STMicroelectronics – Smart power management: an effective way to minimise stand-by power

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This Design Note describes power-management techniques implemented in the VIPerPlus series of AC-DC converter ICs which may be used to minimise stand-by power consumption in household appliances. Use of the VIPerPlus converters enables designers of appliance Switch-Mode Power Supplies (SMPS) to comply with the most demanding energy-saving regulations, through:
• Advanced light-load management
• A clever Zero-Power Mode (ZPM) which enables the appliance to shut down automatically at the end of the operating cycle.

Examples of the implementation of such techniques are presented, using evaluation boards for two VIPerPlus high- voltage converters.

Advanced light-load management
VIPer01, part of the VIPerPlus series, combines an 800V avalanche- rugged power MOSFET with a fixed-frequency PWM current-mode controller in a single chip. An integrated high-voltage start-up, sense FET, error amplifier and oscillator with frequency jitter enable the implementation of a complete AC-DC converter application, using a flyback, buck or buck-boost topology, with few external components.

Important features of the VIPer01 help users to comply with energy- saving regulations governing light-load power consumption:
• The low threshold of both the power MOSFET and internal logic circuitry allow the IC to be supplied from a power rail as low as 4.5V.
• The low gate charge of the power MOSFET and low power consumption of the internal logic circuitry mean that the IC draws an extremely low quiescent current.
• Pulse Frequency Modulation (PFM) decreases the switching frequency under light loads, which minimises frequency-related losses.

The way that these specifications affect real-world performance is illustrated in the following sections, which report measurements of VIPerPlus boards’ performance in light-load conditions.

VIPer01 application performance
An STMicroelectronics applications engineering team tested the operation of the VIPer01 converter using the STEVAL-ISA177V1 evaluation kit, a wide input-range flyback converter which supplies 4.25W to a single 5V output.

In no-load conditions, the board consumes less than 10mW at 230V AC, and its efficiency when supplying a load of 250mW is higher than 60%, as shown in Figure 1.

Input power (mW)Input power (mW)
Output Power (mW)At an Input Voltage of 115V ACAt an Input Voltage of 230V AC
04.48.6
2548.657.4
5089.0100.1
250361.2398.5

Fig. 1: No-load and light-load performance of the STEVAL-ISA177V1 evaluation kit

Advanced architecture for zero-power consumption
A different VIPerPlus part, the VIPer0P, may be used to design an SMPS for periodic household equipment, that is, devices such as washing machines that have finite operating cycles and that normally remain switched off between cycles. The VIPer0P automatically puts the SMPS into the Off mode at the end of the operating cycle, and consumes less than 5mW in this Off state.

This eliminates the need for the bi-stable electromechanical switches that are commonly used in such designs, increasing system reliability, and reducing the cost of the system. In fact:
• The SMPS can be shut down by a microcontroller supervising the operation of the appliance, and enter a special state in which it supplies no power at its output terminals.
• Once in this state, the SMPS is ready to be manually re-started by the user while consuming less than 5mW from the power line at 230V AC.

This capability is a zero-power function, shown in Figure 2: that is, it enables a system to achieve zero-power consumption as defined by the IEC 62301 standard, clause 4.5. It consists of a special idle state, the ZPM, in which the control IC is totally shut down except for the circuitry needed to exit ZPM. The only parts of the control IC that remain alive are the zero-power logic block, and the 4V regulator which provides the bias voltage to it.

pg12_STMicroelectronics_fig2

Fig. 2: Schematic of the zero-power function of the VIPer0P converter IC

Overall power consumption in ZPM consists of two areas: consumption on the branch ZD1, RG, ZD2, M3; and consumption due to the quiescent current, Iq, of around 1.5μA absorbed by the 4V regulator and the zero- power logic block, plus the output current delivered to an external circuit.

This consumption may be estimated as follows:

pg12_STMicroelectronics_equation

At an input voltage of 230V AC and assuming worst-case values, RG = 28MΩ, VZD1 + VZD2 = 20V, Iq = 2μA, ZPM power consumption is 4.2mW plus 0.325mW/μA of current delivered to an external circuit.

A practical example of the zero-power architecture
The demonstration board STEVAL-ISA174V1 is based on the VIPer0P converter IC, and implements a wide input-range, non-isolated, dual- output flyback converter supplying a total of 6.8W, as shown in Figure 3.

pg12_STMicroelectronics_fig3

Fig. 3: The STEVAL-ISA174V1 evaluation board for the VIPer0P converter

It supplies 4W from a -5V output tightly regulated through a voltage divider connected to the non-inverting input of the error amplifier available at the Feedback pin; and 2.8W to a 7V output, semi-regulated by magnetic coupling through the turn ratio of the two output windings.

The completely characterised board is described in ST’s application note AN4836. The application described here complies with the tightest regulations for energy-conscious designs, such as the European Code of Conduct (CoC) version 5 requirements for external power supplies.

Input VoltageZPM Input Power Consumption (mW)No-load Input Power Consumption (mW)
115V AC0.86.5
230V AC3.59.1
Conditions• power line connected
• IC not switching, most internal blocks disabled
• Iout1 = Iout2 = 0
• Vout1 = Vout2 = 0
• power line connected
• IC switching (in burst mode)
• Iout1 = Iout2 = 0
• Vout1 and Vout2 regulated at their nominal values

The data shown in Figure 4 show that the application when in ZPM has zero-power input consumption in accordance with IEC 62301 clause 4.5. It is also five-star energy efficient when operating with no load.

The data in Figure 5 show that the equivalent 12V/6.8W SMPS, which is made by connecting the load across the Vout1 and Vout2 lines, complies with the ErP Lot 6 Tier 2 requirements in the Off mode, and with the 10% load efficiency target envisaged by the European CoC version 5.

Input VoltageEfficiency (%)Efficiency (%)Efficiency (%)Efficiency (%)
At an output power of 25mWAt an output power of 50mWAt an output power of 250mWAt an output power of 680mW
115V AC55.660.872.278.0
230V AC51.357.066.371.4

The STEVAL-ISA174V1 is a demonstration board and does not include an MCU, so the On and Off pins are activated by the user with push-buttons. Evaluation kits which include an MCU are the STEVAL- ISA181V1 and STEVAL-ISA192V1. In these boards, the MCU may be configured to activate the On and Off pins.

Conclusion
Today’s power supply units require more sophisticated methods for improving performance to meet the requirement of the latest energy- saving regulations.

ST’s VIPerPlus high-voltage converters use advanced technologies and clever power architectures to meet the need for increasingly efficient electrical power in smart household appliances that have to be connected with an advanced user interface.

VIPer01 applications demonstrate how easy it can be to meet the most stringent energy regulations for continuous-operation household appliances such as refrigerators. On the other hand, VIPer0P applications demonstrate how to build a clever stand-by architecture, including easy interaction with an MCU, to reduce the bill-of-materials cost of a power supply for a household appliance for periodic operation such as a washing machine.

Such an SMPS design also offers high reliability and flexibility, and has a low component count.

FTM-Board-Club_Stacked
7W dual-output offline flyback converter for home appliances using VIPer0P.

Orderable Part Number: STEVAL-ISA174V1