Improving AC Input Switching Efficiency and Reliability in Smart Home and Smart Building (HBA) Applications Using Lossless Zero-Crossing Detection

“Optimizing the performance of devices that turn AC input power on and off is an important consideration in a growing number of applications including smart home/smart building (HBA), Internet of Things (IoT) enabled appliances , smart switches and plugs, dimmers and human-sensing sensors, especially for designs that use relays or thyristors for power control. When the AC power supply is turned on or off asynchronously regardless of the voltage it is at, efficiency and reliability are adversely affected, and circuitry must be added to protect the switch from high transient currents.

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By: Adnaan Lokhandwala, Product Marketing Manager, Power Integrations

Optimizing the performance of devices that turn AC input power on and off is an important consideration in a growing number of applications including smart home/smart building (HBA), Internet of Things (IoT) enabled appliances , smart switches and plugs, dimmers and human-sensing sensors, especially for designs that use relays or thyristors for power control. When the AC power supply is turned on or off asynchronously regardless of the voltage it is at, efficiency and reliability are adversely affected, and circuitry must be added to protect the switch from high transient currents.

When the AC power supply is turned on asynchronously, the inrush current may exceed 100A. Repeated exposure to high inrush currents can negatively affect the reliability and lifetime of relays and SCRs. The life expectancy of electrical contacts is shortened by inrush current requirements and is usually limited to 100,000 operations, although mechanical life may be 1 million, 10 million, or even 100 million. Considering thyristors, progressive gate degradation can also occur when controlling loads that exhibit low initial impedance. In both cases, minimizing or eliminating inrush current can help extend life expectancy and improve reliability.

For relays, if the device is opened (turned off) when the input voltage sine wave is at the high point of the cycle, arcing will develop on the contacts, corroding the contact surfaces. Using semiconductor line switches, switching losses can be reduced by setting the turn-off at the AC zero-crossing. This will also reduce device stress by eliminating arcing and eliminates the need for surge limiting circuitry.

A discrete circuit can be used to detect the zero-crossing of the AC input to control the switch on and off of the main power device, thereby reducing switching losses and inrush current. This approach is more efficient, but requires additional components, takes up valuable board space, and still has losses, in some cases almost consuming half the standby power budget.

The best zero-crossing detection scheme

To address these challenges, Power Integrations’ LinkSwitch-TNZ off-line switch IC integrates a 725V power MOSFET switch, power controller, and lossless zero-crossing detector (ZCD) into the same package. Use the LinkSwitch-TNZ product family to design AC/DC power supplies with high efficiency, low standby power consumption, minimal component count, and AC zero-crossing detection.

The ZCD of the LinkSwitch-TNZ IC generates an output signal, which is then sent to the microcontroller and controls the power switch relay or thyristor, ensuring that the device turns on and off at zero crossings every time. The ZCD signal logic follows the AC input, with the signal switching at zero crossings every half cycle (see Figure 1). LinkSwitch-TNZ devices consume less than 5mW of power and meet zero-power standards. This is very advantageous compared to the 50 to 90mW power dissipation in a typical discrete ZCD scheme.

Figure 1: ZCD Signal Logic Follows AC Input, Signal Every Half Cycle
Switch once at zero voltage

The LinkSwitch-TNZ buck or flyback converter features a switching frequency of 66kHz, which helps reduce the size and cost of the Inductor and output capacitor, and allows the use of off-the-shelf, low-cost inductors in the buck scheme. Frequency modulation technology reduces electromagnetic interference, thereby simplifying filter circuits. The device has comprehensive safety protection features to protect the device and the entire system in the event of input and output overvoltage faults, device overtemperature faults, voltage offsets, and power supply output overload or short circuit faults.

The LinkSwitch-TNZ IC reduces standby power consumption by 60% and consumes less than 100μA in standby mode, making it easy for power supply designs to comply with global no-load and standby power consumption standards. Using this highly integrated device family, the component count can be reduced by more than 40%, with greater applicability than discrete solutions.

The device family supports flyback, buck, and buck-boost topologies, and includes devices with and without integrated X-capacitor discharge. When used as an auxiliary power supply in a large system, the X-capacitor discharge function can eliminate the permanently connected bleeder resistance, further reducing standby power consumption. LinkSwitch-TNZ devices are suitable for all common topologies, with or without optocoupler-based feedback.

Non-isolated buck converter

Figure 2 shows the typical output current of the LinkSwitch-TNZ device in a non-isolated buck converter, operating at the default current point and with sufficient heat dissipation, and the output current range is 63mA to 575mA. The working modes are mainly discontinuous conduction mode (MDCM) and continuous conduction mode (CCM).

Figure 2: In LinkSwitch-TNZ part numbers, x = 0 indicates a device with zero-crossing detection only,
x = 1 indicates a device with both zero-crossing detection and X-capacitor discharge

Choose between MDCM and CCM operating modes

When choosing between MDCM and CCM modes of operation, designers should choose the LinkSwitch-TNZ device, freewheeling diode, and output inductor to achieve the lowest overall solution cost while delivering adequate power. Typically, MDCM provides the lowest cost, highest efficiency converter. All CCM designs require the use of larger inductors and ultrafast (maximum t_RR 35ns) freewheeling diode.

Isolated Flyback Converter

The maximum practical continuous output power in a flyback design is shown in Figure 3.

Figure 3: Maximum practical continuous output power that LinkSwitch-TNZ can deliver at different input voltages

In flyback designs, LinkSwitch-TNZ can provide more than 80% efficient power conversion. In addition, the use of on/off control enables extremely low light load power consumption, enabling more functions (such as displays, wireless connectivity, sensors, etc.) to be active while the system is in standby.

Evaluation boards for starting projects

To help designers accelerate projects, Power Integrations offers a range of evaluation boards for the LinkSwitch-TNZ IC product family, including 0.5W and 2.5W non-isolated buck converters and 6W and 10W isolated flyback designs. All of these include lossless ZCD functionality, and the 10W flyback evaluation board also includes X-capacitor discharge. Examples of available evaluation boards include:

• 0.5W DER-874 C non-isolated buck converter with 6V/80mA output;
• 2.5W RDK-866 C Non-isolated buck converter with 5V/500mA output; very low audio noise (
• 6W RDK-877 C isolated flyback converter with 12V/0.5A output; 10W DER-879 C isolated flyback converter with 12V/0.75A and 5V/0.2A output;
These low-component-count evaluation boards all support an AC input voltage range of 90V to 305V and meet EN55022 and CISPR-22 Class B conducted EMI limits.

Summarize

Efficient turn-on and turn-off of AC mains is an important consideration in a growing number of applications, especially designs employing relays or thyristors for power control. Repeated exposure to high stress caused by unsynchronized switching can negatively impact the reliability and lifetime of relays and thyristors. In both cases, AC input zero-crossing detection can be used to control main power devices on and off switching to reduce operating stress and help increase life expectancy and reliability.

Designers can choose LinkSwitch-TNZ from Power Integrations to improve reliability and reduce standby power consumption by 60% in applications with integrated lossless AC zero-crossing detection and X-capacitor discharge. Use the LinkSwitch-TNZ product family to design 0.5W to 18W AC/DC power supplies with excellent light-load efficiency, low standby power consumption, minimal component count, and AC zero-crossing detection.