Low-Cost Power Supplies Perform In Hot Environments

Publication date: 19 January 2010

Low-Cost Power Supplies Perform In Hot Environments

By Silvestro Fimiani, Product Marketing Manager
Power Integrations, Inc. (San Jose, CA)

The home is generally considered a benign environment for electronic systems, but the kitchen is an area of the domestic scene that can be particularly challenging. Cooking not only involves heating, but often releases steam and condensing water vapor. A power supply for the electronic controls in a cook-top must perform reliably at an ambient temperature of up to 105°C and under high-humidity conditions. In addition to meeting international standards for EMI and safety, the power supply must contribute to increasingly tough energy-saving goals.

The consequences of energy consumption go far beyond the use of electricity. Additional factors that must be considered include the use of other materials and resources, generation of waste, and release of pollutants to the environment. It is estimated that over 80% of all product-related environmental impacts are determined during the design phase of a product(1). Against this background, the European Commission has issued an Ecodesign Directive to ensure that all energy-using products (EuPs) sold in Europe are designed from the start to have a minimum environmental impact. In particular, the Ecodesign Directive aims to minimize energy consumption throughout the product’s entire life cycle. For auxiliary power supplies in domestic appliances, the EuP Lot 6 requirement applies (see Table 1).

To demonstrate how the EuP Lot 6 requirement can be economically met, Power Integrations has developed two power supplies for kitchen appliance applications with the following goals:

• Low system BOM cost
• Accurate output voltage for microcontrollers and digital logic
• High reliability in extreme operating conditions: Ta = 85ºC /105ºC, 85% humidity
• Easy to design: Reduce time to market, reduce development cost
• Meet international standards: EMI, safety, energy efficiency

The first power supply application delivers an isolated 9 V, 250 mA output from a 175 to 265 VAC input over 0 to 105°C operating ambient. This application is described in Design Engineering Report DER-214(2); the schematic is shown in Figure 1 below.

Figure 1. Schematic of 9 V power supply using LNK623PG

The design is built around the LNK623PG device from Power Integrations’ LinkSwitch^(R)-CV product family(3). The device incorporates a 700 V MOSFET and all the control and protection circuitry required to build a switch-mode power supply. A special feature of the LinkSwitch device is that it provides primary side control of output voltage regulation. The feedback required to guarantee 5% voltage control is provided via the bias winding (Pins 4-5) within the transformer.

The primary side control technique has a dramatic effect on BOM cost and overall reliability through the elimination of feedback and control loop compensation circuitry. In Figure 2, the components eliminated by using primary side control are highlighted in red.

Figure 2. Component count reduction resulting from the use of primary side control

The circuit shown in Figure 1 employs a flyback configuration. The rectified DC voltage from the AC mains is switched through the primary of the transformer T1 by the high-voltage MOSFET within U1. At each OFF transition, the collapsing magnetic field in T1 transfers energy to the secondary (Pins 8-10), which is rectified to create a stable DC output voltage. Regulation is achieved by ON/OFF control circuitry within U1 in response to the voltage developed at the FB pin. The voltage on the FB pin is proportional to the voltage across the bias winding (Pins 4-5) which in turn is proportional to the voltage across the main output winding (Pins 8-10).

While the FB voltage remains below a defined level V_FBTH (typically 1.84 V), U1 switches continuously, providing the maximum energy transfer to the output. This causes the output voltage to rise, hence the voltage on the FB pin rises. When the FB voltage exceeds V_FBTH, the subsequent switching cycle is inhibited until the voltage falls. By adjusting the ratio of enabled-to-disabled switching cycles (illustrated in Figure 3), U1 maintains ±5% voltage regulation. At light loads, the MOSFET current limit is also reduced to decrease transformer flux density and to prevent audible noise. 

Figure 3. ON/OFF control of switching cycles

The ON/OFF control technique offers major advantages when seeking to comply with tight specifications, such as the EuP Lot 6 requirement. The 9-volt power supply described achieves a typical efficiency of 70% at full output, and this level is maintained across a wide load range. This is contrary to PWM supplies, where efficiency falls with the load. For a standby load (Io = 25 mA), the power drawn from the AC input is only 115 mW at 230 VAC, easily meeting the tightened EuP Lot 6 requirement. An even lower standby consumption may be achieved by adding a diode, capacitor, and resistor to provide the internal power for U1 from the transformer bias winding.

An important consideration for kitchen appliance applications is safety and reliability. In hot and humid conditions, there can be a significant build-up of contamination on component surfaces over time. This could result in leakage or even arcing between wires carrying high voltages. Power Integrations designed both the plastic DIP and plastic SO-8 LinkSwitch package options with an extended creepage distance between the Drain pin and all other pins to avoid leakage problems (see Figure 4).

Figure 4. Extended clearance between Drain and other connections

The second example application from Power Integrations illustrates how a dual-output power supply can be developed with very few additional components. This application is described in Design Engineering Report DER-213(4). The schematic is shown in Figure 5 below.

Figure 5. Schematic of a dual-output power supply with primary side regulation

Despite the low component count, this circuit also meets all current and proposed energy efficiency standards. The feedback control utilizes the same primary side regulation technique as for the single output voltage circuit. The currents drawn from the 12 V and 5 V outputs both impact on the flux in the transformer, which in turn influences the voltage developed across the bias winding. This enables U1 to easily maintain ±5% output voltage tolerance on both outputs.

Both of the application circuits described are fully developed and incorporate hysteretic thermal shutdown, auto-restart output circuit protection, EMI filtering, and other safety features. Details can be found in the relevant Design Example Reports (DER-214 and DER-213). These applications illustrate how a design engineer can develop a power supply suitable for use in kitchen appliances using very few components, yet easily meeting current and proposed efficiency standards.

References

1. European Commission Eco-design Directive (2005/32/EC) http://ec.europa.eu/energy/efficiency/ecodesign/eco_design_en.htm
and Commission Regulation (EC) No 1275/2008, Ecodesign requirements for standby and off mode electric power consumption of electrical and electronic household and office equipment.
2. Design Example Report DER-214, 2.25 W Low Cost Cooktop Controller Using LNK623PG. Power Integrations, Inc.
3. LinkSwitch-CV Design Idea DI-201, 6 W, High Efficiency, Adapter Power Supply. Power Integrations, Inc.
4. Design Example Report DER-213. 3.8 W 2-Output Supply Using LNK623PG. Power Integrations, Inc.

About the Author

Silvestro Fimiani is product marketing manager of appliance and industrial applications at Power Integrations (PI). Prior to joining PI in 2005, Silvestro served as director of engineering of high-power products at International Rectifier. He holds a Bachelor of Arts in Physics from the University of Naples, Italy.