APPLICATION NOTE SiC Power Schottky Diodes in Power-Factor SiC Power Schottky Diodes in Power Factor Correction CircuitsCorrection Circuits by Ranbir Richmond BySingh Ranbir and SinghJames and James Richmond Introduction conditions; and complex EMI filtering Electronic systems operating in the systems. On the other hand, CCM circuits 600-1200 V range currently utilize silicon offersilicon low RMS are stable Electronic systems operating in the 600- to 1200-V range currently utilize (Si) PiNcurrents, diodes, which tend to during store PiN diodes, store large state. The operation underhas light load condition, and large amounts(Si) of minority carrier which charge tend in theto forward-biased stored charge to be removed by carrier minority the long storage recombinationamounts before theofdiode can becarrier turned charge off. This in causes and turn-off times. Power devices offer good synchronization with SMPSmade PWM forward-biased state.performance The stored chargeas compared with silicon carbide (SiC) show great advantages to those with other circuits, butmade require an semiconductors. ultrafast diode. The prime benefits theremoved SiC Schottky barrier recombination diode (SBD) lie in its ability to switch fast fast (<50recovery ns), with diodes almost zero has toofbe by carrier Silicon (Si) ultra have reverse-recovery charge, even at high junction temperature operation. The comparable silicon PiN diodes (Si SBDs before the diode can be turned off. This highdrops) Qrr (~ nC), which charge increases are not viable in the 600 V range because of their large on-state voltage have100 a reverse-recovery of causes long storage and turn-off times. significantly di/dt,switching forward elements current and 100-500 nC and take at least 100 ns to turn-off. This places a tremendous burdenwith on other in madeforward with Silicon Carbide temperature. On the contrary, the Qrr of SiC the system inPower terms ofdevices the required safe operating area and the switching losses incurred. (SiC) show great performance advantages SBDs is relatively independent of these compared to those made other and telecom In traditional as off-line AC-DC power supplies used with in computer applications, the ACbiggest input sees a large parameters. One of the applications inductive (transformer) load, which power of factor lower than 1. A PFC circuit allows semiconductors. Thecauses prime the benefits the to be substantially for SiC SBDs in the near future is in the the AC input line see near-unity power factor, as required requirements. The power-factor correction SiCtoSchottky Barrier Diode (SBD) lie in itsby new legal CCM power correction (PFC) circuit. (PFC) circuits can be divided in two broad categories: Boost-converter driven in (1)factor discontinuous-conduction mode ability to switch fast (<50 ns), with almost (DCM) and (2) continuous-conduction mode (CCM). DCM circuits do not require high-speed rectifiers but suffer zero recovery charge, even at high from: de-rating of reverse circuit components; instability under light load conditions; and complexDiodes EMI filtering systems. SiCoperation Schottky junction temperature operation. Thestable during On the other hand, CCM circuits offer low RMS currents, are under light load condition, and comparable Silicon PiN PWM diodes (Si SBDs offer good synchronization with SMPS circuits, but are require an ultrafast diode. Silicon (Si) ultrafast-recovery Characteristics of SiC SBDS diodes have high Q (~ 100 nC), which increases significantly not viable in the 600 V range because of with di/dt, forward current and temperature. On the rr 600 V One SiC of SBDs are presently available contrary, the Q of SiC SBDs is relatively independent of these the biggest applications for SiCin their large on-state voltage drops) have a parameters. rr SBDs in the near future is in the CCM power-factor correction (PFC) circuit. the 1 A, 4 A, 6 A, 10 A and 20 A ratings reverse recovery charge of 100-500 nC and from Cree (www.cree.com). Figure 1 shows take at least 100 ns to turn-off. This places a typical temperature dependent forward a tremendous burden on other switching SiC Schottky Diodes characteristic of a 4 A / 600 V SiC SBD elements in the system in terms of the (CSD04060). required forward safe operating area and Characteristics of SiC SBDS theare switching losses incurred. 600-V SiC SBDs presently available in the 1-A, 4-A, 6-A, 10-A Introduction applications, the AC input sees a large inductive (transformer) load which due causes The on-resistance increases with temperature to the reduction in the elevated temperatures. theelectron power mobility factor toat be substantially lower The diode carries than 4 A at1.a VA of 1.52 V at 25°C. The current reduces F PFC circuit allows the AC input to approximately at thenear-unity same VF atpower 200°C.factor, This negative line 2toA see as temperature coefficient of forward current allows us to parallel required by new legal requirements. The more than one die in a package, or many in a circuit, without Power Factor Correction circuits canhighany unequal current-sharing issues. This (PFC) behavior is unlike divided in two broad Boost voltage Si PiNbe diodes. Figure 