 150^◦(see Figure 4.36i and j), the polarities of the source voltages v_saand v_sbare positive with that of v_scnegative. When the bidirectional switch S_bis on, the source current i_sbflows through S_band diodes D₁and D₂are on. The other diodes not shown in Figure 4.36i are off. When the bidirectional switch S_bis off, the current i_sbflowing through the input inductor continues through diode D₃and diodes D₁and D₂are still on.

The other diodes not shown in Figure 4.36j are off. The current commutation from S_bto D₃is at a certain moment determined by HCC. Diodes D₁and D₂offer a conventional rectifying wave. Switch S_band diode D₃turn on exclusively, and offer the active compensation current.

In the interval between 150^◦and 180^◦(see Figure 4.36k and l), the polarities of the source voltages v_saand v_sbare positive with that of v_scnegative. When the bidirectional switch S_ais on, the source current i_saflows through S_aand diodes D₃and D₂are on. The other diodes not shown in Figure 4.36k are off. When the bidirectional switch S_ais off, the current i_saflowing through the input inductor continues through diode D₁and diodes D₃and D₂are still on.

The other diodes not shown in Figure 4.36l are off. The current commutation from S_ato D₁is at a certain moment determined by HCC. Diodes D₃and D₂offer a conventional rectifying wave. Switch S_aand diode D₁turn on exclusively, and offer the active compensation current.

An active compensation circuit is composed of one of the bidirectional switches and an off-diode in the rectifier bridge legs, but the other legs act as a conventional rectifier. So there are two circuits in the VIENNA rectifier, namely the conventional rectifier circuit and the active compensation circuit. Thus, the load average real power is supplied by the source (the same as a conventional rectifier) and the active compensation circuit does not provide or consume any real power.