Non-isolated buck, boost, and buck-boost topologies are widely used in AC-DC power supply and non-isolated DC-DC converter designs. Although many patents have been filed as early as the 70s of the 20th century, their simple lines, low cost and high efficiency have made them continue to be used today. New technologies such as digital control, improved components and more efficient magnetic materials are also used in new products.
The main difference between isolated and non-isolated dc-dc converters is the presence or absence of a transformer. In isolated converters, a transformer is used to provide a safety barrier between the DC input (primary) and the DC output (secondary). Non-isolated converters typically use a low-voltage battery or an AC-DC power supply with a safety barrier as the input source.
There are three main types of non-isolated converters—buck, boost, and buck-boost. For ease of understanding, a switch is used instead of a transistor, and a diode is used instead of a synchronous rectification circuit.
A buck converter, also known as a buck converter, has an output voltage lower than the input voltage, as shown in Figure 1.
Figure 1 Buck converter.
When the transistor(s) is turned on, current flows through the inductor L and stores energy in the inductor L while charging capacitor C. When the transistor S is disconnected, the energy stored in the inductor L is discharged through the load and the diode D loop, and the capacitor C also provides part of the energy to the load. The switching frequency is usually greater than 100kHz. The magnitude of the output voltage depends on the time when the transistor is on and off.
A boost converter, also known as a boost converter, has an output voltage higher than the input voltage. Refer to Figure 2.
Figure 2 Boost converter.
When transistor S is turned on, the current is returned to the input through the inductor L and transistor S loop. During this time, energy is stored in the inductor. When the transistor S is disconnected, the inductor L acts as a voltage source in series with the input voltage. The energy storage of the inductor is released through the diode d and the load loop. The capacitor (c) can be charged to a higher level than the input voltage, and the output voltage depends on the time when the transistor S is turned on and off.
The boost converter topology is applied to the power factor correction (PFC) portion of most AC-DC power supplies. Of course, the control ICs used are different, and the purpose of the PFC is to ensure that the input current waveform is sinusoidal. When the input voltage is greater than 250VAC, the DC input may be higher than the voltage on capacitor C. This will reduce the performance of the PFC boost converter and the power factor will be slightly reduced since the converter is not operating in boost mode.
A buck-boost converter is a combination of a buck and boost converter. The output voltage can be higher or lower than the input voltage. Refer to Figure 3
Figure 3 Buck-boost converter.
As shown in the diagram above, buck-boost converter circuits are more complex and require more components. S2, L, and D2 make up the boost conversion part, and S1, L, and D1 make up the buck conversion part.
TDK-Lambda offers buck and buck-boost converters as standard. Compared with buck-boost converters, buck converters have the advantages of fewer components, low complexity, low cost, high efficiency, small size and larger output power.
Figure 4 i7C series 300W buck-boost dc-dc converters.
Figure 5 I3A series 100W BUCK DC-DC converters.
Figure 6 I6A series 250W BUCK DC-DC converters.
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