2 shows the categories: reverse characteristics of the 4-A / 600-V SBD. The typical leakage is less than 20 converter driven in (1) current Discontinuous μA at 600 V atConduction 25°C which increases 50 μA at 200°C a very Mode to (DCM) and —(2) nominal increase for such a wide temperature range. Continuous Conduction Mode (CCM). DCM circuits do not require high-speed rectifiers, but suffer from: de-rating of circuit components; instability under light load CPWR-AN01 Rev - 8 Forward Current (Amperes) , Rev. A e: CPWR-AN01 Application Not and 20-A ratings from Cree (www.cree.com). Figure power 1 shows a In traditional off-line AC-DC typical temperature dependent forward characteristic of a 4-A / supplies used in computer and telecom 600-V SiC SBD (CSD04060). o 25 C o 50 C 6 o 100 C o 75 C o 125 C o 150 C 4 o 175 C o 200 C 2 0 0.0 SiC Schottky Diode 600 V, 4 Amp CSD04060 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Forward Voltage Drop (Volts) Figure 1: The forward characteristics of a 4 Figure 1: The forward characteristics of a A/600 V SiC SBD. 4-A/600-V SiC SBD. Subject to change without notice. www.cree.com/power Page 1/9 200 C o 100 C 40 20 o 25 C 0 0 100 200 300 400 500 600 700 Reverse Bias (Volts) Figure 2: The reverse characteristics of a 4 A/600 V Figure 2:SiC TheSBD. reverse characteristics of a 4-A / 600-V SiC SBD. The devices were packaged in plastic TO220 packages. These parts are rated for a The devices were packaged in plastic TO-220 packages. maximum junction temperature of temperature 175°C. These parts are rated for a maximum junction For aForcase temperature of up the the of 175°C. a case temperature of to up150°C, to 150°C, junction temperature remains below 175°C at full junction temperature remains below 175°Crated current. at full rated current. The turn-offThe characteristics of the 10-A / 600-V turn-off characteristics of theSiC 10SBD are compared with a Si FRED at different temperatures A/600 V SiC SBD are compared with a Si (Figure 3). The SiC diode, being a majority carrier device, APPLICATION NOTE FRED at different temperatures (Figure 3). does not have any stored minority carriers. The SiC diode, being a majority carrier device, does not have any stored minority 10 carriers. Therefore, there is no reverse 8 recovery current associated with the turn-off transient of the SBD. However, there is a 6 4 2 0 SiC 10 A/600 V SBD TJ = 25, 50, 100, 150C -2 600V, 10A Si FRED Rev CPWR-AN01 -4 TJ = 25C TJ = 50C TJ = 100C TJ = 150C Therefore, there is no reverse-recovery current associated with the amount turn-off transient of the SBD.current However, there small of displacement is a small amount of displacement current required to required to charge the Schottky junction charge the Schottky junction capacitance (< 2 A), which (< the 2 A), which is current independent iscapacitance independent of temperature, level and di/dt. of the temperature, current level and exhibits di/dt. a large In contrast to the SiC SBD, the Si FRED amount of the reverse-recovery charge, which In contrast to the SiC SBD, the Si FRED increases dramatically temperature, and reverse di/ exhibits a with large amount on of current the reverse dt. For example, the Qrr of the Si FRED is approximately recovery charge, which increases 160 nC at room temperature and increases to about 450 dramatically withexcessive temperature, current nC at 150°C. This amounton of Q increases the rr and reverse di/dt. For example, the rr of switching losses and places a tremendousQburden on the the Siand FRED 160 nC at switch diode is in approximately typical PFC applications. room temperature and increases to about 450 nC at 150°C. This excessive amount of PFC Qrr Circuits increases the switching losses and places a tremendous burden on the switch Aand simple CCM circuit is shown in Figure 4. This circuit diode inPFC typical PFC applications. achieves near-unity power factor by chopping the full wave-rectified input with a fast switch (MOSFET) and then PFC Circuits stabilizing the resulting DC waveform using a capacitor. When Athe MOSFET is ON, is necessary to prevent the simple CCM PFCit circuit is shown in current the achieves output capacitor or the load Figure to4. flow Thisfrom circuit near-unity through the MOSFET. Hence, when the Diode is ON, the power factor by chopping the full wave FET is OFF, and vice versa. rectified input with a fast switch (MOSFET), and then stabilizing the resulting DC waveform using a capacitor. When the L Diode MOSFET is ON, it is necessary to prevent the current to flow from the output capacitor or the load through the MOSFET. Hence, when the Diode is ON, the FET is OFF, and vice versa. FET Cout DC Output Leakage Current (µA) o 60 Time (s) Figure 3: Turn-off switching waveform of the 10 A / 600 V SiC SBD in comparison to Si FRED (IXYS DSEI 12-06A). Bridge Rectifier SiC Schottky Diode 600 V, 4 Amp CSD04060 80 -10 -1.0E-07 -5.0E-08 0.0E+00 5.0E-08 1.0E-07 1.5E-07 2.0E-07 AC Input 100 Current (A) with in the atures. 