Overview of Home Green Power Supply Design

The cooking process not only generates heat, but also releases and condenses a lot of water vapor. The power supply in the electronic control device of the cooker must perform stably in ambient temperatures up to 105°C and high humidity conditions. In addition to meeting international EMI and safety standards, the power supply must also help promote the realization of increasingly stringent energy-saving goals.

The consequences of energy consumption extend far beyond the use of electricity. Other factors that must be considered include the use of materials and resources, the generation of waste, and the emission of environmental pollutants. It is estimated that more than 80% of all environmental impacts associated with a product are determined at the product design stage (1). It is against this background that the European Commission issued the Ecodesign Directive to ensure that all energy-using products (EuP) sold in Europe are designed to minimize environmental impact from the beginning. It is worth mentioning that the Ecodesign Directive aims to reduce energy consumption levels throughout the product's life cycle. For auxiliary power supplies in household appliances, the EuP Lot 6 requirements are applicable (see Table 1).

To illustrate how EuP Lot 6 requirements can be met economically, Power Integrations has designed two cost-effective power supplies for kitchen appliance applications that achieve the following goals: low system BOM cost; accurate output voltage for microcontrollers and digital logic; high reliability under extreme operating conditions: Ta = 85℃ /105℃, 85% humidity; easy to design in: shorten time to market and reduce development costs; meet international standards: EMI, safety and energy efficiency.

The first power supply application provides an isolated output of 9V, 250mA from an input voltage of 175 to 265VAC in an operating environment of 0°C to 105°C. Specific application information is detailed in Design Engineering Report DER-214(2), and the circuit diagram is shown in Figure 1 below.

The design is based on the LNK623PG device from the LinkSwitch-CV family from Power Integrations (3). This device integrates a 700V MOSFET and all the control and protection circuitry required to create a switching power supply. One of the unique features of the LinkSwitch device is that it can provide primary-side control of the output voltage regulation. The feedback function that ensures 5% voltage control is provided by the bias winding (pins 4-5) within the transformer.

Primary-side control technology can eliminate the need for feedback and control loop compensation circuits, thereby significantly reducing BOM cost and improving overall reliability. In Figure 2, the components that are eliminated by using primary-side control technology are highlighted in red.

The circuit shown in Figure 1 uses a flyback topology. The rectified DC voltage from the AC mains is switched by the high voltage MOSFET in U1 through the primary side of transformer T1. At each OFF transition, the collapsed magnetic field in T1 transfers energy to the secondary (pins 8-10), which is rectified to produce a stable DC output voltage. The ON/OFF control circuit in U1 provides regulation for voltage changes appearing at the FB pin. The voltage at the FB pin is proportional to the voltage on the bias winding (pins 4-5) and, in turn, the voltage on the main output winding (pins 8-10).

When the FB voltage is continuously below a specified voltage level, VFBTH (typically 1.84 V), U1 will switch continuously to transfer maximum energy to the output. This causes the output voltage to increase, and therefore the voltage at the FB pin. When the FB voltage exceeds VFBTH, subsequent switching cycles are disabled until the voltage drops. By adjusting the ratio of enabled switching cycles to disabled switching cycles (as shown in Figure 3), U1 can maintain a ±5% voltage regulation rate. Under light load conditions, the MOSFET current limit is also reduced to reduce the transformer flux density, thereby preventing the generation of audible noise.

ON/OFF control techniques offer significant advantages in meeting stringent energy efficiency regulations, such as EuP Lot 6 requirements. The 9V power supply mentioned above typically achieves 70% efficiency at full load and maintains this efficiency level over a wide load range. PWM power supplies, on the other hand, tend to decrease in efficiency as the load increases. For the standby load (Io=25mA), the input power is only 115mW at 230VAC, easily meeting the stringent EuP Lot 6 requirements. If a diode, capacitor, and resistor are added separately to provide internal power to U1 from the transformer bias winding, the standby power consumption can be even lower.

An important consideration for kitchen appliance applications is safety and reliability. In hot and humid environments, thick contaminants accumulate on the surface of components over time. This can easily lead to leakage and even arcing between high-voltage wires. Power Integrations has designed both DIP and SO-8 plastic LinkSwitch package options to extend the creepage distance between the drain pin and all other pins to avoid leakage problems (see Figure 4).

The second power application example from Power Integrations shows how to design a dual-output power supply using only a few additional components. For more information on this application, see Design Engineering Report DER-213(4). The circuit diagram is shown in Figure 5 below.

Although the circuit uses a relatively small number of components, it still meets all current and proposed energy efficiency standards. Feedback control uses the same primary-side regulation technique as the single output voltage circuit. The current drawn from both the 12V and 5V outputs affects the magnetic flux in the transformer, which in turn affects the voltage that appears on the bias winding. This allows U1 to easily maintain a ±5% output voltage tolerance on both output circuits.

Both application circuits have been carefully designed to integrate hysteretic thermal shutdown, auto-restart output circuit protection, EMI filtering, and other safety features. Detailed information can be found in the related design example reports (DER-214 and DER-213). The introduction of these two power supply applications proves that design engineers can use very few components to design power supplies that are suitable for kitchen appliance applications and can easily meet current and proposed energy efficiency standards.