hard switched silicon igbts? cut switching losses in half ...€¦ · sbd at 150°c. figure 6 shows...

APPLICATION NOTE

CPWR-AN03, Rev A /9

Hard Switched Silicon IGBT’s?

Cut Switching Losses in Half with SiliconCarbide Schottky Diodes

By Jim Richmond

Replacing the Si Ultrafast soft recoverydiode used as the freewheeling componentin hard switched IGBT applications with aSilicon Carbide (SiC) Schottky diodereduces the switching losses in the diode by80% and the switching losses in the IGBTby 50%.

IntroductionThe Silicon IGBT, which combines the

output and switching characteristics of abipolar transistor and the ease of control ofa MOSFET, has become the power switchof choice for hard switched, high voltage(greater than 500V) and high power (greaterthan 500 watts) applications. Typicalapplications include motor control inverters,uninterruptible power supplies, weldingequipment and switched mode powersupplies (SMPS).

The ever-increasing demand in powerelectronics for improved efficiency, reducedcooling, decreased size and weight, andstricter EMI/RFI and power qualityrequirements present new challenges to thedesigner. All of these requirements aregreatly influenced by the high transientlosses during IGBT turn-on when switchingthe inductive load found in hard-switchedtopologies. The reverse recovery currentpresent at turn-off of the siliconfreewheeling diode directly affects this IGBTturn-on transient. To compound matters, thediode reverse recovery current increaseswith increasing operating temperature,diode current, and di/dt.

The diode reverse recovery current and theIGBT switching losses can be drasticallyreduced by replacing the siliconfreewheeling pin diode with a SiC SchottkyBarrier Diode (SBD). Due to the materialproperties of silicon, Silicon Schottky diodesare not possible in the 200 plus volt range.

SiC Schottky DiodesThe SiC SBD is commercially available

with 600 volt and 1200 volt ratings. The 600volt diodes are available with 1, 4, 6, 10,and 20 amp current ratings. The 1200 voltdiodes are available with 5 and 10 ampcurrent ratings. The main advantage of ahigh voltage SiC SBD lies in its superiordynamic performance. The reverse recoverycharge in the SiC SBD is extremely low andis the result of junction capacitance, notstored charge. Furthermore, unlike the SiPiN diode, it is independent of di/dt, forwardcurrent and temperature. The maximumjunction temperature of 175°C in the SiCSBD represents the actual useabletemperature. The ultra low Qrr in SiC SBDsresults in reduced switching losses in atypical hard switched IGBT basedapplication. This lowers the casetemperature of the IGBT improving thesystem efficiency and possibly allowing for areduction in size of the silicon IGBT. Inorder to measure the benefit of these highperformance rectifiers, an inductiveswitching test circuit was used to measurethe IGBT and diode switching losses. Thisallowed for a switching loss comparisonbetween an ultrafast soft recovery silicondiode and the Cree ZERO RECOVERY

SBD,as well as the impact their reverse recoveryhas on the switching losses of a IGBT.

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Switching Measurement Figure 1 shows the inductive test circuitused for making the switchingmeasurements. During operation, a doublepulse is used to drive the IGBT gate. For the600 volt device testing a 10 ohm gate driveresistor is used to set the di/dt to 750 A/µs.A 22 ohm resistor was used with the 1200volt devices for a di/dt of 250 A/µs.

Figure 1. Inductive Test Circuit withoperating waveforms.

At time T1, the IGBT is turned on andcurrent through the inductor ramps up until itreaches the desired test current at time T2.At time T2, the IGBT is turned off and theinductor current is transferred to the diode.The IGBT turn-off losses and diode turn-onlosses are measured at the T2 transition.The inductor current continues to flowthrough the diode until the IGBT is turnedback on at time T3. Now inductor current is

transferred from the diode back to the IGBT.The IGBT turn-on losses and diode turn-offlosses are measured at the T3 transition.

600 volt Switching Comparison Switching parameters were measured fora 15 A, 600 V ultrafast soft recovery silicondiode (similar to what would be co-packaged in a 40 A ultrafast IGBT) and a 10A, 600 V SiC SBD, along with the switchinglosses of a 40A, 600 V Silicon IGBT. Thelosses were measured at a voltage of 500 Vand current of 20 A.

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Figure 2. 600 volt Si Ultrafast diode turn-offvoltage, current and instantaneous power at150°C.

Figure 2 shows the turn-off voltage,current and instantaneous power measuredat a junction temperature of 150°C of the Siultrafast diode. This shows a peak reverserecovery current of 23 amps, a recoverytime of 100 ns, and a peak instantaneouspower of 7 kW. Also shown is the 200 voltovershoot caused by the high di/dt duringthe reverse recovery snap-off.

