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A Synchronous Rectification Method for Bidirectional LLC to Improve No-load or Light Load Control Performance

IP.com Disclosure Number: IPCOM000248982D
Publication Date: 2017-Jan-25
Document File: 6 page(s) / 250K

Publishing Venue

The IP.com Prior Art Database

Related People

DaWei Yao: AUTHOR [+4]

Abstract

This disclosure proposes a new modulation method, in which the controllability of the secondary power devices are utilized to Improve the no-load/light load control performance, which simplifies the control algorithm design and eliminate the potential audible noise issue during idle mode of the converter. Besides, the method also improve the ZVS condition of power devices at primary side.The method is a general method for bidirectional LLC converter application.

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A synchronous rectification modulation method for bidirectional LLC converter to increase the no-load or light load

control performance Principal Applications/Uses of the Disclosure

This disclosure proposes a new modulation method, in which the controllability of the

secondary power devices are utilized to Improve the no-load/light load control performance,

which simplifies the control algorithm design and eliminate the potential audible noise issue

during idle mode of the converter. Besides, the method also improve the ZVS condition of

power devices at primary side.

The method is a general method for bidirectional LLC converter application.

Problem Overcome by the Disclosure

Background

Bidirectional LLC converter (as shown in Figure 1) has high efficiency and symmetric circuit

structure, which makes it very attractive for bidirectional power flow application, such as

V2G EV charger, energy storage converter, PE transformer, DC micro grid etc.

Usually, the bidirectional LLC converter consists of two full bridges. One of the full bridge

works as inverter, while the other one will work as a rectifier by its antiparallel diode.

Q11

Q12

Q13

L1

T1

L2

C2C1

Uin

Q21

Q22

Q23

Q24Q13

+

-

Lm1 Co Uo

+

-

Cin

Bidiretional

Figure 1 The topology of bidirectional LLC

Existing Problems

Different from fast recovery diodes, the switching devices (IGBTs, MOSFETs) have larger

output capacitance (see figure 2, represented by Coes), which result in serious overvoltage

issue during no-load condition.

Q11

Q12

Q13

L1

T1

L2

C2C1

Uin

Q21

Q22

Q23

Q24Q14

Cp Cs

+

-

Lm1

Coes1

Coes1

Coes1

Coes1

Coes2

Coes2

Coes2

Coes2

Co Uo

+

-

Cin

Bidiretional

Figure 2 The topology of bidirectional LLC (with parasitic capacitors)

Although this problem is a common issue for all the LLC converter, it getting serious for

bidirectional LLC because of the larger output capacitance at switching devices at rectifier

side. For example, the Coes of IGBT IKW40N120H3 (1200V, 40A) is 185pF, while the

junction capacitance of diode DESP30-12A (1200V, 30A) is only 12pF.

Figure 3 compares the gain characteristics of the unidirectional LLC converter (Figure 2a)

and the bidirectional LLC converter with parasitic capacitors (Figure 2b). It can be observed

that due to the presence of the parasitic capacitors, another resonant peak appears at

higher frequency zone, which results in the gain distortion and over voltage during no-load

or light load condition.

a) Gain curve of the unidirectional LLC b) Gain curve of bidirectional LLC

Figure 3 Gain characteristics comparison (fs=40 kHz, Lr=7 uH, Cr=2.262uF)

A simulation result is shown in to illustrate this issue.

Figure 4 Comparison of output voltage at light load: Red- unidirectional LLC; Blue-

bidirectional LLC (Rload=2500Ω, Nominal output voltage=566V)

It has known that several methods have been proposed to resolve the DC over voltage

issue, such as 1) increase the excitation inductance or decrease the resonance inductance,

2) add a dummy load, 3) using burst mod...