ST VIPower VIPer22A-E Instructions d'installation et d'utilisation

November 2014 DocID10240 Rev 2 1/17

AN1897

Application note

VIPower™: low-cost universal input DVD supply

with the VIPer22A-E

Introduction

In the past few years, many consumer products have been provided to the end user, such

as DVD or VCD players. Generally, their power supply requires multiple outputs to supply a

variety of control circuits: MCU, motor, amplifier, VFD.

Offline switch mode power supply regulators from ST’s VIPer

family combine high voltage,

avalanche rugged vertical power MOSFET with current mode control PWM circuitry. The

result is the innovative AC-DC converter, simpler, quicker, with reduced component count

and cheap.

The VIPer family complies with the “Blue Angel” and “Energy Star” norms, with very low total

power consumption in standby mode, thanks to the burst operation. This document presents

the application on DVD player power supply with the VIPer22A-E meeting the specifications

in Table 1.

Figure 1. VIPer22A-E evaluation board

Table 1. Output specifications

Input Output

Output

Universal

line 5 V+/-

(1)

1. The accuracy of +/-5% is reached for a range of load combination only. See Section 3.2 for cross-

regulation results.

+12V+/-

(1)

-12 V+/-

(1)

-26 V+/-

(1)

3.3 V+/-

(1)

5 V

stb

+/-

(1)

Min.

85 V

Max.

265 V

Imin.

20 mA

Imax.1.5

Imax.

Imax.50 mA Imax.

150 mA Imax.100 mA

www.st.com

Contents AN1897

2/17 DocID10240 Rev 2

Contents

1 Application description and design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.1 Start-up phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.2 Auxiliary supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.3 Burst mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.4 Feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.5 Primary driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Transformer consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Layout recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Transformer specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

DocID10240 Rev 2 3/17

AN1897 Application description and design

1 Application description and

design

1.1 Schematics

The overall schematic is shown in Figure 3.

1.1.1 Start-up phase

The VIPer22A-E has an integrated high voltage current source linked to the drain pin. At the

start-up converter, it charges the V

capacitor until it reaches the start-up level (14.5 V),

and the VIPer22A-E starts switching.

1.1.2 Auxiliary

supply

The VIPer22A-E has a wide operating voltage range from 8 V to 42 V, respectively minimum

and maximum values for undervoltage and overvoltage protections.

This function is very useful to achieve low standby total power consumption. The feedback

loop is connected to 5 V output by D12 to regulate 5 V output. +5 V

stb

output is blocked by

Q3, so +5 V

stb

regulation is neglected. When the standby signal is present, the Q3 V

cannot provide enough voltage to maintain D12 conducted, so the 5 V output is blocked,

and the +5 V

stb

output is connected to the feedback loop. In this condition the +5 V

stb

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. These features are below-indicated.

– In normal full load, the VDD voltage must be lower than the overvoltage protection.

– In short-circuit, the VDD voltage must be lower than the shutdown voltage.

Actually, this condition leads to the well-known hiccup mode.

– In no-load condition, the VDD voltage must be higher than the shutdown voltage.

1.1.3 Burst mode

The Viper22A-E integrates a current mode PWM with a power MOSFET and includes the

leading edge blanking function. The burst mode allows the VIPer22A-E to skip some

switching cycles when the energy drained by the output load goes below E = (T

)2 *

/2L

= blanking time, V

= DC input voltage, f

= switching frequency, Lp = primary

Inductance). The consequence is the reduction of the switching losses in case of low load

condition by reducing the switching frequency.

1.1.4 Feedback loop

The 5 V output voltage is regulated by a TL-431 (U3) via an optocoupler (U2) to the

feedback pin. If the output voltage is high, the TL-431 draws more current through its

cathode to the anode and the current increases in the optocoupler diode. The current in

optocoupler NPN increases accordingly and the current into the VIPer22A-E FB pin

increases. When the FB current increases, the VIPer22A-E skips some cycles to decrease

turn-on time and lower the output voltage to the proper level (see Figure 1). The 5 V output

voltage is regulated thanks to the TL-431 reference voltage and the R8 and R9 resistive

dividers.

