Exclusive Technology Feature. A Practical Primer On Motor Drives (Part 8): Power Semiconductors. Drives And Other Power Converters

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1 A Practical Primer On Mtr Drives (Part 8): Pwer Semicnductrs by Ken Jhnsn, Teledyne LeCry, Chestnut Ridge, N.Y. ISSUE: September 2016 The preceding parts f this article series have discussed the varius winding cnfiguratins assciated with single-phase and three-phase electrical systems, defined the assciated electrical parameters (vltage, current and pwer) and demnstrated hw t measure these parameters using Teledyne LeCry s Mtr Drive Analyzer. These definitins and measurements are essential knwledge fr evaluating a mtr drive s perfrmance. Many f the measurements discussed were perfrmed at the input t a mtr drive. Here in part 8, we begin t lk inside the mtr drive t understand its peratin by intrducing the pwer semicnductrs (als referred t as pwer switches) that cntrl the flw f pwer t the drive. This sectin n pwer semicnductr device physics will be a review fr mst pwer electrnics engineers. Hwever, fr thse new t the mtr drive field, this sectin lays the grundwrk fr a discussin f the pwer cnversin tplgies and circuits discussed in subsequent parts f this series. Drives And Other Pwer Cnverters Pwer cnversin is the cnversin f electric pwer frm ne frm f pwer (ac r dc) t anther (ac r dc), frm ne vltage t anther, r frm ne frequency t anther, r sme cmbinatin f these cnversins. This primer is mainly cncerned with ne very specific type f pwer cnverter the mtr drive. Typically, the term drive refers t any pwer cnverter that transfrms ne ac r dc line vltage t a different ac vltage r frequency. Mtr drives, variable frequency drives, variable speed drives, and inverter drives are all different ways t say the same thing. Drives are smetimes used in nn-mtr applicatins such as cnverting the vltage r frequency prduced by a wind turbine. In additin t drives, there are pwer cnverters that perfrm ac-dc, dc-ac and dc-dc cnversins as utlined in the table belw. While these pwer cnverter categries are beynd the scpe f this primer, there are many design techniques and issues that are cmmn acrss the different pwer cnverter categries. Therefre much f the material discussed in this primer will be relevant t designers f ther pwer cnverter types. That includes the discussin that fllws n pwer semicnductrs, which are the basic building blcks f any pwer cnversin and drive system. Table. Pwer cnverter types. Type Functin Examples Ac-ac Cnversin f ac line vltage t a different ac vltage r frequency. This is cmmnly referred t as a drive. Mtr drives, variable frequency drives (VFDs), variable speed drives (VSDs) and inverter drives. Ac-dc Dc-ac Dc-dc Cnversin f ac line vltage t a specified dc vltage. Cmmnly referred t as a cnverter, pwer supply r ac adapter. Cnversin f dc vltage t a specified ac vltage and frequency. Cmmnly referred t as an inverter. Cnversin f dc vltage t a different specified dc vltage. Cmmnly referred t as a dc-dc cnverter. Internal and external pwer supplies fr cmputing and ther electrnic equipment. Inverters used in autmtive and ther mbile applicatins as well as slar pwer inverters. These culd als be mtr drives (e.g., a battery-pwered drill r electric vehicle prpulsin mtr drive). Includes islated and nnislated dc-dc cnverters used in cmputing, telecm, netwrking and many ther applicatins Hw2Pwer. All rights reserved. Page 1 f 8

2 Fr ur purpses, pwer cnversin invlves use f fast pwer semicnductr devices as switching devices t enable efficient cnversin. In mst applicatins, these devices run at typical switching frequencies frm 1 t 100 khz. In this cntext, we d nt cnsider a 50-/60-Hz cre/cil device such as a line-frequency stepup r stepdwn transfrmer t be a pwer cnversin device. Pwer Semicnductr Device Operatin One may cnsider a pwer semicnductr device, as used in a pwer cnversin system, as a very fast switch with the fllwing characteristics: A rated withstand (blcking) vltage that is the maximum pen-circuit vltage A current-carrying capability Lw lsses when carrying current (lw frward-vltage drp, r lw resistance) A fast switching capability (typically measured in the tens r hundreds f kilhertz). Fig. 1a shws an insulated-gate biplar transistr (IGBT) with terminal ntatins. A metal-xide semicnductr field-effect transistr (MOSFET) is similar, but has different terminal ntatins, which are shwn in Fig. 1b. (a) (b) Fig. 1. An IGBT symbl (a) and a MOSFET symbl (b) and sme f their assciated characteristics Hw2Pwer. All rights reserved. Page 2 f 8