2 V at mately egative current ie in a ut any This i PiN everse D. The µA at µA at such a diodes. Figure 2 shows the reverse characteristics of the 4 A / 600 V SBD. The typical leakage current is less than 20 µA at 600 V at 25°C which increases to 50 µA at 200°C – a very nominal increase for such a wide temperature range. Figure 4: A simple CCM PFC boost circuit for Figure 4:applications. A simple CCM PFC boost circuit off-line Page 2/9 for off-line applications. During the switching transient when the Diode is turning OFF when and the is During the switching transient the MOSFET Diode is turning -8 OFF and the MOSFET is turning ON, the reverse-recovery turning ON, the reverse recovery current currentfrom from the the diode thethe MOSFET, in addition Diodeflows flowsinto into MOSFET, in -10 to the rectified input current. This results in a large inrush -1.0E-07 -5.0E-08 0.0E+00 5.0E-08 1.0E-07 1.5E-07 2.0E-07 addition to the rectified input current. This current into the MOSFET, necessitating its substantial deTime (s) results in a tolarge inrush current into had the no rating as compared the case where the diode Figure 3: Turn-off switching waveform of the MOSFET, necessitating its substantial dereverse-recovery current. This large MOSFET represents a 10 A /3:600 V SiC switching SBD in comparison Figure Turn-off waveformto of Si rating asincompared the switching case where thealso substantial cost this circuit.to These losses FRED (IXYS DSEI SiC 12-06A). the 10-A / 600-V SBD in comparison limit the frequency of reverse operation, and thecurrent. efficiency of the Diode had no recovery This to Si FRED (IXYS DSEI 12-06A). circuit, and hence its cost, size, weight and volume. A higher large MOSFET represents a substantial small amount of displacement current frequency would allow the size of the passive components cost in this circuit. These switching losses required to charge the Schottky junction to be correspondingly smaller. Many fast silicon rectifiers also limit the frequency of operation, and capacitance (< 2 A), which is independent the efficiency of the circuit, and hence its of the temperature, current level and di/dt. cost, size, weight and volume. A higher In contrast to the SiC SBD, the Si FRED frequency would allow the size of the exhibits a large amount of the reverse passive components to be correspondingly Cree, Inc. recovery charge, which increases 4600 Silicon Drive smaller. Many fast silicon rectifiers also Copyrightdramatically © 2002-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree Durham, NC 27703 with temperature, on current and the Cree logo are registered trademarks of Cree, Inc. USA Tel: +1.919.313.5300 show “snappy” reverse recovery, which and reverse di/dt. For example, the Qrr of Fax: +1.919.313.5778 results in a large EMI signature, which are www.cree.com/power CPWR-AN01, A the Si FRED Rev. is approximately 160 nC at also unacceptable to the new European room temperature and increases to about -6 req reve effic with nea rect sho of c ma rect curr flow Hig and (>2 req circ p-ireco elim circ dehigh snu 600 pow sec –2 Figure 4: A simple CCM PFC boost circuit for off-line applications. also show During “snappy” which results thereverse-recovery, switching transient when the in a large EMI signature, which is also unacceptable to the new Diode is turning OFF and the MOSFET is European requirements. A fast rectifier with near-zero turning ON, recovery PFC current reverse-recovery willthe allowreverse for high-efficiency circuits, the Diode flowslegal intorequirements. the MOSFET, in which from also comply with new addition to the rectified input current. This A SiC results diode isinsuch a rectifier. near-zero a large inrushThis current into reversethe recovery SiC Schottky rectifier offers low switching losses MOSFET, necessitating its substantial dewhile still showing comparable on-state performance of rating as compared to the case where the conventional silicon rectifiers. Due to the majority carrier Diode had no of reverse This transport properties these recovery rectifiers, current. they show only a largecurrent MOSFET a transient, substantialwhich capacitive duringrepresents their turn-off flows through MOSFET. cost in the thispower circuit. These switching losses also limit the frequency of operation, and the efficiency of the circuit, and hence its High-Power Circuits cost, size,PFC weight and volume. A higher frequency would allow the size of the SiC SBDs offer substantial cost, efficiency benefits passive components tosize be and correspondingly in higher power (>250 watts) PFC circuits. Suchalso circuits smaller. Many fast silicon rectifiers require the use of passive or active snubber circuits when show “snappy” reverse recovery, which operated even with ultrafast Si PiN rectifiers in order to in a large EMI signature, which are negateresults its reverse-recovery charge. SiC SBDs are expected also unacceptable to the new European to eliminate the requirement for these snubber-circuit showing comparable on-state performance of conventional silicon rectifiers. Due to the majority carrier transport properties of these rectifiers, they show only a capacitive current during their turn-off transient, which flows through the power MOSFET. The schematic diagram of the PFC stage of this power HighisPower supply shown in PFC Figure Circuits 5. This PFC stage uses two 500V / 