Bandwidth limits inPWM switching amplifiers

PWM buckswitching powerconverters aregood frequencyfs onedecade overthesignal bandwidthf, (in theorder candidates forhighefficiency poweramplification of oftensofMHz)renders thetechnique impractical because ofthe arbitrary band-limited signals. Thecorrelation between the dramatic increase inswitching losses thatresults inseriously signal bandwidth andtheswitching frequency, inturn reduced efficiencies. related toswitching losses, imposes practical limits forhigh Inthispaper, we examine design criteria fortheswitching bandwidth applications. Stringent specifications appearin frequency tosignal bandwidth ratio fslf, that guarantees agiven such applications as audioamplifiers (kHz signal signal fidelity, defined bybounding themodulation error. Note bandwidths), andlately, infastenvelope tracking power that this paper concentrates onthein-band error duetoaliasing amplifiers (MHz signal bandwidths) fortheEnvelope components inherent tothePWM modulation. Signal distortion Elimination andRestoration technique inpolarRF power duetononlinearities ofthepowerconverter andout-of-band amplification. Bandwidth limitations inPWM amplifiers are errors associated withswitching ripples areoutofthescopeof explored inthisworkby proposing designcriteria for this paper. Figure 1qualitatively showstheaddressed problem, obtaining theswitching frequency tosignal bandwidth ratio illustrating howdifferent ratios fslfx result indifferent in-band (f,/f) soastoguarantee a given aliasing error.Toachieve interference powerlevels. thatpurpose, PWM spectra arereviewed forsingle tone, two- toneandmultitone signals. Subsequently, bytaking into ; accounttheanalogybetweenPWM and FM spectra, M bandwidths aroundtheswitching frequency areestimated by extending Carson's ruletoanarbitrary error. Thisallows obtaining anextension oftheNyquist criterion forpulse- widthmodulation. System-level simulation results are reportedto validate theanalysis, showingthatthe conventional f/f. factor usedinthepowerconverter design Figure 1.Evolution ofPWMspectra ofamultitone signal fordifferent field might betooconservative. switchingfrequency tobasebandfrequency ratios.