ST AN2228 APPLICATION NOTE

AN2228

APPLIC ATION NOT E

STD1LNK60Z-based Cell Phone Battery Charger Design

Introduction

This application note is a Ringing Choke Converter (RCC)-based, step-by-step cell phone battery charger
design procedure.

The RCC is es sential to the self-oscillating fly -back converter, and operates within the Discontinuous
Conduction Mode (DCM) and Continuous Conduction Mode (CCM) boundaries without noticeable
reverse recov ery of the output rectifying diodes. RCC control is achieved by using discrete components to
control the peak c urrent mode, so the overall RCC cost is relatively low compare d to the conventional
Pulse Width Modulation (PWM ) IC fly-back converter. As a result, RCC is widely used for low power
applications in industry and home appliances as a simple and cost-effective solution.

Figure 1. STD1LNK60Z-based RCC Printed Circuit Board

Top View

September 2005 1/26

Bottom View

Rev 1.0

http:/www.st.com

26

AN2228 - APPLICATION NOTE

Table of Contents

1 Power Transformer Design Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1 Switching Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 STD1LNK60Z MOSFET Turn Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3 Primary Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4 Primary Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.5 Magnetic Core Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.6 Primary Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.7 Secondary Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.8 Auxilia ry Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.9 Gap Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2 STD1LNK60Z-based RCC Control Circuit Components . . . . . . . . . . . . . 12

2.1 MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2 R3 Startup Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3 Optocoupler Power Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4 R7 Sense Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.5 Constant Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.6 Zero Current Sense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.7 Constant Voltage And Constant Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3 Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Appendix A: STD1LNK60Z-based RCC Circuit Schematics . . . . . . . . . . 22

Appendix B: STD1LNK60Z-based RCC Circuit Bill of Materials . . . . . . . 23

4 Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2/26

AN2228 - APPLICATION NOTE

Figures

Figure 1. STD1LNK60Z-based RCC Printed Circuit Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. Optocoupler Fly-back Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 3. Optocoupler Forward Power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 4. Current Sense Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 5. CV and CC Curve at 110V
Figure 6. CV and CC Curve at 220V
Figure 7. Drain To Source Voltage Operation Waveform, 85V
Figure 8. Drain To Source Voltage Operation Waveform, 110V
Figure 9. Drain To Source Voltage Operation Waveform, 220V
Figure 10. Drain To Source Voltage Operation Waveform, 265V
Figure 11. RCC Control Circuit Components Schematic (see

Figure 12. STD1LNK60Z-based RCC Schematic (full view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

AC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

AC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

AC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

AC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

AC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

AC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Section on page 1

). . . . . . . . . . . . . . . 22

3/26

AN2228 - APPLICATION NOTE

Tables

Table 1. Line and Load Regulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Table 2. Efficiency Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Table 3. Standby Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Table 4. BOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4/26

AN2228 - APPLICATION NOTE 1 Power Transformer Design Calculations

1 Power Tr ansformer Design C al culat i ons

● The specifications:

–V

● Line frequency: 50~65Hz

– V
– I

Taking transient load into account, the maximum output current is set as

1.1 Switching Frequency

The system is a variable switching frequency system (the RCC switchi ng frequency v aries with
the input voltage and output load), so there is some degree of freedom in switching frequency
selection. However, the frequency must be at least 25kHz to minimize audible noise.

Higher switching frequencies will decrease the transformer noise, but will also increase the
level of switching power dissipated by the power devices.

= 85~265V

AC

= 5V

O

= 0.4A

O

I

Omax()

1.2I

4.8A==

O

The minimum switching frequency and maximum duty cycle at full load is expressed as

f

Smin()

D

max

50kHz=

0.5=

where the minimum input voltage is 50kHz and 0.5, respectively.

5/26

1 Power Transformer Design Calculations AN2228 - APPLICATION NOTE

1.2 STD1LNK60Z MOSFET Turn Ratio

The maximum MOSFET drain voltage must be below its breakdown voltage. The maximum
drain voltage is the sum of:

● input bus voltag e,

● secondary reflected voltage, and

● voltage spike (caused by the primary parasitic inductance at maximum input voltage).

