Application Note 5324

Transcription

1 Desaturaton Fault Detecton Optocoupler Gate Drve Product wth Feature: APLJ, APL1J and HPLJ Applcaton Note 1. Introducton Desaturaton fault detecton crcut provdes protecton for power semconductor swtches (IGBT or MOSFETs aganst shortcrcut current events whch may lead to destructon of these power swtches. Ths desaturaton of the nverters can also occur due to an nsuffcent gate drve sgnal from the nverter gate drver msperformance or drver supply voltage ssues. Other falure modes that can potentally cause excessve currents and excessve power dsspatons n the nverters can be due to phase and/or ral supply short crcuts due to user msconnect or bad wrng, control sgnal falures due to nose or computatonal errors, over load condtons nduced by the load, and component falures n the gate drve crcutry. The drastcally ncreased power dsspaton very quckly overheats the power nverter and destroys t. To prevent catastrophc damage to the drve, desaturaton fault detecton and protecton must be mplemented to reduce or turnoff the overcurrents durng the fault condton. Ths applcaton note covers the desgn of desaturaton fault detecton feature provded by Avago Intellgent Gate Drver. How does desaturaton fault detecton feature work n Avago drver?. Fault Detecton The IGBT collectoremtter voltage, ESAT, s montored by the pn of the Gate Drve Optocoupler (Pn of Fgure 1. When there s short crcut n an applcaton and a very hgh current flow through the IGBT, t wll go nto desaturaton mode; hence ts ESAT voltage wll rse up. A fault s detected by the optocoupler gate drver (whle the IGBT s ON once ths ESAT voltage goes above the nternal desaturaton fault detecton threshold voltage whch s typcally.0. Ths fault detecton trggers two events: a. out of the optocoupler gate drver s slowly brought low n order to softly turnoff the IGBT and prevent large d/dt nduced voltage spkes and b. Also actvated s an nternal feedback channel whch brngs the Fault output low for the purpose of notfyng the mcrocontroller of the fault condton. At ths pont, the mcrocontroller must take the approprate acton to shutdown or reset the motor drve.. Soft turnoff Ths feature exsts n Avago gate optocoupler, (e.g. APL J, APL1J and HPLJ. When fault s detected by the feature, a weak pulldown devce n the output drve stage wll turn on to softly turn off the IGBT and prevents large d/dt nduced voltages. Ths devce slowly dscharges the IGBT gate to prevent fast changes n collector current that could cause damagng voltage spkes due to stray nductances. Durng the slow turn off, the large output pulldown devce remans off untl the output voltage falls below olts, at whch tme the large pull down devce clamps the IGBT gate to (Refer to Fgure 1.. Off State and Reset Durng the IGBT off state, the drver fault detecton crcutry s dsabled to prevent false fault sgnals. The fault output, Pn of Fgure 1 s pulled down and output Pn goes low for the duraton of the fault. In APLJ and APL1J, the fault s reset at the next postve nput sgnal to the drver after a fxed mute tme. For HPL J, t has to be reset externally through a Reset pn (Pn of Fgure. For both case, t wll only be cleared when detecton has gone to low (Shortcrcut s cleared. v. Under oltage LockOut protecton (ULO wth hysteress The Output of the optocoupler gate drver and the status are controlled by a combnaton of IN, ULO, and the detected IGBT Desat condton. Durng power up, the ULO feature prevents the applcaton of nsuffcent gate voltage to the IGBT, by forcng the output of the optocoupler gate drver low. Once the power supply of the optocoupler gate drver are above the postve ULO thresholds the detecton feature s the prmary source of the IGBT protecton. The output of the optocoupler s safely brought low once the power supply of the optocoupler falls below the negatve ULO threshold level. A hysteress n the Postve ULO and negatve ULO threshold levels provdes an approprate nose margn for the ULO detecton and output shutdown feature.