Figure 3 is the turn-off waveforms for theSiC SBD at 150°C. This shows a peakreverse recovery current of 4 amps, areduction of 83%, a recovery time of 33 ns,a reduction of 67% and a peakinstantaneous power of 0.5 kW, a reductionof 93%. The drastic reduction in switching

APPLICATION NOTE

CPWR-AN03, Rev A /9

power is due to the SiC SBD only having todissipate a small capacitive charge, whichhappens while the diode voltage is low. Thevoltage overshoot seen in the Si diode iscompletely eliminated with the SiC SBD.

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Figure 3. 600 volt SiC SBD turn-off voltage,current and instantaneous power at 150°C.

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Figure 4. 600 volt IGBT turn-on w/ Si Ultrafastdiode, voltage, current and instantaneouspower at 150°C.

Figure 4 shows the turn-on voltage,current and instantaneous power measuredat a junction temperature of 150°C of theIGBT with a Si ultrafast diode. During theIGBT turn-on the diode reverse recoverycurrent is added to the IGBT current,resulting in a peak current of 44 amps. A

peak instantaneous power of 15 kW isdissipated in the IGBT. Also shown are highfrequency oscillations in the IGBT voltagecaused when the Si diode snaps off. This isa major cause of RFI/EMI generation.

Figure 5 shows the turn-on voltage,current and instantaneous power measuredat a junction temperature of 150°C of theIGBT with a SiC SBD. The use of the SiCSBD results in a peak current of 22 amps, a50% reduction, and a peak instantaneouspower of 7.5 kW, a 50% reduction. The highfrequency oscillations in the IGBT voltageare also eliminated with the SiC SBD,resulting in reduced RFI/EMI generation.

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Figure 5. 600 volt IGBT turn-on w/ SiC SBD,voltage, current and instantaneous power at150°C.

A comparison of the switchingparameters of the SiC SDB with the Siultrafast diode are shown for measurementstaken at a junction temperature of 25°C inTable 1 and for measurements taken at ajunction temperature of 150°C in Table 2.The total switching loss reduction (IGBT +Diode) is calculated to be 52% at 25°C and56% at 150°C.

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CPWR-AN03, Rev A /9

Ic = 20A, Vcc = 500V, Rg = 10 ohmParameter Units Si Pin SiC % Reduction

Peak reverse current Ipr (A) 13 4 69%Reverse recovery time Trr (nS) 83 30 64%

Recovery charge Qrr (nC) 560 78 86%Diode loss turn-off Eoff Diode (mJ) 0.11 0.02 82%Diode loss turn-on Eon Diode (mJ) 0.03 0.02 33%

Diode loss total Ets Diode (mJ) 0.14 0.04 71%IGBT loss turn-on Eon IGBT (mJ) 0.63 0.23 63%IGBT loss turn-off Eoff IGBT (mJ) 0.46 0.32 30%

IGBT loss total Ets IGBT (mJ) 1.09 0.55 50%loss total Ets (mJ) 1.23 0.59 52%

Table 1: 600 volt switching parametercomparison between Si Ultrafast and SiCSBD at 25°C.



Recovery charge Qrr (nC) 1220 82 93%Diode loss turn-off Eoff Diode (mJ) 0.23 0.02 91%Diode loss turn-on EonDiode (mJ) 0.03 0.02 33%




Figure 6 shows the turn-off currents ofthe Si ultrafast diode and the SiC SBD at25°C and 150°C superimposed on one plot.The SiC SBD is unchanged withtemperature, with a peak reverse current of5 amps. The Si ultrafast diode shows strongtemperature dependence, increasing from13 amps at 25°C to 23 amps at 150°C.

Figure 7 shows the turn-on currents ofthe IGBT with a Si ultrafast diode and a SiCSBD at 25°C and 150°C, superimposed onone plot. The peak current in the IGBT withthe SiC SBD is unchanged withtemperature. The peak current of the IGBTwith the Si ultrafast diode shows strongtemperature dependence due to the reverserecovery temperature dependence of thediode.

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Figure 6: 600 volt turn off current of the SiUltrafast diode and the SiC SBD at 25°C and150°C.

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Figure 7: 600 volt turn-on current of the IGBT w/the Si Ultrafast diode and the SiC SBD at 25°C and150°C.

Figure 8 shows the total diodeswitching losses (turn-on and turn-off) inwatts at switching frequencies from 10 kHzto 100 kHz and temperatures of 50, 100 and150°C. The SBD has significantly lowerswitching losses (up to an 85% reduction)and shows no change with increasedtemperature.

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Figure 8: 600 volt switching power loss ofthe Si Ultrafast diode and the SBD at 50°C,100°C, and 150°C.

