ST AN1729 Application note

AN1729

APPLICATION NOTE

L6565-BASED LOW COST SMPS FOR TV WITH

LESS THEN 1W STANDBY CONSUMPTION

by: R.D. Stracquadaini, C. Leonardi

The need for energy consumption reduction brings to the design of more and more efficient power sup­plies. High efficiency is required not only when the system supplied is fully operating but also when it is
in standby condition and absorbs very little power. A common design target for many home appliances
is that their power consumption in standby mode be less than 1W.

In this paper a cost-effective solution for the power supply of a 60W 14" TV, with a standby power con­sumption of less then 1W is presented.

The topology used for t his Switch Mode Power Supply is a Quasi-resonant Flybac k Converter (See
AN1326) in order to reduce the switching losses (ZVS at turn-on) and then to increase efficiency. In or­der to increase the performance in standby condition the start up network was designed with particular
care and the 2
burst mode operation of the power supply when the system is in standby condition. The power MOS
STP7NK80 (BV

nd

level of overcurrent protection of the L6565 QR controller is used to obtain an enhanced

= 800V, R

DSS

= 1.5Ω ) is used as the power switch.

DSon

QR ZVS Flyback topology

In figure 1 a typical Flyback topology is illustrated. CD is the total capacitance of the drain node. It is the sum of
the Mosfet' s C
well as other contributions reflected from the secondary side.

Figure 1. Flyback topology

transformer intrawinding capacitance, st ray capacit ance due to the layout of t he ci rcuit as

OSS,

Vin

Lm Ls

Lp

Llk

Cin

Rp

Cd V

Vout

DS

D03IN1462

June 2003

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AN1729 APPLICATION NOTE

The Power Mos is turned off when the drain current reaches a threshold that is fixed by the controller depending
on the input and the out put v oltages. A s the secondary wi nding has run dry of energy, t he secondary rectif i er
no longer conducts and the Power Mos is still off, the tank circuit, made up L

It is an RLC circuit (considering the sum of dissipative effects concentrated in a equivalent resistor R
drain voltage follows the natural evolution of such circuit starting from the condition of C
= 0 (see waveforms in figure 2). R

is normally by far less than the critical damping impedance of the tank circuit.

P

At the point where the drain voltage has a valley (a minimum) we can turn on the Power MOS and we can have
a zero voltage turn on (if V

≤

VR) or a turn on where VD is as close as possible to zero compared w ith a square

in

wave flyback (see figure 2). This is Quasi-resonant (QR) operation.

Figure 2. Typical waveform s of QR operation

Llk & C

d

V

DS

V

DSs

V

V

in

L

& C

p

d

R

and CD, resonates.

P

VDS @ V

in2 >Vin

V

in2

charged at V

D

) and the

P

DSs

at t

in2 >Vin

Ip @ V

D03IN1463

ZVS

t

in2 >Vin

I

I

I

PKs

PKp

NO ZVS!

p

P

ri

T

ON

T = 1/f

Sec

Is @ V

T

sw

FW

T

V

The main advantages of this kind of control technique are:

1) At turn-on of the Power Mosfet the energy stored in the capacitor C
itself. Being the energy stored in C

proportional to V

D

2

, turning on the Mosfet when VD is minimum means

D

is dissipated into the power Mosfet

D

minimizing this kind of losses.

2) Probably the main benefit concerns the conducted EMI emission. In mains-operated applications, due to the
ripple appearing across the input bulk capacitor, the sw itching frequency is modulated at twice the mains
frequency, with a depth depending on the ripple amplitude. This causes the spectrum to be spread over fre­quency bands, rather than to be concentrated on single frequency values. Especially when measuring con­ducted emissions, with the average detection method, the level reduction can be of several dB

µ

V.

3) Another important be nefit is a high safety degree under short circuit conditions: since t he conduction cycles
of the Mosfet are inhibited until the transformer is full y demagnetised, flux run away and, therefore, trans­former saturation are not possibl e. Moreover, as during a short ci rcuit the demagnetisation voltage i s v ery
low, th e s y ste m w ill be led to w o rk at v ery lo w fr equenc y, with a very s mall duty cycle. As a result, the power
that the converter will be able to carry is very low.

4) Finally, the way the system processes power does not change, thus designer's experience with standard
Flyback can be fully exploited and there is very little additional know-how needed.

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AN1729 APPLICATION NOTE

For further information concerning a QR Flyback converter based on controller L6565 you can refer to the Ap­plication Note AN1326 and to the datasheet of the controller.

Standby Consumption Issues

When the TV set is in standby we need to supply the m icro controller, the LED and the IR receiver for the remote
control. This supply voltage can be obtained through a linear voltage regulator from one of the low voltage out­puts of the converter. Under such conditions the load i s very light also because we have to consider that the
micro controller is in low consumption working mode.
Most of power consumption in a lightly loaded switching converter is due to the switching losses, thus the lower
the switching frequency the lower the losses. In most of the modern Pow er S upply circuits, when the converter
is very lightly loaded, a low frequency working mode is often used. The switching frequency cannot be too low
to avoid audible noise. In order to further lower switching frequency the converter can be operated in a "burst"
mode, where there are short periods of time where the MOS switches at the normal operation switching frequen­cy spaced out by long periods of time where the MOS does not switch; in this way the average switching fre­quency can be very low and switching losses can be minimized.

Burst-mode operation

In a normal converter realized using the L6565 controller, when the load is very light, a burst-mode operation
automatically takes place thanks to the "frequency foldback function" (See App. Note AN1326 and the datasheet
of the L6565 controller for details). It is important to notice that with this kind of burst- mode operation the control
loop is still active and the output voltages are still regulated.
In a TV power supply, when the system is in s tandby mode, generall y we do not need to have al l the out put
voltages regulated, we need only to guarantee a minimum voltage at the i nput of the l inear voltage regul ator
that supplies the micro controller. Moreover, the loss reduction offered by the natural burst-mode described be­fore is not enough to meet the "less than 1 Watt" target in this application. With a simple and low-cost additional
circuitry it is possible to have an enhanced burst-mode operation that decreases the average switching frequen­cy to a very low value, hence considerably reducing the total losses in the converter. In that case the regulation
loop is skipped but the required minimum voltage at the input of the linear voltage regulator can be guaranteed.

Description of the TV Set Power Supply circuit

The complete schematic of the realized circuit is shown in figure 3. It is a two output switch mode power supply
with a third output for the micro controller, obtained through a voltage regul ator (LE50C), from the secondary
output (V

=14V). The electrical specification is listed in table 1:

out2

Table 1.

Input Voltage range 88 to 264 Vac
Mains frequency 50-60 Hz
Maximum output power 60 W
Output voltages Vout1 = 114V; Pout1 = 55 W

Vout2 = 14V; Pout2 = 4.2 W
Output of the voltage regulator Vout
Minimum switching frequency 70 kHz
Target efficiency η > 80%
Maximum standby consumption (@ 220 Vac) < 1W

=5V; Iout

3

= 70mA; Iout

wup

stand-by

=10mA

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