VIPer12AS025 - STMicroelectronics

November 2001 1/11

G. Augustoni - F. Salanitri

AN1484

APPLICATION NOTE

A 3.6 WATTS TRAVEL ADAPTOR USING VIPer12A



Abstract

The VIPer12A isan integratedPWM and MOSFET

circuit for low power application in the 5W range,

typically in Cellular Phone Adapters. It is housed in

surface mount SO-8 and DIP8 packages.

With the availability of VIPer12A in SO-8 package

and the limited number of external components for

a real PWM operation, building a compact and

performant power supply becomes simple.

The travel adaptor design presented here, has

been made with the aim of minimising overall cost

for a secondary voltage and current regulated

adapter, topology widely used in Cellular Phone

Adapters.

The VIPer12A presents itself as the answer for

lower consumption in standby, like in home

appliances where it will be required to limit non

negligible ”off the use” consumptions, as

recommended by the “European Commission of

Energy”. Today regulations demand less than 1

Watt. Thanksto VIPer12A low power consumption,

it is possible to achieve 100mW standby power in

a wide range of operations.

1.0 PRINCIPLE OF OPERATION

The circuit is a standard Flyback converter with

secondary current and voltage regulation driving

the VIPer12A feedback pin through an

optocoupler.

Table 1: System Performances

1.1 The VIPer12A

1.1.1 Start-up phase

VIPer12A, asany member of the VIPer family, has

an integrated highvoltage currentsource to charge

C3 Vdd capacitor until it reaches its startup level

(15V). When Vdd gets to 15V, the VIPer switches,

supplied by the energy stored in C3, until it is

supplied through the auxiliary winding.

1.1.2 Auxiliary Supply

VIPer12A hasa wide operatingvoltage range from

9V to 40V, respectively maximum and minimum

values for undervoltage and overvoltage

protections. This wide voltage supply range

simplifies the design of the VIPer12A supply but, to

ensure proper operation of the application in any

case, it is advised:

- in normal operation, tosupply VIPer12A within its

operating range;

- in standby mode, to minimize auxiliary

consumption to achieve very low standby power;

- in short circuit, to limit output power by going into

an hiccup mode;

- in constant current mode, to ensure current

regulation below 2V before going into hiccup

mode.

Parameters 100VDC 380VDC

Standby Power 90mW 119mW

Efficiency at 3.6W 62% 66%

Short Circuit Power 1W 1.3W

Load Regulation 3% 2%

AN1484 - APPLICATION NOTE

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1.1.3 Burst Mode

The VIPer12A integrates a current mode PWM

with a Power MOSFET and includes the leading

edge blanking function. The burst mode is a

feature which allows VIPer12A to reduce its

average switching frequency when the energy

drained by the output load goes below

E=(tb*Vin)2*fsw/2Lp (tb Blanking time, Vin DC

input voltage, fsw Switching frequency, Lp Primary

Inductance). This is obtained with a small ripple

current around shut down current of feedback pin

and maintaining the Vdd voltage above 9V. If Vdd

goes below 9V there is the “bad burst mode” in

which VIPer12A repeats the restart cycle

continuously, with a worst standby consumption

and a higher secondary ripple voltage.

1.1.4 Compensation and Duty cycle control

The internal structure of VIPer12A feedback and

compensation pin (FB pin 3) is shown in Figure 1.

The current injected on the FB pin is added to the

one coming from the SenseFet in R2 and then

compared to an internal 0.23V Vref. When FB

voltage is closed to ground, the VIPer12A delivers

its full power. On the other side, when FB voltage

is above 0.23*(R1+R2)/R2, the VIPer12A stops

switching.

The FB pin is directly driven by the emitter of the

optocoupler, behaving as a current source. This

current is filtered by a small 47nF capacitor C5 to

guarantee cycle to cycle stability.

Important

: It is necessary to keep C5 very close to

the VIPer12A feedback pin to avoid high frequency

instability on the compensation loop.

1.1.5 Primary drive

In a flyback power supply, the transformer is used

as anenergy tank fuelled duringthe ONtime of the

Mosfet. When theMosfet turns off, its drain voltage

rises from low value to the Input Voltage +

Reflected Voltage when the secondary diode

conducts, regaining on the secondary the

magnetic energy stored in the transformer. As

primary and secondary windings are not perfectly

magnetically coupled, there is a serial leakage

inductance that behaves like an open inductor

charged at Ipeak that makes the Mosfet drain

voltage reach higher values.

If the peak voltage is higher than the Vdss of the

VIPer12A Mosfet, the device will be destroyed. So

the drain voltagemust be kept below its avalanche

voltage of 730V.

Commonly a clamper based on a RCD network or

a diode with a transil to clamp the rise of the drain

voltage is used.

