CALCULATION OF MAJOR IGBT OPERATING PARAMETERS

Transcription

1 CALCULATION OF MAJOR IGBT OPERATING PARAMETERS This applicati note covers how to calculate major IGBT operating parameters - power dissipati; - ctinuous collector current; - total power losses; - juncti temperature & heatsink; - pulsed collector current in a user specified envirment using the datasheet as a source for device characteristics CONTENTS Calculati of power dissipati 2 2 Calculati of maximum ctinuous collector current3 3 Calculati of power losses 6 3 Cducti losses7 32 Switching losses 9 33 Total power losses6 4 Calculati of juncti temperature and heatsink 7 5 Calculati of juncti temperature and power losses 9 6 Calculati of pulsed collector current20 7 Safe operating area23 wwwinfinecom August-99

2 CALCULATION OF POWER DISSIPATION This secti explains how to calculate the maximum allowable power dissipati in the IGBT for a specific case temperature using the datasheet parameters Input data from the datasheet: R thjc - thermal resistance juncti-case; T j(max) - maximum juncti temperature Additial input informati: T C - case temperature Soluti: The juncti temperature rises due to power losses in the device T P R tot thjc The difference between juncti and case temperature is T T T j c () (2) Results: The expressi (3) shown below describes how to calculate the allowable power dissipati in an IGBT for desired juncti and case temperatures P tot where R thjc T c T j T R thjc T j T c R thjc - thermal resistance juncti to case; - case temperature; - juncti temperature Example: Assuming that T j T j( max ) the maximum power dissipati can be calculated for different values of T C (3) T j ( max ) P tot ( max ) R thjc T c (4) wwwinfinecom 2 August-99

3 Figure shows the maximum power dissipati for an IGBT as a functi of case temperature Parameters in this example: R thjc = 07 K/W; T j(max) = 50 C Figure : Power dissipati of SGP20N60 2 CALCULATION OF MAXIMUM CONTINUOUS COLLECTOR CURRENT This secti illustrates how to calculate the maximum ctinuous collector current of IGBT for a specific case temperature using the datasheet parameters Input data from the datasheet: R thjc - thermal resistance juncti-case; T j(max) - maximum juncti temperature; output characteristic at T j(max) Additial input informati: T C - case temperature Soluti: The cducti power losses during the -state of IGBT is the product of the collector current and the collector-emitter voltage drop at this desired current level P cd V (2) ce Collector-emitter saturati voltage depends the collector current flowing through the IGBT The output characteristic of IGBT at maximum juncti temperature (Figure 2) can be used to calculate the cducti losses for different current levels In order to simplify the analysis the output characteristic for a given gate-emitter voltage will be linearly interpolated (Figure 22) wwwinfinecom 3 August-99

4 2 RCE V ce Figure 2: Typical output characteristic of SGP20N60 at T j = 50 C VT0 Figure 22: Linear interpolati of typ output characteristic of SGP20N60 at T j = 50 C The next equati (22) describes the interpolated curve of typical output characteristic V ce VT0 RCE I (22) c The VT0 parameter of the interpolated curve can be defined directly from the figure 22 The following equati (23) describes how to determinate the RCE parameter V ce V ce ( 2 ) V ce ( ) RCE (23) I c ( 2 ) ( ) Using the equati () for juncti temperature increase due to power losses and equatis (2) and (22) we will become the following equati (24) T P R I V R I VT0 RCE I R (24) cd thjc c ce thjc c c thjc This equati (24) outlines the juncti temperature increase in dependence of collector current Solving it for and using equati (2) we become R thjc VT0 2 4 RCE T j ( max ) 2 R thjc RCE T c VT0 2 RCE In order to calculate the maximum collector current we have to use the worst case output characteristic of IGBT Usually ly typical output characteristic can be found in the datasheet The worst case output characteristic can be determined using the typical output characteristic and the typical and maximum values of collector-emitter saturati voltage in electrical characteristic table of the datasheet The typical characteristic has to be moved to the right in directi of higher collector-emitter voltages (Figure 23) (25) wwwinfinecom 4 August-99

