Switching Regulators

If Vi is the supply voltage, and the on and off times for the electronic switch are foN and foFF. then it can be shown that the output voltage V2 is given by 

It is also clear that the ripple depends on the frequency - at higher frequency, the ripple is less. However, each tum-on and turn-off involves the loss of some energy, so the frequency should not be too high. A control circuit is needed to adjust foN to achieve the desired output voltage - such circuits are readily available from many manufacturers. The main energy losses in the step-down chopper circuit are 

In practice all these can be made very low. The efficiency of such a step-down chopper circuit should be over 90%. In higher voltage systems, of about 100 V or more, efficiencies as high as 98% are possible. 

Current supplied by fuel cell during the On time 

Current circulating through diode during the Off time 

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More than 30 years ago Jacob and Monod introduced the Escherichia coli lac operon as a model for gene regulation. The lac repressor molecule functions as a switch, regulated by inducer molecules, which controls the synthesis of enzymes necessary for E. coli to use lactose as an energy source. In the absence of lactose the repressor binds tightly to the operator DNA preventing the synthesis of these enzymes. Conversely when lactose is present, the repressor dissociates from the operator, allowing transcription of the operon. 

From general design requirements, choose a switching regulator topology... 

Small GTPases are not isolated molecular switches regulating cellular processes. Signalling cascades within one subfamily as well as cross-talk between... 

Nuoffer, C. and Balch, W. E. GTPases multifunctional molecular switches regulating vesicular traffic. Annu. Rev. Biochem. 63 949-990,1994. 

Lawrence, R.J., Earley, K., Pontes, O., Sdva, M., Chen, Z.J., Neves, N., Viegas, W. and Pikaard, C.S. (2004) A concerted DNA methylation/histone methylation switch regulates rRNA gene dosage control and nucleolar dominance. Molecular Cell, 13, 599-609. 

Hantschel O, Nagar B, Guettler S et al. A myristoyl/phosphotyrosine switch regulates c-Abl. Cell 2003 112 845-857. 

Figure 5 shows a typical FET LC circuit. The FET acts as a switch regulating charge transfer between the column line and the LC cell. Electri-... 

Ho Y-K, Hingorani VN, Navon SE, et al. (1989) Transducin A signalling switch regulated by guanine nucleotides. In Curr. Top. Cell. Regul. 30 171—202. 

We have seen that one reason why switching regulators have such a high efficiency is because they use a switch (rather than a transistor that thinks it is a resistor, as in an LDO). Another root cause of the high efficiency of modern switching power supplies is their effective use of both capacitors and inductors. 

Note also that in either of these RC-based bucket regulator implementations, the switch ultimately ends up being toggled repetitively at a certain rate — and in the process, a rather crudely regulated stepped down output dc rail is created. By definition, that makes these regulators switching regulators too ... 

So we attempt to use an inductor instead of a resistor for the purpose — we don t really have many other component choices left in our bag In fact, if we manage to do that, we get our first modern LC-based switching regulator — the buck regulator (i.e. step-down converter), as also presented in Figure 1-2. 

Note There are some other capacitor-based possibilities — in particular charge pumps — also called inductor-less switching regulators. These are usually restricted to rather low powers and produce output rails that are rather crudely regulated multiples of the input rail. In this book, we are going to ignore these types altogether. 

In using conventional LC-based switching regulators, we may have noticed that their constituent inductors and capacitors do get fairly hot in most applications. But if, as we said, these components are reactive, why at all are they getting hot We need to know why, because any source of heat impacts the overall efficiency And efficiency is what modern switching regulators are all about ... 

We will also learn that the so-called voltage-mode control switching regulators actually rely on the ESR of the output capacitor for ensuring loop stability — even under normal operation. As indicated previously, loop stability refers to the ability of... 

Figure 1-9 Different Operating Modes of Switching Regulators...

Now we try to understand how our preceding analysis pertains to an actual switching regulator application — in particular, what Vin and Io are, with respect to the topology. 

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