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 (88V (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.

to 265VAC). The adapter

February 2006 Rev . 1 1/33

www.st.com

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 D

1.2.3 C

Output Capacitor Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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

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

2 STEVAL-ISA011V1 Board Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Peak Current . . . . . . . . . . . . . . . . . . . 10

2.1 Start-up Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1.1 Full Load Star t- 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

2/33 Rev. 1

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 88V Figure 14. Steady-state Full Load 265V Figure 15. Steady-state Full Load 115V Figure 16. Steady-state Full Load 230V Figure 17. Steady-state No Load 88V Figure 18. Steady-state No Load 265V Figure 19. Steady-state No Load 115V Figure 20. Steady-state No Load 230V Figure 21. 115V Figure 22. 115V Figure 23. 230V Figure 24. 230V

Line Voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Line Neutral. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Line Voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Line Neutral. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 25. STEVAL-ISA011V1 Schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Rev. 1 3/33

List of Tables AN2272 - Application Note

List of Tables

Table 1. Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 2. Start up Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 3. Component Critical Temperatu re Measurem ent s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Table 4. Steady-state Full Load Condition Measurem ent s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Table 5. Steady-state Output Voltag e Ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Table 6. Bill Of Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Table 7. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4/33 Rev. 1

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

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.

1.1 Primary Side

1.1.1 Step 1, Input Capacitor Selection

Section 2.3: Dynamic

The first design step is to calculate the input capacitor value (C2a + C2b see

ISA011V Demo Board Schematic on page 30

Equation 1

is useful for this purpose:

Equation 1

-----------------------------------------------------------------=

2PINΔT⋅⋅

AC min()pk

–

DC m in()

Where, C

= input capacitor value,

= input power,

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

AC(min)pk

DC(min)

In this case, th e P

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

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

value used is calculated as PO/η, where PO is the maximum output

power and η is the overall ex pect ed efficie ncy (70% in this example).

STEVAL-

Rev. 1 5/33

STEVAL-ISA011V1 Board Design AN227 2 - Application Note

An acceptable value for V

DC(min)

is 80% of V

Equation 2

DC min()

ΔT is expressed as:

Equation 3

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

ΔT

2 π f

⋅⋅

line

Where, ΔT = the time between the two conduction cycles of the input bridge diodes, and f

= line frequency.

line

using

The calculated value of C and C2b, see This means that C

STEVAL-ISA011V Demo Board Schematic on page 30

= 20µF. This value was selected because the tolerance for an

Equation 1

electrolytic capacitor is usually around 20%.

1.1.2 Step 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 (CC M) and Discontinuous Conduction Mode (DCM). The worst case is minimum input voltage and full load. This value is expressed as:

0.8V

AC(min)pk

AC min()pk

π arc

AC min()

DC min()

⎛⎞

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

cos–⋅=

⎝⎠

AC m in()pk

is 16µF. For the board, two capacitors (C2a

) of 10µF were used.

Equation 4

MAX

3.5mH=⇒=

MAX

DC min()DMAX

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

⋅()

2PINf

⋅⋅

Where, L

= maximum inductance for discontinuous mode operation,

MAX

V D P f V

= selected minimum input voltage required for the flyback (converter) stage,

DC(min)

= maximum duty cycle,

MAX

= input power,

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

= reflected voltage (fixed to 90V).

6/33 Rev. 1

AN2272 - Application Note STEVAL-ISA011V1 Board Design

The D

at the boundary between CCM and DCM is expressed as:

MAX

Equation 5

MAX

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

The transformer selected for this application provides an L than the maximum inductance (L

MAX

DC m in()VR

⇒

MAX

0.47==

of 3mH, whic h is a little les s

) calculated in the first equation (3.5mH). This ensures

that the system is not working at boundary and will always func tion in DC M. Using the transformer’s L

● Peak Primary Current, expressed as,

, the designer can calculate the:

Equation 6

⋅

PEAK

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

fSWL

⋅

⇒

PEAK

258m A==

Where, I

= peak primary current,

PEAK

= input power,

= switching frequency, and

= primary inductan ce.

