STMicroelectronics : How to Achieve ‘Zero’ Stand-By Power Consumption in an Offline AC/DC Converter

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By: Giacomo Mercadante, Mirko Sciortino and Vittorio Giammello, STMicroelectronics

Regulations which are in place today, or which are set to come into force in the near future, are forcing manufacturers of electronic equipment to reduce stand-by power consumption in devices sold in almost every developed region of the world.

Many kinds of appliances sit in an idle state most of the time. Today, they normally waste energy simply because the user doesn’t turn off the mechanical switch, leaving the connection with the mains power line open.

Now, however, there is a solution which solves this problem without having to change the behavior of the user: the Zero Power mode in the VIPer0P, an offline AC/DC converter from STMicroelectronics. Its clever on/off management enables extremely low input power consumption even when appliances are connected to the mains in an idle state.

In fact, as soon as the equipment completes its working cycle, it can be automatically turned off and immediately switched on again only when required. The control of the Zero Power mode is automatic with a microcontroller, or manual using a tactile low voltage button or remote-control system. In fact, thanks to the VIPer0P’s Zero Power mode, mechanical switches can be eliminated, saving bill of materials cost.

A simplified block diagram of the VIPer0P converter is shown in Figure 1. It consists of an 800V MOSFET with on-resistance rated at 20Ω, and a fixed frequency current mode PWM controller with overcurrent protection. It operates at a jittered fixed frequency, reducing EMI filtering requirements.

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Figure 1: Simplified block diagram of the VIPer0P AC/DC converter

Two different part numbers are available: the ‘L’ version is optimized for a 60kHz switching frequency, and the ‘H’ version for 120kHz. The user can choose the frequency which best balances efficiency and transformer size. The VIPer0P is also equipped with a light load management scheme which ensures it achieves good efficiency even when supplying a load as small as a few milliamps.

In the steady state, the IC supports either self-supply or an external supply. Since it has a wide input voltage range of 5V to 30V, it can operate from an external supply even when the output voltage is as low as 5V. The upper limit of 30V accommodates large variations in the auxiliary voltage, thus giving the user flexibility in the selection of a transformer.

Zero Power Mode for Stand-By Operation
A key feature of the VIPer0P IC is its Zero Power mode, an idle state in which the device is totally shut down, the output voltage is zero, and the residual power consumption from the mains is kept below 4mW at 230V AC (see Table 1). All the IC’s functional blocks are turned off except those needed to exit the mode. Microcontroller management of the operation of the Zero Power mode is shown in Figure 2.

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Figure 2: The operation of Zero Power mode may be managed by a microcontroller

An evaluation kit, the STEVAL-ISA174V1, shows that a power supply with a built-in VIPer0P easily meets the demand of energy regulations such as the European Union’s Code of Conduct for the efficiency of external power supplies, and the IEC 62301 standard for zero power consumption in stand-by mode. Moreover, the evaluation kit demonstrates pre-compliance with IEC standards for EMC, EMI, surge current, burst-mode current, and in-application ESD. The evaluation board implements a non-isolated flyback topology, delivering about 7W across a dual output: 800mA on the -5V output, 400mA on the +7V output. It uses the VIPer0P L version.

Values for the light-load and no-load power consumption of the equivalent single output 12V/6.8W converter are shown in Table 1. Its active mode efficiency, which is defined as the average of the efficiencies measured at 25%, 50%, 75% and 100% of maximum load, is 81.6% at a nominal 115V AC input voltage, and 81.5% at 230V AC. This means that the single output equivalent converter complies with the EU Code of Conduct and with US Department of Energy standards.

Table 1: Power consumption of the VIPer0P for various loads

Output power (mW)02550250Zero Power Mode
Input power (mW) at 115V AC6.544.982.2346.20.8
Input power (mW) at 230V AC9.148.787.7377.13.5

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