Feature Power & Design Supplement
Reducing power loss
Edward Ong at Power Integrations explores the performance and reliability advantages of using Zero Crossing and how to get the most out of this technique
ero-crossing detectors are used to synchronise switching with the AC wavelength or to extract the timing signal. By using zero crossing techniques, high inrush currents can be avoided.
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There are many different approaches to detecting the zero-crossing point; both traditional and novel methods are well-documented. However, most approaches rely on a circuit such as that shown in Figure 1. A device like this detects the transition of the AC mains signal waveform between positive and negative. It then produces a narrow pulse that coincides exactly with the zero voltage condition. Aside from variations on the theme of Fig 1, alternative approaches include using an exclusive OR gate, a comparator circuit or even a differential line receiver. Each has its merits in particular applications, particularly at higher frequencies (such as audio applications). However, in the domestic appliance market, the traditional approach has the advantages of lower cost and high reliability. This approach does, however, have one major shortcoming: regardless of the exact design of the zero-crossing detector, inevitably a number of energy- consuming passive devices will be required – at least a pair each of resistors and capacitors. And while this has not always been a major issue, the evolution of international legislation and the market's insatiable desire for greener appliances are conspiring against the traditional zero-crossing circuit. The major legislation in question is the European Commission ErP Ecodesign Directive, and specifically
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ErP Lot 6, which applies to standby and off-mode power consumption in domestic appliances. The standard dictates that the maximum allowable off-mode power consumption for any domestic appliance is just 0.5W. Therefore any losses from anything as peripheral as a zero-crossing circuit are (to say the least) unfortunate. However, ErP Lot 6 is just the tip of the iceberg. Such is the public interest in green appliances that manufacturers are playing a game of one-upmanship in which standby power consumption is becoming billed as a key perform- ance indicator. So any source of power saving is welcome.
Enter the CAP Xero IC solution Power Integrations’ CAPZero IC was originally conceived as a means of improving the efficiency of EMI filters. In this application, the device is con- nected in series with the resistors
Figure 1:
Basic 50/60Hz zero- crossing detector Source: Power Integrations
within the filter circuit, and only becomes active when the AC power is removed. So in the off-state (when the AC power is connected), it blocks power losses through the discharge resistors. And in the on-state (when the AC supply is turned off), it switches on the resistors and permits the discharge of the capacitance from the filter within the required time frame, as required by safety standards. In this way, this device allows designers to use higher values of X capacitors, while complying with safety agency approvals for sub-1s dis- charge of the X capacitors. What's more, the IC is self-powered from the AC line, and has a power consumption of less than 5mW at 230VAC. Interestingly, International Standard IEC 62301 on household electrical appliances, which embodies the requirements of ErP Lot 6 among other standards, rounds down any standby power consumption below 5mW to zero. This makes the solution loss-less. However, with just a few external components, this IC can also function as a zero-crossing detector. But unlike the traditional designs of zero-crossing detectors, it does not require a stack of resistors for the high-voltage divider. As these resistors are the major source of losses in traditional zero- crossing designs, the zero-crossing detector is effectively lossless as it runs from the IC’s supply current. The exact circuitry for this zero- crossing detector will depend on the nature of the system, and will vary between isolated and non-isolated appliances. To further improve system efficiency, it is the same device that forms part of the EMI filter within the appliance that is used for AC line volt- age zero-crossing detection when an AC voltage is present. (On the basis that it is only used as an X capacitor discharge circuit when the AC supply is removed).
Edward Ong is Product Manager at Power Integrations
The zero-crossing detection circuit can also act as a loss-of-AC detector, which can be used to provide a time stamp for system settings and memory to be backed up before power is lost. It can also extend to AC fault detection for con- ditions such as temporary line dropouts and load short-circuits that might cause the temporary loss of AC input. Today's appliance manufacturers are driven by seemingly conflicting mandates to reduce power consump- tion, reduce costs and yet maintain or improve reliability. Using a single CAPZero device to reduce power con- sumption, cut component count and improve reliability through efficient zero crossing detection ticks all the boxes. Power Integrations
www.powerint.com Enter 207
SEPTEMBER 2013 Electronics
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