14-A, MOSFETs (IRFP450) in parallel (not shown) and SiC SBDs offer substantial cost, size a dual 600-V / 4-A MURH860CT as PFC diode, and snubber and efficiency benefits in higher power diode. The snubber components re-direct the reverse(>250 Watts) PFC the circuits. Such circuits recovery charge from PFC diode to an alternative bias require the use of passive snubber network. A snubber inductoror (Lactive ) in series with the PFC S diode a snubber capacitor ) in series with circuitand when operated even(C with ultrafast Sia snubber S diode provide the necessary lag foritsthe re-direction of p-i-n rectifiers in order to negate reverse this reverse-recovery current away from the two power recovery charge. SiC SBDs are expected to MOSFETs during its turn-on transient. Switching waveforms eliminate requirement for these i.e. snubber were takenthe under full-load condition, a 2 Ω load for components, as a severely acircuit 28-V output voltage.as Thewell operating frequency was 95 de-rated whilewere stillconducted offering under a kHz, and allMOSFET, measurements room temperature ambient. higher efficiency. A 390 Watt power with passive Switching waveforms usingsupply Si diodes The measured currentand voltage-switching snubber circuits was used for evaluation ofwaveform on diodeSBDs are shown in Figure The 6 (a). These 600theV Si / 4PFC A SiC (CSD04060). measurements takencomposed under full-load and power supplywerewas of condition two an input voltage of 85 V AC. This condition represents the sections: a PFC stage, which takes in 85 V highest duty cycle for the diode, with the snubber. A peak – 265 V as AC input voltage it towhen this reverse-recovery current of 1.8and A isboosts observed CS Snubber Diode PFC Diode LS FET Cout Snubber Bias Network SMPS 390-28 V Bridge Rectifier AC Input 85 V – 265 V LBoost DC Output 390 V Brid FET Figure 5: High Power PFC circuit with snubber components. Figure 4: High Power PFC circuit with snubber components. CPWR-AN01 Rev components, as well as a severely de-rated MOSFET, while still offering a higher efficiency. A 390-watt power supply with passive snubber circuits was used for evaluation of 600-V / 4-A SiC SBDs (CSD04060). The power supply was composed of two sections: a PFC stage, which takes in 85 V – 265 V as AC input voltage and boosts it to 390 V DC output; a step-down SMPS, which steps down the DC voltage from 390-V to 28V output. Copyright © 2002-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree and the Cree logo are registered trademarks of Cree, Inc. CPWR-AN01, Rev. A Page 3/9 diode turned off at a reverse di/dt of 200 A/µsec. A turn-on dv/dt of 3.5 kV/µsec was used when the diode transitions from OFF-state to ON-state. The switching waveforms on the Si snubber diode used in conjunction with the main PFC diode are shown in Figure 6 (b). It can be seen that this snubber diode does not contribute any reverse-recovery current that needs to flow through the MOSFET because it is not carrying any forward current during the PFC diodeswitching transient. This diode provides a small voltage transient to the PFC diode, which prevents it from turning off too fast. Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 Fax: +1.919.313.5778 www.cree.com/power APPLICATION APPLICATION NOTE NOTE -300 -300 -9 -9 -400 -400 -12 -12 -500 -750n -500 -750n -700n -700n -650n -600n -650n -600n Time (s) -550n -550n 0 0 10 10 -100 -100 5 5 -200 -200 0 0 -300 -300 -5 -5 -400 -400 -10 -10 -500 -500-5µ -5µ Excess Reverse Recovery Current 0 0 0 0 -100 -100 -8µ -8µ Figure 7: MOSFET Current and Voltage turn- In order toInascertain the of the the snubber network order to effect ascertain effect of on snubber network on the MOSFET and PFC the MOSFET and PFC diode switching, the snubber network snubber network on thesnubber MOSFET and PFC diode from switching, network was was removed the PFCthe circuit. The measured MOSFET diode switching, the snubber network was from the PFC circuit. The 8. current removed without a snubber network is shown in Figure removed the reverse PFC circuit. The a The peak current from dueMOSFET to diode recovery increases measured current without measured MOSFET current without a the from 1.8 A with the snubber 6.5 A without snubber network is network shown into Figure 8. The snubber network. This is obviously stresses the MOSFET snubber network shown in Figure 8. The peak current due to diode reverse recovery excessively and maydue lead todiode an excessive thermal load to peak current reverse increases from to1.8 A with therecovery Snubber the entire circuit assembly. increases from 1.8 A with the Snubber network to 6.5 A without the network to 6.5 A without the -15 0 -15 0 400 400 condition at an 85 V AC input is shown in condition at an 85 V AC input is shown in Figure 7. This circuit uses a Si PFC diode in Figure This uses circuit. a Si PFC in addition7. to thecircuit snubber A diode turn-on addition to the snubber circuit. A turn-on di/dt of 81 A/ µsec was measured and the di/dt of 81 A/ µsec was measured and the MOSFET turns on with a total current of MOSFET turns on with a total current of 5.96 A, which decays to 3.44 A within a 5.96 A, which decays to 3.44 A within