BSEE 3-1 Past Paper PDF

ORQUEZA, ARJIE E. (202200677) BSEE 3-1 1. DEFINE THE OPERATION OF AN INDUCTOR IN A DC CIRCUIT. IN A DC CIRCUIT, AN INDUCTOR ACTS LIKE A SHORT CIRCUIT W HEN THE CUR RENT IS CONSTANT. THIS MEANS THAT T H E C U R R E N T F L OW S T H R O U G H I T W I T H O U T A N Y V O L T A G E DROP. THE...

ORQUEZA, ARJIE E. (202200677) BSEE 3-1 1. DEFINE THE OPERATION OF AN INDUCTOR IN A DC CIRCUIT. IN A DC CIRCUIT, AN INDUCTOR ACTS LIKE A SHORT CIRCUIT W HEN THE CUR RENT IS CONSTANT. THIS MEANS THAT T H E C U R R E N T F L OW S T H R O U G H I T W I T H O U T A N Y V O L T A G E DROP. THE INDUCTOR PROVIDES NO RESISTANCE TO T HE DC CURRENT AFTER IT STABILIZES, ENABLING IT TO F L OW FREELY. THERE IS NO VOLTAGE ACROSS AN IN DUCTOR AFTER THE CURRENT IS STABLE BECAUSE OF ITS INDUCTANCE, W HI C H O P P O S E S VARIAT IONS IN CURRENT. PUT ANOTHER W AY, W HILE THE CURRE NT IS STEADY, AN IND UCTOR PERMITS DC CURRENT TO GO THROUGH W ITHOUT ANY PRO BLEMS. 2. DEFINE THE OPERATION OF A CAPACITOR IN A DC CIRCUIT. IN A DC CIRCUIT, A CAPACITOR ACTS LIKE AN OPEN CIRCUIT W HEN IT IS F ULLY CHARGED OR DISCHARGED. THIS M E A N S T H A T N O C U R R E N T F L OW S T H R O U G H I T. A S S O O N A S T H E P OW E R S O U R C E F U L L Y C H A R G E S T H E C A P A C I T O R , D C C U R R E N T N O L O N G E R F L OW S T H R O U G H I T. B E C A U S E T H E CAPACITOR’S ELECTRODE PLATES ARE SEPARAT ED BY AN INSULATOR (AIR OR A DIELECTRIC), NO DC CURRENT CAN FLOW UNLESS THE INSULATION DISINTEGRATES. IN OTHER W ORDS, A CAPACITOR B LOCKS DC CURRENT.  AS S I G N M E N T 2 1. E X P L AI N B O O S T C O N V E R T E R D P S T M E T H O D In a boost converter using the DPST method, t he operation alternates between two states. When the switch is closed, the inductor stores energy from the input voltage. When the switch opens, the inductor releases this stored energy through a diode to the output, boosting the voltage. The timing of the se states is controlled by a pulse -width m o d u l a t i o n (P W M ) s i g n a l , w h i c h r e g u l a t e s t h e d u t y c y c l e. T h e duration the switch stays closed versus open determines the voltage boost level. This process happens continuously, stepping up the input voltage to a hig her output for the load. 2. D I S C U S S T H O R O U G H L Y T H E D U T Y C Y C L E C AN B E C R E AT E D B Y A LOGIC CIRCUIT. The duty cycle in a logic circuit is generated by using a p u l s e - w i d t h m o d u l a t i o n (P W M ) t e c h n i q u e. A l o g i c c i r c u i t , s u c h as a timer or a flip-flop, can create a square wave signal, where the duty cycle is the ratio of the "on" time to the total period of the signal. This cycle is adjusted by controlling the t i m i n g o f t h e h i g h (1 ) a n d l o w (0 ) s t a t e s , w h i c h i n t u r n c o n t r o l s the power delivered to a load. O U T P U T R E S E AR C H 1. C H AR AC T E R I S TI C S AN D C O M P O S I T I O N O F T H E F F. S E M I C O N D U C T O R DEVI CES B i p o l a r J u n c t i o n T r a n s i s t o r (B J T ) C u r r e n t - c o n t r o l l e d d e v i c e w i t h t h r e e r e g i o n s (e m i t t e r , b a s e , collector). Operates in active, saturation, and cutoff modes. Made of p-n-p or n-p-n semiconductor layers. F i e l d E f f e c t T r a n s i s t o r (F E T ) Voltage-controlled, with high input impedance and fast s w i t c h i n g. H a s t h r e e t e r m i n a l s (s o u r c e , d r a i n , g a t e ) , and comes in JFET or MOSFET types. MOSFET A type of FET with high switching speed and efficiency, controlled by gate voltage, isolated by an oxide layer. Used in analog and digital circuits.  I n s u l a t e d G a t e B i p o l a r T r a n s i s t o r (I G B T ) Combines MOSFET’s high input impedance with BJT’s low saturation voltage. Ideal for high -voltage/current applications. 