Use Advanced Switching ICs to Implement Efficient, Feature-Rich, Low-Power AC/DC Supplies

Low-power AC/DC supplies of approximately 10 W or less are widely used in home dimmers, switches, sensors, appliances, Internet of Things (IoT), and industrial controls. Their duty cycle is relatively low, with their load being in standby mode for long periods, yet the supply must “wake up” quickly when the device is activated.

Designing such supplies is conceptually easy: start with a few diodes for line rectification, add a controller IC, put filter capacitors at the output, insert a transformer if isolation is needed, and the task is done. However, despite the apparent simplicity, the reality of creating these supplies differs significantly.

They must provide the basic function of delivering a stable DC output rail and meet multiple stringent regulatory mandates for user safety, efficiency under load, and standby efficiency. In addition, there are physical layout, support component, reliability, performance evaluation, certification, and packaging issues designers must consider as they also work to minimize footprint and cost while meeting short time-to-market cycles.

This article introduces a family of highly integrated offline switching controller ICs from Power Integrations and shows how it can be used to address these challenges.

Integrated MOSFET and controller IC

The LinkSwitch-TNZ family of eight distinct offline switching controller ICs from Power Integrations combines a 725 V power MOSFET switch with a power-supply controller in a single device housed in an SO-8C package. Each monolithic IC offers excellent surge-withstand capability, an oscillator, a high-voltage switched-current source for self-biasing, frequency jittering, a fast (cycle-by-cycle) current limit, hysteretic thermal shutdown, and output and input overvoltage protection circuitry.

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

Figure 1: This typical non-isolated buck converter design using a LinkSwitch family member is just one of many possible topologies that can be implemented using these devices. (Image source: Power Integrations)

While loads that are double insulated or are otherwise protected from AC-line wiring faults do not need galvanic isolation, some devices do require it. Using the LinkSwitch-TNZ devices in a universal-input isolated flyback design is a better choice in such a situation. The devices offer up to 12 W of output power in that topology.

The ICs in the LinkSwitch-TNZ family offer different output currents and power capacities, depending on topology (Table 1).

Table 1: The LinkSwitch-TNZ family supports multiple configurations, topologies, and operating modes. Each arrangement has a different maximum output current or power limit. (Image source: Power Integrations)

From concept to implementation

The high integration and flexibility of the LinkSwitch-TNZ family simplifies the designer’s task. Among the many challenges of developing a certified, shippable power-supply design are:

1. Stringent mandated requirements associated with efficiency and safety. These are made more difficult by the need to provide power in a standby mode while still meeting strict standby-power efficiency regulations. LinkSwitch-TNZ ICs provide best-in-class light-load efficiencies, enabling more system features to be powered while meeting standby regulations that include:
   • The European Commission (EC) standard for home appliances (1275), which requires equipment to consume no more than 0.5 W in standby or off mode
   • Energy Star version 1.1 for Smart Home Energy Management Systems (SHEMS), which limits standby consumption of smart lighting control devices to 0.5 W
   • China’s GB24849, which limits the off-mode power consumption in microwave ovens to 0.5 W

While meeting these requirements, LinkSwitch-TNZ ICs also reduce component count by 40% or more compared to discrete designs. These switching power-supply ICs enable ±3% regulation across line and load, have a no-load power consumption of less than 30 mW with external bias, and have an IC standby current of less than 100 µA.

2. Safely supporting two-wire AC-line connections without a neutral wire and three-wire connections. Many loads, such as dimmers, switches, and sensors do not have this third wire, so there is a risk of excessive and potentially dangerous leakage current. The standard defines the maximum leakage current under various circumstances, and the LinkSwitch-TNZ leakage under 150 µA in two-wire no-neutral designs is under this maximum.
3. Not exceeding electromagnetic interference (EMI) emissions limits. To meet this objective, the LinkSwitch-TNZ oscillator uses a spread-spectrum technique that introduces a small amount of frequency jitter of 4 kilohertz (kHz) around the nominal 66 kHz switching frequency (Figure 2). The modulation rate of the frequency jitter is set to 1 kHz to optimize EMI reduction for both average and quasi-peak emissions.

Figure 2: To keep EMI emissions under the regulatory limit, the LinkSwitch-TNZ oscillator uses a spread-spectrum technique with a 4 kHz spread around the nominal 66 kHz switching frequency. (Image source: Power Integrations)

4. Detecting AC-line zero-crossings with minimal additional components or power consumption. This detection is needed for light switches, dimmers, sensors, and plugs, which connect and disconnect the AC line periodically using a relay or triac.