● actual Maximum Duty Cycle (D

), expressed as,

MAX

Equation 7

MAX

⋅⋅⋅

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

SWLP

⋅

MAX

0.42=⇒=

2PINfSWL

and

● the primary side Root Mean Square (RMS) current value (I

PRMS(max)

), which is the

current that flows through the main switch and primary winding. It is expressed as:

Equation 8

PRMS max()IPEAK

MAX

------ ----------- -- I

PRMS max()

97m A=⇒⋅=

Where, I

PRMS(max)

= Primary Current root mean square,

Rev. 1 7/33

STEVAL-ISA011V1 Board Design AN227 2 - Application Note

The conduction losses in the main switch depend on the VIPer12A I resistance, and are expressed as:

Equation 9

Where, P

VIPer12A

ds(on)

= VIPer12A conduction losses, and

= VIPer12A ON resistance.

1.2 Secondary 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

page 30

Equation 10

). V

is calculated as follows:

R(max)

VIPer12Ards on()

Rmax()VOUT

PRMS(max)

⋅=

PRMS max()

STEVAL-ISA011V1 Schematic on

OUT

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

⋅+=

DC m ax()

and ON

Where, V V V V

= maximum reverse vo ltage,

R(max)

= output voltage,

OUT

= reflected voltage, and

R DC(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 V

calculation when a Schottky diode is used, or a safety

R(max)

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 V the D

value is about 34V. This makes the STPS340U (with 40V breakdown voltage) an

is 23V and a Schottky diode is used (adding a 50% safety margin),

R(max)

excellent choice for this application.

8/33 Rev. 1

AN2272 - Application Note STEVAL-ISA011V1 Board Design

1.2.1 D11 Current and Power Dissipation

● The average current flowing through D

is the output current while the I

expressed as:

Equation 11

DRMSIPKS

⋅=

Where, I

= current root mean square,

DRMS

= peak current at secondary winding, and

PKS

D For one output flyback, I

= conduction duty cycle of the secondary diode.

s_cond

(peak current at the secondary winding) can be calculated

PKS

as the primary peak current multiplied by the turns ratio. Note: This formula applies only to DCM operation.

● D

power dissipation is calculated as follows:

Equation 12

lossDVdDIDavg()rdD

Where, P

= diode power dissipation,

lossD

= drop voltage (when the diode is forward-biased),

= diode average current, and

D(avg)

= dynamic resistance.

Note: The formula and the correct values for V

DRMS

s cond

–

-------------------- 3

⋅+⋅=

DRMS

and rdD are in the diode datasheets.

value is

Rev. 1 9/33

STEVAL-ISA011V1 Board Design AN227 2 - Application Note

1.2.2 Transformer 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, V

expressed as:

Equation 13

DROP(avg)

DROP avg()VdDrdD

Where, V

DROP(avg)

= drop voltage (when the diode is forward-biased),

= dynamic resistance,

= diode output current, and

● Using the calculated V

= average voltage drop (across the output diode)

DROP(avg)

Equation 14

------N

Where, N

= Primary Turns,

= Secondary Turns,

= reflected voltage, and

= output voltage.

● Using the calculated turns ratio, I

Equation 15

+ I

⋅=

value, the turns ratio is expressed as:

----------------------------------------------= VOV

is then expressed as:

PKS

DROP avg()

-------

⋅=

PKP

Where, I

= peak current at secondary winding, and

PKS

= peak power current

PKP

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

● The D

conduction duty cycle is expressed as:

Equation 16

scond

–

Where, D L f

10/33 Rev. 1

= Secondary Diode conduction duty cycle,

s_cond

= primary inductan ce, and

= switching frequency.

⋅⋅

PKPLPfSW

------ ----------- ----------- ----------- ----- -=

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