A Novel Droop Method for Converter

A Novel Droop Method for Converter

Parallel Operation

Jung-Won Kim, Member, IEEE, Hang-Seok Choi, Student Member, IEEE, and Bo Hyung Cho, Senior Member, IEEE

Abstract— For the converter parallel operation, the current sharing between modules is important for the reliability of the system. Among several current sharing schemes, the droop method needs no interconnection between modules, which implies true redundancy. But the droop method has poor voltage regulation and poor current sharing characteristics.

In this paper, a novel droop method is proposed for the converter parallel operation, which adaptively controls the reference voltage of each module. This greatly improves the output voltage regula-tion and the current sharing of the conventional droop method. The analysis of the proposed method and design procedure are pro-vided and experimental results verify the excellent performance of the proposed method.

Index Terms— Converter parallel operation, current sharing, droop method, sample and hold.

NOMENCLATURE

Output voltage of module #[pic 1].

[pic 2][pic 3][pic 4][pic 5]

Output voltage set-point of module #[pic 6].

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Output current of module #[pic 15].

[pic 16][pic 17][pic 18][pic 19]

Conventional droop gain.

[pic 20]

Nominal output voltage.

[pic 21][pic 22][pic 23][pic 24][pic 25][pic 26][pic 27][pic 28]

Output voltage set-point accuracy.

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Maximum output voltage variation.

[pic 37][pic 38][pic 39][pic 40][pic 41][pic 42]

Rated output current of one module.

[pic 43][pic 44][pic 45][pic 46][pic 47][pic 48][pic 49][pic 50]

Maximum output current sharing error.

[pic 51][pic 52][pic 53][pic 54][pic 55][pic 56]

Peak output current of module #[pic 57].

[pic 58][pic 59][pic 60][pic 61][pic 62][pic 63][pic 64][pic 65][pic 66][pic 67]

Output current breakpoint set value.

[pic 68][pic 69][pic 70][pic 71][pic 72][pic 73][pic 74]

Droop gain of the proposed method.

[pic 75][pic 76]

Output voltage offset adjusting gain.

[pic 77][pic 78]

Output voltage sensing gain.

[pic 79][pic 80]

I.  INTRODUCTION

GENERALLY, the paralleling of lower-power converter modules offers a number of advantages over a single, high-power, centralized power supply. Performance-wise, the advantages include higher efficiency, better dynamic response and better load regulation. System-wise, parallel scheme allows redundancy  implementation,  expandability  of  output  power

and ease of maintenance.

Manuscript received October 17, 2001; revised November 6, 2001. Recom-mended by Associate Editor F. D. Tan.

J.-W. Kim is with Fairchild Korea Semiconductor, Puchon, Kyonggi 420-711, Korea.

H.-S. Choi and B. H. Cho are with the School of Electrical Engineering, Seoul National University, Seoul 151-742, Korea (e-mail: lonelycloud@hanmail.net; hangseok@hitel.net; bhcho@snu.ac.kr).

Publisher Item Identifier S 0885-8993(02)02177-4.

To improve the reliability of the parallel-connected con-verter system, load current must be distributed equally among the converter modules. Many current sharing methods with different complexities and current sharing performances were proposed [1]. Most of the proposed methods need interconnec-tions between modules. The added interconnections increase complexity and may cause single point failure, and the share line may be sensitive to noise. In order to overcome the prob-lems mentioned above, methods without any interconnection between the parallel connected modules are proposed [2], [3]. Among these methods, the simplest is the droop method.

The droop method relies on the internally and/or externally added resistance of the paralleled modules to maintain a rel-atively equal current distribution between the modules. Gen-erally, the droop method is simple to implement, and it does not require any communication between modules. However, it achieves the current sharing accuracy at the penalty of poor output voltage regulation.

In this paper, a novel droop method is proposed for the con-verter parallel operation, which adaptively controls the refer-ence voltage of each module. This greatly improves the output voltage regulation and the current sharing of the conventional droop method. The analysis of the proposed method and the de-sign procedure are provided and experimental results verify the excellent performance of the proposed method.

II.  LIMITATIONS OF THE CONVENTIONAL DROOP METHOD

The current sharing techniques based on the droop method rely on the slope of the load regulation characteristic of the par-allel-connected modules. The output voltage of each module droops as the output current increases by

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