a microsecond. This represents an excess microsecond. This represents an excess current of 2.52 A during turn on of the current of 2.52 A during turn on of the MOSFET. MOSFET. Page 4/9 Page 4/9 MOSFET Voltage (V) 200 200 6 6 100 100 3 3 0 0 0 -100 10.2µ -100 10.2µ MOSFET Current (A) voltage transient to the PFC diode, which Measurements were also to performed on diode, both MOSFETs voltage transient the PFC which (in prevents it from turning off too fast. parallel) in the PFC circuit. It was found that the current prevents it from turning off too fast. sharing was fairly uniform between theperformed two MOSFETs. Measurements were also on The Measurements were also performed MOSFET voltageand current-switching waveformson under both MOSFETs (inV AC parallel) in theinPFC full-load condition at an 85(in input is shown Figure 7. both MOSFETs parallel) in the PFC circuit. It was thatinthe current This circuit uses a Si found PFC diode addition to sharing the snubber circuit. It was found that the current sharing fairly di/dt uniform between the two and circuit.was A turn-on of 81 A/µsec was measured was turns fairly uniform between of the two the MOSFET with a total current 5.96 and A, which MOSFETs. onThe MOSFET voltage Thea microsecond. MOSFET voltage and decaysMOSFETs. to 3.44switching A within This represents current waveforms under full load an waveforms under full load excesscurrent currentswitching of 2.52 A during turn-on of the MOSFET. -2 -3 -4 12 0 10.3µ 10.4µ 10.3µ 10.4µ Time (s) MOSFET Current (A) voltage switching waveforms. -1 -5 MOSFET Switching 12 Si Diode; No Snubber MOSFET Switching 9 Full Load; 85 V Input Si Diode; No Snubber 9 Full Load; 85 V Input 300 300 Figure 6(b): 6 (b) Snubber Snubber diode diodecurrentcurrent and Figure Figure 6 (b) Snubber diode current and and voltage switching waveforms. voltage-switching waveforms. -3 10.5µ -3 10.5µ Time (s) Figure 8: MOSFET Switching waveform Figure 8:Snubber MOSFET Switching waveform waveform without network. Figure 8: MOSFET switching without Snubber network. without snubber network. snubber network. This obviously stresses snubber network. This obviously stresses thewaveforms MOSFET excessively, and may lead to Switching using SiC Diodes the MOSFET excessively, and may leadentire to an excessive thermal load to the After these measurements were completed using Si diodes, an excessive thermal load to the entire the PFCcircuit diode assembly. was replaced with a SiC SBD (CSD04060), circuit assembly. and all components of the snubber network, including snubber inductor, capacitor and the bias network, were removed and measurements were repeated. The measured current- and voltage-switching waveforms on the Si PFC CPWR-AN01 Rev CPWR-AN01 Rev - Copyright © 2002-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree and the Cree logo are registered trademarks of Cree, Inc. CPWR-AN01, Rev. A -3 -6µ -3 -6µ -7µ Time-7µ (s) Figure 7: current and turnFigure 7: MOSFET MOSFET andvoltage Voltage on waveforms forCurrent the case with Si turndiodes on waveforms for the case with Si diodes onand waveforms for the case with Si diodes Snubber network. and network. and snubber Snubber network. Time (s) 3 In order to ascertain the effect of Snubber Snubber Diode Diode Current Current (A)(A) Snubber Snubber Diode Diode Voltage Voltage (V)(V) characteristics w/snubber. 15 15 3 Excess Reverse Recovery Current Time (s) Figure 6(a): 6: (a) SiSi PFC PFC Diode turn-off turn-off Figure Figure 6: (a)w/snubber. Si PFC diode Diode turn-off characteristics characteristics w/snubber. 100 100 6 6 100 100 -15 -500n -15 -500n Time (s) Snubber Diode Switching Snubber Full Load;Diode 85 V Switching Input Full Load; 85 V Input -4µ -3µ -2µ -1µ -4µ -3µTime (s)-2µ -1µ 200 200 com comp wa was and and inca includ the the me meas me meas wa wave Fig Figur rev rever of oftran 2. trans PFC Diode Voltage (V) PFC PFC Diode Diode Voltage Voltage (V)(V) -6 -6 9 9 MOSFET Current (A) PFC Diode Switching PFC Diode85 Switching Full Load; V Input Full Load; I PRR = 1.8 A85 V Input I PRR = 1.8 A -200 -200 -3 -3 PFC PFC Diode Diode Current Current (A)(A) -100 -100 12 MOSFET Switching Si Diode + Snubber MOSFET Switching Full Load; 85 V Input Si Diode + Snubber Full Load; 85 V Input 300 300 0 0 Sw Switc 12 MOSFET Current (A) 0 0 PFC PFC wn in wn 00 Vin/ 00 V / el (not el (not 4 A 4 A ubber ubber -direct -direct e PFC e PFC ork. A ork. A e PFC e PFC series series essary essary everse everse power power nsient. nsient. er full er full 28 V 28 V uency uency were were rature rature oltage oltage diode diode These These l load l load V AC. V AC. t duty t duty er. A er. A 8 A is 8 A is f at a f at a dv/dt dv/dt diode diode e. The e. The ubber ubber main main It can It can es not es not nt that nt that SFET SFET orward orward tching tching 3 3 MOSFET Voltage (V) MOSFET Voltage (V) 100 100 MOSFET Voltage (V) which which to 28 to 28 400 400 Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 Fax: +1.919.313.5778 www.cree.com/power Fig Figur wav wave inp input swi switc con cond Fig Figur with with A tu A and tur and cur curre Sw Switc SIN SING recS recov size size the there diodes to ON-state. 