A growing trend in the power conversion world is “green”. Governments and industry organisations are introducing more stringent regulations every year, as people become increasingly aware of the need to use less energy.
Improving power conversion efficiency is important for saving energy in power supplies. There are many discussions in the field about achieving higher efficiency by implementing techniques such as a digital control loop, using better components, innovative packaging, or optimising thermal management.
Although digital control holds much promise in improving efficiency and dynamic response, it is in its infancy. Major efficiency improvements are expected on the overall system through digital power management, such as communication between digital controllers.
Many soft-switching techniques have been introduced for higher efficiency by reducing power losses during the on/off transition of power switches. For an example, an LLC resonant topology is now a mainstream topology for flat panel TV and telecom power supplies, and even for high-end game console power supplies.
Synchronous rectifier
For the secondary side of switch-mode power supplies, a synchronous rectifier becomes an essential building block for greater efficiency. It replaces rectifiers in terms of providing better efficiency. Power losses can be reduced when the product of a mosfet’s on resistance and drain current is less than the diode forward voltage drop. It is popular in applications ranging from high-end servers to laptop adapters.
This synchronous rectification is also used in desktop PC power supplies that have never been considered “high-end” applications. With new 80PLUS silver and gold guidelines, both soft-switching topology and synchronous rectifier are required to make the power supplies meet the target efficiency level.
Silicon carbide
With an innovative new device, sometimes designers can go back to conventional hard switching topologies, which are usually less complex. Silicon carbide (SiC) is coming into the spotlight as a material for power semiconductors. It is very useful for a higher voltage and extended operating temperature. Schottky diodes based on SiC are already commercially available.
By applying the SiC Schottky diode, designers can improve efficiency by several per cent and eliminate many components. A lot of research is being done to make SiC JFET and even SiC mosfets commercially available. A drawback of SiC-based devices is cost, but the price gap is quickly getting smaller. Innovative new silicon devices also enable better efficiency.
Good examples in the area of high-voltage devices are field-stop IGBTs and charge-compensated mosfets. The field-stop IGBTs have lowered VCE(sat), a voltage drop across the switch at on-state, and are perfect for motor drives and induction heating applications.
Faster switching versions of the field-stop IGBTs are being introduced too. Deep trench filling mosfets utilising charge-compensation theory have ultra low RDS(on) and extremely fast switching speed. Their RDS(on) is less than one fourth of standard power mosfets. This is almost the ideal switch, and greatly improves efficiency.
Figure 1 shows specific RDS(on) improvement of a high-voltage power mosfet. Having more active cells in the same area allows smaller chip size for a given RDS(on) value, which means a more cost-efficient device. It also provides much smaller parasitic input capacitance.
Component integration
New package developments are usually related to component integration, while packages for discrete power semiconductors are focused on cost reduction. Integrating components into a single package reduces board space, offers a smaller number of components, accelerates time to market, provides higher reliability, and enhances performance. Subsequently, these benefits help to reduce the overall cost of the system.
Thermal performance is another important factor in the integrated solution because multiple power components are closely located in a single package. This can be addressed by selecting optimised power devices, substrate material and size, and internal layout.
The substrate material and internal layout are also critical to EMI, which greatly affects embedded controllers. During switching on and off transitions of power devices, displacement current is induced across the parasitic capacitance of the substrate. The induced current interferes with the sensitive signals for the controller. Several techniques are commercially available, such as endmills, low dielectric materials, or lamination of FR4 on IMS.
Power semiconductor suppliers have introduced many technologies and integrated solutions to the market, and these products are widely accepted in displays, power supplies, motion control, and automotive applications. These innovative power semiconductors are able to relieve designers’ green headaches, making systems more efficient.
Author is Sungmo Young, an applications engineer at Fairchild Semiconductor