Randomized modulation schemes for digital modulators of switched-mode DC-DC converters
Date of Issue2018
School of Electrical and Electronic Engineering
Conventional switched-mode dc-dc converters usually operate based on the Pulse Width Modulation scheme (PWM). Nonetheless, the frequent switching-activities of the PWM result in strong harmonics at multiples of the switching-frequency in both the input current and output voltage spectra. An effective way to suppress the switching-frequency harmonics is to replace the PWM with randomized modulation schemes. The randomized modulation schemes can spread the harmonic power over the frequency spectrum to reduce the peak spectral power. Nonetheless, due to the randomization, the noise floor in the input current and output voltage spectra are usually increased. This noise floor will translate to ripple noise current at the input and ripple noise voltage at the output. This thesis pertains to the proposal, analysis, and verification of randomized modulation schemes that feature low-harmonic and low-noise for single- and multi-phase switched-mode dc-dc converters. The intended applications of the converters are portable devices where the circuit area is scarce, low level of Electromagnetic Interference is mandatory, and power source is limited. There are three proposed randomized modulation schemes in this thesis. The first proposed scheme is a noise-shaped randomized modulation scheme for applications in single-phase switched-mode dc-dc converters with low-harmonics and low input noise current requirements. An analytical expression for the input current spectrum of the proposed scheme is derived to analyze the harmonics and low-frequency noise therein. A novel pulse generator structure is proposed in order to realize a dc-dc converter that embodying the proposed scheme. Experimental measurements of the input current spectrum, the output voltage spectrum, the transient-response, and the operating range are carried out on the realized converter. On the basis of the measurements, the proposed scheme features very low peak spectral power in the input current spectrum (18.1 dB lower than the PWM at 3.3 V input voltage, 0.5 duty cycle, and 100 kHz average switching-frequency). The input noise current of the proposed scheme, obtained at ~73mA rms (integrated over a 200 kHz bandwidth without an input filter), and is comparable to that of the PWM. The second proposed randomized modulation scheme is the Randomized Wrapped-Around Pulse Position Modulation scheme (RWAPPM) with adjustable limits for applications in single-phase switched-mode dc-dc converters that require relatively low load current, low harmonics, and low output ripple noise voltage. This scheme can trade-off the output ripple noise voltage and the peak spectral power by varying/adjusting the limits on the randomized pulse position of the RWAPPM. An analytical expression of the output voltage spectrum is derived to provide an analytical means of calculating the trade-offs between the peak spectral power and the output ripple noise voltage. SPICE simulations and measurements on a dc-dc converter realized with discrete electronic components are conducted to verify the derived analytical expression. The results show that when the minimum (or maximum) limit is increased (or decreased) by 1% of the switching-period, the peak spectral power increases by 1 dB, whereas the ripple noise decreases by 0.6 mV. The ripple noise is improved by 9.1 mV and 10.7 mV respectively for the minimum limit value at 15% of the switching-period and the maximum limit value at 85% of the switching-period, as compared to that for the RWAPPM without adjustable limits. The third proposed randomized modulation scheme is denoted as RWAPPM scheme with Wrapped-Around Phase-Shift (RWAPPM+WAPS). It is proposed for applications in multi-phase dc-dc converters that require relatively high load current, low harmonics, and low output ripple noise voltage. Unlike other randomized modulation schemes, it does not negate the ripple cancellation effect of the multi-phase converters. A general N-phase analytical expression for the input current spectrum of the RWAPPM+WAPS is derived to analyze the harmonics therein. The derived analytical expression is verified by means of SPICE simulation on two- and three-phase dc-dc converters. The results (using a three-phase converter with 3.3 V input, 0.75 duty cycle, and 100 kHz switching-frequency) show that the RWAPPM+WAPS has the attribute of low peak spectral power in the input current spectrum (–29.9 dBFS and lower by ~19 dB compared to that of the PWM). It also features a low output ripple noise voltage at 4.9 mV, and is comparable to that of the PWM.
DRNTU::Engineering::Electrical and electronic engineering::Applications of electronics