Syam Kumar Pidaparthy

Average Current Mode Control for LLC Series Resonant DC-to-DC Converters

An average current mode control scheme that consistently offers good dynamic performance for LLC series resonant DC-to-DC converters irrespective of the changes in the operational conditions is presented in this paper. The proposed control scheme employs current feedback from the resonant tank circuit through an integrator-type compensation amplifier to improve the dynamic performance and enhance the noise immunity and reliability of the feedback controller. Design guidelines are provided for both current feedback and voltage feedback compensation. The performance of the new control scheme is demonstrated through an experimental 150 W converter operating with 340 V to 390 V input voltage to provide a 24 V output voltage.

Stability analysis of PWM converters connected to general load subsystems

This paper investigates stability of dc-to-dc converters deriving another dc-to-dc converter through a filter stage. The impacts of the downstream filter/converter stage on the absolute and relative stabilities of the upstream converter are analyzed. The absolute stability is examined using the minor loop gain, defined as the ratio of the impedances seen at the output port of the upstream converter. The relative stability is assessed using the loop gain expression of the upstream converter. This paper reveals that the downstream filter/converter stage rarely destabilizes the upstream converter, but usually dictates the loop gain characteristics of the upstream converter and frequently determines the phase margin and 0 dB crossover frequency of the loop gain. This paper proposes a simple method to determine the stability margins of the converter from the minor loop gain. The theoretical predictions are supported by both small-signal simulations and experimental measurements.

Designing control loop for PWM converters in dc-to-dc power conversion systems

This paper presents a methodology to deal with the control design for PWM converters in dc-to-dc power conversion systems. The concept of uncoupled dc-to-dc converter is established, which allows dc-to-dc converters to be designed independently from the source and load impedances. The control design strategy of the uncoupled dc-to-dc converters is formulated to offer good and robust closed-loop performance when the converters are coupled with the actual source and load subsystems. A peak current-mode controlled buck converter is used as an example throughout this paper to verify the proposed control design technique. We compare the control design of the uncoupled dc-to-dc converters with that of the converters coupled with resistive and non-resistive loads. The validity of the conventional design intended for a resistive load is also be discussed.

Control Design and Loop Gain Analysis of DC-to-DC Converters Intended for General Load Subsystems

DC-to-DC converters are usually intended for general applications where the load impedance characteristics are unknown or undefined. This paper establishes the control design procedures for DC-to-DC converters in the absence of any prior knowledge on their load impedance. The proposed control design can be universally adapted to all the DC-to-DC converters regardless of the impedance characteristics of their actual load. This paper also presents the loop gain analysis of the converter combined with an actual load whose impedance characteristics are only available afterward. A graphical analysis method is proposed, which enables us to predict the loop gain of the converter in the presence of an arbitrary load impedance. The validity of the analysis method is demonstrated using a current-mode controlled buck converter coupled with an inductive load, capacitive load, and converter load. Theoretical predictions are verified with both computer simulations and experimental measurements.

Performance of an interleaved boundary conduction mode boost PFC converter with wide band-gap switching devices

This paper presents the performance evaluation of an interleaved boundary conduction mode (BCM) boost power factor correction (PFC) converter employing three different switching devices: the super junction silicon (Si) MOSFET, silicon carbide (SiC) MOSFET, and gallium nitride (GaN) high electron mobility transistor (HEMT). An experimental 500 W interleaved BCM boost PFC converters is built and its performance is evaluated while adopting each of the three semiconductor devices as the main switch. The overall efficiency and thermal performance of the experimental boost PFC converter are analyzed under three different operational conditions: one operational condition with the baseline switching frequency of fs base = 65 kHz and the other two conditions with fs base = 130 kHz and 200 kHz. The three baseline frequencies are selected in consideration of IEC 61000-3-2 and CISRR 22 EMC standards for conducted emission noise. This study confirms that the GaN HEMT exhibits the superior switching characteristics and pronounces its merits at high frequency operations. The current paper highlights the efficiency improvement with the GaN HEMT and demonstrates its application potentials to high power density/low profile BCM boost PFC converters.

Stabilizing effects of load subsystem in multi-stage dc-to-dc power conversion systems

This paper investigates the impacts of the load subsystem on stability and performance of the upstream converter in multi-stage dc-to-dc power conversion systems. The paper demonstrates that an appropriate load subsystem could offer both stability and predictable performance for the upstream converter that was at the brink of instability before being coupled with the load subsystem. This stabilizing effects of the load subsystem are theoretically analyzed and experimentally validated using a two-stage power conversion system, consisting of an upstream boost converter, downstream buck converter, and two filter stages. The results of this paper can be used to stabilize the upstream converter that was initially well designed but later became destabilized due to the detrimental interaction stemming from unknown source subsystems.

Input Impedances of PWM DC-DC Converters: Unified Analysis and Application Example

The input impedances of pulse width modulated (PWM) dc-to-dc converters, which dictate the outcomes of the dynamic interaction between dc-to-dc converters and their source subsystem, are analyzed in a general and unified manner. The input impedances of three basic PWM dc-to-dc converters are derived with both voltage mode control and current mode control. This paper presents the analytical expressions of the 24 input impedances of three basic PWM dc-to-dc converters with the two different control schemes in a factorized time-constant form. It also provides a comprehensive reference for future dynamic interaction analyses requiring knowledge of the converters’ input impedances. The theoretical predictions of the paper are all supported by measurements on prototype dc-to-dc converters. The use of the presented results is demonstrated via a practical application example, which analyzes the small-signal dynamics of an input-filter coupled current-mode controlled buck converter. This elucidates the theoretical background for the previously-reported eccentric behavior of the converter.

A load impedance specification of dc power systems for desired dc link dynamics and reduced conservativeness

This paper proposes a new load impedance specification for multi-stage dc power conversion systems. The proposed specification avoids drawbacks of the existing specifications, such as the lack of explicit connections to the dc link dynamics and unnecessary conservativeness. The proposed specification offers a direct command/supervision of the frequency- and time-domain dynamics of the intermediate dc link, while being less conservative than the existing specifications. This paper also presents procedures of redesigning ill-conditioned load impedances to comply with the specification. The validity and utility of the proposed specification are demonstrated using an illustrative example.

Stabilizing Effects of Load Subsystem in Multistage DC-to-DC Power Conversion Systems

This paper investigates the impacts of the load subsystem on stability and performance of the upstream converter in multistage Dc-to-Dc power conversion systems. This paper demonstrates that an appropriate load subsystem can be constructed to provide both stability and intended performance for the upstream converter at the brink of instability before being coupled with the load subsystem. These stabilizing effects of the load subsystem are theoretically analyzed and experimentally validated using a two-stage power conversion system, consisting of an upstream boost converter, downstream buck converter, and two filter stages. This paper also presents the design procedures for the stabilizing load subsystem, along with the performance verification of the upstream boost converter and the downstream buck converter.

Design and performance evaluation of digital control for LLC series resonant dc-to-dc converters

This paper presents the design and performance evaluation of the digital control adapted to an LLC series resonant dc-to-dc converter operating with wide input and load variations. The main theme of this paper is to highlight the advantage of using the push-pull mode of digital pulse width modulation (DPWM) operation over the complementary mode operation for the implementation of digital control to the resonant converters using a low cost digital signal controller (DSC). Conventionally, the complementary mode of DPWM operation is used for resonant converter. However, the push-pull mode of DPWM operation is chosen for the resonant converter circuit to improve closed-loop performance. Design procedures and performance evaluation are illustrated for both DPWM operational modes using an experimental 150 W LLC converter.