3 Applying a vltage frm the gate t an IGBT s emitter (V GE ), r, in a MOSFET, frm the gate t the surce (V GS ) cntrls the device s switching behavir. This vltage is knwn as the gate-drive vltage. Typically, V GE = 0 V (in an IGBT) means the pwer semicnductr is nt cnducting, r is pen. In pwer cnversin applicatins, the V GE signal is a pulse-width mdulated (PWM) gate-drive signal that switches frm 0 V t sme upper vltage (typically 3 t 20 V, depending n whether is it is an IGBT r MOSFET, is Si, SiC r GaN, and the intended applicatin). When the V GE vltage is 0 V, the pwer semicnductr is pen frm the cllectr (C) t the emitter (E). When it reaches a switching threshld, it cnducts (i.e., the switch clses) and cnducts a high level f current at the defined biased dc bus vltage. Fig. 2 shws the gate-drive PWM signal as applied at the gate terminal. Fig. 2. Gate drive signal fr an IGBT. Nte that the pwer semicnductr device, as implemented in a pwer-cnversin design, is nt referenced t grund ptential. The gate-drive signal (V GE ) and the cllectr-emitter signal (V CE ) are therefre flating at half r full dc bus vltage (depending n the design), and this culd be several hundred vlts. Therefre, take care when prbing these signals with an scillscpe and a prbe that has a grund reference (e.g., a typical nn-islated scillscpe and a typical passive prbe). Fr safety s sake, ensure prvisin f islatin at up t the flating vltage rating in either the scillscpe r the prbe. A high-vltage differential prbe prvides such islatin. While sme lw-vltage pwer cnversin designs (<50 V) may be safely prbed with an rdinary passive prbe and a nn-islated scillscpe, a highvltage differential prbe may still be desirable fr ther in-circuit measurements, such as line-t-line vltage ac input r line-t-line drive utput prbing. When the gate-drive signal is high, current cnducts thrugh the pwer semicnductr. When the gate-drive signal is lw, the pwer semicnductr switch pens, blcking current flw. If the pwer semicnductr device in this example is supplied with a 170-Vdc supply vltage (derived frm a 120-Vac single-phase full-wave rectified and filtered line vltage) acrss V CE, then the lw-vltage PWM gatedrive signal creates a higher amplitude PWM signal that switches frm 0 t 170 Vdc at a switching frequency in the kilhertz range. The gate-drive signal s PWM mdulatin is determined thrugh a variety f different mdulatin algrithms prgrammed int a cntrl system. The PWM gate-drive signal turns the device n, the device delivers current at the rated dc bus vltage, and then the PWM gate-drive signal turns the device ff. The device s utput vltage is linearly related t the dc bus vltage and the gate-drive pulse width duratin (r duty cycle), and the device s utput frequency is related t the duty-cycle crssver pint (duty cycle reducing t 0%, and then increasing again). If this PWM device utput signal were filtered t remve the higher harmnics, the fundamental (rectified) sinewave wuld be apparent. See Fig. 3 belw fr an example f the utput PWM and a representative mdulating sinusidal wavefrm fr a single pwer semicnductr device cntrlled as described abve Hw2Pwer. All rights reserved. Page 3 f 8

4 Fig. 3. PWM signal using single pwer semicnductr with mdulating sinewave signal. Practically speaking and fr varius reasns, the utput PWM fundamental sinewave s (r rectified fundamental sinewave s) peak vltage never appraches the full dc bus amplitude. Fr the PWM signal, the peak vltage amplitude is typically 85% (r less) f the dc bus vltage, depending n the mdulatin and cntrl scheme. Fr the fundamental ac, the peak amplitude wuld be much less than shwn in Fig. 3 abve. N-Channel And P-Channel Devices Cntrl ver the pwer semicnductr s electrical prperties is achieved via additin f dpant impurity materials t the base semicnductr material (e.g., silicn). These additives cause the semicnductr t cntain an excess f free current carrying charge carriers. When dped with a material that supplies mre free charge-carrying electrns, the semicnductr becmes n-type. When dped with a material that supplies mre free charge-carrying hles, it becmes p-type. The charge-carrying material defines the majrity carrier r channel f the pwer semicnductr, and the directin f current flw: thus, we refer t devices as either n-channel r p-channel. Hwever, when we emply MOSFETS in pwer cnversin applicatins, the lwer n-resistance and imprved efficiency f n-channel devices makes them a heavy favrite ver p-channel devices. Fr IGBTs, p-channel (minrity-carrier) devices are the mre efficient f the tw. Pwer-cnversin designs are simpler when using a mix f n-channel and p-channel devices, based n circuit lcatin and activity, but it is typically desirable t trade ff design cmplexity fr greater verall efficiency by using just ne type f device in the entire design. Pwer Semicnductr Device Materials Pwer semicnductrs are made with silicn (Si), r, mre recently, a wide bandgap material such as silicn carbide (SiC) r gallium nitride (GaN). The term bandgap refers t the energy between the cnductin and valence bands f the semicnductr material, which is the energy required t generate electrn and hle mvement. Wider bandgap devices impart useful attributes t a pwer cnversin design, but add cst, design cmplexity, and EMI/RFI issues. They als may decrease system reliability. Traditinally, pwer semicnductr devices have been based n Si. Hwever, Si has limited blcking vltage when deplyed in a (majrity-carrier) MOSFET, and R DS(ON) increases with blcking vltage, making MOSFETs mstly unsuitable fr 600-V applicatins. This generally relegates MOSFETs t 300-V applicatins. In (minrity-carrier) IGBTs, the blcking vltage is higher (1200 V r mre) with an apprximately cnstant nstate vltage, but there are significant switching lsses and high tail and reverse-recvery currents after switching, which reduces the cnverter s efficiency. This restricts IGBTs mstly t 600-V class equipment applicatins. Cmpared t Si, wide-bandgap materials prvide higher (breakdwn) vltage ratings, faster switching speeds, lwer leakage currents at high temperatures, and lwer thermal resistance (fr SiC). Faster raw device switching speeds translate t rise times in the lw-nansecnd range, thugh deplyment in pwer cnversin 2016 Hw2Pwer. All rights reserved. Page 4 f 8

