Buck converter

Figure 3-1 A basic forward-mode converter (buck converter shown).

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.

A Board-level 10 Watt Step-down Buck Converter... 

Figure 3-64 The gain and phase Bode plots for the design example 3.15.1 (a) the phase plot for the buck converter (b) the phase plot for the buck converter.

Figure 4-3 Major parasitic elements within converters (a) buck converter (b) flyback converter.

The zero current switching (ZCS) quasi-resonant (QR) switching power supply forces the current through the power switch to be sinusoidal. The transistor is always switched when the current through the power switch is zero. To understand the operation of a ZCS QR switching power supply, it is best to study in detail the operation of its most elementary topology—the ZCS QR buck converter (and its waveforms) as seen in Figure 4-10. 

Figure 4-10 The schematic and waveforms of a ZCS quasi-resonant buck converter.

A second type of quasi-resonant converter is the zero voltage switching (ZVS) quasi-resonant family. A ZVS QR buck converter and its waveforms are shown in Figure 4-11. Here the power switch remains on most of the time and performs resonant off periods to decrease the output power. Actually, the ZCS and the ZVS families mirror one another. If you were to compare the switch voltage and current waveforms between the two families, and if one inverts both the voltage and current waveforms in order to reference them to the power switch, the waveforms would have a striking resemblance to one another. 

Figure B-10. Figure B-10 shows the presence of a transformer. For the buck converter, the designer can assume that the turns ratio is 1 1.

Figure 2-11 Input Ripple of a Buck Converter Showing the Effect of a Non-ideal Bulk Capacitor...

The following arguments apply to a Synchronous Buck converter, too (with the Schottky diode placed across the lower Fet), but the effects can be much more severe in a Boost because of the typically higher voltages involved. 

The first question you need to ask is, is your efficiency really bad For example, if you have a worldwide input Flyback of around 70W, you should not be expecting much better than 70% at an input of 90VAC (for the common 5V/12V output rail combinations). For a Synchronous Buck converter, you can expect around 90% at max load, but at very light loads the efficiency will fall much lower. So first assure yourself you really have a problem. And don t forget that this measurement needs Kelvin sensing as described previously (see Chapter 2). 

By the time-sharing principle, we see that in a Buck converter if Vsw is close to VD, the conduction losses do not change with duty cycle or input voltage. But the switching losses progressively increase, and so the efficiency falls off smoothly (almost linearly) with increasing input. See Figure 10-7 for the curve marked Vsw = VD. An example of this is the... 

Figure 10-7 Possible Variations of Efficiency Curves for Buck Converters...

Figure 3.4 Ideal voltage and current waveforms for the buck converter operating in the continuous conduction mode.

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