2. B AS E O N T H O S E S E M I C O N D U C T O R COMPONENTS GI VE THE S WI T C HI N G R E AL I Z A T I O N. BJT - Switches with base current control; moderate speed. FET/MOSFET - Voltage at the gate controls switching; very fast. IGBT - Gate voltage controls switching, handling larger currents, perfect for power applications. R E F E R E N C E : “ F u n da m e n t al s o f P o w e r E l e c tr o n i c s ” b y R ob e r t W. E r i c k s o n a n d D r a ga n M a k s i m ov i c (2 0 0 1 )  A S S I G NM E N T 3 C H A P T E R 4 : S W I T CH R E A L I Z A T I O N 1. W h a t is sw i t c h r e a l i z a t i o n ? Switch realization refers to the p r o c e ss of i m p l em e n t i n g and designing s w i t c h i ng elements within power electronic circuits. These s w i t c h i n g e l e m e n t s c a n b e t r a n s i s tor s , t hy r i s to r s , o r d i o d e s , w h i ch a r e u s e d t o c o n t r o l t he f l o w o f e l e c tr i c al e n e r g y. T h e g o a l i s t o e f f i c i e n t l y convert p o w er by turning on and off the s e m i c o nd u c t o r devices at specific i n t e r v a l s, allowing fo r e ne r g y transfer w i th minimal lo ss e s. S w i t c h r e a l i z a t i o n e n s u r e s t h e p r o p e r f u n c t i o n i n g o f c i r c u i t s s uc h a s DC - D C co n v e r t e r s, i n v er t e r s, a n d r e c t i f i e r s. 4. 1 S W I T CH A P P L IC AT I O N S 1. W h a t ar e t h e sw it c h a p p l i c a t i on s ? S w i t c h a p p l i c a t i o ns i n v o l v e t h e u s e o f s e m i c on d u c t or d e v i c e s as switches in v a r i o us power electronics c ir c u i t s, s u ch as c o n v e r t e r s, i n v e r t e r s , a n d r e c t if i e r s , w h e r e t h e y c o n t r o l t h e f l o w an d c o n v e r s io n o f e l e c t r i c a l e n e r g y. Power s w i tc h e s ar e essential in powe r e l e c t r o n i c s, managing e l e c t r i c a l p o w e r f lo w i n c i r c u i t s. T h e y e n h a n c e e f f i c i e n c y , r e d u c e e n e r g y l o ss e s , a n d im p r o ve r e l i a b i l i t y. Co mm o n t y p e s i nc l u d e t h y r i s to r s (SCRs a n d T r i a c s ), p o w e r M O S F E T s , I G B T s , a n d p o w e r B J T s, w it h a p p l i c a t i o n s i n power s u p p l i e s, mo t o r dr i v e s , renewable energy systems, and e l e c t r ic vehicles. A C s w i t ch e s m a n a ge a l t e r n a t i n g c u r r e n t c i r c u i t s, co n t r o l l i n g po w er f l o w , p r o v i d i n g o v e r c u r r e n t p r o t e c t i on, a n d f a c i l i t a t i n g e f f i c i e n t t r a n s f e r. Examples include T r i a c s, t h y r is t or s , IGBTs, and r e l ay - b a s e d s w i t c h e s, c o mm o n ly u s e d i n l i g h t i n g c o n tr o l, mo t o r s t a r t e r s , a n d p o w e r d i s tr i b u t i o n s y s t e m s.  DC s w i t c h es control d ir e c t c ur r e n t c i r c u i ts , p e r fo r mi n g s i m i l ar functions such as power f lo w m a n ag e m e n t and o v e r cu r r e n t pr o t e c t i o n. P o w e r M O S F E T s, I G B T s , B J Ts , a n d F E T - b a s e d s w i t c h e s a r e p r e v a l e n t , w i t h applications in DC power s u p p l i e s, m o t or d r i v e s, and automotive e l e c t r o n i cs. H i g h - f r e q u e n c y s w it c h e s a r e d e s i g n e d f o r a p p l i c a t io n s r e q u i r i n g f a s t s w i t c h i n g , lo w l o s se s , a n d r e l i a b i l i t y , s u c h a s i n s w i t c h i n g p o w e r s u p p l i e s a n d R F c i r c u i t s. E x a m p l e s i n c l u d e G a N t r a n s i s to r s , S iC M O S F E T s, h i g h - f r e q u e n c y I G B T s , an d R F F E T s w i t c h e s , e s s e n t i a l fo r RF a m p l i f i e r s a n d m i c r o w a v e c ir c u i t s. Soft switches m in i m i z e switching l o ss e s and e l e c tr o m a g n e t i c interference in power c i r cu i t s through techniques like z e r o - vo l t a g e switching (ZVS) an d zero-current switching ( Z C S). Examples include r e s o n a n t, q u a s i - r e so n a n t , a n d m u l t i - r e s o n a n t s w i t c h e s, a s w e l l a s s o f t - s w i t c h i n g D C - DC c o n v e r t e r s, u s e d i n h i g h - e f f i c i e n c y p o w e r s u p p l i e s a n d r e n e w a b l e e n e r g y sy s t e m s. S o l i d- s t a t e s w i t c he s replace electromechanical r e l a ys , offering higher reliability, faster switching, and lower maintenance. C o m mo n e x a m p l e s i n c l u d e th y r i s to r - b a s e d , pow e r M O S F E T - b a s e d, a n d I G B T - b as e d s w i t c h e s , w i d e l y u se d i n