Martin  Oßmann
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Small DC/DC-Converters

DC-DC converters under the magnifying glass

Small DC/DC-Converters
Switch-mode power supplies are not among the hobbyist’s favourite circuits. The specialised components are often hard to obtain, and mistakes are rewarded with noisy and expensive results. If, however, we stick to low voltages and very low power, there are a few universal and readily-modifiable circuits that illustrate the most important principles.The function of a DC-DC converter is to change one DC voltage into another. The resulting voltage can be higher, lower, inverted and possibly isolated with respect to the input voltage. There are many types of converter, of which we shall examine a few here.The basic circuits are shown together in Figure 1. The input voltages are labelled Uin, the output voltages Uout. All the circuits shown here have the common feature that they employ a switch S, a diode D and a coil L to carry out the essential functions of the circuit.
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In Figure 3, point ‘P’ (mentioned in the text) is not indicated. The relevant point is the junction of the collector of T1, choke L1, resistor R1, the cathode of diode D1 and the base of T2.
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