ST AN1897 Application note

AN1897

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- APPLICATION NOTE

VIPower: LOW COST UNIVERSAL INPUT

DVD SUPPLY WITH VIPer22A

Jun-fen g Zhang

INTRODU CTI ON

In the past few years, many consumer products have been provid ed to the end user, such as DVD or VCD players. Generally their power supply require multiple outputs to supply a variety of control circuits: MCU, Motor, Amplifier, VFD.

ST VIPer series of off-line switch mode power supply regulators c ombines an optimized, high voltage, avalanche rugged Vertical Power MOSFET with current mode c ontrol PWM circuitry. The result is truly innovative AC to DC conversion that is simpler, quicker and - with component count halved - less expensive.

The VIPer family al so represents th e ea sies t s olution to com ply with the "Blue Angel" and "Energy Star" Eco norms, with extremely low total power consumption at stand-by mode, thanks to the burst operation.

This document would present the ap plication on DVD player pow er supply with VI Per22A satisfying the specification See table 1 below.

Table 1: Output Specification

INPUT OUTPUT 1 OUTPUT 2 OUTPUT 3 OUTPUT 4 OUTPUT 5 OUTPUT 6

Universal

mains lin e

Min: 85Vac

Max:

265Vac

Note 1: The accuracy of + /- 5% is reached only for a cer tain range of loads combination. See paragraph 3. 2 for cross regulation results.

5 V +/- 5%

(See note 1)

Imin: 20mA Imax: 1.5 A

+12 V +/- 5%

(See note 1)

Imax: 30 mA Imax: 30 mA Imax: 50mA Imax: 150mA Imax: 100mA

-12 V +/- 5% (See note 1)

-26 V + /- 5% (See note 1)

3.3 V +/- 5% (See note 1)

+/- 5%

stb

(See note 1)

March 2004 1/11

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AN1897 - APPLICATION NOTE

1. APPLICATION DESCRIPTION AND DESIGN

1.1 Schematics

The overall schematic is shown in figure 2.

1.1.1 Start-up Phase

As any member of the VIPer family, VIPer22A has an integrated high voltage current source linked to Drain pin . At the startup converter, it will charge the V

(14.5V), and then the VIPer22A starts switching.

1.1.2 Auxiliary Supply

VIPer22A has a wide operating voltage range from 8V to 42V, respectively minimum and maximum values for under-voltage and over-voltage protections.

This function is very useful for achievin g low stand-by total power consum ption. During norma l working, the feedback loop is connected to 5V output by D12 to regulate 5V output. At the mean time, +5Vstb output is blocked by Q3, so +5Vstb regulation is neglected. When the stand-by signal is present, the Vce of Q3 can not provide enough voltage to maintain D12 conducted, so the 5V output is blocked, and the +5Vstb output is connected to the feedback loop. In this condition the +5 Vstb is regulated. Thanks to the transformer structure, all the other secondary outputs and the auxiliary voltages are pulled down to a very low level, also pulling down the total power consumption.

All these contents can be summarized by the following list:

• in normal full load, the V

• in short circuit, the V

to the well known hiccup mode in practice;

• in no load condition, the V

voltage of the device must be lower than the over-voltage protection;

voltage must be lower than the shutdown vol tage. Actually, this condition leads

voltage must be higher than the shutdown voltage.

capacitor until it reaches VIPer startup level

1.1.3 Burst Mode

The Viper22A integrates a current mode PWM with a Power MOSFET and includes the leading edge blanking function. The burst mode is a feature which allows VIPer22A to skip some switching cycles

when the energy drained by the output load goes be low E =(T input voltage, f

It has the consequence to redu ce the switching losses when working in low load cond ition by reducing the switching frequency.

1.1.4 Feedback Loop

The 5V output voltage is regulated with a TL-431 (U3) via an optocoupler (U2) to the feedback pin. If the output voltage is high, the TL-431 will draw more current through its cat hode to t he anode and the current increases in the optocoupler diode. The current in optocoupler NPN increases accordingly and the current into the VIPer22A FB pin increases. When the FB current increases, the VIPer22A will skip some cycles to decrease turn on time and lower the output voltage to the proper level (see figure 1).

The 5V output voltage is regulated thanks to the reference voltage of TL-431 and the resistive divider R8 and R9.