Figure 9 shows the total IGBT switchinglosses (turn-on and turn-off) in watts atswitching frequencies from 10 kHz to 100kHz at 50, 100 and 150°C. The IGBTswitching loss with the SiC SBD is abouthalf that of the IGBT with the Si ultrafastdiode. The IGBT with the SiC SBD alsoshows less increase in switching losses withtemperature. The temperature dependenceof the switching losses in the IGBT with theSiC SBD is due to the increase in IGBTturn-off time since the turn-on losses areunchanged with temperature. This dramaticimprovement in the IGBT switchingperformance is due solely to the absence ofreverse recovery in the SiC SBD.

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Figure 9: 600 volt IGBT switching power lossw/ the Si Ultrafast diode and the SiC SBD at50°C, 100°C, and 150°C.

1200 volt Switching Comparison��The switching parameters were

measured for an 8A, 1200 V ultrafast softrecovery silicon diode (similar to what wouldbe co-packaged in an 11 A ultrafast IGBT)and a 5 A, 1200 V SBD, along with theswitching losses of a 11 A, 1200 V IGBT.The losses were measured at a voltage of1000 V and current of 5 A. The maximumtemperature used in this testing was 125°Csince the IGBT started going into thermalrunaway when biased at 150°C.

Figure 10 shows the turn-off voltage,current and instantaneous power measuredat a junction temperature of 125°C for the Siultrafast diode. This shows a peak reverserecovery current of 6 amps, a recovery timeof 148 ns, and a peak instantaneous powerof 2.8 kW. The voltage overshoot seen withthe 600 volt Si diode is not pronounced heredue to the 1200 volt testing being done at alower di/dt (250 A/µs vs. 750 A/µs).

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Figure 10. 1200 volt Si Ultrafast diode turn-off voltage, current and instantaneous powerat 125°C.

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Figure 11. 1200 volt SiC SBD turn-off voltage,current and instantaneous power at 125°C�

Figure 11 is the turn-off waveforms forthe SiC SBD at 125°C. This shows a peakreverse recovery current of 1 amp, areduction of 83%, a recovery time of 30 ns,a reduction of 80% and a peakinstantaneous power of 0.3 kW, a reductionof 89%. The drastic reduction in switchingpower is due to the capacitive charge of theSBD dissipating while the diode voltage islow.

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Figure 12. 1200 volt IGBT turn-on w/ SiUltrafast diode, voltage, current andinstantaneous power at 125°C.

Figure 12 is the turn-on voltage, currentand instantaneous power measured at ajunction temperature of 125°C for the IGBT

with a Si ultrafast diode. During the IGBTturn-on the diode reverse recovery currentis added to the IGBT current, resulting in apeak current of 11.7 amps. A peakinstantaneous power of 11 kW is dissipatedin the IGBT.

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Figure 13. 1200 volt IGBT turn-on w/SiCSBD, voltage, current and instantaneouspower at 125°C.

Figure 13 shows the turn-on voltage,current and instantaneous power measuredat a junction temperature of 125°C for theIGBT with a SBD. The use of the SBDresults in a peak current of 6.7 amps, a 42%reduction and a peak instantaneous powerof 6.2 kW, a 44% reduction.

Figure 14 shows the turn-off currents ofthe Si ultrafast diode and the SiC SBD at25°C and 125°C superimposed on one plot.The SiC SBD is unchanged withtemperature, with a peak reverse current of1 amp. The Si ultrafast diode shows astrong temperature dependence, increasingfrom 5 amps at 25°C to 6 amps at 150°C.The reverse recovery time of the Si ultrafastdiode increases from 100 nS at 25°C to 148nS at 125°C while reverse recovery time ofthe SiC SBD is unchanged withtemperature.

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Figure 14: 1200 volt turn-off current of the SiUltrafast diode and the SiC SBD at 25°C and125°C.

Figure 15 shows the turn on currents ofthe IGBT with a Si ultrafast diode and theSiC SBD at 25°C and 125°C superimposedon one plot. The peak current in the IGBTwith the SiC SBD is unchanged withtemperature. The peak current and reverserecovery time of the IGBT with the Siultrafast diode shows a strong temperaturedependence due to the reverse recoverytemperature dependence of the diode.

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Figure 15: 1200 volt turn-on current of the IGBT w/the Si Ultrafast diode and the SiC SBD at 25°C and150°C.

A comparison of the switchingparameters of the SiC SBD with the Siultrafast diode are shown for measurementstaken at a junction temperature of 25°C inTable 3, and for measurements taken at ajunction temperature of 125°C in Table 4. Allmeasured parameters show a majorimprovement with the SiC SBD. The valueof the SBD parameters are effectivelyunchanged with increased temperaturewhile the Silicon Ultrafast diode parametersincrease. The total switching loss reduction(IGBT + Diode) is 51% at 25°C and 58% at125°C.