100 SiC PFC Diode Switching No Snubber -200 -3 -300 -6 -400 0 -100 Diode Voltage (V) 0 -9 -500 10.3µ 10.4µ 10.5µ -12 10.6µ Time (s) 400 MOSFET Voltage (V) -100 -100n SiC Diode Full Load; 85 V Input 0 50n 0 -300 -2 -400 -4 Time (s) MOSFET MOSFET removed removed from from previous previous circuit) circuit) under full load and input This diode off at a reverse of under full turned load condition condition and an andi/dt input voltage of85 85A AC. voltage of VVAC. 510 A/µsec. turn-on dv/dt of 1.9 kV/ µsec APPLICATION NOTE was usedcurrentwhen and the voltage-switching diode transitions waveforms from The measured OFF-state to ON-state. The MOSFET on the Si PFC diode are shown in Figure 10 (a). This diode turned off at a reverse di/dt ofswitching 510 A/µsec. A turn-on dv/dt and current waveforms The voltage measured current and voltage of 1.9 kV/µsec was used when the diode transitions underwaveform full load condition 85 Vdiode AC input from switching on -the at Sian PFC CPWR-AN01 Rev OFF-state to ON-state. The MOSFET voltage- and currentis shown in Figure 10 (b). are shown in Figure 10 (a). switching waveforms under full-load condition at an 85-V -3 9 -6 APPLICATION NOTE The measured current and voltage Figure 10(a): diode switching Figure 10: (a) SiC SiC PFC PFC diode switching switching waveform on the Si PFC diode waveforms with aa single single MOSFET (one waveforms with MOSFET (one are shown in Figure 10 (a). 12 A turn-on di/dt of 362 A/µsec was measured and the MOSFET turns on at near-nominal 0 0 400 12 current level. MOSFET Switching 2 -600 -8 10.20µ 10.25µ 10.30µ 10.35µ 10.40µ 10.45µ 10.50µ Figure 9(a): 9: (a)SiCSiCPFC PFCdiode diodeturn-off turn-off Figure waveforms under under full waveforms full load load condition conditionand andan input voltage of 85 V AC. an input voltage of 85 V AC. Switching waveforms using SiC Diodes100n with AC input are shown (b). This diode turned inoffFigure at a 10 reverse di/dt of Time (s) SINGLE FET 200 6 510 A/µsec. A turn-on dv/dt of 1.9 kV/ µsec Since 9SiC no and reverse Figure (b) diodes MOSFEToffer current voltage was used400when the diode transitions from 12 100 3 condition turn-on under full load recovery, it waveforms may be possible to reduce the OFF-state to ON-state. The MOSFET SiCMOSFETs PFC diode.used in the circuit, 300 9 sizeusing of the voltage and current switching waveforms 0 thereby saving cost, weight and size 0of the under full load MOSFET condition at an 85 V AC input Switching circuit even further. This is because 200 6 1 FET, 10 SiC Diode is shown in Figure (b). -100 -3 severely MOSFETs used in PFC circuits are Full Load; 85 V Input -100n -50n 0 50n 100n de-rated due to the additional switching 100 3 Time (s) losses created by the reverse recovery Figure 9 (b) that MOSFET current them and voltage voltage turn400 12 0 0 current flow through during Figure 9(b): MOSFET current and Page 5/9 turn-on waveforms under full load loadcondition condition on waveforms transients. under As mentioned earlier, in this turn-on full usingcircuit, SiC PFC PFC diode. 300 9 -100 -3 using SiC diode. two MOSFETs were used in parallel 10.1µ 10.2µ 10.3µ 10.4µ 10.5µ in order to achieve this de-rating. After the Time (s) MOSFET Switching circuit even using further. This with is above becauseFET Switching waveforms SiC Diodes SINGLE 200 6 1 FET, SiC Diode measurements presented were Since MOSFETs SiC diodes used offer in noPFC reverse recovery, it may be circuits are severely Full Load; 85 V Input a MOSFET was removed, Figure 10(b): Single MOSFET current- and possible tocompleted, reduce the MOSFETs used in and the (b) Single MOSFET current and voltage de-rated duethetosize theof operated additional switching 100 3 voltage-switching waveforms under full the circuit was with the SiC circuit, thereby saving cost, weight and size of the circuit switching waveforms under full load losses created by the reverse recovery load condition using SiC PFC diode. Schottky diode without anyused snubber even further. This is because MOSFETs in PFCcircuit. circuits condition using SiC PFC diode. 0 0 currentde-rated that flow them during turn-losses are severely duethrough to the additional switching on transients. As mentioned earlier, in this -100 -3 circuit, two MOSFETs were used in parallel 10.1µ 10.2µ 10.3µ 10.4µ 10.5µ 100 6 in order to achieve this de-rating. After the Time (s) Diode Switching were measurements presented above Efficiency and Temperature 0 4 SiC Diode; 1 FET Cree, Inc. completed, a MOSFET was Full removed, and Load; 85 V Input 4600 Silicon Drive Measurements (b) Single MOSFET current and voltage Copyright © 2002-2006-100 Inc. All rights reserved. The information in this document is subject to change without notice. Cree Durham, NC 27703 the circuitCree,was operated with the SiC2 and the Cree logo are registered trademarks of Cree, Inc. Tel: +1.919.313.5300 switchingIn waveforms under fullUSA load the test circuit, it was difficult to Fax: +1.919.313.5778 Schottky diode without any snubber circuit. 