ST AN2228 APPLICATION NOTE

AN2228

APPLICATION NOTE

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 essential to the self-oscillating fly-back converter, and operates within the Discontinuous Conduction Mode (DCM) and Continuous Conduction Mode (CCM) boundaries without noticeable reverse recovery of the output rectifying diodes. RCC control is achieved by using discrete components to control the peak current mode, so the overall RCC cost is relatively low compared 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	Bottom View

	Rev 1.0
September 2005	1/26

http:/www.st.com

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	Auxiliary 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

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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 110VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 6. CV and CC Curve at 220VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 7. Drain To Source Voltage Operation Waveform, 85VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 8. Drain To Source Voltage Operation Waveform, 110VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 9. Drain To Source Voltage Operation Waveform, 220VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 10. Drain To Source Voltage Operation Waveform, 265VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 11. RCC Control Circuit Components Schematic (see Section on page 1). . . . . . . . . . . . . . . 22

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

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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

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AN2228 - APPLICATION NOTE

1 Power Transformer Design Calculations

1Power Transformer Design Calculations

●The specifications:

–VAC = 85~265V

●Line frequency: 50~65Hz

–VO = 5V

–IO = 0.4A

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

IO(max) = 1.2IO = 4.8A

1.1Switching Frequency

The system is a variable switching frequency system (the RCC switching frequency varies 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.

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

fS(min) = 50kHz Dmax = 0.5

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

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1 Power Transformer Design Calculations

AN2228 - APPLICATION NOTE

1.2STD1LNK60Z MOSFET Turn Ratio

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

●input bus voltage,

●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:

Vfl = V(BR)DSS – Vm argin – VDC(max) – Vspk = 600 – 50 – 375 – 95 = 80V

The Turn Ratio is given as
Np	Vfl	=	80	= 14
N = ------	= ----------------------------		-----------------
Ns	VOUT + VF		5 + 0.7

where,

Vfl = Secondary reflected voltage

V(BR)DSS = MOSFET breakdown voltage

Vmargin = Voltage margin

VDC(max) = Maximum input bus voltage

Vspk = Voltage spike

Vf = Voltage drop

N = Turn Ratio

Np = Primary Winding Turns

Ns = Secondary Winding Turns

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AN2228 - APPLICATION NOTE

1 Power Transformer Design Calculations

1.3Primary Current

● Primary Peak Current is expressed as:

	Ippk	=	2VOIO(max)	=	2 × 5 × 0.48			=	0.152A
			η--------D---------------maxV---------------DC(min----)		0.7---------------------	× 0.5-------------	× 90-

●Primary Root Mean Square (RMS) Current is expressed as

	Iprms	= Ippk	Dmax	= 0.152 ×	0.5	= 0.062A
			-------------3		-------3

where,

Ippk = Primary peak current

VO = Voltage output

IO(max) = Maximum current output

η = Efficiency, equal to 0.7 Dmax = Maximum duty cycle

VDC(min) = Minimum input bus voltage

Iprms = Primary RMS current

1.4Primary Inductance

Primary Inductance is expressed as

Lp	VDC(min)Dmax	=	90 × 0.5		=	5.92mH
	= --------------------------------------		0.152--------------------------	× 50-
	fs(min)Ippk

where,

VDC (min) = Minimum Input DC voltage fs (min) = Minimum switching frequency Dmax = Maximum duty cycle

fs(min) = Minimum switching frequency Ippk = Primary peak current

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

fs(min)	VIN DC(min)Dmax	=	90 × 0.5		=	57kHz
	= --------------------------------------------		0.152-------------------	×---------5.2-
	LpIppk

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1 Power Transformer Design Calculations

AN2228 - APPLICATION NOTE

1.5Magnetic 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 cm4) is shown as

AP =		LpIprms		1.316	× 10	3
	----------------------------------
		kuBmax T0.5

where,

Lp = Primary Inductance

Iprms = Primary RMS current

ku = Window utilization factor, equal to:

–0.4 for margin wound construction, and

–0.7 for triple insulated wire construction

Bmax = Saturation magnetic flux density

T = Temperature rise in the core

1.6Primary Winding

1.6.1Winding 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

Np	VDC(min)Dmax		=	90	× 0.5	=	179
	= --------------------------------------	BAe		0.22--------------×---------------20.1---------------×	10–----------------6 × 57---------------× 103-
	fs(min)

where,

Np = Primary Winding Turns

VDC (min) = Minimum Input DC voltage

Dmax = Maximum duty cycle

fs(min) = Minimum switching frequency

B = Flux density swing

Ae = Effective area of the core

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