5 Parameters in this example: RCE = 0056 Ω; VT0 = 28 V; VT0 (max) = 78 V VT0 (max) Figure 23: Typical output and worst case interpolated output characteristics of SGP20N60 The RCE parameter remains the same But the VTO parameter has to be increased by the value of tolerance between typical and maximum values of collector-emitter saturati voltage at maximum juncti temperature VTO VTO V V (26) ( max ) ce ( sat ), ( max ) ce ( sat ), ( typ ) Results: Using this equati (26) and (25) the maximum ctinuous collector current can be determined for different case temperatures 2 R thjc VT0 ( max ) 4 RCE T j( max ) ( max ) 2 R thjc RCE T c VT0 ( max ) 2 RCE (27) where R thjc T C T j(max) VT0 (max), RCE - thermal resistance juncti to case; - case temperature; - maximum juncti temperature; - parameters of interpolated output characteristic Example: Figure 24 shows the maximum ctinuous collector current for different values of case temperature wwwinfinecom 5 August-99

6 Parameters in this example: RCE = 0056 Ω; R thjc = 07 K/W; T j(max) = 50 C; VT0 (max) = 78 V Figure 24: Ctinuous collector current of SGP20N60 3 CALCULATION OF POWER LOSSES This secti explains how to calculate the cducti and switching power losses in the IGBT from the actual circuit, including the current waveform, voltage and operating frequency using the datasheet parameters Input data from the datasheet: output characteristic at T j(max) ; collector-emitter saturati voltage vs juncti temperature; switching losses vs collector current at T j(max) ; switching losses vs gate resistor at T j(max) ; switching losses vs juncti temperature Additial input informati: D - duty cycle; di c dt f Q rr, t rr T C T j t p R G V DC() V DC(off) - collector current turn transient rate; - switching frequency; - parameters of the user specific diode at these operati cditis; - case temperature; - juncti temperature; - pulse length; - gate resistor; - DC voltage at IGBT during the off state before the beginning of the turn- transiti; - DC voltage at IGBT after the end of the turn-off transiti wwwinfinecom 6 August-99

7 Soluti: The energy dissipated in the IGBT can be obtained with the following expressi t p E (3) tot v ce i c d t 0 where t p is the pulse length Power is obtained by multiplying by frequency, for repetitive switching waveforms P tot E tot f (32) In order to simplify the analysis the total power losses can be divided into cducti and switching losses P tot P cd P (33) switch The losses during the off state of transistor are negligible and will be not discussed 3 Cducti losses Cducti losses occur between the end of the turn- transiti and the beginning of the turn-off transiti Using the equati 3 and interpolated output characteristic at T j(max) (equati 22) the cducti losses can be calculated for different waveforms of collector current Usually the juncti temperature in the actual operati envirment is lower as the T j(max) With the help of datasheet (figure 3) the output characteristic of the IGBT can be scaled to a given juncti temperature Parameters in this example: = 20 A; T j(max) = 50 C (from datasheet); T j = 00 C (user specific); V ce(sat) (T j(max) ) = 24 V; V ce(sat) (T j ) = 225 V Scale factor for output characteristic for T j = 00 C is 225 V V Figure 3: Collector-emitter saturati voltage vs juncti temperature for SGP20N60 wwwinfinecom 7 August-99

8 The next expressi describes how to obtain the output characteristic at a given juncti temperature: V V ce VT0 RCE ce ( sat ) (T j ) sat j(max) V ce ( ) (34) Results: Collector current waveform: Mathematical expressi: i c 0 t p Cducti energy losses for given pulse length: E cd V ce t p I c A B t (35) p Cducti power losses for periodical signal with given duty cycle: P cd V ce D I c A B D (36) Collector current waveform: Mathematical expressi: () (2) i c ( ) t ( 2 ) ( ) t p 0 t p Cducti energy losses for given pulse length: E cd 2 A I I c ( ) c ( 2 ) 3 B I 2 2 I I I c ( ) c ( ) c ( 2 ) c ( 2 ) t p (37) Cducti power losses for periodical signal with given duty cycle: P cd 2 A I I c ( ) c ( 2 ) 3 B I 2 2 I I I c ( ) c ( ) c ( 2 ) c ( 2 ) D (38) wwwinfinecom 8 August-99

