Increase DC-DC Efficiency Across Load and Frequency

Feb 1, 2008 12:00 PM
By Spiro Zefferys, Senior Manager Market Development, and Michael-Hyung Mook Choi, Director Market D

Integrating a Schottky diode onto a MOSFET chip reduces the power loss attributed to the low-side switch in a buck converter, improving efficiency at light loads and high frequencies.

To help improve dc-dc conversion efficiency at lighter loads and higher switching frequencies, a Schottky diode can be integrated onto a MOSFET chip. The MOSFET-Schottky combination reduces the power loss attributed to the MOSFET when it operates as the low-side switch in a buck converter.

Bringing the two components onto one monolithic chip, instead of copackaging the devices or designing them in discrete form on the board, enhances the effect of the Schottky diode. When compared with the discrete or copackaged approaches, the monolithic device reduces parasitics associated with package or board-level interconnects.

The improvement in buck converter efficiency can be demonstrated using a recently introduced 30-V MOSFET with integrated Schottky as the low-side switch. In this example, the performance of this device in a simple buck converter circuit is compared with that of a standard trench MOSFET.

Demand for Better MOSFETs

Point-of-load (POL) buck converters and voltage regulator modules used in computing and fixed telecom applications are driving the demand for higher-performance MOSFETs that can improve efficiency. One area where there's an opportunity to raise efficiency is at light current loads. Efficiency at light loads is important because most servers and notebook PCs are not at maximum load for most of the time they are turned on. So the dc-dc converter that powers the CPUs in these applications is normally operating well below its rated output.

In server systems, the maximum current level can be greater than 120 A. But during normal operation, current draw falls into the range of 30 A to 40 A. For one or two servers, this inefficient operation might not have much impact on the user's electric bill. However, the impact can be quite substantial when all the servers in a large company or within a server barn are taken together.

A second opportunity to improve efficiency is to target power losses at frequencies of 500 kHz and above. As form factors for POL converters are reduced and as ultramobile personal computers (UMPCs) become more widely used, switching frequencies will increase as a strategy for minimizing the size of power conversion circuits.

But efficiency levels can decrease drastically at higher frequencies if dc-dc components such as the MOSFETs aren't optimized accordingly. MOSFETs that were designed to handle typical motherboard frequencies of 250 kHz may not be ideally suited for higher-frequency POL applications. Therefore, reducing power losses throughout the load range with elevated frequencies is becoming much more important than in the past.

Better Device Performance

Integrating the Schottky diode with the MOSFET improves the device performance in two major ways. The first performance improvement results from the reduction in total reverse-recovery charge (Q_RR) compared to that of the MOSFET's body diode.

When the high-side MOSFET turns on in a buck converter circuit, a reverse-recovery current flows. 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 associated with this reverse-recovery current in the low-side MOSFET is defined as V_IN × Q_RR × f_SW. Therefore, a reduction in Q_RR correlates to a reduction in power loss that is proportional to the increase in switching frequency.

Second, the forward voltage drop (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 of the device with the integrated Schottky is 0.44 V, compared to 0.72 V for the standard MOSFET.

This 38% reduction in V_F results in substantially less power loss (P = V × I) when the MOSFET is turned off during dead time. This is 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.

Current Distribution

During low-current operation, the Schottky diode is able to handle the total inductor current in the system, preventing the body diode of the MOSFET from turning on. Therefore, there is no reverse-recovery loss due to the body diode. The Schottky's reverse-recovery charge, which is theoretically zero, results in minimal losses.

During high-current operation, the Schottky handles a major portion of the inductor current, but is unable to handle the total inductor current. The portion that it can't handle passes through the body diode of the MOSFET. This is why the Q_RR rating for the integrated device is not zero.

The two curves in Fig. 1 and Fig. 2 illustrate what is happening at T_J = 25°C. In Fig. 1, which is the V_F characteristic of the body diode of a standard trench MOSFET (SI4336DY), the V_F is 0.72 V at 3 A. In Fig. 2, which is the V_F characteristic of a MOSFET with the integrated Schottky (SI4642DY), the V_F is 0.49 V at 3 A, which is due to the V_F of the Schottky. This is why efficiency is improved at light loads.

However, once the current increases to 10 A, the V_F becomes 0.72 V, which is similar to the body diode of the MOSFET that is now conducting. At the 10-A condition, we can estimate that roughly 7 A conducts through the Schottky and 3 A through the body diode. So at heavy loads, as long as a major portion of the current conducts through the Schottky, efficiency levels are increased.

Application Performance

Consider the efficiency improvement in a simple buck converter application with a Vishay SI4642DY SkyFET, a 30-V MOSFET with integrated Schottky as the low-side switch. With a V_IN of 19 V and V_OUT of 1.3 V, which is similar to a standard notebook PC power topology, a standard high-side MOSFET is used to evaluate the performance of the low-side switches.

In this evaluation, one high-side MOSFET and two low-side MOSFETs are used. A gate-drive voltage of 4.5 V is applied to both devices. To gauge the performance of the MOSFET with the integrated Schottky, a similar buck converter circuit is tested using an industry-standard trench MOSFET with a similar R_DSON of 4.7 mΩ at 4.5 V. (Though it would be interesting to compare the SI4642DY to that of a copackaged, two-die solution, such a device is not available with comparable R_DSON.)

The integrated device outperforms the trench MOSFET at light loads. This improvement is magnified as the switching frequency increases from 300 kHz to 550 kHz. The light-load improvement in efficiency is roughly 2% at 300 kHz (Fig. 3) and 4% at 550 kHz (Fig. 4).

Therefore, the SI4642DY leverages the advantage of an integrated Schottky diode on the MOSFET chip to balance the limitations of the intrinsic body diode of the MOSFET. The end result is lower power losses in systems such as servers, notebooks and VRMs.