Peak reverse current Ipr (A) 5.5 1 82%Reverse recovery time Trr (nS) 100 30 70%











��Figure 16 shows the total diodeswitching losses (turn-on and turn-off) inwatts at switching frequencies from 10 kHzto 100 kHz for temperatures of 25, 75 and125°C. The SBD has significantly lowerswitching losses (up to a 79% reduction)and shows no change with increasedtemperature. Figure 17 shows the totalIGBT switching losses (turn-on and turn-off)in watts at switching frequencies from 10kHz to 100 kHz for temperatures of 25, 75and 125°C. IGBT switching loss with theSBD is about half that of the IGBT with the

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CPWR-AN03, Rev A /9

Si ultrafast diode. The IGBT with the SBDalso shows less increase in switching losseswith temperature. The temperaturedependence of the switching losses in theIGBT with the SBD is due to the increase inthe IGBT turn-off time, since the turn-onlosses are unchanged with temperature.This dramatic improvement in the IGBTswitching performance is due solely to theabsence of reverse recovery in the SiCSBD.

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Figure 16: 1200 volt switching power loss ofthe Si Ultrafast diode and the SBD at 25°C,75°C, and 125°C.

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Figure 17: 1200 volt IGBT switching powerloss w/ the Si Ultrafast diode and the SBD at25°C, 75°C, and 125°C.

Conduction and Total LossesFigure 18 shows the forward IV of the

1200 volt Si ultrafast diode and the SiC SBDat 25°C and 125°C. At 5 amps the SiC SBD

diode has 0.75 volt lower forward drop at25°C and 0.18 volt lower forward drop at125°C. This results in reduced conductionlosses for the SiC SBD.

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Figure 18: Forward voltage and current of the1200 volt Si Ultrafast diode and the SiC SBDat 25°C and 125°C.

Table 5 shows the calculation of totallosses for a 100 kHz converter operating ata 50% duty cycle with an average current of2.5 amps using the 1200 volt devices. Adevice junction temperature of 125°C wasused. The conduction loss for the IGBT isthe data sheet value of 2.9 volt at 5 amps.With the SiC SBD, the total diode losses arereduced by 50% and the total IGBT lossesare reduced by 51%. This gives a 51%, totalloss reduction for the 1200 volt converterby simply changing the Si ultrafast diode toa SiC SBD.

Si PiN SiC SBD % ReductionDiode Switching loss (watt) 19 4 79%

Diode conduction loss (watt) 12.5 11.7 6%Total Diode loss (watt) 31.5 15.7 50%

IGBT Switching loss (watt) 155 69 55%IGBT conduction loss (watt) 14.5 14.5 0%

Total IGBT loss (watt) 169.5 83.5 51%Total loss (watt) 201 99.2 51%

Table 5: Comparison of calculated losses ina converter with the 1200 volt Si Ultrafastdiode and the SBD at 125°C.

APPLICATION NOTE

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ConclusionsThe turn-on switching losses of the

IGBT are strongly dependent on the reverserecovery characteristics of its freewheelingdiode. The impact of the SiC SBD on theswitching performance of the freewheelingdiode and the IGBT is of great importanceto the hard switched circuit designer. Basedon the measurements presented above,there are significant advantages offered bySiC Schottky diodes. While the reverserecovery current of the Si ultrafast diodeshows a strong temperature dependence,the SiC SBD is unaffected. At a high di/dtthe Si ultrafast diode exhibits a voltageovershoot on turn-off due to snap off duringreverse recovery, but the SiC SBD isunaffected. The snap off in the Si ultrafastdiode causes oscillations in the IGBTvoltage, which generate RFI/EMI. Thisoscillation is not present with the SiC SBD.The 50% reduction in switching losses can

be applied in a number of ways to optimizethe circuit design. The reduction in switchinglosses can be applied to increase efficiency,reduce cooling requirements, or reduce thecurrent rating of the IGBT. The operatingfrequency can be increased in order to allowthe use of smaller passive components, orto achieve acoustic requirements. Theabsence of a voltage overshoot eliminatesthe need for snubber networks. Theabsence of the high frequency oscillationreduces the RFI/EMI filter requirements.The replacement of the Si ultrafast diodewith a SiC Schottky diode such as the CreeZERO RECOVERY

SBD results in a substantialreduction in switching losses in both thediode and the IGBT resulting in a significantsystem level performance improvement.

Cree, Inc.Power Products4600 Silicon DriveDurham, NC 27703 • USAtel: 919-313-5300fax: 919-313-5451��

Copyright © 2003-2005 Cree, Inc. All rightsreserved. Permission is given to reproducethis document provided the entire document(including this copyright notice) isduplicated.

The information in this document issubject to change without notice.

Cree and the Cree logo are trademarksof Cree, Inc.

hard switched silicon igbts? cut switching losses in half ...€¦ · sbd at 150°c. figure 6 shows...

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