0 -200 condition using SiC PFC diode. www.cree.com/power CPWR-AN01, Rev. A separate the efficiency of the PFC circuit -300 -2 and the next stage, which was the voltage MOSFET Voltage (V) -50n MOSFET Voltage (V) MOSFET Voltage (V) MOSFET Current (A) MOSFET Current (A) Diode Current e Voltage (V) (b) S switc cond Effic Mea 4 -200 -500 MOSFET Switching The MOSFET voltageand current-switching waveforms The MOSFET voltage and current SiC Diode 300 waveforms 9 under full-load condition at an 85-V AC input is shown in Full Load; 85full V Input switching under load Figure condition 9 (b). Thisatmeasurement was taken with SiC PFC an 85 V AC input is shown in diode and no snubber circuit. A turn-on di/dt of 362 A/µsec 200 6 Figure 9 (b). This measurement taken was measured, and the MOSFET turns on atwas near-nominal SiC PFC diode, and no snubber circuit. currentwith level. 100 3 6 Diode Switching SiC Diode; 1 FET Full Load; 85 V Input Diode Current (A) PFC Diode Voltage (V) -100 300 - 3 MOSFET Current (A) resses ead to entire 0 MOSFET Current (A) veform circuit was operated with the SiC Schottky diode without any snubber circuit. PFC Diode Current (A) ect of d PFC rk was . The out a 8. The covery nubber de-rated due to the additional switching losses created by the reverse recovery current that flow through them during turnon transients. As mentioned earlier, in this circuit, two MOSFETs were used in parallel in by order achieve this de-rating. After created the to reverse-recovery current that flow the through measurements presented above them during turn-on transients. As mentioned were earlier, in completed, a MOSFET was inremoved, this circuit, two MOSFETs were used parallel inand order to achieve this de-rating. After the measurements the circuit was operated with the presented SiC aboveSchottky were completed, a MOSFET was removed, diode without any snubber circuit.and the MOSFET V T Current (A) including snubber inductor, capacitor and the bias network; were removed, and measurements were repeated. The 0 measured current and voltage switching waveform on the Si PFC diode are shown in -3 Figure 9 (a). This diode turned off at a µ reverse of 5679 A/µsec. turn-on dv/dt diode are showndi/dt in Figure (a). ThisAdiode turned off at a reverse di/dtkV/µsec of 567 A/µsec. A turn-on 2.7 kV/ of 2.7 was used whendv/dt the of diode used when theOFF-state diode transitions from OFF-state transitions from to ON-state. e turn- µsec was 3 I sepa and t stephere as w stage impa efficie these comp the e diode The l to 10 Si diode, the entire snubber network was included in the measurement, while in the case of SiC diode, it was removed. The case where only a single MOSFET was used is also plotted in this figure. As in most PFC circuits, the efficiency of the circuit increases as the load is Measurements increased from 10 Efficiency and Temperature % to 100 %. the case of single FET. At 100% load condition, the total efficiency improves from 78.9 % for the Si diode case to 81.77 % for the case of SiC diode with 2 FET; which decreases to 80.7 % for the case with a single FET. The slightly higher on-state losses in SiC Schottky diode results in the relatively smaller gain in the overall circuit efficiency under full load operating In the test circuit, itThe was measurements difficult to separate at 85theVefficiency input of the PFC circuit and the next stage, which was the voltage condition. of In the general, the efficiency of the step-down. Thevoltage efficiency numbers reported include the efficiency PFC stage as well as the SMPS voltage are presented, which here represents maintains a more uniform profileiswith step-down stage of the power supply. Hence, the impact of SiCcircuit on the PFC stage circuit efficiency somewhat underthe highest stress for the active components estimated in these measurements. Figure 11 shows the comparison of thediode measured efficiencytoof the the entire power a SiC PFC as compared At 10 % load, the circuit supply betweeninSithe andcircuit. SiC diode. traditional design using Si diodes. efficiency increases from 51.4 % for the Si diode between case to 57.5 % for SiC The load was varied 10 percent (20diode Ω) towith 100 2percent (2 Ω) for its 28-V output, in 10 percent increments. As mentioned earlier, in the case of the Si diode, the entire snubber network was included in the measurement, while in the case of the SiC diode, it was removed. The case where only a single MOSFET was used is also plotted in this figure. As in most PFC circuits, the efficiency of the circuit increases as the load is increased from 10 percent to 100 percent. 90 Power Efficiency (%) 80 SiC 70 60 Si 50 85 V Si Diode with Snubber 85 V SiC Diode with 2 FETs 85 V SiC Diode with 1 FET 40 30 0 20 40 60 80 100 Load (%) Figure 11: Efficiency comparison of PFC circuit with Si and SiC diodes. The efficiency of the circuit with single FET remains better than that with two FETs. Figure 11: Efficiency comparison of PFC circuit with Si and SiC diodes. The efficiency of the circuit with single FET remains better than that with two FETs. The measurements at 85-V input voltage are presented, which represents the highest stress for the active components in the circuit. At 10-percent load, the circuit efficiency increases from 51.4 percent for the Si diode case to 57.5 percent for SiC diode with two FETs and increases further to 58.5 percent for