Bench Talk

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The horsepower (hp) is the name of several units of measurement of power. It expresses the rate at which work is done. Metric horsepower is defined as the power to raise a mass of 75 kilograms against the earth’s gravitational force over a distance of one meter in one second. My weight is about 75 kilograms. The force I would exert if I propelled my entire body against gravity for one meter in one second would be the strength of one horsepower—but I'm not as strong as a horse. Ideally, a 2400W power supply should deliver slightly more than 3hp. This blog discusses Infineon Technologies’ (Infineon) 2400W power density evaluation board for power factor correction (PFC).

Successful designs must be super-efficient across a broad load range, be very reliable, and—more than ever—be in a tiny package. Modern MOSFET transistors, when properly integrated, can yield efficiencies that were merely a dream two decades ago. Today, 95 percent efficiency is well within sight, including PFC and AC rectification. The design engineer must do all this while keeping costs down. The key to success is knowing the latest and best semiconductors and optimizing tradeoffs between various circuit areas.

PFC is a must for modern off-line power conversion. PFC reduces the load on the electrical grid, increases power quality, and reduces electricity system costs. PFC is added to a unit to meet the specifications of IEC 61000-3-2, which sets limits to the harmonic currents drawn by an electrical apparatus.

The CoolMOS^TM S7 from Infineon is a 600V device that uses the superjunction (SJ) principle. This family of MOSFETs enables excellent price-performance for low-frequency switching applications while also allowing for fast switching that increases the associated efficiency. The device uses a 4-pin kelvin source package design to complement its best-in-class Rds specification.

The EVAL2K4WACTBRDS7TOBO1 evaluation board exposes this device to a continuous conduction mode (CCM) 2.4kW power supply operating from a 90–265V_rms 50/60Hz with a nominal output voltage of 390V. This design will yield more than 98.6 percent efficiency with a 230V_AC input and 6A output.

The evaluation board is 127mm long, with a width of 85mm and a height of 44mm. Located at the input to the board is a fuse and a Metal-Oxide Varistor (MOV) surge protector, followed by a two-stage electromagnetic interference (EMI) line filter. After that comes the bridge rectifier and the PFC/main converter. The primary inductor is located in the center of the board (Figure 1).

Figure 1: The EVAL_2K4W_ACT_BRD_S7 CCM PFC converter demo board with 600V CoolMOSTM S7 for active-line rectification and inrush current control from Infineon Technologies. (Source: Infineon Technologies)

The Active Bridge Rectifier

In a typical modern supply design, the input bridge rectifier’s power dissipation dominates losses with about 35 percent of the total. The active bridge design uses four IPT60R022S7 FETs that parallel a standard diode bridge. On the evaluation board, these FETs are on a plug-in daughter card along with two gate drivers (Figure 2). It is essential to ensure that the FETs are properly driven to prevent false triggering.

Figure 2: Infineon Technologies KIT_ACT_BRD_60R040S7 is an extremely compact daughter card that aims to replace standard diode bridge rectifiers with an active full-bridge for line rectification in standard CCM PFC Converters. (Source: Mouser Electronics)

With a 1200W load, this active bridge circuit gains about a 0.5 percent efficiency increase over the standard bridge. A peak efficiency improvement of 1.3 percent is reached at low-line with an active-bridge line rectification of 22mΩ MOSFET. At high-line, the delta efficiency peak is around 0.7 percent.

The Conversion and PFC Controller

An Infineon ICE3PCS01G controller IC provides voltage regulation and PFC in a boost topology in CCM. In a CCM design, there is always a current flow in the inductor. The voltage across the inductor reverses as the inductor current rises and falls, and the current flow is continuous. The chip provides voltage loop compensation and fast output dynamic response during load jump and external current loop compensation. The switching frequency is set at 65kHz on this evaluation board, but it can go as high as 250kHz.

Having a very low reverse recovery time PFC diode is an essential factor in this regulator circuit design. A 650V G6 CoolSiC^™ Schottky barrier diode (IDH12G65C6) from Infineon employs leading-edge technology, fully leveraging all advantages of silicon carbide over silicon. A proprietary soldering process is combined with thin-wafer technology and a novel Schottky metal system. The result is a diode with a V_F of 1.25V at 12A and a total capacitive charge (Q_C) of 17.1nC, typical.

Startup Inrush Limiting

All power converters must deal with a large input current inrush when AC power is first switched on. A typical design method to limit this surge is to have a power resistor just after the bridge rectifier. That power resistor, or often a negative temperature coefficient thermistor, is then bypassed with a mechanical relay after a time delay to save power. This reference design is clever and removes bulky components to improve power density significantly.

Various power conversion efficiency requirements, such as 80 PLUS or EuP, are defined for multiple power conversion applications. Meeting the best-in-class 80 PLUS titanium requirement is no easy task. In the platinum class, the PSU must have a peak efficiency above 94 percent at high-line and 92 percent at low-line, while for a titanium design, these values increase to 96 percent and 94 percent, respectively. The addition of the active-bridge solution in this reference design puts it solidly into the platinum class, and the outstanding packaging solutions of the entire scheme make it a power density champion and a sure-to-be industry favorite.

Although it is now possible to obtain 80 PLUS Titanium certification, power supply designers must improve the efficiency of the entire power range, including the thermal performance of the power devices used in high-power supply units. Although I do not possess the strength of a horse, you are hearing it directly from the horse's mouth. The EVAL_2K4W_ACT_BRD_S7 Evaluation Board from Infineon Technologies offers a solution to achieve such efficiency in the form of a 2400W CCM PFC converter with active-line rectification and solid-state relay. It offers designers the ability to tackle their next high-efficiency, high-power-density SMPS design quickly.

Jim Harrison is an electronics engineer and has held senior design engineering positions with industrial automation and scientific instrumentation companies since 1989. In 2004 he moved to writing and was a Sr. Editor with Hearst Business Media, Electronics Products Magazine for 14 years. He is now a consultant with Lincoln Technology Communications.

Paul Golata joined Mouser Electronics in 2011. As a Senior Technology Specialist, Paul contributes to Mouser’s success through driving strategic leadership, tactical execution, and the overall product-line and marketing directions for advanced technology-related products. He provides design engineers with the latest information and trends in electrical engineering by delivering unique and valuable technical content that facilitates and enhances Mouser Electronics as the preferred distributor of choice.

Before joining Mouser Electronics, Paul served in various manufacturing, marketing, and sales-related roles for Hughes Aircraft Company, Melles Griot, Piper Jaffray, Balzers Optics, JDSU, and Arrow Electronics. He holds a BSEET from the DeVry Institute of Technology (Chicago, IL); an MBA from Pepperdine University (Malibu, CA); an MDiv w/BL from Southwestern Baptist Theological Seminary (Fort Worth, TX); and a PhD from Southwestern Baptist Theological Seminary (Fort Worth, TX).