2 1 S LED 1 BLANK R F R D F S 1 ATHODE 1 ANODE ANODE LAMP ATHODE Fgure 1. Desaturaton Detecton rcut for APLJ and APL1J. 1 IN IN LED 1 BLANK, (0pF 0 Ω D 1 LED1 LED1 1 Q HD PHASE A HD Fgure. Desaturaton Detecton rcut for HPLJ

3 . Basc detector crcut component selecton For typcal applcaton, the three external components requred to buld the crcut are the dode, D, resstor, R and blank capactor, BLANK. Blankng Tme The fault detecton crcutry should reman dsabled for a short tme perod followng the turnon of the IGBT to allow the collector voltage to fall below the threshold. The tme perod, called the blankng tme ensures that there s no nusance trppng durng IGBT turnon. Ths tme also represents the tme t takes for the drver to a fault condton. The blankng tme s controlled by nternal charge current, I HG of 0mA (typ, the voltage threshold, and the external blank capactor, BLANK. Durng operaton, blank capactor s dscharged when drver output s low (IGBT off. That s, the detecton features becomes actve only when the output of the gate drver optocoupler s n the hgh state drvng the IGBT n saturaton. When the IGBT s turned on, the capactor starts chargng and protecton becomes effectve only f the threshold s exceeded after the blankng tme. Blankng Tme apactor Szng Blankng tme s determned usng formula (1: t = BLANK BLANK * I HG The recommended value s 0pF whch gves a blank tme of.µs (ondton: I HG = 0mA and =.; Page of HPLJ datasheet A0EN. H Blockng Dode and Threshold The dode functon s to conduct forward current, allowng sensng of IGBT s SAT. In hgh power applcaton, pn may be pulled low due to reverse recovery spkes of the freewheelng dode. Ths reverse recovery spke tend to forward bas the substrate dode (1 of HPLJ, whch may respond by generatng false detecton sgnal. In order to mnmze ths chargng current and avod false trggerng, t s best to use very fast reverse recovery tme dodes wth very small reverse parastc capactance. Lsted n the table below are fastrecovery dodes that are sutable for use as a dode, D. The detecton threshold voltage of (typcal can be reduce by placng a strng of dodes n seres or place a low voltage zener dode n seres. For the strng of dode method, =.0 n * ( New Threshold F ( For the dode wth Zener Dode method, =.0 ( New Threshold F Z where n s the number of dodes, z s the zener voltage value and f s the forward votlage of dode. Ths allow the desgner to choose the approprate threshold voltage. Resstor The antparallel dode of the IGBT can have a large nstantaneous forward voltage transent whch exceeds the nomnal forward voltage of the dode. Ths may result n large negatve voltage spke on the pn whch wll draw a substantal amount of current out of the drver. To lmt the current level drawn from the gate drver, a resstor can be added (0Ω recommended n seres wth the dode. The added resstor wll not apprecably alter the threshold or the blankng tme. Output Pn The pn (Pn of APLJ/1J and Pn of HPLJ s an open collector output and requres a pullup resstor, RF (.1kΩ for APLJ and 1J,. kω for HPLJ to provde a hgh level sgnal. In order to prevent the pn from beng trggered by hgh MR nose, a flter capactor, F s ncluded between pn and ground (Fgure 1. ( Part Number Manufacturer Trr (ns Max. Reverse oltage Ratng, RRM (olts Package Type ERA Fuj Semconductor 1 00 Axal Leaded MUR00E Motorola 00 0 (Axal leaded UF00 General Sem. 00 DO0AL (Axal leaded BYME Phlps 00 SOD (axal leaded BYE Phlps 00 SOD (axsal leaded BY Phlps 00 SOD (surface mount MURS0T Motorola 00 ase 0A (Surface Mt

