ST AN2272 APPLICATION NOTE

AN2272

Application Note

VIPer12A-based Low Power AC/DC Adapter

Introduction

This application note describes a low power, (output power of 4.1W) general purpose adapter which is able to handle a wide range input voltages (88VAC to 265VAC). The adapter (Order Code STEVAL-ISA011V1) is based on the Viper12A monolithic device that has the power switch as well as the basic control function needed to implement a current mode flyback converter.

February 2006

Rev. 1

1/33

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Table of Contents	AN2272 - Application Note

Table of Contents

1	STEVAL-ISA011V1 Board Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			5
	1.1	Primary Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
		1.1.1	Step 1, Input Capacitor Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
		1.1.2	Step 2, Transformer Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	1.2	Secondary Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		8

1.2.1 D11 Current and Power Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2.2 Transformer Turns Ratio and D11 Peak Current . . . . . . . . . . . . . . . . . . . 10 1.2.3 C11 Output Capacitor Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.3 Completed Transformer Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.4 Feedback Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2	STEVAL-ISA011V1 Board Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
	2.1 Start-up Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
	2.1.1	Full Load Start-up Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
	2.1.2	No Load Start-up Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18

2.2 Temperature Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.3 Dynamic Load Regulation Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.4 Steady-State Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.4.1 Steady-State Full Load Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.4.2 Steady-State No Load Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

	2.5 EMI Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		28
Appendix A		STEVAL-ISA011V Demo Board Schematic . . . . . . . . . . . . . . . . . . .	30
Appendix B		STEVAL-ISA011V1 Bill of Materials . . . . . . . . . . . . . . . . . . . . . . . . .	31
3	Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		32

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AN2272 - Application Note	List of Figures

List of Figures

Figure 1. Full Load Start-up Waveforms at 88V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 2. Full Load Start-up Waveforms at 265V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 3. Full Load Start-up Waveforms at 115V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 4. Full Load Start-up Waveforms at 230V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 5. No Load Start-up Waveforms at 88V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 6. No Load Start-up Waveforms at 265V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 7. No Load Start-up Waveforms at 115V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 8. No Load Start-up Waveforms at 230V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 9. Step Load Change Stability Tests at 88V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 10. Step Load Change Stability Tests at 265V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 11. Step Load Change Stability Tests at 115V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 12. Step Load Change Stability Tests at 230V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Figure 13. Steady-state Full Load 88VAC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 14. Steady-state Full Load 265VAC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 15. Steady-state Full Load 115VAC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 16. Steady-state Full Load 230VAC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 17. Steady-state No Load 88VAC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 18. Steady-state No Load 265VAC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 19. Steady-state No Load 115VAC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Figure 20. Steady-state No Load 230VAC Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Figure 21. 115VAC Line Voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Figure 22. 115VAC Line Neutral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Figure 23. 230VAC Line Voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Figure 24. 230VAC Line Neutral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Figure 25. STEVAL-ISA011V1 Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

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List of Tables	AN2272 - Application Note

List of Tables

Table 1. Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Table 2. Start up Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Table 3. Component Critical Temperature Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Table 4. Steady-state Full Load Condition Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Table 5. Steady-state Output Voltage Ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Table 6. Bill Of Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Table 7. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

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AN2272 - Application Note	STEVAL-ISA011V1 Board Design

1 STEVAL-ISA011V1 Board Design

In order to improve regulation, the feedback loop is designed to have enough bandwidth so the converter can react on time to load changes. As is shown in the Section 2.3: Dynamic Load Regulation Tests on page 20, the board is able to handle high load step changes with very low variations in the output voltage.

The flyback converter is designed to work in Discontinuous Conduction Mode (DCM) in all operating conditions (i.e. Minimum Input Voltage, Maximum Load), because it provides better dynamic performance.

1.1Primary Side

1.1.1Step 1, Input Capacitor Selection

The first design step is to calculate the input capacitor value (C2a + C2b see STEVALISA011V Demo Board Schematic on page 30). Equation 1 is useful for this purpose:

Equation 1

C 2 PIN T

IN = -----------------------------------------------------------------

V2AC(min)pk – V2DC(m in)

Where,

CIN = input capacitor value,

PIN = input power,

T = the time between the two conduction cycles of the input bridge diodes,

VAC(min)pk = sinusoidal input waveform peaks (when AC voltage is at its minimum), and

VDC(min) = selected minimum input voltage required for the flyback (converter) stage.

In this case, the PIN value used is calculated as PO/η, where PO is the maximum output power and η is the overall expected efficiency (70% in this example).

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STEVAL-ISA011V1 Board Design	AN2272 - Application Note

An acceptable value for VDC(min) is 80% of VAC(min)pk:

Equation 2

VDC(min) = 0.8VAC(min) pk = 2VAC(min)

T is expressed as:

Equation 3

1			VDC(min)
T = 2----------π---------------fline			π – arc cos V-----------------------------AC(m in)pk

Where,

T = the time between the two conduction cycles of the input bridge diodes, and

fline = line frequency.

The calculated value of CIN using Equation 1 is 16µF. For the board, two capacitors (C2a and C2b, see STEVAL-ISA011V Demo Board Schematic on page 30) of 10µF were used. This means that CIN = 20µF. This value was selected because the tolerance for an electrolytic capacitor is usually around 20%.