Application description and design AN1897

4/17 DocID10240 Rev 2

Figure 2. VIPer22A-E FB pin internal structure

1.1.5 Primary driver

In a flyback power supply, the transformer is used as an energy tank during the on-time of

the MOSFET. When the MOSFET turns off, its drain voltage rises from a low value to the

input voltage while the secondary diode conducts, transferring to the secondary side the

magnetic energy stored in the transformer. Since primary and secondary windings are not

magnetically coupled, there is a serial leakage inductance that behaves like an open

inductor charged at Ipk, causing the voltage spikes on the MOSFET drain. These voltage

spikes must be clamped to keep the VIPer22A-E drain voltage below the BV

dss

(730 V)

rating. If the peak voltage is higher than this value, the device is destroyed. The RCD clamp

(see Figure 4) is a very simple and cheap solution, but it impacts on the efficiency and on

the power dissipation in standby condition. Besides, the clamping voltage varies according

to the load current. RCD clamp circuits may allow the drain voltage to exceed the

breakdown rating of the VIPer22A-E during the overload operation or during turn-on with

high line AC input voltage. A Zener clamp is recommended (see Figure 5). However this

solution gives higher power dissipation at full load, even if the clamp voltage is exactly

defined.

1.2 Transformer consideration

On the electrical specifications 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 allows a lower power clamp to reduce the input power. It l

leads 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.

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DocID10240 Rev 2 5/17

AN1897 Application description and design

Designing transformers for low leakage inductance involves several considerations:

– Minimizing the number of turns

– Keeping ratio of winding height to width small

– Increasing width of windings

– Minimizing the insulation between windings

– Increasing coupling between windings

Application description and design AN1897

6/17 DocID10240 Rev 2

Figure 3. Application schematic

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DocID10240 Rev 2 7/17

AN1897 Application description and design

For safety requirements, a leakage inductance value is 1 to 3% of the open circuit primary

inductance. A high efficiency transformer should have low inter-winding capacitance to

decrease the switching losses. Energy stored in the parasitic capacitance of the transformer

is absorbed by the VIPer22A-E cycle-by-cycle during the turn-on transition. Excess

capacitance also rings with stray inductance during switch transitions, causing noise

problems. Capacitance effects are usually the most important in the primary winding, where

the operating voltage (and consequent energy storage) is high. The primary winding should

be the first winding on the transformer. This allows the primary winding to have a short

length per turn, reducing the internal capacitance. The driven end of the primary winding

(the end connected to the drain pin) should be the beginning of the winding rather than the

end. This takes advantage of the shielding effect of the second half of the primary winding

and reduces capacitive coupling to adjacent windings. A layer of insulation between

adjacent primary windings can cut the internal capacitance of the primary winding by a four

factor, with consequent reduction of losses. A common technique for winding multiple

secondaries with the same polarity sharing a common return, is to stack the secondaries

(see Figure 6). This arrangement improves the load regulation, and reduces the total

number of secondary turns. Commonly a clamper based on an RCD network or a diode with

a Zener to clamp the rise of the drain voltage is used.

Figure 6. Multiple output winding

Figure 4. RCD clamp topology Figure 5. Zener clamp topology

Layout recommendation AN1897

8/17 DocID10240 Rev 2

2 Layout recommendation

Since EMI issues are strongly related to layout, a basic rule has to be taken into account in

high current path routing, (the current loop area has to be minimized). If a heatsink is used it

has to be connected to ground to reduce common mode emissions, since it is close to the

floating drain tab. Besides, in order to avoid any noise interference on the VIPer22A-E logic

pin, the control ground has to be separated from power ground.

DocID10240 Rev 2 9/17

AN1897 Experimental results

3 Experimental results

3.1 Efficiency

Figure 7. Efficiency at 230 V

(load on 5 V) Figure 8. Efficiency at 260 V

(load on 5 V)

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Figure 9. Efficiency at 85 V

(load on 5 V) Figure 10. Load regulation (load on + 5 V)

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Experimental results AN1897

10/17 DocID10240 Rev 2

3.2 Regulation

Figure 11. Cross-regulation

Table 2. Line regulation

Output 85 V

85 V

260 V

5 V/ 0.1 A 5.15 V 5.15 V 5.15 V

5 V

stb

/ 0 A 5.15 V 5 15.15 V 5.15 V

12 V/ 0 A 12.08 V 12.11 V 12.12 V

-12 V/ 0 A -11.98 V -11.99 V -12.00 V

-26 V/ 0 A -25.82 V -25.85 V -25.86 V

3.3 V/ 0 A 3.87 V 3.87 V 3.88 V

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Table 3. Standby model

Output 85 V

230 V

260 V

5 V 2.05 V 2.05 V 2.07 V

5 V

stb

(100 mA)

5.08 V 5.11 V 5.14 V

12 V 4.00 V 3.99 V 3.98 V

-12 V 3.99 V 3.99 V 3.98 V

-26 V 9.12 V 9.10 V 9.08 V

3.3 V 1.70 V 1.50 V 1.51 V

PDIs0.8W1W1.1W

Table des matières

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