m o t o r c o n t r ol , p o w e r d i s t r i b u t ion , a n d i n d u s t r i a l a u t o m a t i o n. Bidirectional s w i t ch e s control c u r r en t flow in both d i r e c t i o n s, m a k i n g t h e m s u i t ab l e f o r reversible power f l o w a p p l ic a t i o n s. Examples i n c l u d e b i d i r e c t i o n al t h y r i s to r s , fo u r - q u a d r a n t I G B T s , b id i r e c t i o n a l DC - DC c o n v e r t e r s , a n d m at r i x c o n v e r t er s , w i th a p p l i c a t i o n s i n e l e c t r i c v e h i c l e s, r e n e w a b l e e n e r g y sy s t e m s , a n d po w e r g r i d i n t e r f a c e s. 4. 2 A B R I E F S U R V EY O F PO W E R S EM I CO N D UC T O R D E V I C E S 1. C l a ss i f y e a c h s em i c o n d u c to r d e v i c e t o w h i c h s w i t c h a p p l i c a t i o n i t b e l o n g s ac c o r d i n g t o i t s c h ar a c t e r i s t ic s. I n p o w e r s w i tc h e s , P o w e r M O S F E T s a n d I G B T s e x c e l in h i g h - p o w e r h a n d l i n g a n d h i g h - s p e e d s w i t c h i n g , w h i l e t h y r i s t or s ( SC Rs a n d T r i a c s ) a r e better suited for h i g h - p o w er , low-frequency a p p l i c a ti o n s. P o w er B J Ts a l s o h a n d l e h i g h po w e r b u t o p er a t e a t l o w er s p e e d s. F o r h i g h - f r e q u e n c y a p p l i c a t i o n s , G a N t r a n s i s t or s a n d Si C M O S F E T s o p e r a t e e f f i c i e n t l y w i t h m i n i m a l p o w e r l o ss. H i g h - f r e q u en c y I G B T s a n d R F MOSFETs ar e a ls o designed fo r high -frequency use, o p t i m i z i ng p e r f o r m an c e i n R F a p p l i c a t i o n s. In AC sw i t c h e s , Tr i a c s enable b i d ir e c t i o n a l c ur r e n t fl o w at low f r e q u e n c i e s , w h i l e I G B T - b a s e d A C s w i tc h e s o f f e r h i g h pow e r h a n d l i n g a n d s p e e d. S C R s ar e ut i l i z e d f o r t h e i r h ig h p o w e r h a n d l i n g i n l o w - f r e q u e n c y A C a p p l i c a t io n s. D C s w i t c h e s i n c lu de P o w e r M O S F E T s f o r h i g h - s p e e d s w i t c h i n g w i t h l o w - vo l t a g e d r o p, w h i l e B J T s a r e s l ow e r a n d h a n d l e l e s s p o w e r. D C - DC c o n v e r t e r s w i t c h e s, l i k e b u c k a n d boo s t co n v e r t e r s, p r o v i d e h i g h - s p e e d s w i t c h i n g a n d e n h an c e e f f i c i e n c y. Bidirectional s w i t ch e s , such as b id i r e c t i o n a l t h y r is to r s, al l o w c u r r e n t f lo w i n b o t h d i r e c t i o ns b u t o p e r a t e a t l o w f r e q u e n c i e s. F o u r - q u a d r a n t I G B T s w i tc h e s p r o v i d e b i d i r e c t i o n a l cu r r e n t f l ow w i t h h i g h po w er handling, and m a tr i x co n v e r t e r s m an a g e h i g h - p o w er applications with r e v e r s i b l e c ur r e n t fl o w. Finally, so f t s w i t ch e s reduce power l o ss through techniques like z e r o - vo l t a g e s w i t ch i n g ( Z V S ) a n d z e r o - c ur r e n t s w i tc h i n g ( Z C S ). R e s o n a n t, q u a s i - r e so n a n t, a n d m u l t i - r e so n a n t s w i t c h e s h i g h l i g h t t h e t r e n d t o w a r d e f f i c i e n c y i n p o w e r p e r f o r m a n c e w i t h i n s o f t s w i t ch i n g a p pl i c a t i o n s. C H A P T E R 5 : T H E D IS CO N T IN O US C O N D UC T I O N M O D E 5. 1 O R I G I N O F T H E D I S CO N T IN O US C O ND U C T I O N M O D E A N D M O D E BOUNDARY D i s c o n t i n uo u s C o n du c t i o n M o d e ( D CM ) i n D C - D C c o n v e r t e r s o c cu r s when the inductor current drops to zero during part of the switching p e r i o d. T h i s t y p i c al l y h a p p e n s w h e n t h e o u t p u t c u r r e n t i s l o w , t h e d u t y c y c l e i s s m a l l, o r t h e i n d u c t a n c e i s l o w. D u r i n g D CM , a f t e r t h e s w i t c h t u r n s o f f , t h e i n d uc t o r c ur r e n t s t a y s a t z e r o u n t i l t h e ne x t c y c l e b e g i n s. The transition p o in t between C o n t i nu o u s C o n d uc t i o n Mode (C C M ) a n d D C M i s c al l e d t h e M o d e Bo u n d a r y , i n f l u e n c e d b y f ac t o r s l ik e o u t p ut l o a d r e s i s t a n c e, i nd u c t a n c e , s w i t c h i n g f r e q u e n c y , a n d d ut y c y c l e. 5. 