The presence of the clamper is an extra

consumption in standby mode, especially with

RCD clamper respect to the transil clamper.

Because the power consumption is manageable

with transil clamper, this solution has been chosen

here.

Figure 1: VIPer12A Internal Structure

1.2 Secondary Regulation

1.2.1 Voltage Regulation

The Voltage regulation is achieved with a zener

diode D6 directly driving the optocoupler. The

resistor R3 limits the current in both the zener and

the opto in case of overvoltage.

The VIPer12A feedback pin is current controlled

and its requirement goes from fewuA at full load to

1mA in standby. The same current change is

experimented by the regulating zener on the

secondary side of the converter leading to around

5% load regulation.

It is possible to improve the load regulation, by

connecting a resistor between the zener and the

Vout. Of course, this will degrade the standby

power consumption.

1.2.2 Current Regulation

The current regulation uses the drop voltage

across a shunt resistor R6/R7/R8 to bias the T1

transistor base-emitter junction. The T1 collector

drives the optocoupler limiting the output power.

60kHz

OSCILLATOR

PWM

LATCH

0.23V

DRAIN

SOURCE

FB R1

+Vdd

Secondary

feedback

IFB Is

Ω

230Ω

AN1484 - APPLICATION NOTE

3/11

Figure 2: Application Schematic

+VOUT

6V/600mA

5.1V

VDD DRAIN

SOURCE

CONTROL

IC2

VIPer12A

S1ZB6D

0.6A,600V

4.7uF

400V

4.7uF

400V

INDUCTOR

47nF

TR1

TRANSFORMER

BAV103

10uF

63V

IC1

SFH517

SMBYW01-200

2.7

BC847B

AC IN

GND

470uF

25V

100nF

2.7

47uF

16V

1.5nF

2KV

1mH

SMAJ188A

BGY20G

TP3TP2

TP1

TP4

TP5

TP6 TP7

AN1484 - APPLICATION NOTE

4/11

The accuracy of this circuit is limited especially in

temperature but is unrivalled in terms of cost. The

addition of R5 base resistor is necessary in short

circuit to avoid destroying T1 Base-Emitter

junction. R3 is also necessary in short circuit

otherwise T1 collector current directly flows

through D6 and the optocoupler is not driven

anymore, leading to an increase of short circuit

power consumption. C8limits the gain infrequency

of T1, stabilising the loop.

2.0 THE TRANSFORMER

An important part of a SMPS design lies in the

transformer. Its performances are a key to the

system performances.

The requirements for this application are: small

size and limited voltage on the drain.

Table 2: Transformer Target Specification

2.1 Primary inductance

A simple calculation gives the range of values of

primary inductance suitable for this application.

VIPer12A has a drain current limitation of 360mA

min. The energy transferred is E=1/2LpIp2*fsw in

discontinuous mode. Emin=5W, Ip=360mA and

fsw=50kHz giving Lp>1.54mH.

The transition mode is when Ton*Vin=Toff*Vr

(Ton*Vin=Lp*Ip). The expression of Lp is:

Lp=1/2*(Ton*Vin)2*fsw/E

With Vin=150V, Vr=50V, Ton=5us, E=5W give

Lp=2.8mH.

Vin was chosen to reach the continuous mode at

low input voltage level. Vr is low to limit the drain

peak voltage.

The transformer optimisation has led to a final

value of 2.5mH partly to reduce the primary turns

and their power dissipation with an E12.5 bobbin.

2.2 Transformer Structure

A standard transformer structure (so called with

windings orderPrimary/Auxiliary/Secondary) gives

the following results on VIPer12A supply:

Table 3: VIPer12A Vdd with a Standard

Transformer

Two concerns can be seen from this table:

- the VIPer12Ais not going in hiccup modein Short

Circuit

- The supply voltage is too low in Standby with the

risk of a ”Bad burst mode” with higher standby

consumption and poor regulation (VIPer12A

undervoltage is at 9V max with 8V Typical).

The solution, implemented in the demoboard, is

the optimized structure (so called with windings

order: Primary/Secondary/Auxiliary) shown on

figure 3.

Figure 3: Transformer Structure

Parameters Value

Power 5W

Saturation Current > 400mA

Primary Inductance 2.5mH

Reflected Voltage 50V

Leakage Inductance < 100µH or <3%

Primary Capacitance > 20pF

Conditions 100VDC 380VDC

Stand-by 10V 8V

Load 6V/100mA 15V 15V

Load 6V/600mA 25V 26V

Short Circuit 9V 10V

CORE

Bobin

Primary

Secondary

Auxiliary

CORE

Bobin

Primary

Secondary

Auxiliary

Optimisation

AN1484 - APPLICATION NOTE

5/11

The position of the auxiliary winding on top gives

the following benefits:

- better coupling of primary and secondary

windings thus lower leakage inductance and

energy stored in the ringing circuit

- no coupling betweenprimary /secondary leakage

inductance and auxiliary windings

- less capacitive coupling between primary and

auxiliary windings

Figure 4a and Figure 4b show the drainvoltage of

the VIPer12 (Trace 1) and Vdd voltage before R2

(Trace 2) at full load and in short circuit with the

auxiliary in sandwich (1) and on top (2) of the

windings.

