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Boost DC/DC converter efficiency at higher frequencies

Posted: 02 Jun 2008 ?? ?Print Version ?Bookmark and Share

Keywords:MOSFET? Schottky integration?

By Spiro Zefferys, Michael-Hyung Mook Choi and Misha-Seung Lee
Vishay Siliconix

How can you increase the efficiency of your DC/DC converter design across its load range for higher switching frequencies?

Depending on the current level and the available space on the PCB, you can use faster high-side MOSFETs with ultra low gate charge (QG) ratings to reduce the switching losses. It'll work, but the downside is that this approach limits the ability to use a MOSFET with lower on-resistance (RDS) to handle large load currents. Another approach is to reduce all parasitics within the circuit by integrating the driver and MOSFETs into one package. This technique will conserve board area. On the other hand, it reduces flexibility since the power engineer is constrained to use the MOSFETs supplied with the driver, which may or may not be optimized for the particular application.

A third method is to focus on reducing the recovery charge (QRR) and forward voltage drop (VF) 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 500kHz and above for the physically smaller POL converters and ultramobile PCs (UMPCs) now coming into wider use. A MOSFET that was designed to handle typical motherboard frequencies of 250kHz 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 120A, but when the CPU is not in use, current draw falls to the range of 30-40A. 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 VF 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 0.44V compared to 0.72V 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 QRR 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 (VIN) 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 VIN ? QRR ? fSW. Therefore, a reduction in QRR 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.

Figure 1: Low-side switch performance, 300kHz.

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 30V product, which is the low-side switch in a standard buck converter with a frequency range from 300kHz to 1MHz. The test set-up is based on a notebook PC's CPU core VR power topology with VIN = 19, VOUT = 1.3, and IOUT = 3-21A on a single-phase evaluation board. We take a look at the performance at 300kHz, 550kHz and 1MHz. 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.

Figure 2: Low-side switch performance, 500kHz.

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 4.5V.

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 300kHz (Figure 1), 4 percent at 550kHz (Figure 2), and up to 6 percent at 1MHz (Figure 3).

Second, the integrated device displays better efficiency than the standard MOSFET at peak current (21A) 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 300kHz (Figure 1), 2 percent at 550kHz (Figure 2), and up to 4 percent at 1MHz (Figure 3).

Figure 3: Low-side switch performance, 1MHz.

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 and is responsible for technology platform definition and product definition for Vishay's low-voltage MOSFETs in the computing and fixed telecom segments.

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.

Misha-Seung Lee is a staff device engineer, where she is involved in research, design, and development of the company's power MOSFETs.

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