2 AN A L Y S I S O F TH E CO N V E RS I O N R AT I O M , ( D , K ) The c o n v e r s io n r a ti o M ( D, K ) in DC-DC c o n v e r t er s represents the r e l a t i o n s h i p b e t w e e n t h e o u t p u t v ol t a g e , V o u t , a n d t h e i n p u t v o l t a g e , V in , a n d i t i s i n f l u e nc e d b y t h e d u t y c y c l e D a n d t h e t r a n s f or m e r t u r ns r a t i o , K. Th e d u t y c y c l e , D , is t h e f r a c t i o n o f t h e s w i t ch i n g p e r i o d d u r i ng which the power switch is o n, w h i le K accounts for the t r a n s f o r m er e f f e c t i n i s o l a t e d c o n v e r t e r s l ik e t h e f l y b a ck co n v e r t e r.  F o r i d e a l c o n v er t e r s , t h e co n v e r s io n r a t i o d e p e n d s o n t h e s p e c i f ic t o p o lo g y : Buck converter: M (D)=D The output vo l t a ge is d i r e c tl y pr op o r t i on a l to the duty c y c l e, m e a n i n g t h a t w h e n t h e s w i t c h i s o n f o r a l on g e r p or t i o n o f t h e c y c l e, t h e o u t p u t vo l t a g e i n c r e a s e s. B o o s t co n v e r t e r : M ( D ) = 1 / (1 - D) In this case, as the duty cycle i nc r e a s e s , the o u t pu t voltage b e c o m e s h i g h e r t h a n t h e i n p u t v o l t a g e , m a k i n g t h e b o o st c o n v e r t e r i d e a l f o r s t e p - u p a p p l ica t i o n s. B u c k - Boo s t c o n v e r te r : M ( D )= − D/ ( 1 − D ) T h i s t o p o lo g y i n v e r t s t h e o u t p u t v o l t a g e a n d c a n b o t h s t e p u p o r step down depending on the duty cycle, producing a negative output voltage. F l y b a c k c o n v e r t er : M ( D , K )= ( D / (1 − D )) ( K ) In the flyback t o p o l o g y, the tu rns ratio K of the transformer modifies the output voltage. Increasing K (the ratio of primary to secondary w i n d i n gs) a l lo w s f or gr e a t e r voltage step -up or s t e p - do w n capability. N o n- i d e a l i t i e s A f f e c t i n g M (D , K ) I n p r a c t i c a l c ir c u i ts , t h e c o n v er s i on r a t i o M ( D, K ) d e v i a t e s f r o m i ts i d e a l f o r m d u e to s e v e r a l no n - i d e a l i t ie s : Switching losses  Losses o c c ur d u r in g the switching t r a n s i t i o ns of the MOSFET or IGBT, reducing the overall efficiency and slightly altering the expected output voltage. I n d u c t o r r e s is t a n c e Real inductors ha v e parasitic r e s is t a n c e , which c au s e s power d i s s i p a t i o n a n d r e d u c e s t h e e f f e c t i v e v o l t a g e d e l i v e r e d to t h e o u t p u t. C a p a c i t o r E q u i v a l e nt S e r i e s R e s i s t a n c e ( E S R ) ESR in output capacitors can lead to voltage rip ple and additional p o w e r l os s e s, a f f e ct i n g t h e c o n v er t e r 's ou t p u t. T r a n s f or m er lo ss e s ( i f a p p l i c a bl e ) I n i s o l a t e d t o p o lo g i e s l i k e t h e f l y ba c k co n v e r t e r , t h e t r a n s fo r m er i n t r o d u c e s co r e l o s s e s a n d w i n d i n g r e s i s t a n c e , im p a c t i ng t h e c o n v er s io n ratio. REFERENCES: A l l A b o u t C i r c u i t s. ( n. d. ). P o w e r e l e c t r o n i c s s w i t c h e s. A l l A bo u t C i r c u i t s. https://www.allaboutcircuits.com Baliga, B. J. (2015). Switching devices for power electronics. IEEE T r a n s a c t i o n s o n I n d u s t r y A p pl i c a t i o n s , 51 ( 4 ) , 3 4 8 3 - 3 4 9 0. https://doi.org/10.1109/TIA.2015.2495141 E r i c k s o n , R. W. , & M a k s i m o v i ć , D. ( 2 0 0 1 ). F u n d a m en t a l s o f p o w e r electronics (2nd ed.). Kluwer Academic Publishers. ISBN: 978 - 0387953956 Kwon, J. M., Choi , J. H., & Lee, K. B. (2018). Power electronic switches f o r r e n e w a b l e e n e r g y s y s t e m s. I E E E T r a n s a c t i o n s o n I n du s t r i a l Electronics, 65(8), 6352-6363. https://doi.org/10.1109/TIE.2017.2772178 M o h a n , N. ( 2 0 0 2 ). P o w e r e l ec t r o n i c s : C o n v er t e r s , a pp l i c a t i o n s , a n d design (3rd ed.). Wiley. ISBN: 978 -0471745995 R a s h i d , M. H. ( 2 0 1 3 ). P o w er el e c t r o n i c s : C i r c u i t s , d e v i c es , a n d applications (4th ed.). Pearson. ISBN: 978 -0132542423 Singh, S. K., Sinha, A. K., & Rai, V. (2017). Power electronic switches: A r e v i e w. J o u r n a l o f P o w er El e c t r o n i c s , 1 7 ( 4 ) , 8 4 9 - 8 5 7. https://doi.org/10.6113/JPE.2017.17.4.849 The dc component of a converter waveform is given by its average value, or the integral over one switching period, divided by the switching period. Solution of a dc-dc converter to find its dc, or steady state, voltages and currents therefore involve averaging the waveforms. 