Yungtaek Jang

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Yungtaek Jang is an electrical engineer at Delta Products Corporation in Fremont, California. He was named a Fellow of the Institute of Electrical and Electronics Engineers (IEEE) in 2016 [1] for his contributions to efficiency optimization of AC-DC power supplies. He obtained BE from Yonsei University in 1988 in South Korea and then got his MS from the University of Colorado in 1991. In 1995, after four years of studying under mentorship from Robert W. Erickson and Dragan Maksimovic, Jang got his Ph.D. following successful defending of his thesis on "Application of Resonant Technique for Three-Phase High Power Factor Rectification and Integrated Magnetic Converters". [2]

Dr. Jang is known worldwide as an expert in performance optimization of switch-mode power supplies for data processing and telecommunications equipment. He is the inventor of a number of power conversion circuits and techniques that have enabled significant efficiency and power-density improvements of ac-dc power supplies in a cost effective manner. Since 1998, power supplies that incorporate Dr. Jang's inventions have been massively deployed in server, networking, and telecom equipment made by IBM, HP, Dell, and Cisco, as well as in datacenters of major internet companies such as Google and Facebook. In the late-nineties, a typical efficiency and power density of ac-dc computer power supplies was below 80% and 5 W/in3, respectively. Prompted by the explosive growth of the Internet, large computer manufacturers started challenging power supply makers to improve performance of their power supplies.

Dr. Jang's research was instrumental in enabling his company to implement power supplies with across-the-load efficiencies in the 90-95% range and power densities in the 20-35-W/in3 range. In addition, his pioneering effort has inspired many industry and university researchers to start working on performance optimization of ac-dc power supplies. Dr. Jang's research focus has been on the performance optimization of single-phase ac-dc boost power-factor-correction (PFC) front-end converters that are an integral part of every ac-dc computer power supply. Specifically, in 1999 he proposed a novel soft-switching technique for the boost PFC front end that has enabled 40-50% loss reduction compared to its conventional counterpart. The invention employs an active-snubber approach to eliminate reverse-recovery-related losses of the boost Si fast-recovery rectifiers. In addition, due to zero-current-switching of the boost switch, this technique made possible cost-minimization by using IGBT devices instead of more expensive MOSFETs. This approach has been employed in all high-performance server power supplies built by Dr. Jang's company until the introduction of virtually reverse-recovery-charge-free SiC rectifiers in 2008. It is still used today in cost-sensitive applications and those requiring low EMI designs. With millions of power supplies deployed, this invention has contributed to significant cumulative energy savings.

Dr. Jang has also made a key contribution to improving power density of high-frequency ac-dc computer power supplies by proposing a unique approach for minimizing the size of the energy-storage (bulk) capacitor of the front-end boost PFC converter. Generally, ac-dc computer power supplies are required to maintain the regulated output(s) for 12-20 ms after a line-voltage drop out. In conventionally optimized power supplies, approximately 1-2 uF/W of storage capacitance is required to meet the hold-up-time requirement since only approximately 40% of stored energy is utilized.

In 2002, Dr. Jang introduced the hold-up time extension concept that reduces the required capacitance and, therefore, the bulk-capacitor size by approximately 50% without adversely affecting the conversion efficiency. In the proposed concept, an auxiliary boost converter, which is only active during line voltage drop outs, is employed to discharge approximately 80% of the store energy. This invention that has been extensively implemented in power supplies manufactured by Dr. Jang's company not only has enabled significant increases in power density, but also has brought about significant material savings. The hold-up time extension technique will gain even more importance in the next generation of computer power supplies implemented with GaN switches that will enable efficient switching at much higher frequencies. With the further reduction of power-stage components enabled by significantly increased switching frequencies, the size of the energy-storage capacitors that are required to handle hold-up time will become the major obstacle in increasing the power density to 100 W/in3 and beyond.

Recently, Dr. Jang has developed a low-cost three-phase low-harmonic rectifier that uses only two switches and yet matches the efficiency and input-current total harmonic distortion (THD) of the rectifiers that require at least six switches. This converter does not employ active input-current shaping which further reduces its cost and, due to zero-voltage-switching of the switches, it exhibits improved EMI performance. Dr. Jang also introduced a light-load efficiency optimization technique that significantly improves the efficiency of power converters at loads below 20% of full load. In addition, he is one of the pioneers in wireless charging of portable electronic devices by having demonstrated 4.5-W and 30-W battery chargers in 2000. Moreover, in 2014, the control concept of the wireless charger was extended to a series resonant converter and makes it possible to operate as a bi-directional battery charger for electric vehicles.

Dr. Jang is a prolific inventor who has been awarded 29 U.S. patents. In addition, he has published 90 papers, 33 of them in various IEEE Transactions. [3] He received the IEEE Transactions on Power Electronics Prize paper awards for the best paper published in 1996, 2009, and 2013, respectively. The best evidence of the relevance and impact of Dr. Jang's work is that his work is highly cited by other researchers.