5 devices are at much slwer speeds t limit harmnics issues and reduce the risk f sht-thrugh (shrt circuiting, which leads t device failure). The high blcking vltage and higher-temperature perfrmance prvides fr better reliability and mre cmpact designs and smaller heat sinks. Higher switching frequencies reduce lsses during switching and reduce the size f dc bus/link filter cmpnents (e.g., capacitrs and inductrs). Thus, pwer cnversin systems that use wide bandgap pwer semicnductrs have reduced weight, higher pwer density (due t smaller size) and higher efficiencies. Design challenges include mitigatin f higher manufacturing cst, lack f lng-term data n field reliability, a small knwledge base fr implementatin, and mre parasitic and EMI effects in bard layut. SiC and GaN are the tw wide-bandgap materials seeing cmmercial usage in MOSFETs and IGBTs. Cmpared t Si, SiC has 10x the blcking vltage, lwer n-resistance, higher-temperature perfrmance, and greater inherent cling. We nw see mre use f SiC in IGBTs, which makes pssible 15-kV devices fr utility applicatins, pening new applicatins fr slid-state distributin transfrmers and ther utility grid-cnnected equipment in the 15-kV class. GaN has perfrmance similar t SiC, but breakdwn vltages that will likely limit it t <600-V applicatins. With the advent f wide-bandgap materials, rise times are becming faster and switching lsses are falling in pwer cnversin designs. Hwever, faster rise times increase the risk f catastrphic failure and prduce mre EMI/RFI emissins. In general, the wide bandgap materials faster rise times impact the switching and cnductin-lss measurements fr individual devices mre than the verall drive system utput. While it is helpful t understand fully a pwer semicnductr device s capabilities, it may ften be prudent nt t push these devices t their limits in a pwer cnversin system fr reasns f cst, reliability, and emissins. Pwer Semicnductr Device Types One may cnstruct a pwer cnversin circuit using a variety f pwer semicnductr device types, depending n the input/utput vltages, surge pwer ratings, cntinuus pwer ratings, and applicatin. The main types f pwer semicnductr devices in use tday are: Pwer metal-xide-semicnductr field-effect transistr (pwer MOSFET) Insulated-gate biplar transistr (IGBT) High-vltage (HV) IGBT Insulated-gate cmmutated thyristr (IGCT) r gate-turn-n thyristr (GTO) Silicn-cntrlled rectifier (SCR, r thyristr). Fig. 4 illustrates the tradeffs in switching frequency, breakdwn vltage, and current-carrying capability between the different silicn pwer semicnductr devices. In the cntext f pwer cnversin, all f these devices may alternatively be described as pwer switches Hw2Pwer. All rights reserved. Page 5 f 8