9 Collector current waveform: Mathematical expressi: i c t t p 0 t p Cducti energy losses for given pulse length: E cd 6 t 2 A I 3 B (39) p c Cducti power losses for periodical signal with given duty cycle: P cd 6 D 2 A I 3 B (30) c where A RCE D = t p / T t p VT0, RCE V ce ( sat ) j V ce ( ) sat j(max) - duty cycle; - pulse length; - parameters of interpolated output characteristic B VT0 V ce ( sat ) j V ce ( ) sat j(max) These expressis (35-30) describe the cducti losses of the IGBT with typical output characteristic Sometimes it is necessary to calculate the worst case cducti losses with worst case output characteristic The same method as described in secti 2 (equati 26) can be used to obtain the worst case cducti losses The VT0 parameter has to be changed to VT0 (max) 32 Switching losses No simple expressi can be found for the voltage and current during a switching transient The datasheet parameters ccerning the switching losses have to be used in this case These parameters are referenced to a specific test circuit that simulates a clamped inductive load operated with a specific diode In actual power circuit the diode used might be different than the diode specified in the datasheet In this case the calculati of turn- losses (equati 3) has to be made using the parameters of this particular diode Soluti: The switching losses in IGBT during e period of a periodical signal include the turn- and turn-off losses P switch f (3) In order to calculate the switching losses the dependence from the collector current has to taken into account This informati can be found in the datasheet (Figure 32) wwwinfinecom 9 August-99

10 Parameters in this example: A = mj/a; B = -049 mj; A off = 0026 mj/a; B off = 002 mj Figure 32: Typical switching losses at T j(max) of SGP20N60 The values at the desired current level can be easily found from these curves or the linear interpolati of these curves can be made to simplify the analysis for different current levels The same methodic as by interpolati of output characteristic (equati 22) can be used in this case Next equati (32) describes the interpolated curve of turn- losses E A B (32) with A ( 2 ) ( ) I c ( 2 ) ( 2 ) B ( ) The turn-off losses curve can be interpolated in the similar way A off B off with 2 A ( 2 ) (33) A off ( 2 ) ( ) B ( 2 ) I off ( ) c ( 2 ) 2 A off ( 2 ) If the gate resistor of a users gate drive does not have the same value as the gate resistor in the test circuit specified in the datasheet some correcti may be necessary This can be de with the help of datasheet (figure 33) wwwinfinecom 0 August-99

11 Parameters in this example: R G = 6 Ω (from datasheet); R G = 30 Ω (user specific); (R G,datasheet ) = 2 mj; (R G,user specific ) = 3 mj; (R G,datasheet ) = 05 mj; (R G,user specific ) = 065 mj Scale factor for is 3 mj mj Scale factor for is 065 mj 3 05 mj Figure 33: Typical switching losses as a functi of gate resistor for SGP20N60 In order to obtain the switching losses at desired gate resistor the switching losses from the equatis 32 and 33 have to be scaled using the informati from figure 33 for the user specific R G and the R G used in datasheets test circuit (figure 32): E E G, user specific A B (34) G, datasheet A off B off G, user specific (35) G, datasheet Since the switching energy is proportial to voltage, the result is scaled by ratio of the actual circuit voltage to the test voltage in the datasheet: E V E DC(), user specific A B G, user specific V G, datasheet DC, datasheet G, user specific A off B V DC(off), user specific off V G, datasheet DC, datasheet E off Finally, the actual juncti temperature has to be taken into account Figure 34 shows the dependence of switching losses the juncti temperature (36) (37) wwwinfinecom August-99

