Green line, Blue Angel, Energy Star. Today’s key word in the appliance industry is Energy Savings

The worldwide technical community recognizes that the energy consumption due to stand-by losses in houses and offices is a major contributor to environmental pollution. When systems like TV receivers, monitors, printers and VCRs are in the stand-by mode, the question is, "How much power are they consuming"? The energy consumption of appliances is in two distinct areas: The "active mode" of the apparatus and the "waiting mode," or stand-by mode. While in the active mode energy savings can be achieved by lowering the power demand while maintaining an identical system performance. The power supply efficiency will directly affect the savings in the global system energy. In the stand-by mode the approach is somewhat different: Generally, a "wake-block" of components is permanently powered in order to be ready to re-activate the whole system.

This paper describes a new controller recently introduced by Motorola’s Semiconductor Components Group (now known as ON Semiconductor) which adresses the concerns in energy savings.

1 Watt in stand-by

The tendency is to aim at a 1-W consumption for any apparatus supplied by a wall outlet when in the stand-by mode, and several power management options are available to achieve this goal. The classical technique, consisting of disabling the secondary loads while keeping the power supply running, is no longer preferred. In fact, even in a disabled mode, the loads are generally presenting some 100 mW of leakage in the case of monitors or TV receivers. A solution to cancel this leakage is to totally disconnect the loads. Another technique consists of completely disabling the main power supply during the stand-by mode and installing a micropower side-power supply ready to operate the wake-up block.

The ON Semiconductor controller, the MC44608, is designed to address the difficulties encountered in these two known techniques, to allow a high efficiency switched-mode power supply (SMPS) to be built. Thanks to an efficient "SMPS status detection" technique a "secondary reconfiguration" can be used to perform the leakage suppression. This loss suppression is achieved by a sharp reduction of the secondary supply rail voltages, with the exception of the low-voltage rail that feeds the wake-up block.

SMPS secondary reconfiguration

The SMPS transformer exists to partition the primary/secondary energy transfer, of course, with the ratio between the primary winding and each of the secondary windings determining the output voltages. The regulation is on the outputs, which must present the best stability during the normal mode of operation.

The principle of the secondary reconfiguration lies in the modification of the winding turns ratio for the desired regulated output, activated by the switch located on the secondary side of the SMPS. The switch arrangement (see Fig. 1) establishes the connection between a high-voltage winding (high turns ratio) and the supply rail for the wake-up block. In stand-by the switch is closed. In this configuration, the current which was stored in the primary winding of the transformer during the ON period is no longer delivered to the corresponding output rail, but is injected into the low-voltage rail. The result is to stop supplying the high-voltage output and to rapidly charge up the low-voltage output. The normal-mode regulation (through the TL431) reacts with a drastic increase of the energy demand and a simple zener diode in parallel with the TL431 ensures regulation of the low-voltage rail.

Pulsed Mode operation allows SMPS load disabling.

This reconfiguration has a second impact on SMPS behavior: The high-voltage winding, which is behaving as a current source, is biased at a low-voltage level. Through magnetic coupling, each transformer winding reflects this low voltage relative to its turns ratio. Thus the voltage developed on the 112V output becomes 11.2V, the 24V output becomes 2.4V and Vcc reduces to 1.2V. It is clear that under such conditions the controller, the MC44608, will stop working. In fact the time out is related to the amount of energy present in the Vcc smoothing capacitor (C7 in Fig. 1, again.) When reaching the under-voltage lock-out level (UVLOL) the chip enters into a waiting phase. At the end of that time the chip tries to restart the power supply by activating a Vcc capacitor (C7) re-charge process and a new start-up phase. If the secondary reconfiguration is still activated, then the same shut-down sequence will repeat: Regulation of the low-voltage rail, UVLOL of the chip, waiting phase and then another re-start phase. This sequence will repeat so long as the secondary is reconfigured. On the load side of the SMPS, the low-voltage capacitor is charged by successive packets of energy (Pulsed Mode.)

The 1000 µF bulk capacitor (C15 in Fig. 1, again) smooths these packets of energy and the resulting ripple can be smoothed on the supply for the wake-up block logic.

Pulsed Mode Specifics

Several SMPS controllers offer a burst mode while the power supply is lightly loaded. To some extent, the Pulsed Mode here can be associated with the burst mode. The major difference is that the Pulse Mode is the result of a well-defined modification of the power supply’s working mode: Quasi-disabling the non-used secondary rails while designating a specific rail for the stand-by mode. In classic burst modes there is a drawback with the possibility of entering a transformer mechanical resonance, producing an audible noise. This phenomenon is governed by electro-mechanical laws which are difficult to analyse. When the designer faces this difficulty the available tools are few: Mechanical behaviour modification with special cushions inserted in the transformer air-gap and modification of the burst-frequency spectrum. Both of these difficult to control.

The MC44608

The device is a Power MOSFET driver imbedded in a DIP-8 plastic package. It contains all the basic functions of a flyback SMPS controller: An integrated start-up current source with 500-V voltage capability, an internal fixed-frequency oscillator (available at 40, 75 or 100 kHz), a transformer demagnetisation-detection system to ensure a discontinuous current mode of operation (can also work in self-oscillating power supply -- SOPS -- or Quasi-Resonant Mode, a shunt regulator allowing an opto closed-loop regulation, a fully programmable over-current sensing feature used for both modes (normal and Pulsed Mode), and an over-voltage protection against regulator run-away.

