547 Pet 0809
2
56 have their limitations and cost burdens. And when tr ying to drive the high-side rectier in a orward converter where the duty cycle is low, timing becomes a major issue. Adding a special gate-drive winding to the power transormer can be an eective way to drive the gate, except when the bus volt- age or load varies signicantly, as the resulting gate-drive voltage will vary with both duty cycle and bus voltage. Most recently , circuits that reside on the secondary side, with no communication to the primary side, are be- coming popular. Tese syn- chronous rectiier circuits measure voltage across (and, indirectly, current through) the switch, and determine when to drive the gate on or o to behave like a diode. More importantly, they can work with a variety o con- trol methods and topologies, because there is no direct link to the primary side. Te major challenge or these secondar y-side circuits is how to measure ast low-level analog signals accurately in the presence o noisy high-current signals, all while sur- viving periodic high-voltage transients (while the diode is o). Moreov er, the gate-driver section has to be extremely ast (propagation delay) and able to quickly charge and discharge the FE gate capacitance. Finally, all o this perormance has to be implemented in a technology and package that meets the overall cost goals o the power-supply designer. While today’s synchro- nous rectier circuits are suitable or buck, resonant and critical-conduction-mode yback topologies, additional impr ovements in speed are necessary to address the needs o hard-switched low-duty-cycle applications. I am con- dent that our semiconductor industr y will apply innovative technologies to provide cost-eective synchronous rectier solutions or uture power-supply generations. Power Electronics Technolo gy September 2008 www.powe relectronics.com I n looking at any buck converter point-o-load regulat or designed today, chances are that it uses synchronous rectiication rather than a conventional diode or reewheeling. Synchronous rectication, using a switch (FE) along with a control circuit to emulate ideal di- ode behavior, completely dominates this application now. With the world’s ocus on energy efciency now more than ever, it is not surprising that replacing a simple diode with a rectication circuit makes economic sense in many cases. Te primary benet o synchronous rectica tion cir- cuits is to dramati cally reduce diode orward-vol tage drop, providing a corresponding reduction in conduction power loss. FEs with on-resistance in the milliohms are readily availab le and are becoming increasingly cost competitive. But Schottky and conventional diode sales have not allen o a cli just yet, so why aren’t synchronous recti- ers replacing diodes in every application? It turns out that designing a cost-e ective circuit to make a better dio de isn ’t always so simple and is highly applicatio n dependent. ake, or example, the synchronous buck converter. In years past, a Schottky rectier was commonly used as the reewheeling diode in a buck converter . As outpu t voltages dropped ro m 5 V to 3.3 V and n ow approach 1 V , the ha l- volt drop o a Schottky diode be comes a large portion o the total output. By replacing the Schottky with an n-channel FE (and switching it on and o at the correct time), the rectier orward voltage can be reduced by an order o magnitude or more. But timing the gate-drive signals to switch the FE on or o at the correct moment turns out to be a difcult design challenge in synchronous rectication. urning the synchronous FE on early results in a high-current shoot- through event in which two FE s are both on, shorting the input voltage and causing large power dissipation. On the other hand, turning it on late only incurs incre- mental loss — due to the body diode conducting with a higher voltage drop — until the FE is turned on. o avoid the shoot-through condition, synchronous buck converters insert a dead time between switch control signals, so as to guarantee no overlap o their on-time. Both FEs can be controlled by a single control IC. Consider an isolated version o this same circuit: the common orward converter. Te controller is on the pri- mary side, and the rectie rs are on the secondary side. It is a challenge to get accurate timing signals across the isolation boundary. Magnetic and optical isolation methods both Eric Persson has more than 20 years experience in power electronics design. He has presented more than 50 tutorials and papers at international conferences, as well as power electronics short courses and lectures at several U.S. univer- sities. He holds 12 U.S. and foreign patents, is a recipient of the IEEE Tird Millennium Medal and is a member of Power Electronic echnology’ s editorial advisory board. ExEcutIvE vIEWPOINt By Eric Persson, Executive Director o Global FAE, International Rectifer, El Segundo, Cali. Are Schottky and Conventi onal Diodes Obsolete ?
