How to boost DC/DC converter efficiency at higher frequencies

A third method is to focus on reducing the recovery charge (Q_RR) and forward voltage drop (V_F) of the low-side MOSFET. Chips that integrate a Schottky diode with the MOSFET have several benefits at higher frequencies when compared to a standard trench MOSFET. The combination reduces parasitics, takes advantage of the Schottky's performance compared to the intrinsic body diode of the MOSFET, and cuts board real-estate requirements. In addition, very low RDS ratings are possible, even with Schottky integration, to reduce conduction losses at the heaviest loads. Above all, this combination boosts efficiency significantly at the higher switching frequencies. Here's what you can expect from such a design.

Market trends
Point-of-load (POL) DC/DC conversion and voltage regulator modules are driving the demand for higher-performance MOSFETs that can improve the overall efficiency of the power conversion at higher frequency and light loads. Indeed, the key is to optimize the MOSFET to improve DC/DC efficiency at 500 kHz and above for the physically smaller POL converters and ultra-mobile personal computers (UMPCs) now coming into wider use. A MOSFET that was designed to handle typical motherboard frequencies of 250 kHz isn't really suited for this type of POL application.

The second technology objective is to improve efficiency at light current loads. Most servers and notebook PCs are not at maximum load for most of their operating time, and CPU current is usually low. In server systems, the maximum current level can be greater than 120 amps, but when the CPU is not in use, current draw falls to the range of 30 to 40 amps. The impact on the user's electric bill is not too much of a concern for a system with, say, two servers, but it multiplies when all the servers in a large company or within a server barn are taken together.

The technology
MOSFETs with a Schottky diode integrate the diode at the wafer level. More specifically, today's generation of MOSFETs incorporate a junction trench MOS barrier Schottky (JTMBS). This Schottky technology is a combination of a trench MOS barrier Schottky (TMBS) and a junction barrier Schottky (JBS). The combination structures keep the forward voltage drop across the Schottky low and also reduce the leakage current. Current flows through the integrated Schottky junction instead of the body diode of the MOSFET because of its lower voltage drop.

The TMBS design allows for a lower Schottky barrier height, which in turn keeps the forward voltage drop low. Additionally, the JBS structure is designed such that the depletion layer of the MOSFET's p-n junction intersects under the Schottky metal barrier. During depletion-mode operation, a barrier is formed in the channel, which shields the Schottky from further applied voltage while suppressing the leakage current. This design helps distribute the current evenly across the silicon die for optimum performance.

Integrating the Schottky diode with the MOSFET improves the device performance in three major ways compared to using separate or co-packaged components. First, the V_F across the Schottky diode is much lower than the voltage drop across the intrinsic body diode of the MOSFET. The typical forward voltage for the device with the Schottky is 0.44 volt compared to 0.72 volt for the standard MOSFET. That's a reduction of 38 percent. As a result, there's substantially less power loss when the MOSFET is turned off during dead time (the interval when both MOSFETs are off and the main inductor current is conducting through the Schottky instead of the body diode of the MOSFET) in a buck converter application.

In addition, the Schottky diode's Q_RR is much less compared to that of the body diode of the MOSFET. During the time the high-side MOSFET turns on in a buck converter circuit, the body diode or integrated Schottky diode of the low-side MOSFET conducts the reverse recovery current. This current flows from the input source (V_in) through the high-side MOSFET and through both the low-side MOSFET's body diode and the integrated Schottky diode. The power loss from the reverse recovery current in the low-side MOSFET is V_IN × Q_RR × f_SW. Therefore, a reduction in Q_RR correlates to reduced power loss, and the improvement becomes more significant as the switching frequency increases.

The benefit of using a MOSFET with an integrated Schottky is particularly noticeable at light loads. During such operation, the inductor current in the system flows through the Schottky portion rather than the body diode of the MOSFET. Since the reverse recovery charge of the Schottky approaches zero, there's minimal power loss. When the loads are heavy and at maximum load conditions, the Schottky portion can't conduct the total inductor current. The amount it can't handle conducts through the body diode of the MOSFET, and losses increase somewhat.

Integrating the Schottky into the MOSFET also eliminates the parasitic inductances that would be present due to interconnects between individual components. The integration also helps the device achieve much lower RDS(on) levels compared to its co-packaged counterpart.

Enhanced performance
Let's look at some curves to illustrate the high-frequency advantages of using such integrated devices, in this case the SI4642DY, a 30-volt product, which is the low-side switch in a standard buck converter with a frequency range from 300 kHz to 1 MHz. The test set-up is based on a notebook PC's CPU core VR power topology with V_IN = 19, V_OUT = 1.3, and I_OUT = 3 to 21 amps on a single-phase evaluation board. We take a look at the performance at 300 kHz, 550 kHz, and 1 MHz. The passive components (inductor and capacitors) remain the same for all three frequencies, which accounts for some of drop in peak efficiency as frequency increases.

A standard trench MOSFET in an SO-8 package is used for the high-side control switch and two MOSFETs in an SO-8 package for the low-side switches. For the low-side switch, we compare the MOSFET with integrated Schottky diode to a standard trench MOSFET with similar RDS(on) specifications at a gate drive of 4.5 volts.

Note that the integrated device improves efficiency at light loads compared to the standard MOSFET at all three frequencies. Efficiency increases at light load conditions as the frequency increases. More specifically, results show a gain of roughly 2 percent at 300 kHz (Fig. 1), 4 percent at 550 kHz (Fig. 2), and up to 6 percent at 1 MHz (Fig. 3).

Second, the integrated device displays better efficiency than the standard MOSFET at peak current (21 amps) at all three frequencies. This result is especially notable since it is during heavy current loads that designs are most thermally challenged. Specifically, efficiency increases roughly by 1 percent at 300 kHz (Fig. 1), 2 percent at 550 kHz (Fig. 2), and up to 4 percent at 1 MHz (Fig 3).

Clearly in this application, the MOSFET-integrated Schottky approach delivers a significant advantage over the standard trench MOSFET approach.

About the authors
Spiro Zefferys is senior manager for market development. In that role, he is responsible for technology platform definition and product definition for Vishay's low-voltage MOSFETs in the computing and fixed telecom segments. Zefferys holds a bachelor's degree in electrical engineering from Johns Hopkins University and a master's in business administration from Thunderbird School of Global Management.

As director of market development, Michael-Hyung Mook Choi is responsible for technology platform definition, and product definition for the company's low-voltage MOSFETs. Choi holds patents in Taiwan and Korea, and has bachelor's and master's degrees in electrical engineering, with a major in power electronics, from Seoul National University.

Misha-Seung Lee is a staff device engineer, where she is involved in research, design, and development of the company's power MOSFETs. She holds a master's degree and a doctorate from the University of Southern California.