Low power AC/DC power supplies with a power of about 10 W or lower are widely used in household dimmers, switches, sensors, household appliances, the Internet of Things (IoT), and industrial control devices. The duty cycle of this power supply is relatively low, and the load is in standby mode for a long time, but when the device is started, the power supply must be able to be quickly 'woken up'.

Conceptually, designing this type of power supply is very simple: first prepare several diodes for line rectification, then add a controller IC, install a filtering capacitor at the output terminal, and if isolation is required, insert a transformer to complete the design. However, despite appearing simple on the surface, the manufacturing processes of these power supplies are actually different.

These power supplies must have the basic function of providing stable DC output rails and meet multiple strict regulatory requirements such as user safety, load efficiency, and standby efficiency. In addition, designers must also consider issues such as actual layout, supporting components, reliability, performance evaluation, certification, and packaging, while minimizing packaging and costs to shorten the product launch cycle.

This article introduces the highly integrated offline switch mode controller IC series of Power Integrations and explains how to utilize it to address these challenges.

Integrated MOSFET and controller IC
The LinkSwitch TNZ series of Power Integrations includes eight different offline switch mode controller ICs, which integrate 725 V power MOSFET switches and power controllers into SO-8C packaged monolithic devices. Each single-chip IC has excellent surge tolerance, oscillator, high-voltage switch current source for self bias, frequency jitter, fast (cycle by cycle) current limiting, hysteresis thermal shutdown, and output and input overvoltage protection circuits.

These devices can form the core of a non isolated layout, such as the step-down converter design using LNK3306D-TL (Figure 1), with an output current of 225 mA or 360 mA (depending on the selected conduction mode). These devices can also be configured as non isolated "buck boost" power supplies, achieving an output current of up to 575 mA.

Design of LinkSwitch TNZ Non Isolated Buck Converter for Power Integrations
Figure 1: This typical non isolated buck converter design using LinkSwitch series devices is just one of many possible topologies implemented through these devices. (Image source: Power Integrations)

Although loads that use double insulation or other methods to prevent AC line faults do not require electrical isolation, some devices require electrical isolation. In this case, using LinkSwitch TNZ devices in a general input isolated flyback design would be a better choice. In this topology, the output power of these devices can reach up to 12 W.

The ICs in the LinkSwitch TNZ series can provide different output currents and powers based on their topology (Table 1).

The LinkSwitch TNZ series of Power Integrations supports multiple configurations
Table 1: The LinkSwitch TNZ series supports multiple configurations, topologies, and operating modes. Each layout has different maximum output current or power limits. (Image source: Power Integrations)

From concept to implementation
The highly integrated features and flexibility of the LinkSwitch TNZ series simplify design work. Developing certified and marketable power supply designs will face many challenges, including:

Meet strict regulatory requirements related to efficiency and safety. Due to the need to provide power in standby mode while meeting strict standby efficiency regulations, these tasks become even more difficult. The LinkSwitch TNZ IC has the best in class light load efficiency, providing power for more system functions while meeting standby requirements including the following:
The European Commission (EC) Household Appliances Standard (1275) requires that the power consumption of equipment in standby or shutdown mode should not exceed 0.5 W
The Smart Home Energy Management System (SHEMS) Energy Star 1.1 limits the standby power consumption of smart lighting control devices to within 0.5 W
According to GB24849 regulations in China, the non mode power consumption of microwave ovens shall not exceed 0.5 W
Compared to discrete design, LinkSwitch TNZ IC can meet these requirements while reducing the number of components by 40% or more. These switching power supply ICs can achieve ± 3% regulation of the circuit and load, with no-load power consumption less than 30 mW and IC standby current less than 100 µ A under external bias.

Can safely support two-wire AC line connections and three wire connections without neutral wires. Many loads, such as dimmers, switches, and sensors, do not have a third wire, which poses a risk of excessive and potentially dangerous leakage current. This standard specifies the maximum leakage current under different conditions. In a dual line design without a neutral wire, the leakage current of LinkSwitch TNZ is less than 150 µ A, which is lower than the maximum value mentioned above.
Not exceeding the electromagnetic interference (EMI) radiation limit. To achieve this goal, the LinkSwitch TNZ oscillator adopts spread spectrum technology, introducing a small amount of 4 kHz frequency jitter around the nominal 66 kHz switching frequency (Figure 2). The modulation rate for frequency jitter is set to 1 kHz to optimize the EMI reduction effect under average and quasi peak radiation conditions.