Feature Power Making the most of MOSFETs


Won-suk Choi, Sung-mo Young and Dong-wook Kim, Application Engineering at Fairchild Semiconductor Korea, explain how a new solution in MOSFETs is responding to challenges for greater power density


Table 1: Critical


specification comparison of DUTs


age across the switch during switching transient, the ‘Miller effect’ does not happen here.


As shown in Table 1, QSYNC of the


3.6mohm MOSFET is reduced by 22% and 59% compared to 4.5mhom and 3.0mohm of the competitor’s part. The competitor’s driving loss of 3.0mohm is more than twice the conduction loss at 10% load condition.


Therefore, in practice, the gate charge value in synchronous switch, QSYNC QSYNC


is close to QG B


uilding a modern power system with higher efficiency and power density is an important focus for system designers, since making a small and high efficiency power system means saving space and energy bills. From a topology point of view, a syn- chronous rectifier is an essential build- ing block for switch-mode power supplies. From the device point of view, trench gate MOSFETs are the most preferred power devices for medium to low voltage power applica- tions. The specific on-resistance improved about 30% with the new technology developed.


Power losses in synchronous rectifi- cation can be lowered when the MOSFET’s on resistance and drain cur- rent is less than the diode forward volt- age drop. However, low on-resistance is not the only requirement for power switches in terms of synchronous recti- fication levels.


Critical parameters of MOSFETs, such as RDS(ON)


, QG , QOSS , Qrr and


reverse recovery characteristics directly affect the system efficiency of synchronous rectifications.


Fairchild Semiconductor has desi- gned new PowerTrench MOSFETs after deep analysis of power loss in synchro- nous rectification, which combines a smaller gate charge and soft reverse recovery body diode with fast switch- ing, aimed at achieving better effi- ciency in synchronous rectification. While MOSFET technologies and cell structure have innovated through the years, trench-gated MOSFETs have become the mainstream in medium voltage (BVDSS


< 200V).


With the compelling advantage of the trench structure in the ability to reduce RDS(ON)


, it is possible to increase


cell density without any JFET pinch- off effect.


The conventional trench gate struc- ture enables lower on-resistance by increasing the channel width to length


24


ratio. To improve switching perform- ance and increase the CGS


to CGD ratio,


development of a thick oxide in the trench bottom was added.


Another concept is the use of charge balance technology. The latest medium voltage MOSFETs employ shielded- gate structure. The shield electrode, along with the thicker oxide between electrode and drift region provides a charge balance for the drift region. This enables the use of higher doping in the drift region, resulting in reduced drift resistance. The specific resistance of the new MOSFET has been signifi- cantly improved compared to the previ- ous generation, while improving on the switching characteristic.


Major power loss causes Major power losses in power switches include conduction and switching losses. However, during light load con- ditions, conduction loss is minimal and driving loss from the gate driver is even more important to system efficiency. As new efficiency guidelines, such as; Climate Savers Computing Initiative are introduced, the driving loss becomes critical for light load efficiency. It is well known that the driving loss can be obtained through the following equation:


Pdrive = Qg ·Vgs fs Equation 1


In synchronous rectification, one dif- ference from the diode rectifier is that the MOSFET is a bidirectional device. Generally, current flows through the MOSFET channel from source to drain during conduction time and flows through the body diode during dead time. Because of the soft switching operation at turn-on and turn-off tran- sient, dVds


current from CGD


/dt is zero, the capacitive is also zero.


Because of the sequence, the gate charge value in Equation 1 should be selected carefully. As there is no volt-


-QGD . The exact


is slightly different from the simple estimation because there is a negative bias between drain and source in synchronous rectification. The output charge Qoss reverse recovery charge Qrr


and the also pro-


duce losses while turning off the switch. Since switching converters try to switch the power MOSFET as fast as possible, edge rates such as diF


/dt


synchronous rectification. These increases in Qrr


, can


be up to ten times faster than the datasheet conditions, increasing Qrr


for should be used


to estimate switching loss. Snubbers are widely used to manage the voltage spikes within the maximum drain- source voltage ratings, but it introduces additional power losses.


In synchronous rectification, a major device-related parameter affecting volt- age spikes is the softness of the body diode. When all situations are the same, snappy diodes always have higher volt- age spikes, which cause additional losses in the snubber circuits. The lower peak voltage of the soft device leads to smaller power loss in the snubber and 0.5% better system efficiency; 94.81% vs 94.29% at 20% load, although the soft device has 25% higher RDS(on)


. At full load, both devices have the same efficiency.


The total power loss with the 3.6- mohm MOSFET is 43% less than that of the 3.0mohm due to lower driving loss and output capacitive loss at 10% load condition. Also, compared to the 4.7mohm, the 3.6mohm MOSFET can reduce conduction losses at full load condition.


From the loss summary, it is clear how the 3.6mohm MOSFET can signif- icantly reduce power loss under both full load and light load conditions. The latest MOSFETs from Fairchild provide designers the opportunity to significantly increase system effi- ciency and power density. Fairchild Semiconductor www.fairchildsemi.com Enter 230


JULY 2011 Electronics


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