Column: DC/DC converters
The difference between the operating limits of a buck and buck-boost converter By By Lars Foerster, DC/DC Business Development Manager EMEA, TDK-Lambda
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uck and buck-boost DC-DC non-isolated converters are widely used in power supply designs. Due to their simplicity, low cost and
high efficiency, the topologies have remained extremely popular. No transformer is used in the design
as there is no requirement for isolation between the input and output voltages, with the only large wire-wound component being an inductor. A buck converter provides a voltage lower than the input voltage, whereas a buck-boost can deliver an output voltage that is either higher, equal to or lower than the input voltage; see Figures 1 and 2. Compared to an isolated DC-DC
converter, the package size for a given power level is smaller. For many users, the lack of input-to-output isolation is not an issue, as the AC-DC power supply will have an isolation barrier between the AC input and the DC output.
Limitations As with any AC-DC power supply or DC-DC converter, there are limits to their operation. A certain level of care must be given to ensure they are appropriately operated, according to the manufacturer’s datasheets. The main restriction of a non-isolated
(step-down) buck DC-DC converter is that the input voltage has to be higher than the output voltage. An example is TDK-Lambda’s 250W-rated i6A4W010A033V-001-R, with an input range of 9-53Vdc
, 3.3-40V output
adjustment range, and a maximum output current of 10A. The relationship between the input and the allowable output voltage is shown in Figure 3.
12 April 2021
www.electronicsworld.co.uk TDK-Lambda’s i7C Series 300W buck-boost DC-DC converter Figure 1: Buck (step-down) converter
Figure 2: Buck-boost (step-up/step-down) converter
Note that it is the maximum output power that restricts the amount of output current that it can produce. A non-isolated buck-boost (step-
up/step-down) DC-DC converter has different constraints to its operation, besides the rated output power,
current and voltage. The TDK-Lambda i7C4W008A120V-001-R, for instance, has an input voltage range of 9-53V, 9.6-48V output adjustment range, and a maximum output current of 8A. Its operating range is defined by two plots (Figures 4 and 5), which show the valid
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