Buck converter 1

ТЛПР

ShcA: cortverter 2

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Fig, 6.11 Deiivution ol bi itlgc inveitcr (H-bridge); (a) differemial connection of load across ontpiils of buck converters, (b) bypassing load by capacitor, (c) combining series inductors, (d) circuit (c) redrawn in its usual form.

+ V -V\r-

Fig, 6.11 Continued

1 D

Fig, 6.12 Conversion ratio of the H-bridge inverter circuit.

6.1 Circuit Manipiuations

age V2 = DV The differential load voltage is

(6.12)

Slitiplifixation leads to

V = i2D- l)V,

(f,.B)

This equation is plotted in Fig. (1.12. It tan be seen the output voltage is positive for£> > 0,5, and negative for D < 0.5. If the duty eyele is varied sinusoidally about a quiescent operating point of 0.5, then the otit-put voltage v/ill be sinusoidal, with node bias.

The circuit of Fig, (i.l 1(a) can be simplified. It i.s usually desired to bypass the load direcl:ly with a capacitor, as in Fig. 6.11(b). The two inductors are now effectively in series, and can be combined into a single inductor as in Fig. fj. 11(c). Figure 6.11(d) is identical to Fig. 6.11(c), but is redrawn for clarity. This circuit is commonly called the H-bridge, or bridge inverter circuit. Its use is widespread in servo amplifiers and single-phase inverters. Its properties are siinilar to those of the buck, converter, from which it is derived.

Polyphase inverter circuits can be derived in a similar manner. A three-phase load can be connected differentially across the outptits of three dc~dc converters, as illustrated in Fig. 6.12. If the three-phase load is balanced, then the neutral voltage K will be equal to the average ofthe three converteroutput voltages:

(6.14)

dc source

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Converter 1

Converter 2

Converter 3

1 ТГ

I-

Зас load

-CZh

Fig. 6.12 tieneiatioii of dt;-30ac itiverter by differential connection of Щ load.