4 . Advanced desaturaton detecton topc Internal hargng urrent Source Wde araton, I HG The blankng capactor charge current parameter n the data sheet (page of HPLJ datasheet, ts values are lsted as: Blankng apactor Mn Typ Max Unts hargng urrent, I HG 0 0 ma Based on desaturaton voltage threshold and the above chargng current, we wll get three dfferent blankng tme; mnmum, typcal and maxmum value. I HG = * t ( Usng the above formula, the I HG, BLANK = 0pF (Fgure 1 and =, we wll have the followng blankng tme, 0 pf * t (max = =. A 0 pf * t (typ = =. 0 A 0 pf * t (mn = =.1 0 A For some applcaton, ths varaton may not be a problem. However, to mnmze the above varaton, several external blankng crcuts are suggested n ths Applcaton Note. These are shown n fgures, and. Prevent False Fault Detecton Due to Negatve oltage Spkes durng Power Semconductor Swtchng Operaton One of the stuatons that may cause the drver to generate a false fault sgnal s f the substrate dode of the drver s forward based. Ths can happen f the reverse recovery spkes comng from the IGBT free wheelng dodes brng the pn below ground. Hence the pn voltage wll be brought above the threshold voltage. Ths negatve gong voltage spkes s typcally generated by nductve loads or reverse recovery spkes of the IGBT/MOSFETs freewheelng dodes. In order to prevent false fault sgnal, t s hghly recommended to connect a zener dode and schottky dode across pn and E pn (e.g. for HPLJ, between pn and. Ths crcut soluton s shown n Fgure. The schottky dode wll prevent the substrate dode of the gate drver optocoupler from beng forward based whle the zener dode (alue around. to s used to prevent any postve hgh transent voltage to affect the pn. µa µa µa Other Methods of Tweakng detecton Blankng tme Besdes the recommended crcut to adjust blankng tme n Fgure 1 and, two other methods are ntroduced n ths applcaton note. The frst method shown n Fgure and uses addtonal capactors, resstor and a FET. The second smpler method shown n Fgure requres only one addtonal resstor plus scalng of the blankng capactor, BLANK. The fgures show how ths can be connected usng HPLJ. Ths crcutry s applcable wth other smlar drvers lke APLJ. In Fgure and, the blankng tme s controller by wth tme constant adjusted by capactor value of 0pF and resstor 1kohm. Desgners may choose to adjust ths value accordng to ther desred blankng tme. The * tme constant wth 0pF and 1 kω provde for a blankng tme of. µs. Fgure shows another concept for an external blankng crcut. Ths method uses one addtonal external resstor RB connected from the output to the pn. Ths allows an addtonal blankng capactor chargng current component from the output of the gate drver optocoupler through RB and adds to the nternal current source of the gate drve optocoupler. Ths hgher blankng capactor chargng current allows a desgner greater flexblty n choosng both an approprate value of the blankng capactor and an approprate current through a choce of the external resstor RB. By adjustng the capactance of the blankng capactor and the addtonal current through RB a desgner can set a specfc precse blankng tme, and an example calculaton of the blankng tme s shown below: ( t = (1 e ondtons are = = 1 R B = 00kohm BLANK = 00pF At t= 0, At t= nfnty, ( 0 = = f ( = = = 1 f t

5 Hence, ( t = (1 e (t = (1 e where t = tblank and c(tblank = (base of HPLJ threshold voltage t = BLANK = (1 = e f t t e t BLANK = 1 = 0. t BLANK = ln(0. Wth R B = 00, BLANK = 00pF t BLANK =. µs 1 IN IN LED 1 0pF 0 Ω D 1 LED1 LED1 Protecton on Pn Zener 1NA 1 0pF MBR00 Schottky Dode 0pF 1 R1 1kΩ 0.1uF TMOS N00LT1 External Blankng rcut Q HD PHASE A HD Fgure. External blankng crcut wth.µs nomnal blankng delay usng HPLJ

6 1 IN IN LED 1 0pF 0 Ω D 1 LED1 LED1 Protecton on Pn 1 0pF. Ω 0.1uF TMOS N00LT1 nf. Ω Q HD PHASE A HD Zener 1NA MBR00 Schottky Dode 0pF 1 R1 1kΩ External Blankng rcut Fgure. External blankng crcut wth external buffer for hgh current drve usng HPLJ Blankng rcut 1 IN IN LED 1 00pF 0 Ω D 1 LED1 LED uF R B = 1kΩ Q HD PHASE A HD Fgure. Second Method of External Blankng rcut usng HPLJ For product nformaton and a complete lst of dstrbutors, please go to our web ste: Avago, Avago Technologes, and the A logo are trademarks of Avago Technologes, Lmted n the Unted States and other countres. Data subject to change. opyrght 00 Avago Technologes Lmted. All rghts reserved. A00EN June 1, 00