2. The linear ripple approximation greatly simplifies the analysis. In a well designed converter, the switching ripples in the inductor currents and capacitor voltages are small compared to the respective dc components, and can be neglected. 3. The principle of inductor volt-second balance allows determination of the dc voltage components in any switching converter. In steady-state, the average voltage applied to an inductor must be zero. The principle of capacitor charge balance allows determination of the dc components of the inductor currents in a switching converter. In steadystate, the average current applied to a capacitor must be zero. 5. By knowledge of the slopes of the inductor current and capacitor voltage waveforms, the ac switching ripple magnitudes may be computed. Inductance and capacitance values can then be chosen to obtain desired ripple magnitudes. 6. In converters containing multiple-pole filters, continuous (nonpulsating) voltages and currents are applied to one or more of the inductors or capacitors. Computation of the ac switching ripple in these elements can be done using capacitor charge and/or inductor flux-linkage arguments, without use of the small-ripple approximation. 7. Converters capable of increasing (boost), decreasing (buck), and inverting the voltage polarity (buck-boost and Cuk) have been described. Converter circuits are explored more fully in a later chapter. The dc transformer model represents the primary functions of any dc-dc converter: transformation of dc voltage and current levels, ideally with 100% efficiency, and control of the conversion ratio M via the duty cycle D. This model can be easily manipulated and solved using familiar techniques of conventional circuit analysis. 2. The model can be refined to account for loss elements such as inductor winding resistance and semiconductor on-resistances and forward voltage drops. The refined model predicts the voltages, currents, and efficiency of practical nonideal converters. 3. In general, the dc equivalent circuit for a converter can be derived from the inductor volt-second balance and capacitor charge balance equations. Equivalent circuits are constructed whose loop and node equations coincide with the volt-second and charge balance equations. In converters having a pulsating input current, an additional equation is needed to model the converter input port: this equation may be obtained by averaging the converter input current. A nontrivial step: two SPST switches are not exactly equivalent to one SPDT switch _ It is possible for both SPST switches to be simultaneously ON or OFF _ Behavior of converter is then significantly modified —discontinuous conduction modes (chapter 5) _ Conducting state of SPST switch may depend on applied voltage or current —for example: diode  Chapter 4. Switch Realization 4.1. Switch applications Single-, two-, and four-quadrant switches. Synchronous rectifiers 4.2. A brief survey of power semiconductor devices Power diodes, MOSFETs, BJTs, IGBTs, and thyristors 4.3. Switching loss Transistor switching with clamped inductive load. Diode recovered charge. Stray capacitances and inductances, and ringing. Efficiency vs. switching frequency. 