The zero-crossing signal is used by smart home and building automation (HBA) products and appliances to control switching to minimize switching stress and system inrush current.

Similarly, appliances often use a discrete zero-cross detection circuit to control motor and microcontroller unit (MCU) timing. These applications also require an auxiliary power supply for wireless connectivity, gate drivers, sensors, and displays.

To achieve this, a discrete circuit is usually implemented to detect the AC-line zero-crossing to control the turn-on transition of the primary power device while reducing switching losses and inrush current. This approach requires many components and is very lossy, sometimes consuming almost half the standby power budget.

Instead, the LinkSwitch-TNZ ICs provide an accurate signal indicating that the sinusoidal AC line is at zero volts. LinkSwitch-TNZ’s detection of the zero-cross point consumes under 5 mW, thus allowing systems to reduce standby power losses versus alternative approaches that require ten or more discrete components and dissipate 50 to 100 mW of continuous power.

Then there is the X capacitor

Line EMI filters include Class X and Class Y capacitors to minimize the generation of EMI/RFI. They are directly connected to the AC power input at the AC line and the AC neutral (Figure 3).

Figure 3: EMI filtering requires Class X and Class Y filtering capacitors at the AC line, but the Class X capacitor must be managed after line disconnect to ensure user safety. (Image source: www.topdiode.com)

Safety mandates require that the X capacitor in EMC filters be discharged when the AC line is disconnected to ensure that stored voltage and energy do not remain on the line cord for an extended period after cut-off. The maximum allowable discharge time is governed by industry standards such as IEC60950 and IEC60065.

The traditional approach to ensuring the requisite discharge occurs is to add bleeder resistors in parallel to the X capacitor. However, this approach comes with a power penalty. A better solution is to include an X capacitor discharge function with a user-settable time constant. ICs such as the LNK3312D-TL take this approach. This results in reduced printed circuit (pc) board space, a lower bill of materials (BOM), and increased reliability.

Power supplies and converters need multiple protection features. All devices in the LinkSwitch-TNZ family of ICs incorporate:

• Soft-start to limit system component stress at start-up
• Auto-restart for short-circuit and open-loop faults
• Output overvoltage protection
• Line input overvoltage protection
• Hysteretic overtemperature protection

From IC to complete design

An IC alone, no matter how good or packed with features, cannot be a complete, ready-to-go AC/DC converter, as many components cannot or should not be integrated into that device. These include bulk-filtering capacitors, bypass capacitors, inductors, transformers, and protective components. The need for external components is shown in the non-isolated universal input, 6 V, 80 mA constant-voltage power supply with a zero-crossing detector based on an LNK3302D-TL device (Figure 4).

Figure 4: Shown are the external components needed for a complete and safe non-isolated universal input, 6 V, 80 mA constant-voltage power supply with zero-crossing detector based on an LNK3302D-TL IC. (Image source: Power Integrations)

There are also safety-related minimum dimensions for attributes such as creepage and clearance. The issue then becomes the difficulty of developing a complete design. The LinkSwitch-TNZ IC family eases the task. For instance, by using a 66 kHz switching frequency, the required magnetics are standard, off-the-shelf items from multiple vendors. In addition, Power Integrations provides reference designs.

For those requiring an isolated supply, the RDK-877 reference design (Figure 5) is a 6 W, isolated flyback power supply with zero-crossing detection based on the LNK3306D-TL.

Figure 5: The 6 W RDK-877 reference design provides isolation in a flyback topology and is based on the LNK3306D-TL. (Image source: Power Integrations)

The supply has an input range of 90 V_AC to 305 V_AC, an output of 12 V at 500 mA, and a no-load power consumption of less than 30 mW across the entire AC line range. More than 350 mW of power is available in stand-by mode, while active-mode efficiency meets DOE6 and EC CoC (v5) requirements with greater than 80% full-load efficiency at nominal loads. The design also meets EN550022 and CISPR-22 Class B requirements for conducted EMI.

Conclusion

Designing and implementing a low-power AC/DC supply may seem trivial. Still, the realities of meeting performance and efficiency objectives, safety and regulatory mandates, as well as cost, footprint, and time-to-market demands make it a challenging task. Switching ICs such as those in the Power Integrations LinkSwitch-TNZ family of combined controller and MOSFET greatly ease the task. These ICs support different power levels and can be used with various supply topologies while incorporating essential features such as zero-crossing detection and X-capacitor discharge.