the case of a single FET. At 100-percent load condition, the total efficiency improves from 78.9 percent for the Si diode case to 81.77 percent for the case of SiC diode with two FETs, which decreases to 80.7 percent for the case with a single FET. The slightly higher on-state losses in a SiC Schottky diode results in the relatively smaller gain in the overall circuit efficiency under full-load operating condition. In general, the efficiency diode CPWR-AN01 Rev - of the circuit maintains a more uniform profile with a SiC PFC Page 7/9as compared to the traditional design using Si diodes. Figure 12 shows the measured MOSFET case temperature as a function of time after initial power up. Initially, the devices were in thermal equilibrium at room temperature. This measurement was done when full-load operating condition was Copyright © 2002-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree and the Cree logo are registered trademarks of Cree, Inc. CPWR-AN01, Rev. A Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 Fax: +1.919.313.5778 www.cree.com/power impressed on this circuit. These the case with Si diode, For the 85 V input measurements were taken under two voltage, the MOSFET case temperature extreme input voltages – 85 V and 250 V. decreases from a steady state temperature Since a higher duty cycle is used in the of 45.5 oC to 42.5 oC when the SiC PFC case of 85 V input voltage, the MOSFET diode is introduced for the case of 2 FETs. sees a higher reverse recovery current Whentwo one FET removed, its V. Since a impressed on this circuit. These measurements were taken under extreme input was voltages –o 85 V and 250 causing it to have a higher case temperature stabilizes at 47.6 C, a small higher duty cycle is used in the case of 85-V input voltage, the MOSFET sees a higher reverse-recovery current, causing it to temperature. have a higher case 250-V input the MOSFET case temperature decreases from a steadyFor temperature. 250 V inputFor voltage, the voltage, increase as compared to the original case stateMOSFET temperature of 35.7°C when adecreases Si PFC diode is used to 30.2°C for the case with case temperature from with when a SiC PFCSidiode is introduceddiode. two FETs. When a FET was removed, the case temperature on the single FET was only 32.5°C, which is an improvement over the case with Si diode. For the 85-V input voltage, the MOSFET case temperature decreases from a steady-state temperature of 45.5°C to 42.5°C when the SiC PFC diode is introduced for the case of two FETs. When one FET was removed, its temperature stabilizes at 47.6°C, a small increase as compared to the original case with a Si diode. 50 o MOSFET Case Temperature ( C) 55 Si Diode 2 FETs Snubber SiC Diode 2 FETs SiC Diode 1 FET 85 V Input 45 40 35 30 250 V Input 25 10 100 1000 Time (secs) Figure 12: MOSFET case temperature comparison for the following cases at 85-V and 250-V input: Si diode with two FETs, SiC diode with two FETs and SiC diode with single FET. Figure 12: MOSFET case temperature comparison for the following cases at 85 V and 250 V input: Si diode with 2 FETs, SiC diode with 2 FETs and SiC diode with single Conclusions FET. The realization of the impact of SiC SBDs on the circuit efficiency and MOSFET case temperature is of great importance to a PFC circuit designer. Based on measurements presented above, the most significant advantages offered by SiC Schottky diodes vis-à-vis Si PiN diodes in a PFC circuit are: higher circuit efficiency; lower FET case temperature; and a significant reduction in the number of circuit components due to the elimination of the snubber inductors, capacitors and networks. These advantages can be very effectively harnessed for lowering the cost of the circuit. For a given efficiency, a higher frequency ofRev operation of the circuit can result in smaller (and hence cheaper) inductorsPage and MOSFETs, which CPWR-AN01 8/9 are typically the most expensive components in the PFC circuit. For an identical case temperature, a smaller and cheaper MOSFET and heat sinks can be used in the circuit. Another simple circuit modification to lower the total circuit losses involves reducing the gate resistance of the MOSFET. A higher gate resistor is used in typical PFC circuits in order to limit the di/dt in the Si PiN diode, which might result in excessive reverse-recovery current and EMI emissions. Since SiC Schottky diodes can operate under very high di/dt, a smaller MOSFET gate resistance can be utilized, further reducing MOSFET turn-on losses. Such a modification will also result in lowering the MOSFET turn-OFF losses, which showed little change with direct replacement of SiC Schottky diodes with Si PiN diodes in the PFC circuit described above. The resulting circuit is much more simplified and reduces manufacturing costs, design errors, and the number of components that emit and absorb harmful EMI. Overall, this can also result in improved circuit reliability. Copyright © 2002-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree and the Cree logo are registered trademarks of Cree, Inc. CPWR-AN01, Rev. A Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 Fax: +1.919.313.5778 www.cree.com/power