Synchronous-rectifier DC/DC

14.13 Synchronous-rectifier DC/DC

−1 (t 1 +t 2 +t 3 )

Luo-converters

t 3 +t 4 Synchronous-rectifier (SR) DC/DC converters are called the k =

; T =t 1 +t 2 +t 3 +t 4 ; f = 1/T

fifth-generation converters. The development of the micro- t 1 +t 2 +t 3 +t 4 electronics and computer science requires the power supplies

with low output voltage and strong current. Traditional diode bridge rectifiers are not available for this requirement. Soft- switching technique can be applied in SR DC/DC converters.

14.12.3 Four-quadrant ZVS Quasi-resonant

We have created few converters with very low voltage (5 V, 3.3 V,

DC/DC Luo-converter

and 1.8 ∼ 1.5 V) and strong current (30 A, 60 A up to 200 A) and high power transfer efficiency (86%, 90% up to 93%). In

Four-quadrant ZVS quasi-resonant Luo-converter is shown in this section, few new circuits, different from the ordinary SR Fig. 14.103. Circuit 1 implements the operation in quadrants DC/DC converters, are introduced:

I and II, circuit 2 implements the operation in quadrants III and IV. Circuit 1 and 2 can be converted to each other by aux-

• Flat transformer synchronous-rectifier DC/DC Luo- iliary switch. Each circuit consists of one main inductor L and

converter;

two switches. Assuming that the main inductance L is suffi- • Double current synchronous-rectifier DC/DC Luo- ciently large, the current i L is constant. The source and load

converter with active clamp circuit;

• Zero-current-switching synchronous-rectifier DC/DC There are four modes of operation:

voltages are usually constant, e.g. V 1 = 42 V and V 2 = ±28 V.

Luo-converter; • Zero-voltage-switching synchronous-rectifier DC/DC

Luo-converter.

C r1

14.13.1 Flat Transformer Synchronous-rectifier

DC/DC Luo-converter

2 + D 2 Flat transformer SR DC/DC Luo-converter is shown in Fig. 14.104. The switches S 1 ,S 2 , and S 3 are the low-resistance V 1

C r2

MOSFET devices with very low resistance R −

3 we use a flat transformer, the leakage inductance L m and

resistance R L are small. Other parameters are C = 1 µF, + V L m = 1 nH, R L

ab cd ab cd

O = 10 µF. The input

2 voltage is V 1 = 30 VDC and output voltage is V 2 , the output current is I O . The transformer term’s ratio is N = 12 : 1. The FIGURE 14.103 Four-quadrant DC/DC ZVS quasi-resonant Luo-

repeating period is T = 1/f and conduction duty is k. There converter.

are four working modes.

336 F. L. Luo and H. Ye TABLE 14.13 Switch’s status (the blank status means off)

Circuit//switch or diode

Mode A (QI)

Mode B (QII)

Mode C (QIII)

Mode D (QIV)

State-off Circuit

State-on

State-off

State-on State-off

State-on State-off

FIGURE 14.104 Flat transformer SR Luo-converter.

The natural resonant frequency is

14.13.2 Double Current SR DC/DC Luo-converter with Active

Clamp Circuit

The converter in Fig. 14.104 resembles a half-wave rectifier. The intervals are

Double current (DC) SR DC/DC Luo-converter with active clamp circuit is shown in Fig. 14.105. The switches S 1 –S 4 are

; t 2 ≈ kT ;

(14.368) the low-resistance MOSFET devices with very low resistance

3 and S 4 plus L 1 and L 2 form a double ⎡

current circuit and S 2 plus C is the active clamp circuit, this ⎢ π

converter resembles a full-wave rectifier and obtains strong t 3 = L m C ⎢

; t 4 ≈ (1 − k)T

output current. Other parameters are C = 1 µF, L m = 1 nH,

O = 10 µF. The input voltage is

= 30 VDC and output voltage is V 2 , the output current is

I O . The transformer term’s ratio is N = 12 : 1. The repeating period is T

= 1/f and conduction duty is k. There are four

Average output voltage V 2 and input current I 1 are

working modes.

kV 1 L m

The natural resonant frequency is

The power transfer efficiency is

V 1 I 1 kV 1 /N The interval of t 1 is

When we set the frequency f = 150–200 kHz, we obtained the V 2 = 1.8 V, N = 12, I O

= 0–30 A, Volume = 2.5 in 3 . The

average power transfer efficiency is 92.3% and the maximum

power density (PD) is 21.6 W/in 3 .

