Deep Dive Into a New Offline Switcher IC, a Buoy for Smart Home Power

August 07, 2021 by Ikimi .O

Power Integrations recently released an offline switcher IC. What's unique about this device? And what does it have to do with zero-crossing detection and x-capacitor discharge?

Offline switcher ICs are crucial in a wide range of applications, including smart lighting, appliances, and home building automation. Power Integrations recently released its LinkSwitch Offline Switcher IC, which promises improved current and voltage regulation in power systems.


Power Integration’s LinkSwitch-TNZ IC

Power Integrations' LinkSwitch-TNZ IC. Image used courtesy of Power Integrations

What's the general working principle behind this new IC? The basic features and a reference design including the device can shed some insights. 


Offline Switcher IC: An Overview

An offline switcher IC is a device that can seamlessly regulate voltage or current in electronics. Manufacturers design it to directly receive electrical power from an AC utility power source. The term “offline” stems from the fact that its power source is independent of the mains voltage line. 

Although various manufacturers have their own unique family of offline switcher ICs, they share some common characteristics, including:

  • Convert input ability to accept power at line voltage
  • Peak sine wave voltage range of ±170 V for  120 V RMS and ±339 V for 240 RMS
  • Tolerant to various power fluctuations like voltage spikes, brownouts, and surges


The New LinkSwitch-TNZ Offline Switcher IC

Recently, Power Integrations released its portable LinkSwitch-TNZ switching power supply IC that incorporates lossless zero-cross detection, an optional x-capacitor discharge, and offline power conversion.

Thanks to this new device’s ability to detect the zero-cross point, which consumes less power (< 5 mW), it allows power systems to minimize standby power losses. Thus, the LinkSwitch-TNZ is said to be more compact and efficient compared to other similar devices that require at least ten more discrete components and burn up to 100 mW of constant power.

Power Integrations' LinkSwitch-TNZ can achieve higher levels of efficiency required for buck-boost and non-isolated buck power supply applications, providing about 12 W output power and 575 mA output current for universal-input isolated flyback designs.

It has a wide range of power applications, including home and building automation (HBA), electrical appliances, dimmers, sensors, switches (with or without neutral wires), industrial controls, and IoT. Generally, the IC is appropriate for smart lighting, meters, and industrial applications, appliances, and unstable mains voltage.


Relevant Specifications of the LinkSwitch-TNZ IC

According to Power Integrations, the LinkSwitch-TNZ offline switcher IC offers high levels of design flexibility by supporting buck, buck-boost, and flyback topologies. The company claims it also has high-performance functionalities like reduced EMI filter complexity through frequency jittering, lossless zero-cross signal detection, 66 kHz operation, and optional x-capacitor discharge.

The IC's reliability features include:

  • Auto-restart option in the event of open-loop and short-circuit faults
  • Output overvoltage protection (OVP)
  • Surge withstand through its 725 V MOSFET rating
  • Line input overvoltage protection (OVL)
  • Extended creepage between the DRAIN pin and other pins

The press release for the IC also reports consistent efficiency over a wide load range with its on/off control and its <100μ standby supply current.


What Is Zero-Crossing?

Zero-crossing is the transition of an AC mains signal waveform from positive to negative or vice versa. 

Generally, this type of detection is relevant when synchronizing switching with the AC wavelength, acquiring the timing signal, and avoiding high inrush currents in power systems. The image below shows a common circuit on which most zero-crossing detection approaches rely:


Simple 50/60 Hz zero-crossing detector

Simple 50/60 Hz zero-crossing detector. Image used courtesy of Power Integrations


The circuit in the above figure detects zero-crossing by generating an arrow pulse as soon as there is a zero voltage condition. Although other approaches—including an exclusive OR gate, a differential line receiver, or a comparator circuit—exist, this conventional method is known for its cost effectiveness and reliability.

Many devices, including plugs, dimmers, sensors, and light switches, occasionally connect and disconnect from the AC line via TRIACs or relays. To make this process seamless, these devices can employ zero-crossing detection. While minimizing in-rush current and switching loss, Power Integrations' IC can detect AC line zero-crossing and influence turn-on transition of the main power device.


What Is X-Capacitor Discharge?

X-capacitor discharge in ICs blocks the flow of current in power systems to discharge resistors when AC voltage is applied. This feature reduces of the amount of power wasted in the system down to zero. LinkSwitch-TNZ IC incorporates it for high-power applications in its LNK331x version.

The image below represents a universal power supply that incorporates LinkSwitch-TNZ IC for zero-crossing detection.


Universal constant voltage power supply zero-crossing detection

Universal constant voltage power supply zero-crossing detection. Image used courtesy of Power Integrations


This circuit implements X-capacitor discharge functionality by:

  • Connecting one AC input line with Z1 using an external series resistor
  • Connecting the other AC input line with Z2 using a different external series resistor


Reference Designs for the Offline Switcher IC

Power Integrations includes five reference design examples for LinkSwitch-TNZ IC. However, this article focuses on a reference design for home build automation and IoT. The design features an input AC power range of 90–300 VAC, delivering 5 V/ 500 mA. The schematic is presented below.


RDR schematic

RDR schematic. Image used courtesy of Power Integrations


The RDR-866 is a non-isolated buck design where the LinkSwitch-TNZ in U1 above connects the positive buck rail. It also achieves the standard mode current limit with a 100 nF capacitor C3.

Some other attractive features to electrical engineers include:

  • <50 mW no-load input power
  • Conducted EMI margin > 6 dB
  • Limited dissipation during output short-circuit fault
  • 1.25" x 1.25" x 0.6" compact solution
  • High-quality output voltage line control



What experiences do you have with offline switcher ICs in power designs? Share your thoughts in the comments below.