AN2066 APPLICATION NOTE New packaging concepts for low voltage power MOSFETs lead to performance improvement in advanced DC-DC converters

Transcription

1 AN2066 APPLICATION NOTE New packaging concepts for low voltage power MOSFETs lead to performance improvement in advanced DC-DC converters 1. INTRODUCTION The new faster CPU processor demands a reduced power supply voltage at higher current level compared to the previous designs. In order to supply this large amount of current, some new circuit solutions are currently used by the designers but this requires a new MOSFET generation with improved performances in both electrical fields and thermal management. In order to satisfy the fast current slope required by the new CPU, the converter frequency operation is increased and/or a multiphase topology is chosen. The newest STripFET MOSFET has been specially developed for this kind of applications and in order to satisfy the new requirements. The non-isolated buck converter is the most used topology for the new VRM. The buck topology combines high performance, low cost and design simplicity. The efficiency and the working temperature of the active components are the most important key to measure the converter excellence and to keep the reliability under a safe margin. 2. IDEAL MOSFETs FOR VRMs Even if an ideal MOSFET should have at same time, low on-resistance and low gate-charge values to minimize both static and dynamic losses, due to the physical limit of the silicon, a trade-off between these parameters has to be performed. In a buck converter the two MOSFETs often work asymmetrically or in other words, due to the Vin/Vout ratio, the duty cycle of each MOSFET is different. For this reason and to achieve a higher efficiency, many designers prefer using personalized devices for both high-side and low-side. By using our patented STripFET technology we are able to modulate the single parameter in order to optimize the performance of either high or low side switch. In particular, the on-resistance is the main parameter that is worth considering in a synchronous low-side MOSFET because it stays on longer than the high-side one. On the contrary, the control high-side MOSFET is used to regulate the output voltage by adjusting its duty cycle. It must be capable of very fast switching, therefore the main parameter that will be considered for the high side optimization is the gate-charge. 3. IMPORTANCE OF A NEW PACKAGE The pie chart below shows detailed ON-RESISTANCE contributions in a generic 30V N-channel STripFET MOSFET. As you can see, the higher contribution to the total on-resistance of the devices is due to the silicon, about 66%. But at the same time, a big slice is due to the package contribution, about 34%. So in order to decrease the total on-resistance and improve both current and thermal capability, we have developed a new package family in conjunction to the new silicon technology. With the new package concept we obtain better thermal performances, lower parasitic component and higher current capability. November /9

2 Figure 1: "On-resistance" Contribution in a Generic 30V N-channel MOSFET 4. PACKAGE IMPROVEMENTS Figure 2 shows the SO-8 family evolution, the same concept has been adopted even in DPAK packages. At the top the cross section of the standard SO-8 is reported where standard wires are used to bond both source and gate pads. Figure 2: SO-8 Family Evolution 2/9

3 The first step was the introduction of a metal clip instead of standard wires. We called this new package "Bondless". The advantage of the metal clip is the larger cross section used to carry the current, so the wide surface connected to the source allows the reduction of the package parasitic resistance and gives us higher current capability. We have experienced that by replacing the standard wires with a metal clip about 30% of on-resistance is saved in a DPAK package. Figure 3: Standard Wires Bonding Used in SO-8 Packages Figure 4: Metal Clip Used in "Bondless SO-8" Packages Another step forward has been done with the introduction of the "Bottomless" package. A metal frame has been added on the bottom in order to increase the thermal capability. Thanks to this metal frame the RThj-case has been drastically reduced, and the copper plate of the board can be used as a heatsink. Figure 5 shows a detailed thermal comparison between "SO-8" and "Power SO-8" packages using an FR-4 Demo Board with 2 oz copper thickness. Note that "SO-8" and "Bondless SO-8" have about the same thermal performances. 3/9

4 Figure 5: Thermal Performance Comparison Between SO-8 and Power SO-8 in a Generic Demo Board It is worth underlining that Standard "SO-8", "Bondless SO-8" and "Bottomless SO-8" are pin-to-pin compatible, so, in the same design, the designer can use the device he needs depending on the demanded current. In addition, thanks to the industry standard pin out, a large amount of devices can be replaced on the design without any change in the board. All the same, in order to take advantage of the higher thermal capability of the new Power SO-8 package, a suitable copper island has to be drawn under the metal slug. In any case the pin out compatibility with all SO-8 family is ensured. The last improvement in the package development is what we call "Reverser SO-8". In this package, thanks to the reversed pin out, a heat-sink can be assembled directly on the top of the package, so even a fan can be used in order to further improve the thermal performance and keep the board cold. This package is still under development. 4/9

5 5. EXPERIMENTAL TESTS In order to compare the performance of the new "Bottomless SO-8" package with the "Bondless SO-8", efficiency and thermal measurements were taken in a generic single-phase synchronous buck converter. Figure 6: Schematic of a generic single-phase non-isolated Buck converter The figure shows the main electrical specification comparison of the MOSFETs used in the evaluation. The only difference between the two MOSFETs is the higher thermal capability of the new Power SO-8 package. This difference is due to the metal slug on the bottom of the "Bottomless SO-8". In fact, the onresistance and the gate charge is the same because the same silicon die and the same metal clip are used. We cannot consider an Rthj-case for "Bondless SO-8" because it has no frame, so we can consider only Rthj-lead. Table 1: Specifications Comparison of the MOSFETs Used MOSFET Type A STS25NH3LL Type B STSJ100NH3LL package type SO-8 PowerSO-8 on-resistance V GS =10V, I D =12.5A Rthj-amb [ C/W] Rthj [ C/W] Rthj-leads= 16 Rthj-case=1.8 rated current [A] breakdown voltage [V] threshold voltage I D =250µA >1V >1V gate charge V DS =15V, V GS =4.5V, I D =25A /9

6 The converter works at 300 khz of operative frequency, and 20A of maximum output current. The output voltage is 5V of input voltage. In this example due to the duty cycle value which is very close to 50%, the same part types for both high and low side have been used. The efficiency curve at 25 centigrade degree of working temperature is very close in the whole current range from 1 to 18A, because the same silicon die with the same electrical performances is used. Figure 7: Efficiency Comparison But if we perform a thermal map of the board after 30 minutes of continuous work, we can see that in the worst case the "Bondless SO-8" package works at a temperature that is about 15 degrees higher than the new "Power SO-8" package. Bigger differences in terms of working temperatures can be found between the two SMD packages if a bigger copper island is used to cool down the "Power SO-8". Figure 8: Thermal Map and Temperature Measurements 6/9

7 6.CONCLUSION Thanks to a new powerful package and optimized silicon we can design cooler converters with improved efficiency, reliability and higher power density. The new silicon technology improvements together with the new package solutions allow the design of a high current converter in a compact footprint. 7/9

8 7. REVISION HISTORY Table 2: Revision History Date Revision Description of Changes 5-Nov First Release 8/9

9 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 2004 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 9/9