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AN1660 APPLICATION NOTE

3

ANALYSIS AND DESIGN EQUATIONS

Following assumptions is for simplify the analysis.

1) All circuit components are ideal devices.

2) DT (Fig.2) is very short compared to the resonant time. DT = 0, and I1 = I2.

Fig.3 is the schematic of L6598 resonant converter demoboard.

Figure 3. 180W Half-Bridge resonant with the L6598: electrical schematic

The spec. of the power supply.

Input voltage: DC 370V ~ 420V (output of a PFC stage)

Output voltage: 110V/1.2A , 24V/2A

Output power : 180W

3.1 Resonant frequency and switching frequency

Before the design, we need to decide several operation frequencies of converter. That include the start

switching frequency - Fstart, minimum switching frequency - Fmin and resonant frequency - Fr.

In this application, we choose:

Fstart = 250KHz ( by design Rfstart of L6598)

Fmin = 68KHz ( by design Rfmin of L6598)

Fr = 84KHz ( by design L1, C17, C18)

To calculate Rfstart and Rfmin, using below equations:

(1)

(2)

C10

100nF

C15

220pF

C19

2200

µF

50V

C12

100nF

R7

6.8K

R6

27K

C17

20nF

C18

20nF

C16

100nF

C26 2200pF/3KV

C9

1

µF

C8

47

µF

R5

5.6K

R17

R4

110K

EN1

RFSTART

OPIN-

OPOUT

EN2

VS

VBOOT

OUT

LVG

OPIN+

HVG

R2 160K/2W

R3 10

R13 22

D6 1N4148

C11

1nF

C13

220pF

R9

10K

R8

27K

L1 90

µH

T1

D7

STPR1010CT

D8 BYT03-400

D9

BYT03-400

R15 22

R14

10K

R24

3.3K

R23

150K

Q1

STP8NA50

Q2

STP8NA50

TRANSFORMER

R16

10K

R10

20K

R12 10K

R19

0.2

R18 1K

R11 100K

RFMIN

GND

CSS

CF

C24

100nF

24V

Vin

DC370V-420V

C21

330

µF

160V

C20

1000

µF

C25

100nF

R22

1K

R21

3.6K

R25 1K

U2

4N25

U3

TL431

C22

C23

0.47

µF

115V

C14

0.22

µF

D03IN1427

L6598

Rfm in

1.41

Fmin Cf

---------------------

=

Rfstart

1.41

Fstar t

Fm i n

–

()Cf

--------------------------------------------------

=