1.1.2Step 2, Transformer Selection

The next step is selecting a transformer with a Primary Inductance (LP) that allows the system to work at the boundary between Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM). The worst case is minimum input voltage and full load. This value is expressed as:

Equation 4

LMAX	(VDC(min)	DM AX)2
	= -------------------------------------------------------------- LMAX = 3.5mH
	2 PIN	fSW

Where,

LMAX = maximum inductance for discontinuous mode operation,

VDC(min) = selected minimum input voltage required for the flyback (converter) stage, DMAX = maximum duty cycle,

PIN = input power,

fSW = switching frequency (internally fixed in the VIper12A to 60kHz), and

VR = reflected voltage (fixed to 90V).

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AN2272 - Application Note	STEVAL-ISA011V1 Board Design

The DMAX at the boundary between CCM and DCM is expressed as:

Equation 5

	DMAX =	VR	DM AX	= 0.47
		V---------------------------------------------DC(m in) + VR

The transformer selected for this application provides an LP of 3mH, which is a little less than the maximum inductance (LMAX) calculated in the first equation (3.5mH). This ensures that the system is not working at boundary and will always function in DCM.

Using the transformer’s LP, the designer can calculate the:

●Peak Primary Current, expressed as,

Equation 6

IPEAK	2 PIN			IPEAK	= 258mA
	= ------------------------		LP
	fSW

Where,

IPEAK = peak primary current, PIN = input power,

fSW = switching frequency, and LP = primary inductance.

●actual Maximum Duty Cycle (DMAX), expressed as,

Equation 7

DM AX	2 PIN	fSW	LP	DMAX	= 0.42
	= -------------------------------------------------	LP
	fSW

and

●the primary side Root Mean Square (RMS) current value (IPRMS(max)), which is the current that flows through the main switch and primary winding. It is expressed as:

Equation 8

	IPRM S(max) = IPEAK	DMAX	IPRMS (max ) = 97mA
		------------------3

Where,

IPRMS(max) = Primary Current root mean square,

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STEVAL-ISA011V1 Board Design	AN2272 - Application Note

The conduction losses in the main switch depend on the VIPer12A IPRMS(max) and ON resistance, and are expressed as:

Equation 9

PVIPer12A = rds(on) I2PRMS(m ax)

Where,

PVIPer12A = VIPer12A conduction losses, and

rds(on) = VIPer12A ON resistance.

1.2Secondary Side

In order to select the output rectifier (secondary) diode D11, the designer needs to know the maximum reverse voltage that the diode has to sustain, as well as the average and root mean square of the current flowing through it (see STEVAL-ISA011V1 Schematic on

page 30). VR(max) is calculated as follows:

Equation 10

V V VOUT V

R(max) = OUT + ------------------ DC(m ax)

VR

Where,

VR(max) = maximum reverse voltage,

VOUT = output voltage,

VR = reflected voltage, and

VDC(max) = selected maximum input voltage.

A commonly used selection method is to choose a diode with a 40% to 50% safety margin

from the value given by the VR(max) calculation when a Schottky diode is used, or a safety margin of 20% to 30% if a standard “fast” diode is used. The safety margin prevents diode

breakdown from oscillation caused by circuit parasitic elements (e.g. transformer secondary inductance leakage or parasitic diode capacitance) when the MOSFET is turned ON.

If the calculated VR(max) is 23V and a Schottky diode is used (adding a 50% safety margin), the D11 value is about 34V. This makes the STPS340U (with 40V breakdown voltage) an

excellent choice for this application.

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PlossD

IPKS

AN2272 - Application Note	STEVAL-ISA011V1 Board Design

1.2.1D11 Current and Power Dissipation

●The average current flowing through D11 is the output current while the IDRMS value is expressed as:

Equation 11

			Ds–cond
IDRMS	= IPKS		--------------------
			3

Where,

IDRMS = current root mean square,

= peak current at secondary winding, and

Ds_cond = conduction duty cycle of the secondary diode.

For one output flyback, IPKS (peak current at the secondary winding) can be calculated as the primary peak current multiplied by the turns ratio.

Note: This formula applies only to DCM operation.

●D11 power dissipation is calculated as follows:

Equation 12

Plos sD = VdD ID(avg) + rdD I2DRMS

Where,

= diode power dissipation,

VdD = drop voltage (when the diode is forward-biased), ID(avg) = diode average current, and

rdD = dynamic resistance.

Note: The formula and the correct values for VdD and rdD are in the diode datasheets.

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IPKS

IPKP

STEVAL-ISA011V1 Board Design	AN2272 - Application Note

1.2.2Transformer Turns Ratio and D11 Peak Current

●The turns ratio that is selected for the transformer depends on the output voltage, the chosen reflected voltage, and the average voltage drop across the output diode.

Keeping in mind the voltage drop across its dynamic resistance, VDROP(avg) is expressed as:

Equation 13

VDROP(avg) = VdD + rdD IO

Where,

VDROP(avg) = average voltage drop (across the output diode) VdD = drop voltage (when the diode is forward-biased),

rdD = dynamic resistance,

IO = diode output current, and

●Using the calculated VDROP(avg) value, the turns ratio is expressed as:

Equation 14

	NP	=	VR
	N-------S		+ VDROP(avg)
		VO

Where,

NP = Primary Turns,

NS = Secondary Turns,

VR = reflected voltage, and

VO = output voltage.

●Using the calculated turns ratio, IPKS is then expressed as:

Equation 15

IPKS =	NP
	------- IPKP
	NS

Where,

= peak current at secondary winding, and = peak power current

Note: The worst case (maximum power dissipation) will be in full load condition.

●The D11 conduction duty cycle is expressed as:

Equation 16

Ds – co nd	IPKP LP fSW
	= --------------------------------------------
	VR

Where,

Ds_cond = Secondary Diode conduction duty cycle, LP = primary inductance, and

fSW = switching frequency.

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