Power stage

The first step in determining the component values for this compensation method is to determine the inherent dc gain of the power stages. The calculation should be performed with the maximum value of the input voltage, since this is where the system will have its widest exhibited bandwidth. This is done using Equation B.12. 

Power stage with high voltage amplifiers. 

The overall ability of a power supply to attenuate disturbances at its input is expressed as its PSRR (power supply rejection ratio). In graphs, PSRR is usually plotted as a function of frequency. We will invariably find that the rejection ratio is very low at higher frequencies. One reason for this is that the Bode plot cannot really help because the open-loop gain is very small at these frequencies. The other reason is, even a tiny stray parasitic capacitance (e.g., across the power switch and inductor) presents such a low impedance to noise frequencies (whatever their origin) that almost all the noise present at the input migrates to the output unimpeded. In other words, the power stage attenuation (which we had earlier declared to be Vo/Rin) is also nonexistent for noise (and maybe even ripple) frequencies. The only noise attenuation comes from the LC filter (hopefully). 

For all topologies, that is in fact a key requirement—that the control IC be powered off a clean rail. It is just that in a Buck, the input decoupling capacitor for the power stage and the decoupling capacitor for the control are often the same component. Though in some cases, it may be necessary, even for a Buck controller IC, to add a small RC going to its supply pin (typically a 10Q resistor and an additional 0.1 pF ceramic capacitor). 

Note that in the Buck and the Buck-Boost, the input capacitor is included in the critical path. That implies we need very good input decoupling in these topologies (for the power section). So, besides the necessary bulk capacitor for the power stage (typically a tantalum or aluminum electrolytic of large capacitance), we should also place a small ceramic capacitor (about 0.1 to IpF) directly between the quiet end of the switch (i.e., at the supply side) and the ground—and also as close as possible to the switch. 

Figure 11-11 Two Safety-approved Y-capacitors from Primary Ground to Protective Earth to Help Complete the CM Noise Loop Close to the Power Stage...

Figure 11-15 A High-voltage Decoupling Ceramic Capacitor Close to the Power Stage Helps Complete the Noise Current Loop...

Figure 3.16 Schematic diagram of a SiC gate turnoff thyristor-based, dc-ac inverter power stage.

Figure 5.1 Schematic of electronic load power stage.

A class AB amplifier is defined as an amplifier using a power stage that has output current flow for more than half, but less than all, of the input cycle (Gilbilisco 1994). 

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