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Trimming standby power supply consumption

Posted: 03 Nov 2008 ?? ?Print Version ?Bookmark and Share

Keywords:consumer electronics? energy saving? power supply? power consumption?

By Giacomo Mercadante and Davide Stracquadaini

Energy saving is increasingly becoming a concern not only for technicians but also for the general public. Industrialized countries are introducing ever stricter energy regulations to reduce energy consumption as much as possible.

In this context, consumer electronics and home appliances, and in general all electronic equipment, play a key role in the reduction of the energy consumption. This requires a continuous technological improvement of all electronics products. New advanced solutions must be continuously and rapidly developed with high quality, efficiency and respect for the environment. Understand the role of the electronics in the consumption issue is easy considering that inside a common house the consumer equipments, like LCD TV, DVD reader, STB and the home appliances are connected to the main 24 hour for day. When the equipment is not used it's in stand-by state and there is a continuous consumption from the main in order to keep the equipment ready for remote control. The energy needed during the stand-by is little, few mill watts, but if multiplied for the million of electronics unit the impact on the global energy consumption is high. Minimize the stand-by consumption for equipments connected to the main is become mandatory for the manufacturers.

STMicroelectronics, with these requirements in mind, has introduced the new VIPer plus family that enhances the present family introducing innovation and value for power supply solutions. The new family, VIPerX7, includes VIPer17, VIPer27 and VIPer37. The family addresses power supplies up to 25W with a wide-range input voltage and up to 35W with the European input voltage range. The first product of the family is the VIPer17. It is an innovative high-voltage converter suitable for offline power supplies up to 6W with a wide-range input voltage and up to 10W with the European input voltage range. All the products include the power section and controller in one package.

Due to the refined technology, this product has advanced circuits for minimum consumption in normal mode and significant reduction during standby. Functionalities, reliability and safety are other requirements for power supply with VIPer17. The avalanche ruggedness power section, with a breakdown voltage of 800V, the integrated protections for over-load, over-voltage, hard transformer saturation, brown out and thermal shut down all help to keep the power supply safe and reliable. Soft start up, simple feedback management and frequency modulation jittering are the new functionalities that give the power supply higher performance with few external components. Jittering reduces the EMI and helps meet the standards regarding electromagnetic disturbance. VIPer17 works in current mode and is suitable for fly-back converters. It works with fixed frequency and two versions are available, 60kHz (L type) or 115kHz (H type): the VIPER17LN and VIPER17HN in DIP7 package or VIPER17LD and VIPER17HD in SO16N package.

Consumption and efficiency
Standby consumption issues are very similar to efficiency issues. The main concern is to reduce the losses that in a switch-mode power converter are generally two type: conduction losses and switching losses. In no load or in very light load conditions, the conduction losses usually can be neglected as they are very close to zero, so the most of the losses are Switching losses that could be considered almost proportional to the switching frequency.

Burst-mode operation of VIPer17 is one method to reduce the switching frequency and the related losses. As the load decreases, the feedback voltage also decreases, and in turns the primary current limitation. When the feedback voltage goes below a certain threshold, the power switch is no longer allowed to be switched on. The device stops operation and no more power is delivered to the output. The output voltage starts to drop and the feedback loop reacts increasing the voltage on the feedback pin. When this exceeds a second threshold (higher than the first one), MOSFET switch-on is again allowed and the converter delivers power to the output that, this time, is in excess to load power demand because of a minimum value for the Drain peak current internally fixed. The resulting behavior is an intermittent operation (See Figure 1) where the average power delivered to the load is the requested one, and the average switching frequency is reduced. Burst mode operation is not new, but its effectiveness can be significantly improved if some considerations are taken into account.

Figure 1: Burst mode operation

In the following we refers to a flyback converter realized using VIPer17 device, but some consideration could be applied to others converter topology.

As above described, when device is operating in burst mode the average switching frequency is related to the total load that the converter has to supply, where in the total load have to be included the bias currents of the components at the secondary side that are used for closing the feedback loop and the bias current for the control logic of the VIPer17 provided through the auxiliary winding. Considering that during burst mode operation the drain peak current is constant, it means that the average switching frequency, during burst mode operation, is proportional (see Equation 1) to the power delivered to the secondary and to the auxiliary winding. From this consideration arises the needs to reduce as much it is possible before said bias currents.. Doing so the average switching frequency will be reduced, a so its related losses.

Equation 1

The controller of the VIPer17 has been designed with advanced mixed signal technology in order to minimize the consumptions. The result is a maximum of 2mA when it is switching, that is reduced to 800?A when the device is not switching. During the burst mode with the intermittent switching phase the average switching frequency is lowered, then also the VIPer17 average consumption is lowered. From experimental results, when the converter is no loaded the measured VIPer17 consumption it is always below 1mA.

Cutting consumption tips
Some tips can be used in order to minimize contributes of consumption from the secondary side components. In an isolated fly-back converter, usually a TL431 is used in order to directly sense the output voltage and to generate the input signal for the control loop. The TL431 is a device that has inside an error amplifier and a voltage reference and its minimum bias current is 1mA. Usually in order to guarantee this minimum bias current in every operating condition 1k? resistor is connected in parallel with the photo-diode of the optocoupler (R15 in the schematic of Figure 2).

TL431 can be replaced with a device with a lower minimum bias current reducing the current consumption needed for its function. The minimum bias current in this case is 60?A so the R15 value can be increased up to 15k?. Using the TS431 the partition divider (Figure 2 resistance R8 & R9) that senses the output voltage has to be changed considering that the internal voltage reference changes (2.5V in TL431, 1.24V in TS431). In order to minimize the load at secondary side the sum of these two resistor values as to be as high as possible, taking into account the compensation loop needs. Another contribute on consumption from the secondary side is the optocoupler, which usually in this kind of application has a current transfer ratio close to one. Using an optocoupler with higher gain the current consumption at the secondary will be further reduced helping to improve the stand-by performance.

Figure 2: This shows a typical feedback loop for isolated fly-back converter.

Finally but not less important is the impact of the transformer on the reduction of the switching losses and then on the stand-by performance. The primary parasitic capacitance contribute to switch-on losses being this capacitance fast charged each time the power switch is turned ON and the charging current dissipates energy inside the power switch itself. Reducing as low as possible this capacitance this kind of losses will be minimized.

Looking at equation of the output power it is clear that a further reduction of the average switching frequency can be obtained increasing as much as possible, according also to other design considerations, the primary inductance of the transformer (Lp). In fact, assuming constant the Drain peak current, with a higher primary inductance the energy processed each switching cycle increases reducing the number of switching cycles for unit of time, needed to sustain the load, which means a lower average switching frequency.

The previous consideration were applied in designing a prototype board whose schematic is reported in Figure 3 and the electrical specification in Table 1.

Figure 3: Evaluation board schematic

Table 1: Electrical specification

Following some experimental results regarding the stand-by performance of the board, Table 2 reports the measured input power of the power supply when no external load is applied and when the external load is about 50mW for different line voltage values. For the same operating condition the measured average switching frequency. Figure 4 and 5 show in diagram forms the same measurement results of Table 2.

Table 2: This shows Input power and switching frequency during burst mode.

Figure 4

Figure 5

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