AN2239 APPLICATION NOTE

Transcription

1 AN2239 APPLICATION NOTE Maximizing Synchronous Buck Converter Efficiency with Standard STripFETs with Integrated Schottky Diodes Introduction This document explains the history, improvements, and performance evaluation of low voltage Power MOSFETs with advanced strip technology (STripFET ). The integration of a Schottky diode with the Power MOSFET die (MOSFET-Schottky diode structure) was designed to improve the efficiency and reliability of the Synchronized Buck Converter in mobile applications. The DC/DC converter example that is used is a Voltage Regulator Module (VRM) which will highlight the significant improvement obtained by using the MOSFET-Schottky structure. The DC/DC converter efficiency improvement is demonstrated by comparing the monolithic device performances of the MOSFET-Schottky structure with a standard MOSFET as they are used in a generic VRM application. A thermal comparison (to demonstrate heat reduction) is also performed. The continuously increasing power requirements of new processors have prompted MOSFET manufacturers to redesign new, low voltage Power switches in order to meet the new PC market demands. Several manufacturers have developed a new family of MOSFETs devoted to these types of applications. The first STripFET was introduced by STMicroelectronics in 1997, but significant changes have been introduced in the last four years. With the third generation of the STripFET, improvements include: very low ON resistance (r DS(on), achieved by increasing the Strip density), or very low gate charge value (achieved by using a new silicon layout. The STripFET technology facilitates the modulation of these parameters in order to optimize the performance of either high or low side switches for a buck converter. By optimizing the total process, a new, more efficient power package family was created. The StripFET can be assembled either on bondless or bottomless packages, minimizing the number of parasitic components required due to the standard bond wires, and, consequently, obtaining the best thermal performance. In any case the "pin-topin" compatibility is guaranteed, so a single PCB can be used in a wide range of converters by simply replacing the switches. In VRM applications, the non-isolated buck converter is the most commonly used topology. To reduce converter losses, the Schottky diode is assisted by a low r DS(on), synchronized MOSFET, or is even replaced by the MOSFET altogether. The use of an external or internal Schottky diode is suitable for getting the best application performance. The Schottky diode is generally connected in parallel to the MOSFET body diode in order to reduce losses. The reasons for this include: forward voltage drop (V f ), and body diode turn-on and reverse recovery losses. Schottky diodes are faster and have lower drop voltage with respect to the integrated body diode. Rev 1.0 October /

2 AN APPLICATION NOTE Table of Contents 1 VRM Converter Benefits Static Benefits Figure 1. In Synchronized Buck Converter Typical Waveform Figure 2. Synchronous Buck Converter Circuit with an External Schottky Diode Figure 3. Synchronous Buck Converter Circuit with an Integrated Schottky Diode Dynamic Benefits Why Use an Integrated Schottky Diode? Figure 4. Breakdown Voltage (BV DSS ) vs. Current Figure 5. Forward Voltage (V f ) vs. Current Technology Developments Figure 6. MPS + STripFET III Converter Electrical Schematic Table 1. STS25NH3LL vs. STS20NHS3LL Electrical Comparison Experimental Results Table 2. MOSFETs Used for Each Board Position Table 3. Boards A and B Temperature Comparison Figure 7. Normalized Efficiency vs. Load Current (f = 300KHz) Figure 8. Normalized Efficiency vs. Load Current (f = 500KHz) Conclusion Revision History /14

3 AN APPLICATION NOTE 1 VRM Converter Benefits 1 VRM Converter Benefits 1.1 Static Benefits These kinds of converters prevent the shoot-through phenomenon by putting dead time in between the MOSFETs turning ON and OFF. Even if the new gate drivers are able to reduce the dead time to the lowest possible, it still cannot be zero. During the dead time, the current previously stored on the main coil flows through the body diode of the low side MOSFET, or if an external Schottky diode is connected, the current will flow across it (see Figure 1, and Figure 2 and Figure 3 on page 4). Figure 1, shows that the lower V f provided by the Schottky diode leads to lower static loss. Diode Conduction loss: Where: V f = diode forward voltage; I L = Diode current; and f = frequency V f I L t deadtime f Figure 1. In Synchronized Buck Converter Typical Waveform 3/14