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Transcript of 547 Pet 0809
56
have their limitations and cost burdens. And when trying todrive the high-side rectier in a orward converter where theduty cycle is low, timing becomes a major issue. Adding aspecial gate-drive winding to the power transormer can bean eective way to drive the gate, except when the bus volt-age or load varies signicantly, as the resulting gate-drive voltage will vary with both duty cycle and bus voltage.
Most recently, circuits that reside on the secondary side,with no communication tothe primary side, are be-coming popular. Tese syn-chronous rectiier circuitsmeasure voltage across (and,indirectly, current through)the switch, and determinewhen to drive the gate onor o to behave like a diode.More importantly, they canwork with a variety o con-trol methods and topologies,because there is no direct link to the primary side.
Te major challenge or these secondary-side circuits ishow to measure ast low-level analog signals accurately inthe presence o noisy high-current signals, all while sur- viving periodic high-voltage transients (while the diode iso). Moreover, the gate-driver section has to be extremely ast (propagation delay) and able to quickly charge anddischarge the FE gate capacitance.
Finally, all o this perormance has to be implementedin a technology and package that meets the overall costgoals o the power-supply designer. While today’s synchro-nous rectier circuits are suitable or buck, resonant andcritical-conduction-mode yback topologies, additionalimprovements in speed are necessary to address the needso hard-switched low-duty-cycle applications. I am con-dent that our semiconductor industry will apply innovativetechnologies to provide cost-eective synchronous rectiersolutions or uture power-supply generations.
Power Electronics Technology September 2008 www.powerelectronics.com
In looking at any buck converter point-o-load regulatordesigned today, chances are that it uses synchronousrectiication rather than a conventional diode orreewheeling. Synchronous rectication, using a switch(FE) along with a control circuit to emulate ideal di-
ode behavior, completely dominates this application now.With the world’s ocus on energy efciency now more
than ever, it is not surprising that replacing a simple diodewith a rectication circuit makes economic sense in many cases. Te primary benet o synchronous rectication cir-cuits is to dramatically reduce diode orward-voltage drop,providing a corresponding reduction in conduction powerloss. FEs with on-resistance in the milliohms are readily available and are becoming increasingly cost competitive.
But Schottky and conventional diode sales have notallen o a cli just yet, so why aren’t synchronous recti-ers replacing diodes in every application? It turns out thatdesigning a cost-eective circuit to make a better diode isn’talways so simple and is highly application dependent.
ake, or example, the synchronous buck converter. Inyears past, a Schottky rectier was commonly used as thereewheeling diode in a buck converter. As output voltagesdropped rom 5 V to 3.3 V and now approach 1 V, the hal- volt drop o a Schottky diode becomes a large portion o thetotal output. By replacing the Schottky with an n-channelFE (and switching it on and o at the correct time), therectier orward voltage can be reduced by an order o magnitude or more.
But timing the gate-drive signals to switch the FEon or o at the correct moment turns out to be a difcultdesign challenge in synchronous rectication. urning thesynchronous FE on early results in a high-current shoot-through event in which two FEs are both on, shorting theinput voltage and causing large power dissipation.
On the other hand, turning it on late only incurs incre-mental loss — due to the body diode conducting with ahigher voltage drop — until the FE is turned on. o avoidthe shoot-through condition, synchronous buck convertersinsert a dead time between switch control signals, so as toguarantee no overlap o their on-time. Both FEs can becontrolled by a single control IC.
Consider an isolated version o this same circuit: thecommon orward converter. Te controller is on the pri-mary side, and the rectiers are on the secondary side. It is achallenge to get accurate timing signals across the isolationboundary. Magnetic and optical isolation methods both
Eric Persson has more than 20 years experience in power electronics design. He has presented more than 50 tutorialsand papers at international conferences, as well as power electronics short courses and lectures at several U.S. univer-sities. He holds 12 U.S. and foreign patents, is a recipient of the IEEE Tird Millennium Medal and is a member of PowerElectronic echnology’s editorial advisory board.
ExEcutIvE vIEWPOINt
By Eric Persson, Executive Director o Global FAE,International Rectifer, El Segundo, Cali.
Are Schottky and Conventional Diodes Obsolete?