Related Research Articles

In electrical engineering, the power factor of an AC power system is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit. Real power is the average of the instantaneous product of voltage and current and represents the capacity of the electricity for performing work. Apparent power is the product of root mean square (RMS) current and voltage. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power may be greater than the real power, so more current flows in the circuit than would be required to transfer real power alone. A power factor magnitude of less than one indicates the voltage and current are not in phase, reducing the average product of the two. A negative power factor occurs when the device generates real power, which then flows back towards the source.

<span class="mw-page-title-main">Rectifier</span> Electrical device that converts AC to DC

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation is performed by an inverter.

<span class="mw-page-title-main">Power supply</span> Electronic device that converts or regulates electric energy and supplies it to a load

A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a result, power supplies are sometimes referred to as electric power converters. Some power supplies are separate standalone pieces of equipment, while others are built into the load appliances that they power. Examples of the latter include power supplies found in desktop computers and consumer electronics devices. Other functions that power supplies may perform include limiting the current drawn by the load to safe levels, shutting off the current in the event of an electrical fault, power conditioning to prevent electronic noise or voltage surges on the input from reaching the load, power-factor correction, and storing energy so it can continue to power the load in the event of a temporary interruption in the source power.

<span class="mw-page-title-main">Power inverter</span> Device that changes direct current (DC) to alternating current (AC)

A power inverter, inverter, or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of rectifiers which were originally large electromechanical devices converting AC to DC.

<span class="mw-page-title-main">Switched-mode power supply</span> Power supply with switching regulator

A switched-mode power supply (SMPS), also called switching-mode power supply, switch-mode power supply, switched power supply, or simply switcher, is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently.

<span class="mw-page-title-main">Ćuk converter</span> Type of electrical current converter

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<span class="mw-page-title-main">Power electronics</span> Technology of power electronics

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<span class="mw-page-title-main">Variable-frequency drive</span> Type of adjustable-speed drive

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This is an alphabetical list of articles pertaining specifically to electrical and electronics engineering. For a thematic list, please see List of electrical engineering topics. For a broad overview of engineering, see List of engineering topics. For biographies, see List of engineers.

<span class="mw-page-title-main">Flyback converter</span> Type of voltage converter circuit

The flyback converter is used in both AC/DC, and DC/DC conversion with galvanic isolation between the input and any outputs. The flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation.

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The single-ended primary-inductor converter (SEPIC) is a type of DC/DC converter that allows the electrical potential (voltage) at its output to be greater than, less than, or equal to that at its input. The output of the SEPIC is controlled by the duty cycle of the control switch (S1).

<span class="mw-page-title-main">Active rectification</span>

Active rectification, or synchronous rectification, is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches, usually power MOSFETs or power bipolar junction transistors (BJT). Whereas normal semiconductor diodes have a roughly fixed voltage drop of around 0.5 to 1 volts, active rectifiers behave as resistances, and can have arbitrarily low voltage drop.

A Royer oscillator is an electronic relaxation oscillator that employs a saturable-core transformer in the main power path. It was invented and patented in April 1954 by Richard L. Bright & George H. Royer, who are listed as co-inventors on the patent. It has the advantages of simplicity, low component count, rectangle waveforms, and transformer isolation. As well as being an inverter, it can be used as a galvanically-isolated DC-DC converter when the transformer output winding is connected to a suitable rectifying stage, in which case the resulting apparatus is usually called a "Royer Converter".

<span class="mw-page-title-main">Vienna rectifier</span> Pulse-width modulation rectifier

The Vienna Rectifier is a pulse-width modulation rectifier, invented in 1993 by Johann W. Kolar at TU Wien, a public research university in Vienna, Austria.

A solid-state AC-to-AC converter converts an AC waveform to another AC waveform, where the output voltage and frequency can be set arbitrarily.

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Slobodan Ćuk is a Serbian author, inventor, business owner, electrical engineer, and professor of electrical engineering at the California Institute of Technology (Caltech). The Ćuk switched-mode DC-to-DC voltage converter is named after him.

Switching Control Techniques address electromagnetic interference (EMI) mitigation on power electronics (PE). The design of power electronics involves overcoming three key challenges:

  1. power losses
  2. EMI
  3. harmonics

This glossary of power electronics is a list of definitions of terms and concepts related to power electronics in general and power electronic capacitors in particular. For more definitions in electric engineering, see Glossary of electrical and electronics engineering. For terms related to engineering in general, see Glossary of engineering.

References

  1. ↑ "PELS Members Elevated to Fellow in 2016". IEEE Fellows Directory. Retrieved 2019-12-30.
  2. ↑ Jang, Yungtaek (1995). Application of resonant technique (PDF) (PhD). University of Colorado.
  3. ↑ "Yungtaek Jang". scholar.google.com. Retrieved 2024-07-17.
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