Notebook DC-DC Converter Design

Johan Strydom, PhD Vice President, Applications, Efficient Power Conversion Corporation


Using eGaN® FETs to increase efficiency and reduce size

In the competitive world of notebook computers, the key performance attributes are battery life, weight, and size (particularly height) for a given level of performance. Recent advances in power transistor technology enable significant improvements of the POL converters used to convert the battery and charger voltage (Approximately 19 V in a four-cell system) down to the 1.2 V needed for the microprocessor and graphics processors. Enhancement mode gallium nitride (eGaN®) FETs have been in the commercial market for over two years and have begun to make significant inroads into the territory dominated by the aging power MOSFET. One of the primary benefits of this new technology is that these FETs are capable of much higher switching speed with lower power losses. Pushing frequency higher to reduce the size and cost of energy storage and transmission elements such as capacitors, inductors and transformers has been discussed for many years but, with traditional silicon power devices, the tradeoff between frequency and efficiency has been too costly to implement. eGaN FETs now make it possible for designers to reduce the space occupied by their DC-DC converters in 4 cell and industrial systems by increasing frequency while still exceeding the efficiency of converters based on conventional power MOSFETs. eGaN® FET Performance To drive frequency higher in a Buck Converter, especially at a high input voltage, power devices must have very low dynamic losses. The dominant component to the dynamic losses is the classic hard switching "event" where current commutates to the device turning on before the voltage across that device collapses. The energy of each switching period is approximated by ESW = VIN x IOUT x t where t is determined by the various components of the device gate charge (QG), device series gate resistance (RG), driver impedance, drive voltage, and device transfer characteristics. Due to eGaN FETs requiring much less die area and having a lateral structure, they have ultra-low gate and Miller' (QGD) charges. This, combined with a gate electrode designed to have low RG, switching times for these devices are very short, and energy dissipated due to classical hard switching is very low. There are four additional components that contribute to dynamic losses. These include, (1) diode recovery charge (QRR) where the energy loss (ERR) is equal to the recovery charge times the input voltage, (2) output charge (QOSS) which has an energy loss (EOSS) determined by multiplying one-half of the output charge times the input voltage, (3) the energy loss (EG) associated with the gate charge (QG) is calculated as the gate charge times VGS, and (4) the reverse conduction voltage (VSD) which is determined by the following equation: ESD = VSD x IOUT x tR (where tR is the total reverse conduction time). To determine the total power loss, the sum of these four components is multiplied by the frequency. PDYN = f x (ESW + ERR + EOSS +EG + ESD). eGaN FETs , unlike standard power MOSFETs, have no minority carriers to be stored in a junction, and therefore no QRR. QOSS , VGS , and QG are low compared with a MOSFET, so EOSS and EG are much lower. Finally, due to the reverse current conduction mechanism, eGaN FETs have a high VSD when compared with the body diode forward voltage of a MOSFET. This condition has the potential to increase the energy loss ESD and is influenced by the total reverse conduction time; a condition that can be controlled by the time the rectifier switch is acting like a diode.

eGaN FET - power MOSFET Comparison in a Buck Converter The devices compared were EPC1014 (40 V, 16 mW) on the high side and EPC1015 (40 V, 4 mW) on the low side, and Infineon MOSFETs BSZ130M03MS (30 V, 15 mW) on the high side and BSZ035N03M (30 V, 4.3 mW) on the low side of a Buck Converter. In all cases, a single device was used for each socket. The MOSFETs were chosen as they are state of the art and similar RDS(ON). 40 V eGaN FETs were compared with 30 V MOSFETs because higher overshoot can be expected with the much higher switching speeds of the eGaN FETs. Table 1 shows important characteristics of the switching devices.

The converters were run open-loop with the duty cycle adjusted for the appropriate output voltage. The output filter was kept small to take advantage of the space savings enabled by high frequency conversion (See Table 2). For 800 kHz testing, only one output filter capacitor was used, and for 300 kHz testing, a 470 mF PosCap was added.

Experimental Results The tests show that the circuit with eGaN FETs running at 500 kHz was comparable to the MOSFETs at 300 kHz, while the MOSFET circuit saw an efficiency decrease of roughly 1.5% through most of the current range at 500 kHz.

At low current, the efficiency decreases for the eGaN FET solution because the fixed dead-time was minimized for optimal high current operation. This resulted in the loss of the zero-voltage switching advantage at light loads. However, when the dead-time was increased, as an adaptive type driver would do, the 500 kHz efficiency of the eGaN FET system became comparable to that of the 300 kHz MOSFET system under all load conditions. At these frequencies the eGaN FET system saves 36 mm2 of board space (about 20%) with no efficiency penalty. The frequency was then increased to 800 kHz on the eGaN system, and output filter reduced to maximize board space savings. Even with the higher switching speeds, overshoot was limited to 33 V, and ringing was mostly damped in only a few cycles. The result was impressive. Efficiency over most of the current range stayed within 1% of the 500 kHz MOSFET system. The peak efficiency was over 86%, and the board space saved was 30 mm2 (an additional 20%). Board space requirements are compared in Figure 2. This 33% space savings can be translated into a reduced size and price for the multi-layer printed circuit board, or into increased system performance by using that space for processing power and memory. Conclusions We have demonstrated significant space savings while maintaining high efficiency by driving frequency from the traditional 300 kHz all the way up to 800 kHz. By using eGaN FETs, power conversion system designers now have a new opportunity to reduce system size and enhance efficiency while reducing overall system cost.