3rd part
3.1.8. Full-Bridge DC-DC Converter Full-bridge dc-dc converter provides an opportunity to obtain across the load dc voltage components of the both polarities. For the implementation, the circuits of bridgetype and half-bridge can be applied (Fig.3.16).
a
b
+
V11
VT1
U1
VT3
V13 U1
Ld C V14
–
VT4
uld,ild
+
VT2
V12 –
C1
C2
VT3
V11
Ld VT2
V12
Fig. 3.16. The circuits of reversible DC-DC converters: bridge-type (a), half-bridge (b)
The control of circuit a is possible as separated and coordinated modulation. In circuit b, for providing DC voltage component over load, the both capacitors must be supported by two similar voltage sources, as the capacitors cannot be the dividers of dc-voltage. The separated modulation can be implemented with one pair of transistors, and it does not usually differ from typical step-down circuit operation. If one of the transistors of the pair (for example, VT1) is on all the time but the second one (VT2 in this case) is periodically changing its state from on to off then during the pause time the load current flows through VT1 and diode V13 and the load voltage has two instantaneous values: U1, when both transistors are on, and zero, when VT2 is off. Thus, the positive load voltage (according to the direction defined in Fig. 31.6,a) is obtained with average value DU1, where D is the relative on-time (duty ratio) of a pair of transistors per period. In the case of separated control, the opposite polarity of the load voltage can be obtained with the second pair of transistors VT3, VT4, one of which is permanently on. This type of control is called single-polarity modulation, which is illustrated in Fig. 3.17. In the separated control, double-polarity modulation is also possible when instantaneous values of both voltage polarities are possible across the load. This mode of operation is available if both transistors of a pair are simultaneously off. When turned off the current is conducted by diodes (for example, after the transistors VT1 and VT2 simultaneously turn off diodes V14 and V13 continue to conduct), the current flows
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