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How do non-isolated buck, boost, and buck-boost converters differ?

Posted by Martin on October 21, 2020

Non-isolated buck, boost and buck-boost topologies are utilised in the designs of many AC-DC power supplies and non-isolated DC-DC converters. Although many of the patents were filed as early as the 1970s, their simplicity, low cost and very high efficiencies make them attractive even now. Standard, off-the-shelf products are still being introduced, taking advantage of new techniques like digital control, improved components, and more efficient magnetic materials.

The main difference between an isolated and a non-isolated DC-DC converter is the transformer or lack of it. In an isolated converter, the transformer provides a safety barrier between the DC input (primary) and the DC output (secondary). Non-isolated converters are powered by low voltage batteries or by an AC-DC power supply which already contains the safety barrier.

Non-isolated converters fall into three main categories – buck, boost and buck-boost. To make the following schematics more understandable, they are drawn using switches rather than transistors, and diodes instead of synchronous rectifier circuits.

A buck converter reduces voltage, and the output voltage is lower than the input voltage. See Figure 1.

Figure 1: Buck converter

When transistor (S) is turned on, energy is stored in inductor (L) as the current flows through it to the load and also charges capacitor (C). When S is turned off, the energy stored in L is released and current flows into the load and circulates via diode (D). Capacitor (C) also provides some energy to the load. This is repeated at high frequencies, greater than 100,000 times a second. The length of the time transistor S is turned on and off defines the output voltage.

A boost converter increases voltage and the output voltage is higher than the input voltage. See Figure 2.

Figure 2: Boost converter

When transistor (S) is turned on, current flows through inductor (L), 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. This 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.

This boost converter topology is used in the Power Factor Control (PFC) section of most AC-DC power supplies. The control IC is different, of course, as its purpose is to ensure the AC input current drawn is sinusoidal in shape. At high line voltages greater than around 250Vac the DC input may be higher than the voltage on capacitor (C). This will reduce the PFC boost converter’s performance and the power factor will be degraded slightly as the converter is not operating in boost mode.

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.

Figure 3: Buck-boost converter

As you can see the circuit is more complicated and has more components. S2, L, and D2 is the boost converter section and S1, L and D1 the buck section.

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Figure 4: TDK-Lambda’s i7C Series 300W buck-boost DC-DC converter

Many manufacturers like TDK-Lambda offer both buck and buck-boost converters as standard products. With fewer components and complexity, a buck converter will offer a lower cost, higher efficiency, and either a smaller package or more output power than a buck-boost converter.

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