4 1 VRM Converter Benefits AN APPLICATION NOTE Figure 2 shows that Parasitic Stray Inductance (L p ) decrease the effectiveness of an external Schottky diode, where Figure 3 shows no L p with an integrated Schottky diode. Figure 2. Synchronous Buck Converter Circuit with an External Schottky Diode Figure 3. Synchronous Buck Converter Circuit with an Integrated Schottky Diode 4/14

5 AN APPLICATION NOTE 1 VRM Converter Benefits 1.2 Dynamic Benefits Every time the high-side MOSFET is turned ON, the low side diode (Schottky or the integrated MOSFET body diode) is forced to recover the stored charge. Using the diode recovery loss formula (below), it is apparent that the Schottky diode experiences lower loss because of its lower Reverse Recovery Charge (Q rr ) with respect to the standard diode or internal MOSFET body diode. Diode Recovery loss: Where: V IN = VRM input voltage; Q rr = Diode Reverse Recovery charge; and f = frequency V IN Q rr f 5/14

6 2 Why Use an Integrated Schottky Diode? AN APPLICATION NOTE 2 Why Use an Integrated Schottky Diode? Discrete Schottky diodes are currently used in synchronized buck converters, connected in parallel to the MOSFET body diode in order to reduce voltage loss. The benefits of having an external Schottky diode can be minimal (see Figure 2 on page 4) because of the L p of the PCB and package. Parasitic components (e.g., PCB traces or the package) can be minimized by placing a discrete Schottky diode into the same package as the MOSFET. However, a large amount of space is lost because the two chips have to be positioned on the same package. In this case, the overall MOSFET die size would be reduced by about 50% and the ON Resistance (R ON ) would be enhanced. ST now makes it possible to integrate a Schottky diode inside a STripFET and the silicon die can be assembled in a single package. This structure offers performance similar to a separate MOSFET and Schottky rectifier in one package, but, by using the ST solution, the parasitic inductance has been eliminated because the Schottky diode is interdigitated directly on the STripFET structure (without compromising overall MOSFET die size). The only consequence of using the integrated Schottky diode is that it causes higher leakage current, but with ST s solution it is possible to keep the leakage current under a safe design margin (see Figure 4 and Figure 5 on page 7). 6/14

7 AN APPLICATION NOTE 2 Why Use an Integrated Schottky Diode? Figure 4. Breakdown Voltage (BV DSS ) vs. Current Figure 5. Forward Voltage (V f ) vs. Current 7/14

8 3 Technology Developments AN APPLICATION NOTE 3 Technology Developments Actual power electronic applications demand high-voltage, fast-switching rectifiers that feature low forward-conducting voltage drop, low reverse leakage current, and excellent reverse recovery behavior. Although well-suited for high voltage power converters such as freewheeling diodes, PiN (P-N intrinsic diode, or body/drain diode) rectifiers exhibit very poor switching behavior due to the high reverse recovery time, and large peak reverse recovery current. In fact, to prevent high forward voltage drop values derived from the use of thick drift regions which deliver high voltage capability, PiN rectifiers with high carrier lifetimes have to be used. Schottky diodes, though considered minority carrier devices, are preferred because of excellent performance in terms of reverse recovery (derived from the majority carrier conduction mechanism). Lower V f values are typically achieved with Schottky diodes than with PiN diodes, so Schottky diodes are recommended for those applications which have strict requirements for reduced ON state and commutation loss. However, compared with PiN diodes, Schottky rectifiers exhibit very poor leakage behavior, which limits their use for low voltage applications. Breakdown voltages higher than 100V in silicon-based Schottky diodes are difficult to achieve. Schottky diodes with higher breakdown voltages involve higher surface leakage currents which are generated by defects at the metalsemiconductor interface (during the metal deposition process). The erratic recovery behavior typical of Schottky diodes is another factor that should be considered with reference to the Electro-Magnetic Interference (EMI). Device designers will need to take special measures to achieve soft reverse recovery characteristics in their design. The search for innovative devices with the combined desirable characteristics of the PiN diode and Schottky rectifier as led to the Merged Pin-Schottky (MPS) rectifier, which is suitable for high voltage, fast-switching power applications. By integrating an MPS rectifier with the STripFET III active area, the advantages include (see Figure 6 on page 9): greater trade-off optimization between the Schottky area and source area percentage, and the lowest V f with little R DS(on) is achieved as compared to a standard MOSFET. Table 1 on page 9 shows the electrical comparison between the standard STripFET versus the new Schottky-STripFET structure. No differences were observed in terms of MOSFET switching performances between the STS25NH3LL and STS20NHS3LL. However, the STS20NHS3LL produced about 20% less V f, and about 10% less Q rr. This means that lower loss occurred in both, the switching and static fields. Also worth noticing, is the higher R ON caused by the Schottky diode implementation inside the standard structure. The benefits of applying the Schottky diode are more evident with respect to loss reduction due to the high on resistance value (see Experimental Results on page 10). 8/14