4.4. Summary of key points Fundamentals of Power Electronics 1 Chapter 4: Switch realization  SPST (single-pole single-throw) switches Buck converter SPST switch, with voltage and current with SPDT switch: polarities defined 1 L iL(t) + 1 2 Vg + C R V i – + v – – with two SPST switches: 0 iA A L iL(t) + vA – + – All power semiconductor Vg + vB B C R V – devices function as SPST + iB switches. – Fundamentals of Power Electronics 2 Chapter 4: Switch realization  Realization of SPDT switch using two SPST switches G A nontrivial step: two SPST switches are not exactly equivalent to one SPDT switch G It is possible for both SPST switches to be simultaneously ON or OFF G Behavior of converter is then significantly modified —discontinuous conduction modes (chapter 5) G Conducting state of SPST switch may depend on applied voltage or current —for example: diode Fundamentals of Power Electronics 3 Chapter 4: Switch realization  Quadrants of SPST switch operation 1 Switch i on state A single-quadrant + current switch example: v ON-state: i > 0 – OFF-state: v > 0 0 Switch off state voltage Fundamentals of Power Electronics 4 Chapter 4: Switch realization  Some basic switch applications switch switch on-state on-state Single- current Current- current quadrant bidirectional switch two-quadrant switch off-state voltage switch switch off-state voltage switch switch on-state on-state current current Voltage- Four- bidirectional quadrant two-quadrant switch switch off-state switch off-state switch voltage voltage Fundamentals of Power Electronics 5 Chapter 4: Switch realization  4.1.1. Single-quadrant switches 1 Active switch: Switch state is controlled exclusively i by a third terminal (control terminal). + v Passive switch: Switch state is controlled by the applied current and/or voltage at terminals 1 and 2. – SCR: A special case — turn-on transition is active, 0 while turn-off transition is passive. Single-quadrant switch: on-state i(t) and off-state v(t) are unipolar. Fundamentals of Power Electronics 6 Chapter 4: Switch realization  The diode A passive switch i Single-quadrant switch: 1 on can conduct positive on- + i state current off v v can block negative off- state voltage – provided that the intended 0 on-state and off-state operating points lie on the diode i-v characteristic, Symbol instantaneous i-v characteristic then switch can be realized using a diode Fundamentals of Power Electronics 7 Chapter 4: Switch realization  The Bipolar Junction Transistor (BJT) and the Insulated Gate Bipolar Transistor (IGBT) 1 An active switch, controlled BJT i by terminal C i + C Single-quadrant switch: v on – can conduct positive on- off v 0 state current can block positive off-state IGBT 1 voltage i + C provided that the intended v on-state and off-state – operating points lie on the 0 instantaneous i-v characteristic transistor i-v characteristic, then switch can be realized using a BJT or IGBT Fundamentals of Power Electronics 8 Chapter 4: Switch realization  The Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) An active switch, controlled by i terminal C Normally operated as single- 1 on quadrant switch: i + C off v can conduct positive on-state v current (can also conduct – negative current in some on (reverse conduction) circumstances) 0 can block positive off-state voltage Symbol instantaneous i-v characteristic provided that the intended on- state and off-state operating points lie on the MOSFET i-v characteristic, then switch can be realized using a MOSFET Fundamentals of Power Electronics 9 Chapter 4: Switch realization  Realization of switch using transistors and diodes Buck converter example iA A L iL(t) + vA – + – + vB B Vg C R V – + iB Switch A: transistor – Switch B: diode iA iB Switch A Switch B SPST switch on iL on iL operating points Switch A Switch B off off Vg vA –Vg vB Switch A Switch B Fundamentals of Power Electronics 10 Chapter 4: Switch realization  Realization of buck converter using single-quadrant switches iA vA L + – iL(t) + – vL(t) – Vg + vB – + iB iA iB Switch A Switch B on iL on iL Switch A Switch B off off Vg vA –Vg vB Fundamentals of Power Electronics 11 Chapter 4: Switch realization  4.1.2. Current-bidirectional two-quadrant switches Usually an active switch, controlled by terminal C i 1 on Normally operated as two- (transistor conducts) i quadrant switch: + C off v can conduct positive or v negative on-state current – can block positive off-state on 0 (diode conducts) voltage provided that the intended on- state and off-state operating BJT / anti-parallel instantaneous i-v points lie on the composite i-v diode realization characteristic