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 this Switch Mode Power Supply is a Quasi-resonant Flyback 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 2nd level of overcurrent protection of the L6565 QR controller is used to obtain an enhanced burst mode operation of the power supply when the system is in standby condition. The power MOS STP7NK80 (BVDSS = 800V, RDSon = 1.5Ω ) is used as the power switch.

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 COSS, transformer intrawinding capacitance, stray capacitance due to the layout of the circuit as well as other contributions reflected from the secondary side.

Figure 1. Flyback topology

Lm

Ls

Vout

Lp

Llk

Vin Cin

Rp

Cd VDS

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 output voltages. As the secondary winding has run dry of energy, the secondary rectifier no longer conducts and the Power Mos is still off, the tank circuit, made up LP and CD, resonates.

It is an RLC circuit (considering the sum of dissipative effects concentrated in a equivalent resistor RP) and the drain voltage follows the natural evolution of such circuit starting from the condition of CD charged at VDSs at t = 0 (see waveforms in figure 2). RP is normally by far less than the critical damping impedance of the tank circuit.

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 Vin ≤ VR) or a turn on where VD is as close as possible to zero compared with a square wave flyback (see figure 2). This is Quasi-resonant (QR) operation.

Figure 2. Typical waveforms of QR operation

Llk & Cd
Lp & Cd
VDS
VDSs	VDS @ Vin2 >Vin
VR	Vin2
Vin
NO ZVS!	ZVS
Ip	t

IPKs

Pri Sec

		Is @ Vin2 >Vin
IPKp		Ip @ Vin2 >Vin
TON	TFW	TV
	T = 1/fsw	D03IN1463

The main advantages of this kind of control technique are:

1)At turn-on of the Power Mosfet the energy stored in the capacitor CD is dissipated into the power Mosfet itself. Being the energy stored in CD proportional to V2D, turning on the Mosfet when VD is minimum means 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 switching frequency is modulated at twice the mains frequency, with a depth depending on the ripple amplitude. This causes the spectrum to be spread over fre-

quency 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 dBμV.

3)Another important benefit is a high safety degree under short circuit conditions: since the conduction cycles of the Mosfet are inhibited until the transformer is fully demagnetised, flux run away and, therefore, transformer saturation are not possible. Moreover, as during a short circuit the demagnetisation voltage is very low, the system will be led to work at very low frequency, with a very small 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 micro 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 is 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 Power Supply 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 burstmode operation the control loop is still active and the output voltages are still regulated.

In a TV power supply, when the system is in standby mode, generally we do not need to have all the output voltages regulated, we need only to guarantee a minimum voltage at the input of the linear voltage regulator 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 regulator (LE50C), from the secondary output (Vout2 =14V). The electrical specification is listed in table 1:

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	Vout3 =5V; Ioutwup = 70mA; Ioutstand-by=10mA
Minimum switching frequency	70 kHz
Target efficiency	η > 80%
Maximum standby consumption (@ 220 Vac)	< 1W

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