Lossless switcher slashes standby power

June 25, 2021 // By Nick Flaherty
Lossless switcher slashes standby power
The LinkSwitch-TNZ switcher from Power Integrations combines offline power conversion, lossless zero-cross detection and, optionally, X-capacitor discharge functions in an SO-8C package for lower standby power in smart home devices and LED dimmers

Power Integrations has launched an offline switcher chip that it says can reduce standby power consumption to close to zero in ‘always on devices’.  

The LinkSwitch-TNZ combines offline power conversion, lossless zero-cross detection and, optionally, X-capacitor discharge functions in an SO-8C package.

“In a buck design it can power the system but we’ve added these functions with non-isolated high voltage pins rated to 1000V that can interface directly to the AC line without having to step it down,” said Adnaan Lokhandwala, Senior Product Marketing Manager for home building and IoT products at Power Integrations.

“Typically in a traditional supply with PWM control with fixed switching frequency as you go to a light load the switching losses are more significant, with on off control the switcher only turns on when power is required,” he said. “It is only sensing the point at which the output voltage is lower than the set point. A feedback pin has a threshold and depending on how the resistors are set that determines the voltage. The bias current is under 50uA so this is designed to reduce the power consumption by using higher value resistors.”

The switcher chip can be used for non-isolated buck and buck-boost power supplies with up to 575 mA output current and provides up to 12 W output for universal-input isolated flyback designs.

“There are two key areas we are targeting, one is smart homes and the other is appliances. Products are getting more intelligent and so there are issues with standby consumption,” said Lokhandwala. “The duty cycle is very low but they are constantly consuming power and this is exacerbated with the IoT and more connectivity. Sleeping devices typically account for 10 percent of the household energy use, and estimates are that there are up to 40 devices in the home constantly drawing power,” he said.

“The other area is appliances and the tight regulations they have to follow. They have to increase their functionality in standby as part of the smart home in a standby mode of 500mW. The only way appliance makers can fit into that is to have lower power losses,” he said.

Today the standby switch for consumer equipment tends to be implemented discretely. “Some discrete zero cross detection switching designs use 100mW and we can reduce that to under 20mW and reduce the component count by 40 percent or more,” he said.

The TNZ integrates the lossless switch and an optional cross capacitor discharge switch for backup and safety applications.

“It’s a standard high voltage process that we use with our switchers and the isolation is coming from the casing so an isolated power supply is not needed,” he said. “The other beauty is that we don’t need a transformer, it can use an inductor to get the voltages they need, As we are doing the auxiliary supply we can also cover flyback and buck-boost and they can add isolation.”

The switcher chip is also aimed at 2 and 3 wire dimmers, switchers and occupancy sensors. “With zero crossing you can make smart decisions on how to switch on the triac,” said Lokhandwala. “We have taken a complete system for a two wire dimmer and driving a Bluetooth Low Energy (BLE) module with a Nordic Semiconductor controller.”

The switcher can use leading edge or trailing edge switching as its based on MOSFETs so designers can implement a common design with the zero crossing point for the dimming with under 200uA leakage and this prevents the ghosting that is seen in dimmers. “We have seen flicker issues become very important for customers,” he said. “Now you can do a simple design for global markets. For example we are now designing to 300V AC to cater to India where the input line voltage is not very stable with the larger electrolytic capacitors and off the shelf inductors.”

The detection can also be used to give time to provide a backup for smart devices in the home. “As there is a logic sensor that is synchronised with the AC, in applications when the AC is lost you can backup data battery backup you. If you feed the output to a microcontroller, then you have time to store the state,” he said.

“The accuracy is to 1 percent and this is used to synchronise functions in the power supply, as a timer from the frequency. The key benefit is if you turn on the device at the right time the inrush current is reduced.”

Hot switching is more common for smart home and these tend to use relays which can be vulnerable to high inrush currents. "The challenges we see in our customers is the weak link in many designs is the relay where the inrush can be 10 to 15A and that creates a crater in one contact of the relay or stressing a MOSFET. The zero crossing switch instead can allow a longer lifetime,” he added.

There are eight versions of the switcher chip covering 572mA down to 63mA and there is a range of reference designs for ODM device manufacturers.

“These are designed to show the functionality and customers take this as a base and optimise the design, they may use HV ceramic capacitors if the output ripple is OK as they typically have a DC-DC converter afterwards and we have support teams that can help customers optimise the designs,” said Lokhandwala. “Design tools are available now with a complete system design with wireless control,” he added.

www.power.com

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