characteristic, then switch can be realized as shown Fundamentals of Power Electronics 12 Chapter 4: Switch realization  Two quadrant switches i switch on-state current on 1 (transistor conducts) i + off v v switch – off-state voltage 0 on (diode conducts) Fundamentals of Power Electronics 13 Chapter 4: Switch realization  MOSFET body diode i 1 on (transistor conducts) i + off v C v on – (diode conducts) 0 Power MOSFET Power MOSFET, Use of external diodes characteristics and its integral to prevent conduction body diode of body diode Fundamentals of Power Electronics 14 Chapter 4: Switch realization  A simple inverter iA + Q1 Vg + D1 vA v0(t) = (2D – 1) Vg – – L iL + + Vg + D2 v C R v0 – B Q2 – – iB Fundamentals of Power Electronics 15 Chapter 4: Switch realization  Inverter: sinusoidal modulation of D v0(t) = (2D – 1) Vg Sinusoidal modulation to v0 produce ac output: Vg D(t) = 0.5 + Dm sin (ωt) D The resulting inductor 0 0.5 1 current variation is also sinusoidal: –Vg v0(t) Vg iL(t) = = (2D – 1) R R Hence, current-bidirectional two-quadrant switches are required. Fundamentals of Power Electronics 16 Chapter 4: Switch realization  The dc-3øac voltage source inverter (VSI) ia Vg + – ib ic Switches must block dc input voltage, and conduct ac load current. Fundamentals of Power Electronics 17 Chapter 4: Switch realization  Bidirectional battery charger/discharger D1 L + + vbus Q1 D2 vbatt spacecraft main power bus Q2 – – vbus > vbatt A dc-dc converter with bidirectional power flow. Fundamentals of Power Electronics 18 Chapter 4: Switch realization  4.1.3. Voltage-bidirectional two-quadrant switches Usually an active switch, controlled by terminal C 1 i Normally operated as two- i + on quadrant switch: can conduct positive on-state v v current C off off (diode (transistor can block positive or negative blocks voltage) blocks voltage) – off-state voltage 0 provided that the intended on- state and off-state operating points lie on the composite i-v BJT / series instantaneous i-v characteristic, then switch can diode realization characteristic be realized as shown The SCR is such a device, without controlled turn-off Fundamentals of Power Electronics 19 Chapter 4: Switch realization  Two-quadrant switches 1 i switch + i on-state current v on – 0 v switch 1 off off off-state (diode (transistor voltage i + blocks voltage) blocks voltage) v C – 0 Fundamentals of Power Electronics 20 Chapter 4: Switch realization  A dc-3øac buck-boost inverter iL φa + vab(t) – φb – + Vg + vbc(t) – φc Requires voltage-bidirectional two-quadrant switches. Another example: boost-type inverter, or current-source inverter (CSI). Fundamentals of Power Electronics 21 Chapter 4: Switch realization  4.1.4. Four-quadrant switches switch on-state current Usually an active switch, controlled by terminal C can conduct positive or negative on-state current switch off-state can block positive or negative voltage off-state voltage Fundamentals of Power Electronics 22 Chapter 4: Switch realization  Three ways to realize a four-quadrant switch 1 1 1 i i i 1 + + + i + v v v v – – – – 0 0 0 0 Fundamentals of Power Electronics 23 Chapter 4: Switch realization  A 3øac-3øac matrix converter 3øac input 3øac output ia van(t) + – vbn(t) ib + – – vcn(t) + ic All voltages and currents are ac; hence, four-quadrant switches are required. Requires nine four-quadrant switches Fundamentals of Power Electronics 24 Chapter 4: Switch realization  4.1.5. Synchronous rectifiers Replacement of diode with a backwards-connected MOSFET, to obtain reduced conduction loss i 1 1 1 on i i i + (reverse conduction) + + C off v v v v – – – on 0 0 0 ideal switch conventional MOSFET as instantaneous i-v diode rectifier synchronous characteristic rectifier Fundamentals of Power Electronics 25 Chapter 4: Switch realization  Buck converter with synchronous rectifier MOSFET Q2 is vA controlled to turn on iA L iL(t) + – when diode would Q1 normally conduct – Semiconductor + C Vg vB conduction loss can – C + be made arbitrarily Q2 iB small, by reduction of MOSFET on- resistances Useful in low-voltage high-current applications Fundamentals of Power Electronics 26 Chapter 4: Switch realization  4.2. A brief survey of power semiconductor