12 Parameters in this example: Figure 34: Switching losses as a functi of juncti temperature for SGP20N60 = 20 A; T j(max) = 50 C (from datasheet); T j = 00 C (user specific); (T j(max) ) = 2 mj; (T j ) = 09 mj; (T j(max) )= 05 mj; (T j )= 042 mj Scale factor for at T j = 00 C is 09 mj mj Scale factor for at T j = 00 C is 042 mj mj The next expressis outline how to obtain the switching losses for desired operating cditis (collector current, gate resistor, DC voltage and juncti temperature) from the datasheet: E E E j A B V G, user specific DC(), user specific (38) V DC, datasheet G, datasheet j(max) E A off B V off G, user specific DC(off), user specific off V G, datasheet DC, datasheet E off j j(max) (39) wwwinfinecom 2 August-99

13 Results: Using equatis some modificatis can be de in order to get readable results Collector current waveform: Mathematical expressi: i c 0 t p Turn- energy losses with the diode specified in the datasheet: E A B Turn- energy losses with the user specific diode: 2 2 Q rr t E rr V 2 di DC(), user specific c (320) (32) dt 6 2 Q rr t rr di t c rr dt 4 Q rr 3 I t c V DC(), user specific rr Turn-off energy losses: A2 B2 (322) wwwinfinecom 3 August-99

14 Collector current waveform: Mathematical expressi: () (2) i c ( ) t ( 2 ) ( ) t p 0 t p Turn- energy losses with the diode specified in the datasheet: E A ( ) B (323) Turn- energy losses with the user specific diode: 2 2Q rr ( ) t E rr V 2 di DC(), user specific c (324) dt 6 t rr 2Q rr di t c rr dt 4Q rr 3I t c( V ) DC(), user specific rr Turn-off energy losses: E off A2 ( 2 ) B2 (325) Collector current waveform: Mathematical expressi: i c t t p 0 t p Turn- energy losses is negligible: E 0 (326) Turn-off energy losses: A2 B2 (327) wwwinfinecom 4 August-99

15 where A A G, user specific V DC(), user specific G, datasheet V DC, datasheet B B G, user specific V DC(), user specific G, datasheet V DC, datasheet A2 A off G, user specific V DC(off), user specific G, datasheet V DC, datasheet B2 B off G, user specific V DC(off), user specific G, datasheet V DC, datasheet A, B, A off, B off - parameters of interpolated curve for switching losses; j j(max) j j(max) j j(max) j j(max) di c dt Q rr, t rr T j V DC() V DC(off) - collector current turn- transient rate; - parameters of the diode at these operati cditis; - actual juncti temperature; - DC voltage at IGBT during the off state before the beginning of the turn- transiti; - DC voltage at IGBT after the end of the turn-off transiti wwwinfinecom 5 August-99

16 33 Total power losses Total power losses for periodical signal can be calculated as the sum of cducti (secti 3) and switching (secti 32) losses: Results: P tot P cd f (328) Due to the trade off between cducti and switching losses inherent in IGBT technology it is unlikely that the e and the same IGBT will have the performance of both worst case cducti and worst case switching losses In order to calculate the worst case power losses in the IGBT it is useful to use the worst case cducti losses and the typical switching losses P tot ( max P ) cd ( max E ) f (329) where P cd (max) - worst case cducti losses (calculated in secti 3);, - typical turn- and turn-off switching losses (calculated in secti 32); f - switching frequency Example: Figures 35 and 36 show the total power losses in the IGBT in dependence switching frequency for the following operating cditis: = 20 A - peak of collector current; V DC() = 300 V - DC voltage at IGBT during the off state before the beginning of the turn transiti; V DC(off) = 300 V - DC voltage at IGBT after the end of the turn-off transiti; R G = 30 Ω; - gate resistor; T j = 00 C - juncti temperature Figure 35: Total power losses in case of a square wave collector current of SGP20N60 Figure 36: Total power losses in case of a triangle wave collector current of SGP20N60 wwwinfinecom 6 August-99

