Column: DC/DC converters
The capacitor (C) also provides
some energy to the load. This process is repeated at high frequencies, over 100,000 times a second. The time length at which S is turned on and off defines the output voltage.
Boost converters A boost converter increases voltage, with the output voltage higher than the input voltage; see Figure 2. When transistor (S) is turned on,
current flows through inductor (L), and through transistor S back to the input. During this period, energy is again stored in the inductor. When transistor (S) is turned off, the
inductor acts a voltage source in series with the input voltage.
The inductor’s stored energy is
circulated through diode (D) to the load. Tis charges capacitor (C) to a higher level than the input voltage. Again, the length of the time transistor (S) is turned on and off defines the output voltage. Tis boost converter topology is used
in the power factor control (PFC) section of most AC-DC power supplies. Te control IC is different, of course, since its purpose is to ensure the AC input current drawn is sinusoidal in shape. At line voltages greater than around 250Vac, the DC input may be higher than the voltage across capacitor (C). Tis will reduce the PFC boost converter’s performance and its power factor will be degraded slightly, since the converter is not operating in boost mode.
Buck-boost converters A buck-boost converter is a combination of a buck and a boost converter. The output voltage can be higher or lower than the input voltage; see Figure 3. This circuit’s topology is more
complicated and has more components. S2, L and D2 form the boost converter section, whereas S1, L and D1 are the buck section.
Availability Many manufacturers like TDK-Lambda offer both buck and buck-boost converters as standard products. With fewer components and lower complexity, a buck converter will be cheaper, more efficient and smaller or with more output power than a buck-boost converter.
Figure 1: Buck converter
Figure 2: Boost converter
Figure 3: Buck-boost converter
www.electronicsworld.co.uk December/January 2021 09
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