The maximum input bus voltage is 375V and the STD1LNK60Z MOSFET breakdown voltage is
600V. Assuming that the voltage drop of output diode is 0.7V, the voltage spike is 95V, and the
margin is at least 50V, the reflected voltage is given as:

V

V

fl

BR()DSSVminarg

The Turn Ratio is given as

N

where,

= Secondary reflected voltage

V

fl

V

(BR)DSS

V

margin

V

DC(max)

V

spk

= Voltage drop

V

f

= MOSFET breakdown voltage

= Voltage margin

= Maximum input bus voltage

= Voltage spike

N = Turn Ratio

N

------ ­N

p
s

V

---------------------------­V

DC max()Vspk

OUTVF

––– 600 50– 375– 95 80 V=–==

V

fl

+

80

----------------- -14== = =

50.7+

= Primary Winding T urns

N

p

= Secondary Winding Turns

N

s

6/26

AN2228 - APPLICATION NOTE 1 Power Transformer Design Calculations

1.3 Primary Current

● Primar y Peak Current is expressed as:

2VOI

I

● Primar y Root Mean Square (RMS) Current is expressed as

ppk

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

ηD

Omax()

maxVDC min()

25× 0.48×

----------------------------------- 0.152A===

0.7 0.5× 90×

I

prmsIppk

where,

= Primary peak current

I

ppk

= Voltage output

V

O

= Maximum current output

I

O(max)

η = Efficiency, equal to 0.7

= Maximum duty cycle

D

max

V
I

prms

= Minimum input bus voltage

DC(min)

= Primary RMS current

1.4 Primary Inductance

Primary Inductance is expressed as

V

L

where,

DC min()Dmax

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

p

f

D

max

------------- ­3

smin()Ippk

0.152

90 0.5×

--------------------------- - 5.92mH===

0.152 50×

0.5

------- ­3

0.062A=×==

V

DC (min)

f

s (min)

D

max

f

s(min)

I

ppk

= Minimum Input DC voltage

= Minimum switching frequency

= Maximum duty cycle

= Minimum switching frequency

= Primary peak current

For example, if Primary Inductance is set to 5.2mH, the minimum switching frequency is:

f

smin()

V

IN DC min()Dmax

-------------------------------------------­LpI

ppk

90 0.5×

---------------------------- - 57kH z===

0.152 5.2×

7/26

1 Power Transformer Design Calculations AN2228 - APPLICATION NOTE

1.5 Magnetic Core Size

One of the most common ways to choose a core size is based on Area Product (AP), which is
the product of the effective core (magnetic) cross-section area times the window area available
for the windings.

Using a EE16/8 core and standard horizontal bobbin for this particular application, the equation
used to estimate the minimum AP (in cm

where,

= Primary Indu ctance

L

p

= Primary RMS current

I

prms

= Window utiliz ation factor, equal to:

k

u

– 0.4 for margin wound construction, and
– 0 .7 for tri p le insulat e d wire cons tructi on

= Saturation magnetic flux density

B

max

ΔT = Temperature rise in the core

1.6 Primary Winding

1.6.1 Winding Turns

The effective area of an EE16 core is 20.1mm2 (in the core’s datasheet). The number of turns
of primary winding is calculated as

V

N

DC min()Dmax

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

p

f

smin()

AP

ΔBA

4

) is shown as

ΔT

0.5

1.316

103×=

90 0.5×

6–

× 57× 103×

L

pIprms

---------------------------------- ­kuB

max

---------------------------------------------------------------------------- 179== =

e

0.22 20. 1× 10

where,

= Primary Windi ng Turns

N

p

V

DC (min)

D

max

f

s(min)

= Minimum Input DC voltage

= Maximum duty cycle

= Minimum switching frequency

ΔB = Flux density swing

= Effective area of the c ore

A

e

8/26