17 4 CALCULATION OF JUNCTION TEMPERATURE AND HEATSINK This secti describes how to calculate the juncti temperature of IGBT and how to select the heatsink for a given operating cditi using the datasheet parameters Input data from the datasheet: R thjc thermal resistance juncti to case; Z thjc transient thermal impedance juncti to case Additial input informati: P tot(max) - worst case total power losses (calculated in secti 3); R thcs - thermal resistance case to heatsink; R thjc - thermal resistance juncti to case; R thsa - thermal resistance heatsink to ambient; T A - ambient temperature Soluti: The juncti temperature can be calculated using the thermal resistance R thjc This is suitable for the DC collector current But in case of pulsed collector current the more accurate results can be obtained if the transient thermal impedance juncti-case Z thjc is used The transient thermal impedance is a functi of duty cycle and pulse length or switching frequency: n Z thjc j = 0 R e j e t p τ j t p τ j D n j = 0 R e j e Figure 4 demstrates the transient thermal impedance of SGP20N60 for different values of duty cycle and pulse durati D τ j f τ j f Parameters in this example: R j, K/W τ j, s e e e-5 (4) R R 2 C =τ /R C 2 =τ 2 /R 2 Figure 4: Transient thermal impedance of SGP20N60 wwwinfinecom 7 August-99

18 The difference between juncti and case temperatures can be obtained using the informati of worst case total power losses (secti 33) and transient thermal impedance for desired values of duty cycle and switching frequency T j T (42) c P tot( max ) Z thjc Results: The juncti temperature for the defined case temperature, duty cycle and switching frequency is T P Z T (43) j tot ( max ) thjc c The following expressi describes how to select the heatsink that keeps the juncti at, or below, a given temperature T j T A R thsa P tot ( max ) Z thjc R thcs (44) where P tot(max) - worst case total power losses; R thcs - thermal resistance case to heatsink; R thsa - thermal resistance heatsink to ambient; - ambient temperature; T A T j Z thjc - juncti temperature; - transient thermal impedance juncti to case for given duty cycle and switching frequency Example: Calculati of the juncti temperature for a given operating cditi: D = 05 - duty cycle; f = 75 khz - switching frequency; T C = 80 C - case temperature; P tot(max) = 45 W - total power losses The transient thermal impedance for this duty cycle and this switching frequency from figure 4 (t p = 667 us) is Z thjc = 035 K/W Using the equati 43 the juncti temperature can be calculated: T j 45 W 035 K W 80 C 9575 C In order to keep the juncti below a given temperature the right heatsink has to be chosen The required thermal resistance of a heatsink can be obtained from the equati 44 using input data from above plus some additial informati: R thcs = 045 K/W - thermal resistance case to heatsink; T A = 40 C - ambient temperature; T j = 00 C - juncti temperature 00 C 40 C R thsa 45 W 035 K W 045 K W 053 K W wwwinfinecom 8 August-99

19 5 CALCULATION OF JUNCTION TEMPERATURE AND POWER LOSSES In the previous two sectis we have explained how to calculate the power losses and the juncti temperature In this secti we present an algorithm how calculate both these parameters in a specific applicati envirment Soluti: Since the juncti temperature affects cducti and switching losses, which, in turn, affect the juncti temperature, a direct mathematical soluti is not possible However, if we apply the calculatis introduced in sectis 3 and 4 iteratively, we will get the results for a given operating cditi in very few iteratis Results: Step: Acti: Input: Output: Described in secti: Assume that T j(n-) = T j(max) T j(max) T j (n-) - 2 Calculate the total power losses P tot(n-) T (n-) at this juncti temperature T j(n-) P tot(n-) 3 j(n-) 3 Calculate the juncti temperature T j(n) P (n) at these total power losses P tot(n-) T j(n) 4 tot(n-) Compare difference between the juncti temperature from current iterati T j(n) and juncti temperature from the previous iterati T j(n-) with a given precisi T j (usually 5-0 C) 4 (n) if T j(n-) - T j(n) < T j then stop the iteratis; otherwise - next iterati (n=n+ & go to step 2) T j, T j(n-), T j(n) Decisi to ctinue or to finish the iteratis - wwwinfinecom 9 August-99