The MC44608 automatically selects the working mode.

This feature allows the power supply to self-detect whether the SMPS secondary side is configured in the Normal Mode or in the Pulsed Mode, performed without the need of any specific access pin on the chip and therefore without additionnal components. The principle is based on the storage of the regulator’s status at the end of every packet of energy. Basically, two kinds of burst mode can occur: A "hiccup mode" corresponding to a secondary overload, or a "pulsed mode" corresponding to the secondary reconfiguration activation

During a hiccup mode the important SMPS feature is to "survive" -- no matter if it is noisy. In that case the power components (transformer, MOSFET and diodes) must remain at an acceptable temperature level. To ensure this status, the working duty cycle is only 10% of the burst period.

The different chip status phases (Fig. 2) show that in the overload mode each switching sequence (every burst) is terminated by the detection of an Over-Current. This OC status is memorized and at the next device start-up the mode will be "Normal." In case of secondary reconfiguration, the regulated level is reached before the termination of the working phase and the status memorized at the end of the working phases is No Over Current (NOC.) At the next device start-up the mode will be "Stand-by."

What happens in stand-by mode?

When the stand-by mode decision has been taken, the Over Current level is reduced to a much lower value than the normal mode. The choice of that new peak detection level is fully programmable. The resistor inserted between the sense resistor and the current sense input pin 2 fixes the allowed amount of peak current in the stand-by mode.

So, the amount of available power on the output of the low-voltage rail is determined by three parameters: The peak current during each burst, the duration of the burst and the burst duty cycle.

Dealing with transformer acoustical noise

Power supply design is very often considered an unrewarding task. There are, in fact, some basic reasons why. First, the power supply aspect in a global system is too often studied when the "noble" parts are completed. By definition, the only way to verify compliance of all the system parts is at the end of the project. This is the moment where problems can occur, for example, slight instability on a display or audible noise. If there are insufficient design margins the project can become a nightmare. The MC44608’s design should avoid failure due to acoustical noise.

The burst period is the sum of three distinct phases (Fig. 2, again): start-up phase, the working phase and the latched off phase. There is no choice in the start-up period once the Vcc capacitor has been choosen. The same applies for the working phase period. The latched-off phase allows choice of the burst duty cycle together with the period duration. The MC44608 includes a feature which allows for determination of the latched-off phase perios. The resistor connected between pin 3 and ground defines the effective current that will discharge the Vcc capacitor during this waiting phase. Therefore, a very easy way is offered to the designer to play with the three fundamental parameters governing the Pulsed Mode: peak current, burst duty cycle and burst period.

The application (Fig. 1, again) is a typical wide mains 80-W TV power supply. The use of the MC44608 only requires four low-cost resistors. The discrete power switch is the ON Semiconductor MTP6N60E energy-rated MOSFET. The clamping and snubbing networks are used to lower the EMI radiation and promote a high-breakdown safety margin on the power switch at higher line voltages. The transformer is provided by Thomson-OREGA and is referenced 10642520-P1. The coil former slotted and the ferrite core is ER 29 x 15. The specification is as follows:

The feedback loop uses the opto-technique for primary/secondary isolation purposes. The 112 V output rail is regulated using a resistor divider applied to the TL431. This arrangement produces a current injected in the opto-diode with a value proportional to the d Vout deviation on the 112 V rail. The opto-transistor injects this reflected current into the regulator (pin3.) An internal shunt regulator converts this current into a voltage, and then a voltage-mode PWM controls the power switch on-state duration.

On the secondary side the reconfiguration is performed using a fast diode in series with a TO-92 thyristor MCR22-6. The network 47 kW - 120 pF allows the thyristor firing synchronously with the TMOS switch-off (rising edge of the flyback voltage.) During the SMPS normal mode the current spike produced by the RC network is directed towards the ground via the stand-by switch. The stand-by switch can be controlled by any wake-up block -- for example, a microprocessor.

The way the reconfiguration is performed does not create any additionnal stress in the power supply. The self-thyristor firing is comparable to the secondary diodes switch-on. The effective connection to the low-voltage rail only modifies the level from the high-voltage winding. An additional advantage is that the power switch Drain plateau voltage is significantly reduced during the secondary reconfiguration. In fact, the flyback voltage is divided by the ratio 112/8.

Performance

When working in the active mode, the MC44608 offers the performance of a classical flyback controller. It provides a high safety level thanks to the two distinct Over Voltage Protections: one on Vcc and the other directly sensing the auxilliary winding voltage. This second OVP is not depending on the Vcc capacitor presence and therefore is much quicker. Thanks to the demagnetisation detection, a secure discontinuous current mode is ensured improving the output short-circuit behaviour. The global SMPS efficiency is 83%. The input power achieved with the Pulsed Mode of operation exhibits nearly 1 W while delivering 300 mW on the 8 V output. This power level can potentially be considered as being the total system consumption. In addition, the stand-by mode offers a high safety level by lowering the voltage stress on the power elements. It must be noticed that, thanks to the demagnetisation feature, the power supply could run in the so called SOPS mode by properly choosing the Ton and Toff periods together with the oscillator switching frequency.

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