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Efficient Offline Power for Small Online Appliances

By Richard Lin, AC-DC Business Unit Manager


It is widely acknowledged that, at its heart, the Internet of Things (IoT) comprises countless sensors, actuators, and other devices that add ‘intelligence’ to the connected world. For the IoT to work as intended, many of these devices must be switched on and connected 24 hours a day, seven days a week.

However, there is also increased awareness of the need to moderate energy usage, not least to protect and preserve what will eventually become our ‘smart’ planet. Achieving better environmental protection demands greater emphasis on developing efficient offline power and making that power available to IoT devices while keeping energy consumption as low as possible. The same logic applies to a wide range of devices used in industrial control applications as well as home appliances, such as electric shavers, air conditioning fans, and even rice cookers.

As the semiconductor industry continues to develop ultra-low power processes, fewer small appliances require large power supplies, so the most effective way of making small, low-power, ‘always-on’ appliances more energy efficient is better power delivery. The problem is there may be no practical alternative to running these devices from a main AC supply in many smart buildings. If that is the case, then the power-conversion stage could easily become unnecessarily large and costly.

The benefits of using AC are obvious; it offers as much power as necessary, delivered for as long as required. However, as previously mentioned, budgetary restrictions and design constraints can make efficient AC-DC conversion difficult to deliver. Certainly, in the IoT world, compactness and cost-effectiveness are essential requirements, so any method of power provision that is unwieldy or expensive will not meet requirements.

The solution would be to implement offline power without the expense of an isolated solution or a transformer or, in other words, to provide a non-isolated power supply that offers low-standby current and requires very few external components yet is still able to meet the requirements of small appliances. A direct, non-isolated conversion topology that takes a high-voltage AC source and converts it to a low-voltage DC supply would provide an efficient solution, particularly when it operates with very low-quiescent current.

More than 20 years ago, before the IoT was a viable proposition, it was estimated that around $3bn. of energy was wasted every year in the USA alone by devices that were left in standby mode. As a comparison, consider the billions of devices in the IoT today; while many devices may require being always-on and always-connected, not all of them will be active all of the time, and this presents a potential and significant problem in terms of wasted energy. Product developers face a difficult choice between reliable but large and costly AC-DC conversion and primary cells that will eventually run down. While there is undoubtedly a place for battery-powered devices in IoT networks, they cannot compete in terms of the non-stop longevity offered by direct power sources.

Even when they are active, many of today’s small appliances consume very little power, whether they are connected to the IoT or not. This means the power supply will likely be the main source of inefficiency in the entire system. Furthermore, the size of the power-conversion stage can dictate the overall product profile. In order to bring true design freedom to low-power product design, it is essential to select the most efficient power-conversion topology available. Having access to a small and efficient, direct, non-isolated conversion solution will certainly have a significant impact on the way low-power devices are designed in the future.

Thankfully, efficient power conversion with low-standby quiescent current can be solved by using a non-isolated, universal AC-DC, step-down power switcher that is designed specifically for low-power devices, IoT applications, and home appliances—the AP3917. By taking advantage of operating conditions such as peak current and switching frequency, which reduce as load decreases, delivering superior performance at light loads is possible while improving system efficiency overall. The integration of a 650V or 700V, high-performance power MOSFET into the buck switch reduces the number of external components required and keeps bill of material (BOM) costs down to a minimum.

Combining excellent regulation and high-power efficiency, the devices in this product category also incorporate various features that increase reliability and improve safety. These include overtemperature protection, undervoltage lock function, output short protection, overload protection, open-loop protection, and short-circuit protection. The device will shut down if the peak current exceeds the short-circuit threshold and start up again when the fault is resolved.

Operating from an input voltage between 85VAC and 265VAC, the AC-DC buck switcher delivers a maximal peak current of 280mA, 420mA, or 500mA and a maximum rated output current of 170mA, 270mA, or 300mA depending on the model chosen. The low-noise design also suppresses EMI through frequency modulation.

It is clear that non-isolated offline conversion is now viable for the potentially billions of low-power appliances that will make up the IoT. With the introduction of the AP3917, Diodes Incorporated has delivered a solution to a problem that has been taxing many product developers. Detailed technical details of the AP3917 and how it can enable your next IoT design are available at https://www.diodes.com/part/AP3917