; t 2 ≈ kT ; (14.373)

14 DC/DC Conversion Technique and 12 Series Luo-converters 337

FIGURE 14.105 Double current SR Luo-converter.

The intervals are

Average output voltage V 2 and input current I 1 are

I ; I (14.375)

V 2 I 1 Z r π /2 +α (14.380)

The power transfer efficiency Average output voltage V and input current I are

I O ; I 1 (14.381) When we set the frequency f = 200–250 kHz, we obtained

− R L +R S +

TN 2 =k N

= 1.8 V, N = 12, I 3 = 0–35 A, Volume = 2.5 in . The average power transfer efficiency is 94% and the maximum

the V 2 O

The power transfer efficiency

power density (PD) is 25 W/in 3 .

+R S + (L m /TN )

=1−

I O (14.382)

14.13.3 Zero-current-switching

V 1 I 1 kV 1 /N

Synchronous-rectifier DC/DC

When we set the V 1 = 60 V and frequency f = 200–250 kHz,

Luo-converter

we obtained the V 2 = 1.8 V, N = 12, I O = 0–60 A, Volume =

4 in 3 . The average power transfer efficiency is 94.5% and the maximum power density (PD) is 27 W/in SR DC/DC Luo 3 -converter, we designed ZCS SR DC/DC Luo- .

Since the power loss across the main switch S 1 is high in DC

converter shown in Fig. 14.106. This converter is based on the DC SR DC/DC Luo -converter plus ZCS technique. It employs

14.13.4 Zero-voltage-switching

a double core flat transformer.

Synchronous-rectifier DC/DC

The ZCS resonant frequency is

Luo-converter

ω r = √ (14.377) ZVS SR DC/DC Luo -converter is shown in Fig. 14.107. This

converter is based on the DC SR DC/DC Luo-converter plus ZVS technique. It employs a double core flat transformer.

The normalized impedance is

The ZVS resonant frequency is

Z r = and α = sin −1

(14.378)

(14.383) L r C r

FIGURE 14.106 ZCS DC SR Luo-converter.

FIGURE 14.107 ZVS DC SR Luo-converter.

14 DC/DC Conversion Technique and 12 Series Luo-converters 339 The normalized impedance is

How to investigate the large quantity converters is a vital task. This problem was addressed in the last decade of last

1 century. Unfortunately, much attention was not paid to it. This Z r =

; α = sin −1 (14.384) generation converters were not well discussed, only limited

number of papers was published in the literature. The intervals are

14.14.1 Two Energy-storage Elements Resonant

1 Power Converters

The 8 topologies of 2-element RPC are shown in Fig. 14.108. These topologies have simple circuit structure and least com-

I 1 (1 + cos α)L r

t 1 +t 2 +t 3

(14.386) ponents. Consequently, they can transfer the power from

V 1 (V 1 /V 2 ) −1

source to end-users with higher power efficiency and lower power losses.

Average output voltage V 2 and input current I 1 are

Usually, the 2-Element RPC has very narrow response fre-

kV 1 L m

I O quency bands, which is defined as the frequency width between

V 2 = − R L +R S +

(14.387) the two half-power points. The working point must be selected N

2 I O ; I 1 =k

TN

√ in the vicinity of the natural resonant frequency ω 0 = 1/ LC.

Another drawback is that the transferred waveform is usu- The power transfer efficiency ally not a perfect sinusoidal, i.e. the output waveform THD is

not zero.

Since total power losses are mainly contributed by the

V 1 I 1 kV 1 /N

power losses across the main switches. As resonant conversion technique, the 2-Element RPC has high power transferring

When we set the V 1 = 60 V and frequency f = 200–250 kHz, efficiency. we obtained the V 2 = 1.8 V, N = 12, I O = 0–60 A, Vol-

ume = 4 in 3 . The average power transfer efficiency is 94.5%

and the maximum power density (PD) is 27 W/in 3 .