Figure 4a1:At fullLoad at 100V (Sandwich)

Figure 4a2:At fullLoad at 100V (On Top)

Figure 4b1:In Short Circuit at 100V (Sandwich)

Figure 4b2:In Short Circuit at 100V (OnTop)

The VIPer12A auxiliary supply showsthe following

voltage on Vdd pin:

Table 4: VIPer12A Vdd with an optimized

transformer

Conditions 100V 380V

Standby 12V 11V

Load 6V/100mA 18V 18V

Load 6V/600mA 20V 19.5V

Short Circuit Hiccup Hiccup

AN1484 - APPLICATION NOTE

6/11

In these conditions, the VIPer12A is properly

operating. It draws less than 100mW in standby

and the Hiccup mode is safe in short circuit (Figure

5a and Figure 5b).

Figure 5a: Hiccup Mode at 100V

Figure 5b: Hiccup Mode at 380V

2.3 Peak Drain Voltage

This transformer allows the reduction of Drain peak

voltage in any condition.

The gain of this structure is 70V. With the lower

reflected voltage (100V to 50V), the gain is up to

120V. There is a good voltage margin at full load

under 380VDC. This means that a standard 200V

transil clamper will not take any energy in normal

operation.

The clamper is still necessary during start-up and

short circuit, the drain voltage goes above the

730V VIPer12A avalanche voltage.

Table 5: MOSFET Peak Voltage at 380VDC

2.4 EMC Compatibility

Most of the EMC performances are due to the

”floating” voltage of the secondary winding or to

the voltage across C7 EMC capacitor. This

”floating” amplitude is linked to all the parasitic

capacitances along the wire between primary and

secondary windings. With the optimized

transformer, the EMC performances aredegraded.

But it is possible to turn this problem into an

advantage.

The secondary winding is placed between the

primary and the auxiliary ones. The auxiliary

winding is used to compensate the induction from

primary to the secondary. Figure 6 shows this

compensation.

Figure 6: EMC Compensation Technique

- The cold point is wound close to the secondary

winding, limiting the voltage swing of the closest

one.

Conditions Standard

Transfo Optimized

Transfo

Full Load Vpeak 750V 630V

Reflected Voltage 100V 50V

Leakage Inductance 105µH25µH

Primary Capacitance 22pF 26pF

Primary Inductance 3mH 2.6mH

Full Load Ipeak 260mA 275mA

Auxiliary Winding

Primary Winding

Vin

Secondary

AN1484 - APPLICATION NOTE

7/11

- The voltage variation of the primary and the

auxiliary side of the converter must be opposite.In

this design, the D3 diode has been placed on the

ground so the voltage swing is opposite on the

transition.

2.5 Transformer Specification

Lp = 2.5mH @ 50KHz

Ll =30µH @ 50KHz

Cp = 35pF @ 1MHz

Voltages: 55V-Pri / 7.2V-Sec. / 20.0V-Aux

Isat > 400mA

Pout = 5W

Geometry: E12.5

Winding Order: Primary / Secondary / Auxiliary

Primary Winding: 180 Turns AWG 30

Auxiliary Winding: 60 Turns AWG 30

Secondary Winding: 25 Turns AWG 20

3.0 SYSTEM PERFORMANCES

3.1 Efficiency

The Power losses are distributed at 6V / 600mA

output power as follows:

- 400mW in the output diode

- 700mW in the VIPer12A

- 300mW in the transformer

- 380mW in the shunt resistor

Overall efficiency is 3.6W/ (3.6W+1.78W)=67%.

Ifthelosses in the shunt resistor are considered as

available power, the converter efficiency becomes

3.98W/(3.98W+1.4W)=74%. This is possible using

secondary controller like STM’s TSM101.

Figure 7a: Efficiency at 100V

Figure 7b: Efficiency at 380V

3.2 Regulation

Figure 8: VOUT Vs. IOUT

3.3 Standby Consumption

The demoboard consumes less than 100mW at

100VDC and 120mW at 380VDC. This power level

is far below today regulation’s requirement.

The charts in Figure 9a/9b shows the details of the

charger standby consumption at minimum and

maximum input voltage.

The majorcontribution tothe standby consumption

is the VIPer12A own consumption of just 35mW

and is independent from input voltage.

