Inductor ripple

Inductor ripple current, peak-to-peak value, Steady State PWM Duty Cycle, range limits from 0 to 100 L Ipp 12.0 A Duty Cycld 50.4... 

Average input current 5 [inductor ripple current, peak-to-peak value Jin Avg [i-IPP 0.93 A (0.65 A... 

Inductor ripple current, peak-to-peak value... 

L - Inductance of the supply source L, - Inductor to smooth ripples... 

The major concern of both output and input filter capacitors is the ripple current entering the capacitor. In this application, the ripple current is identical to the inductor ac current. The maximum limits of the inductor current is 2.8 A for I peak and about one-half the maximum output current or 1.0 A. So the ripple current is 1.8 A p-p or an estimated RMS value of 0.6 A (about one-third of the p-p value). 

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). 

We are also seeing another pattern emerge here, that the inductor and its associated traces are not critical in any topology. That is because the inductor smooths out the current through it, so obviously no edges of current pass through it. The slowly undulating inductor current has ripple, but not noise We need not pay very close attention to it, except to keep it away from sensitive nodes, in particular the feedback trace. 

The value of the inductor may be increased above the minimum recommended value to reduce input and output ripple. However, once the ripple current is less than 20% of the average current in the inductor, the benefit to output ripple becomes minimal. 

The results of the breadboard waveforms and the IsSpice waveforms are compared side by side in Figs. 4.15 and 4.16. Figure 4.15 shows the output ripple voltage at the top, with the inductor voltage at the bottom. Figure 4.16 shows the oscillator frequency at the top, with the... 

Figure 4.15a IsSpice LM78S40 waveforms (top, output ripple bottom, inductor voltage).

The transient domain model shown in Fig. 4.33 was used to measure output ripple voltage, transient response, gate voltage, and inductor current. This model properly predicts the cycle-by-cycle switching effects of the regulator. 

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