9 AN APPLICATION NOTE 3 Technology Developments Figure 6. MPS + STripFET III Converter Electrical Schematic Table 1. STS25NH3LL vs. STS20NHS3LL Electrical Comparison STS20NHS3LL Sym Parameter Test Conditions STS25NH3LL (MOSFET + Schottky) Units t rr Reverse Recovery Time di/dt = 100A/µs; T J = 25 C; Id = 25A ns Q rr Reverse Recover Charge di/dt = 100A/µs; T J = 25 C; Id = 25A nc V fec R DS(on) Diode Forward Voltage Drop Static Drain-Source ON Resistance I f = 12.5A; T J = 25 C mv V gs = 4.5V; I d = 12.5A mω 9/14

10 4 Experimental Results AN APPLICATION NOTE 4 Experimental Results Using a high efficiency converter stage (see Figure 6 on page 9 for the schematic), the new STripFET plus Schottky STS20NHS3LL was compared to state-of-the art STripFET III devices (e.g., STS25NH3LL). The analysis was done by measuring the switches' temperatures based on different operating frequencies and current ranges. For the purposes of this experiment (based on Figure 6): no voltage feedback exists, regulation was done manually via a pulse generator, and one switch is used for both, the low and high sides. The mechanical details are: the power stage represents a typical single-phase stage of a multiphase application, the input voltage is 12V while the output is regulated at 1.2V, the driver's supply voltage V REG is 5V, which was derived from a V REG 12V with a series regulator, and the main electrical characteristics of the MOSFETs are in Table 1 on page 9. Table 2 shows the MOSFET combinations used for the comparison. The devices are packed in Bondless SO8 cases. The faster STS12NH3LL MOSFET was used as the high side for both of the boards. Note: On the Board A (with the Schottky MOSFET), the high side device works at a lower temperature due to the smaller recovery pulse current of the Schottky diode. The working temperatures for the low side MOSFETs on both boards is basically the same at 300KHz, but at 500KHz, the Board A runs cooler (see Table 3). Table 2. MOSFETs Used for Each Board Position Device Side Board A Board B High STS12NH3LL STS12NH3LL Low STS20NHS3LL MOSFET + Schottky STS25NH3LL Table 3. Boards A and B Temperature Comparison Board A Board B Frequency STS20NHS3LL STS25NH3LL (KHz) Hs Temp ( C) Ls Temp ( C) Hs Temp ( C) Ls Temp ( C) Note: I LOAD = 16A. 10/14

11 AN APPLICATION NOTE 4 Experimental Results Figure 7 and Figure 8 show converter efficiency given at 300KHz and 700KHz, and the load current range is 2/20A. The results are normalized to the higher efficiency peak. The better performance achieved with the Schottky MOSFET occurs over the whole current and frequency range. Figure 7. Normalized Efficiency vs. Load Current (f = 300KHz) Figure 8. Normalized Efficiency vs. Load Current (f = 500KHz) 11/14

12 5 Conclusion AN APPLICATION NOTE 5 Conclusion Designers who replace a low side standard MOSFET with a STripFET plus a Schottky diode can get 1% to 2% higher efficiency from their applications, depending on load conditions and switching frequency, even if the Schottky produces a small increase in resistance because the diode is integrated in the structure. The advantages outweigh this item with performance enhancements, including greater area optimization, low V f, and cooler running temperatures at higher frequencies. This development is another step forward in achieving increased component efficiency and optimization. 12/14

13 AN APPLICATION NOTE 6 Revision History 6 Revision History Date Revision Changes 13-October First edition 13/14

14 6 Revision History AN APPLICATION NOTE Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners 2005 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America 14/14