20 6 CALCULATION OF PULSED COLLECTOR CURRENT This secti describes how to calculate periodically pulsed collector current for given operati cditis: duty cycle, switching frequency and case temperature Input data from the datasheet: Z thjc - transient thermal impedance juncti-case; T j(max) - maximum juncti temperature; output characteristic at T j(max) ; switching losses vs collector current at T j(max) Additial input informati: D - duty cycle; di c dt f Q rr, t rr T C V DC() - collector current turn transient rate; - switching frequency; - parameters of the diode at these operati cditis; - case temperature; - DC voltage at IGBT during the off state before the beginning of the turn- transiti Soluti: The maximum power dissipati for a given operating cditi can be calculated using the transient thermal impedance at this particular duty cycle and switching frequency T j( max ) T c P tot( max ) Z thjc The total power losses in the IGBT are described by expressis (3) and (32) combining these we will get the following equati t p P tot f (62) v ce i c d t 0 Combining the equatis (6) and (62) the informati about the maximum pulsed collector current can be obtained (6) wwwinfinecom 20 August-99

21 Results: Collector current waveform: Mathematical expressi: i c 0 t p Maximum pulsed collector current of IGBT with the diode specified in the datasheet: ( max ) B 2 4A T j ( ) max T c C B 2A where: A D RCE Z thjc B D VT0 ( max ) A A f Z off thjc C B B off f Z thjc Maximum pulsed collector current of with the user specific diode: ( max ) B 2 4 A T j ( ) max T c C B 2 A where: A RCE D 2 V f Z DC(), user specific di thjc c (63) (64) V B x 2 V DC(), user specific t A DC(), user specific rr off Q rr di c t dt rr 2 C 3 Q rr V DC(), user specific dt x ZthJC f VT0 ( max D Z ) thjc 2 B 2 V DC(), user specific off Q 3 rr di c 2 t dt rr Z thjc f wwwinfinecom 2 August-99

22 Collector current waveform: Mathematical expressi: i c t t p 0 t p Maximum pulsed collector current of IGBT with the diode specified in the datasheet: ( max ) B 2 4 A T j ( max ) T c C B 2 A where: A 3 D RCE Z thjc B 2 VT0 D f A ( max ) off C f B Z off thjc Z thjc (65) where A, B, A off, B off D di c dt f Q rr, t rr T C V DC() VT0, RCE Z thjc - parameters of interpolated curve for switching losses; - duty cycle; - collector current turn- transient rate; - switching frequency; - parameters of the diode at these operati cditis; - case temperature; - DC voltage at IGBT during the off state before the beginning of the turn- transiti; - parameters of interpolated output characteristic; - transient thermal impedance juncti to case for given duty cycle and switching frequency wwwinfinecom 22 August-99

23 Example: The examples of pulsed collector current for SGP20N60 are shown in Figures 6 and 62 Figure 6: Amplitude of a square wave collector current of SGP20N60 Figure 62: Peak of a triangle wave collector current of SGP20N60 7 SAFE OPERATING AREA This secti describes how to evaluate the thermal calculati of the pulsed collector current described above using the datasheet Input data from the datasheet: Safe operating area Additial input informati: Pulsed collector current (calculated in secti 6) Results: The calculatis of pulsed collector current described in secti 6 are based ly the thermal behavior of IGBT The peak collector current must not exceed the safe operating area limits wwwinfinecom 23 August-99

24 Example: Figure 7 shows the safe operating area of SGP20N60 Figure 7: safe operating area of SGP20N60 The maximum peak collector current is 80 A for SGP20N60 The calculated curves do not exceed this limit of 80 A in case of square wave collector current (Figure 72) If the collector current has a triangle waveform (Figure 73) the calculated curves for both case temperatures exceed the limit of SOA at some frequencies In this case the peak collector current has to be limited to 80 A for these frequencies SOA SOA Figure 72: peak of a square wave collector current of SGP20N60 and SOA limit Figure 73: peak of a triangle wave collector current of SGP20N60 and SOA limit wwwinfinecom 24 August-99