Buck-boost converters

Buck/Boost Converter Buck/Boost Converter... 

More sophisticated systems are able to remove charges from cells which have a higher voltage than the others and to bring them to cells which have a lower voltage in the series connection [58], Among the main converter topologies, the bidirectional nondissipative buck-boost converter [59,60] and the flyback converter [61,62] are the most studied. Active systems appear to be sophisticated circuits and thus may be expensive in relation to the cost of the cell they are intended to protect. 

Question 30 Which end of a given input voltage range Vinmin to Vinmax should we pick for starting a design of a buck, a boost, or a buck-boost converter ... 

Buck converter Boost converter Buck-boost converter Cuk converter... 

FIGURE 10.80 Converter types (a) buck converter, (b) boost converter, (c) buck-boost converter, (d) nonisolated Cuk converter. 

The flyback converter (Fig. 10.81) is an isolated version of the buck-boost converter. In this converter, when the transistor is on, energy is stored in the coupled inductor (not a transformer) and this energy is transferred to the load when the switch is off Flyback converters are used in the power range of 20-200 W. 

Figure 3-59 The major current loops within the major switching power supply topology types (a) the nonisolated buck converter (h) the nonisolated boost converter (c) the transformer-isolated converter.

The operation of a discontinuous-mode, flyback converter is quite different from that of a forward-mode converter, and likewise their control-to-output characteristics are very different. The topologies that fall into this category of control-to-output characteristics are the boost, buck/boost, and the flyback. The forward and flyback-mode converters operating under current-mode control also fall into this category. Only their dc value is determined differently. Their representative circuit diagram is given in Figure B-12. 

In Figure 6-5 we carry out analysis for a Flyback PCB, and realize that unlike a Forward Converter, even the output capacitor has to have very short interconnecting leads and trace lengths. That situation is similar to the Buck-Boost in Figure 6-3, from which the transformer-coupled Flyback is essentially derived. 

Off-line converters are derivatives of standard dc-dc converter topologies. For example, the flyback topology, popular for low-power applications (typically <100 W), is really a buck-boost, with its usual single-winding inductor replaced by an inductor with multiple windings. Similarly, the forward converter, popular for medium to high powers, is a buck-derived topology, with the usual inductor ( choke ) supplemented by a transformer. 

Answer Just as the isolated flyback is a derivative of the buck-boost topology, the forward converter is the isolated version (or derivative) of the buck topology. It too uses a transformer (and optocoupler) for providing the required isolation in high-voltage applications. Whereas the flyback is typically suited for output powers of about 75 W or less, the forward converter can go much higher. 

For a buck, we know that at turn-on, the instantaneous switch (and inductor) current is Io x (1 — r/2), where r is the current ripple ratio, and Io is the load current of the dc-dc converter. At turn-off, the current is Io x (1 + r/2). Usually, we can ignore the current ripple ratio and take the current as Io for both the turn-on and the turn-off analysis. So the load current of the dc-dc converter, Io, becomes the same as the Io used so far in the switching loss analysis. Similarly, in a boost and buck-boost, the current Io in our switching loss analysis, is actually the average inductor current Io/(l — D). 

The first requirement for the designer is to understand the flow of power-related currents in the converter. This leads to an identification of the troublesome or critical traces of the PCB we must pay the closest attention to these traces. We will also see that this identification process is very topology-dependent. So we can t, for example, design the PCB for a buck-boost, the same way we would do it for a buck. The rules change significantly We may thus also realize that very few PCB layout persons out there would understand this too well Therefore it really is a good idea for the power supply designer to do the layout personally, or at the very least, closely supervise the PCB person in the act. 

---