0 0.5 1 1.5 2 2.5 3 3.5 4

Pout (W)

Efficiency CV

Efficiency CC

0 0.5 1 1.5 2 2.5 3 3.5 4

Pout (W)

Efficiency CV

Efficiency CC

0 100 200 300 400 500 600 700 800

Iout (mA)

Vout (V)

100V

380V

AN1484 - APPLICATION NOTE

8/11

Figure 9a: Standby Consumption at 100V

Figure 9b: Standby Consumption at 380V

The only change is due to the internal startup

current source of 22uA which consumption goes

from 2mW at 100V up to 8mW at 380V. Another

factor due to the VIPer12A is the current used on

the feedback pin,regulated around 1mA in standby

that leads to a 10mW consumption in the primary

and 6mW in the secondary (with an opto gain of 1).

Note that it is necessary to keep a certain level of

current in the regulating zener to improve the load

regulation. As load increases, the current in the

opto and the zener decreases lowering the output

regulated voltage. Overall, VIPer12Aneeds 50mW

to operatein standby.

It is possible to spare some mW in the auxiliary

supply, especially the 22Ωserial resistor which is

necessary to regulate the transformer ringing

voltage peak. In the demoboard the transformer

voltage has a narrow dynamic so R2 becomes

useless. So the standby consumption is decreased

if the resistor is removed and the transformer is

tuned to set 10V or less on VIPer12A Vdd.

The standby consumption is less than 60mW at

100V and 80mW at 380V: fairly good considering

the 50mW required by VIPer12A.

4.0 DESIGN MATERIAL

4.1 PCB Solder Side

Figure 10a: Bottom view of Charger’s Board.

Figure 10b: PCB Art Work

4.2 Silk Screen Solder Side

Figure 11: SMD components

Clamper

0mW

22Ω

20mW

VIPer12A

37mW

Opto

11mW

Zener

5mW

Opto

1mW

Rectifier

Diode

1mW

Rectifier

Diode

5mW

Switching

Losses

2mW

22uA

2mW

0mW

Transfo.

4mW

VIPer12A

Vin

100V

12V

Vout

Primary : 81mW Secondary : 7m

Clamper

0mW

Ω

31mW

VIPer12A

34mW

Opto

10mW

Zener

5mW

Opto

1mW

Rectifier

Diode

1mW

Rectifier

Diode

7mW

Switching

Losses

12mW

22uA

8mW

10mW

Transfo.

4mW

VIPer12A

Vin

380V

11V

Vout

Primary : 112mW Secondary : 7m

AN1484 - APPLICATION NOTE

9/11

4.3 Silk Screen Component Side

Figure 11a: Top view of the Charger’sBoard Figure 11b: Through Hole Components

AN1484 - APPLICATION NOTE

10/11

4.4 Component List

Ref. Part List Description Supplier

R1 Wirewound Res. 10ohm5% 2W VITROHM

TYOHM

R2 Chip Res. 22ohm 5% 0.125W S0805

R3 Chip Res. 56ohm 5% 0.125W S0805

R5 Chip Res. 1Kohm 1% 0.125W S0805

R6-R7-R8 Chip Res. 2.7ohm 5% 0.25W S1206

C8 Chip Cap. 100nF 25V X7R S0805

C4 Chip Cap. 47nF 50V X7R S0805

C5 Chip Cap. 1.5nF KX

WKP

MURATA

TDK

ROEDERSTEIN

C1-C2 Elect. Cap. 4.7uF 400V 10x12.5 SD

KMG

P/N 2222 151 90021

NHG

SAMWHA

NICHICON/SANYO

NIPPON CHEMI-CON

PHILIPS

PANASONIC

C3 Elect. Cap. 10uF 63V 5x11 LXZ

YXG

WD/WL

NIPPON CHEMI-CON

RUBICON

PANASONIC

NICHICON

SAMWHA

C6 Elect. Cap. 470uF 25V 10x16

C7 Elect. Cap. 47uF 16V 6.3x7

D1 Phase Bridge Rectifier S1ZB60

MB6S SHINDENGEN

G.I.

D2 Diode BAV103 MINIMELF

D5 Diode SMBY01-200 SMA STMicroelectronics

D6 Diode Zener 5.1V 2% MINIMELF

I1 Inductor 1mH series SPS TDK

IC1 Optocoupler SFH617-A3

PS2561L-1D

PC123FY/2

TCET1106G

SIEMENS

NEC

SHARP

TEMIC

IC2 I.C. VIPer12A STMicroelectronics

T1 BC847B SOT23

TR1 Transformer PF0037 PULSE

JP1 Jumper TinnedCopper Wire 0.7

AN1484 - APPLICATION NOTE

11/11

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