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=Switching frequency, Lp=Primary Inductance).

)2 * fsw/2Lp (Tb=blanking time, Vin=DC

b*Vin

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Figure 1: VIPer22A FB pin internal structure

60kHz

OSCILLATOR

AN1897 - APPLICATION NOTE

DRAIN

Secondary feedback

+Vdd

0.23V

1 kΩ

PWM

LATCH

230 Ω

SOURCE

1.1.5 Primary Driver

In a fly-back power supply, the transformer is used as an energy tank fuelled during the ON time of the MOSFET. When the MOSFET turns off, its drain voltage rises from a low value to the input voltage plus the reflected voltage while the secondary diode conducts, transferring on the secondary side the magnetic energy stored in the transformer. Because primary and secondary windin gs are not perfectly magnetically coupled, there is a serial leakage inductance that behaves like an open inductor charged at

that causes the voltage spikes on the MOSFET drain. These voltage spikes must be clamped to keep

the VIPer22A Drain voltage below the BVds s (730Vmin) rating. If the peak voltage is higher than this value, the device will be destroyed. The most used solution is the RCD clamp (see figure 3). This is a very simple and chea p solution, but it impacts on the efficiency and even on the power dissipation in stand-by condition. Also the clamping voltage varies with load current. RCD clamp circuits may allow the drain voltage to exceed the data sheet breakdown rating of VIPer22A during overload operation or during turn on with high line AC input voltage. So, a zener clamp is recommended (see figure 4). However such a solution gives higher power dissipation at full load, even if the clamp voltage is exactly defined.

1.2 Transformer Consideration

On the electrical specification of a multiple output transformer (cross regulation, leakage inductance), the main efforts focused on the proper coupling between the windings. A lower leakage inductance transformer will allow a lower power clamp to reduce the input power. It will lead to lower power dissipation on the primary side.

Auxiliary and secondary windings are swapped in order to decrease the coupling to the primary one. The secondary windings act as a shielding layer to reduce the capacitive coupling. Fewer spikes are generated on the auxiliary windings, the primary and secondary windings have better coupling.

Designing transformers for low leakage inductance involves several considerations:

•Minimize number of turns

•Keep winding build (ratio of winding height to width) small

•Increase width of windings

•Minimize insulation between windings

•Increase coupling between windings

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AN1897 - APPLICATION NOTE

Figure 2: Application schematic

CON4

--12V / 0.03A

470uF/25V

C19

D13

BYW 100/ 2 00

CON5

F- (3.3Vac) / 0.15A

12345

C25

100uF/10V

D11

1N5818

F+ (3.3Vac)

+12V / 0.03A

C17

BYW 100/ 200

470uF/25V

123

--26V / 0.05A

C20

220uF/50V

C20

220uF/50V

C20

220uF/50V

D10

BYW 100/ 200

+5V / 1.5A

STPS5L60

3.3V

GND

Vout

Vin

C15

100uF/10V

LD33V

C13

470uF16V

C12

1000uF/16V

D12

D12 1N5818

1N5818

123456789

1234567

123456789

CON9

+5Vstb / 0.1A

R11

680 ohm

220uF/50V

BYW 100/ 200

8550

STB

5.1K

C10

47pF

U1A

817

TL431

9014

151411131298

47pF/1KV

FR157

100k/1W

47uF/400V

1N4007D21N4007D11N4007

1N4007

RT1

NTC5D-9

2200pF Y1C12200pF Y1

CH1

2.2mH

1 2

0.1uF X2

CON2

0.1uF X2

CON2

250V 1A

1N4937

9.1K

DRAIN

VDD

U1B

817

TX1

TFO EC28---VER3

47uF/50V

SOURCE

CONTROL

VIPER22A

47nF

2JP1

JUMPER

1nF / 1KV

Auxilia ry Volt 10 Vmin

+ 5V 1.5A

Auxilia ry Volt 10 Vmin

+ 5Vstb 0.1A

- 12V 0.03A

+ 12V 0.03A

- 26V 0.05A

3.3Vac 0.15A

+ 5Vstb 0.1A

- 12V 0.03A

+ 12V 0.03A

- 26V 0.05A

3.3Vac 0.15A

+ 5Vstb 0.1A

- 12V 0.03A

+ 12V 0.03A

- 26V 0.05A

3.3Vac 0.15A

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