Juanjuan Sun

Multifrequency Small-Signal Model for Buck and Multiphase Buck Converters

To investigate the high-frequency behaviors of buck and multiphase buck converters, this paper develops a modeling method based on the harmonic-balance approach. Because the nonlinear pulse-width modulator generates sideband components, the sideband effect occurs in a closed-loop converter. Taking this effect into account, the multifrequency small-signal model is proposed, which is applicable beyond half of the switching frequency. In a voltage-mode-controlled buck converter, the introduced model predicts the measured phase delay of the loop gain, while the conventional average model fails to explain this phenomenon. Furthermore, this model is extended to the case of the multiphase buck converter. The influence from the interleaving technique is discussed and the frequency-domain characteristics are clearly explained. Simulation and experimental results are provided to verify the achievements of the proposed model

Bandwidth Improvements for Peak-Current Controlled Voltage Regulators

To ensure the current sharing among the interleaved phases, the peak-current control is widely used in the voltage regulator (VR) applications. Meanwhile, to save the cost and footprint by reducing the output capacitance, high control bandwidths are mandatory. Because of the sample-hold effect in the peak-current loop, there exist stringent challenges to the high-bandwidth designs for VRs. In this paper, the influence from the sample-hold effect is investigated to clarify the difference between the VR and conventional applications. After that, two approaches for higher bandwidth are introduced. To decrease the phase delay due to the sample-hold related double poles, excessive external ramps are inserted to the modulators. To increase the effective sampling frequency, the phase inductor currents are coupled, either by the coupled-inductor structure or through the feedback control. In addition, a small-signal model including the sample-hold effect is derived for the coupled-inductor buck to explain the improvement. High-bandwidth designs are verified by the simulation and experimental results. A bandwidth of one-third switching frequency is demonstrated with a coupled-inductor VR.

High-Frequency Dynamic Current Sharing Analyses for Multiphase Buck VRs

Under high-frequency repetitive load transients, multiphase voltage regulators suffer from beat-frequency oscillations of the phase currents. To investigate this issue, this paper develops a methodology based on the multifrequency model with describing functions. The phenomenon of beat-frequency oscillation in phase currents is explained. The influences from the interleaving operation are identified. Using peak-current control as an example, the magnitude of beat-frequency oscillations is predicted by a proposed model that considers sideband information. To reduce this oscillation without impacting the voltage regulation, several solutions are proposed. Experimental and simulation results are provided as verification of the analyses and solutions.

The multi-frequency small-signal model for buck and multiphase interleaving buck converters

This paper introduces a multi-frequency small-signal model for buck and multiphase interleaving buck converters. Including the influences from the sideband-frequency components generated by the pulse-width modulation (PWM), this model is applicable beyond half of the switching frequency. In voltage-mode-controlled buck converters, the proposed model predicts the measured phase delay, while the conventional average model fails to explain this phenomenon. With this new model, frequency-domain characteristics are clearly explained for buck and multiphase buck converters. Furthermore, the aliasing effect at half of the switching frequency is examined. Simulation and experimental results are presented to verify the proposed multi-frequency model.

Dynamic performance analysis of outer-loop current sharing control for paralleled DC-DC converters

For paralleling DC/DC converters, this paper investigates the fundamental relationship between the outer-loop current sharing control and the voltage regulation control. By using the concept of output impedance, the inherent function of the current sharing control is clarified and its influences on the voltage regulation of paralleling system are revealed. Although there may exist tradeoffs between dynamic current sharing and voltage regulation, it is possible to have good performances on both aspects, as long as the closed-loop output impedances of individual modules are within a certain tolerance range. After that, a design guideline for the current sharing compensator is proposed. The analyses and designs are verified by the simulation and experimental results

A Generic High-Frequency Model for the Nonlinearities in Buck Converters

To investigate the high-frequency behaviors for the switching converters, the modeling and analysis should include the influences from the sideband components, which are generated by the converters nonlinearities. With a buck converter, two non- linearities are identified in this paper: the pulse-width modulator and the switches. Based on the harmonic-balance approach, the multifrequency model is proposed to address these nonlinearities and to predict the high-frequency performances. With the derived model, different modulation schemes are compared based on their influence on the sideband effect and the control bandwidth limitations. The possible benefit from the double-edge modulation and its limitations are clarified. In addition, the audio-susceptibility is studied considering the nonlinearity of the switches. With the combination of the two nonlinearities, a generic high-frequency model is achieved. Simulation and experimental results are presented to verify the achievements of the proposed models.

High-bandwidth designs for voltage regulators with peak-current control

In the multiphase interleaving buck converters with peak-current control, the sample-hold effect limits the bandwidth of the voltage-regulation loop. For higher bandwidth and better transient performances, this paper presents two methods to reduce this limitation. To decrease the quality factor of the sample-hold related double poles, external ramps are inserted to the modulators. To increase the effective sampling frequency, the phase inductor currents are coupled, either through feedback control or by the coupled-inductor structure. In addition, a small-signal model including the sample-hold effect is derived for the coupled-inductor buck to explain the improvement. High-bandwidth designs are verified by the simulation and experimental results.

Dynamic current sharing analyses for multiphase buck VRs

For high-control-bandwidth multiphase buck voltage regulators, this paper addresses the issue of dynamic current sharing among interleaved phases, especially under repetitive high-frequency load transients. The beat-frequency oscillation phenomenon of phase currents is identified and explained by a proposed small-signal model including the sideband information. To reduce the sideband frequency component while not impacting the control loops phase margin, the notch filter solution is proposed. Dynamic current sharing improvements with this method are proven by the experimental results for different control structures

A high power-density, high efficiency front-end converter for capacitor charging applications

This paper introduces a high power-density, high-efficiency isolated full-bridge boost converter, which is used for the front-end converter of a capacitor charger. The design equations, design considerations and practical trade-offs for the converter power stage are summarized. In addition, by fully taking advantage of the pulsed load profile, a transformer design using a high-saturation flux density material is introduced to maximize the power density. The principle of operation for the converter is analyzed and verified on a 15 kW, 100 kHz front-end converter prototype.

A high power density high voltage distributed power system for pulse power applications

In this paper, a high-density high-voltage distributed power system for pulse power applications is designed and implemented. Different topologies are evaluated for two power stages. According to pulse load condition, system power density is optimized through the tradeoff between power loss and magnetic component size. High power density and high efficiency are verified by the experimental result