6 Fig. 4. Cmparing breakdwn vltage, switching frequency and current-carrying capability f silicn pwer switches. Wide-bandgap materials (SiC and GaN) are attractive because they expand the applicatin range f MOSFETs and IGBTs and incur lwer switching lsses (if implemented at maximum capabilities). As a result, widebandgap devices can alter the tradeffs shwn abve. Pwer MOSFETs Pwer MOSFETs made f silicn typically have blcking vltage ratings up t 600 V and are used fr switching vltages at rughly 300 V r less, especially with inductive lads that can impse large vervltage cnditins (up t 2x). Althugh current-carrying capability is in the tens f amps, paralleling devices makes fr higher currents. These devices exhibit high switching frequencies (>100 khz), high efficiencies (especially at lw pwer levels), and reasnable csts. Typical applicatins are in 120-/240-V switch-mde pwer supplies, lighting ballasts, dc-dc cnverters, and lwvltage (<50 V) r lw-pwer 120-/240-V mtr drives. The upper end f the vltage range fr pwer MOSFET use is likely at abut 500 V in slar phtvltaic (PV) inverter applicatins. Pwer MOSFETs have a surce and a drain that are cnnected t individually and highly dped regins that are separated by the bdy regin. Current flw is acrss the drain and surce when the gate-surce vltage (V GS ) is crrectly biased. MOSFETs may be n-channel r p-channel. In an n-channel MOSFET, the surce and drain are n+ regins and the bdy is a p regin; the ppsite is true fr a p-channel MOSFET. This defines the directin f current flw when the device is biased. MOSFETs may be either enhancement-mde r depletin-mde types. Enhancement-mde MOSFETs are nrmally ff at zer gate-surce vltage (V GS = 0 V), whereas depletin-mde MOSFETs are nrmally n at V GS = 0 V. Mst MOSFETs used in pwer cnversin devices are n-channel (majrity carrier) enhancement-mde MOSFETs because n-channel MOSFETs have a third f the n-resistance f p-channel MOSFETs and are thus mre efficient. Fig. 5 shws the electrical symbls fr n- and p-channel enhancement-mde MOSFETs Hw2Pwer. All rights reserved. Page 6 f 8

7 Fig. 5. MOSFET symbls. IGBTs IGBTs have blcking vltage ratings f ~1200 V and switching frequencies f 1.5 t 10 khz (in silicn). Highvltage (HV) IGBTs can reach up t 6000-V blcking vltage (typically implemented in designs at half this vltage fr reliability reasns). Lwer-vltage IGBTs can be cascaded t achieve similar ratings (thugh with higher design cmplexity). Current-carrying capability is in the hundreds f amperes. Due t their higher blcking vltage and ruggedness cmpared t MOSFETs, ne ften finds IGBTs in 600-V class mtr drives, prpulsin ( tractin ) mtr drives, uninterruptible pwer supplies (UPSs), and welding systems. In pwer cnversin applicatins, IGBTs are minrity-carrier (p-channel) devices. Fig. 6 shws the electrical symbl fr a p-channel (minrity-carrier) IGBT. Fig. 6. P-channel IGBT symbl. Nte that in an IGBT, the surce and drain are knwn as a cllectr (C) and emitter (E). IGCTs, GTOs, And SCRs Histrically, IGCTs and GTOs have been the preferred pwer semicnductr fr medium-vltage (2.4 kv t 7.2 kv) ac inductin mtr drives. These devices have blcking vltage ratings up t 6000 V and a simpler and mre-efficient architecture fr higher pwer ratings, but a much slwer (<1 khz) switching frequency cmpared t IGBTs. Hwever, slwer switching matches up well with higher vltages because prblems ften result frm attempts t switch high vltages at high speeds. SCRs can switch nly nce per pwer cycle they can switch n at any pint in a cycle but cannt switch ff until a zer crssing ccurs. This makes their switching frequencies very slw, making them suited fr very high-vltage transmissin and distributin applicatins, but less s fr drives and ther lwer-vltage pwer cnversin applicatins Hw2Pwer. All rights reserved. Page 7 f 8

8 Cnclusin This sectin reviewed the varius types f pwer semicnductr devices r switches that are emplyed in pwer cnverters. The principles f device peratin were explained, their capabilities were discussed, and their typical applicatins were nted. The perfrmance benefits f the newer, wide-bandgap devices versus the mre established silicn devices were als discussed. This material lays the fundatin fr a discussin f pwer stage tplgies in the next sectin (part 9) and subsequent discussins f mtr drive circuitry. Fr a full list f tpics that will be addressed in this series, see part 1. Abut The Authr Kenneth Jhnsn is a directr f marketing and prduct architect at Teledyne LeCry. He began his career in the field f high vltage test and measurement at Hiptrnics, with a fcus n <69-kV electrical apparatus ac, dc and impulse testing with a particular fcus n testing f transfrmers, inductin mtrs and generatrs. In 2000, Ken jined Teledyne LeCry as a prduct manager and has managed a wide range f scillscpe, serial data prtcl and prbe prducts. He has three patents in the area f simultaneus physical layer and prtcl analysis. His current fcus is in the fields f pwer electrnics and mtr drive test slutins, and wrks primarily in a technical marketing rle as a prduct architect fr new slutin sets in this area. Ken hlds a B.S.E.E. frm Rensselaer Plytechnic Institute. Fr further reading n mtr drives, see the Hw2Pwer Design Guide, lcate the Pwer Supply Functin categry, and click n the Mtr drives link Hw2Pwer. All rights reserved. Page 8 f 8