devices G Power diodes G Power MOSFETs G Bipolar Junction Transistors (BJTs) G Insulated Gate Bipolar Transistors (IGBTs) G Thyristors (SCR, GTO, MCT) G On resistance vs. breakdown voltage vs. switching times G Minority carrier and majority carrier devices Fundamentals of Power Electronics 27 Chapter 4: Switch realization  Charge-control in the diode: Discussion The familiar i–v curve of the diode is an equilibrium relationship that can be violated during transient conditions During the turn-on and turn-off switching transients, the current deviates substantially from the equilibrium i–v curve, because of change in the stored charge and change in the charge within the reverse-bias depletion region Under forward-biased conditions, the stored minority charge causes “conductivity modulation” of the resistance of the lightly-doped n– region, reducing the device on-resistance Fundamentals of Power Electronics 34 Chapter 4: Switch realization  Diode in OFF state: reversed-biased, blocking voltage v(t) v + – t p n– n E i(t) – v + 0 { Depletion region, reverse-biased t Diode is reverse-biased No stored minority charge: q = 0 (1) Depletion region blocks applied reverse voltage; charge is stored in capacitance of depletion region Fundamentals of Power Electronics 35 Chapter 4: Switch realization  Turn-on transient v(t) The current i(t) is determined by the converter circuit. This t current supplies: Diode conducts with low on-resistance charge to increase Diode is forward-biased. Supply minority charge to n– region to reduce on-resistance voltage across depletion region Charge depletion region charge needed to i(t) support the on-state On-state current determined by converter circuit current charge to reduce t on-resistance of n– (1) (2) region Fundamentals of Power Electronics 36 Chapter 4: Switch realization  Turn-off transient v i (< 0) + – p n- n + + + + + + + + } Removal of stored minority charge q Fundamentals of Power Electronics 37 Chapter 4: Switch realization  Types of power diodes Standard recovery Reverse recovery time not specified, intended for 50/60Hz Fast recovery and ultra-fast recovery Reverse recovery time and recovered charge specified Intended for converter applications Schottky diode A majority carrier device Essentially no recovered charge Model with equilibrium i-v characteristic, in parallel with depletion region capacitance Restricted to low voltage (few devices can block 100V or more) Fundamentals of Power Electronics 41 Chapter 4: Switch realization  Paralleling diodes Attempts to parallel diodes, and share the i current so that i1 = i2 = i/2, generally don’t work. i1 i2 + + Reason: thermal instability caused by temperature dependence of the diode equation. v1 v2 Increased temperature leads to increased current, or reduced voltage. – – One diode will hog the current. To get the diodes to share the current, heroic measures are required: Select matched devices Package on common thermal substrate Build external circuitry that forces the currents to balance Fundamentals of Power Electronics 43 Chapter 4: Switch realization  Ringing induced by diode stored charge see Section 4.3.3 iL(t) L vi(t) V1 + – iB(t) t vL(t) 0 + vi(t) + silicon –V2 diode vB(t) C – – iL(t) Diode is forward-biased while iL(t) > 0 0 Negative inductor current removes diode area t – Qr stored charge Qr vB(t) When diode becomes reverse-biased, t 0 negative inductor current flows through capacitor C. –V2 Ringing of L-C network is damped by t1 t2 t3 parasitic losses. Ringing energy is lost. Fundamentals of Power Electronics 44 Chapter 4: Switch realization  MOSFET: conclusions G A majority-carrier device: fast switching speed G Typical switching frequencies: tens and hundreds of kHz G On-resistance increases rapidly with rated blocking voltage G Easy to drive G The device of choice for blocking voltages less than 500V G 1000V devices are available, but are useful only at low power levels (100W) G Part number is selected on the basis of on-resistance rather than current rating Fundamentals of Power Electronics 55 Chapter 4: Switch realization  4.2.3. Bipolar Junction Transistor (BJT) Base Emitter Interdigitated base and emitter contacts Vertical current flow n n n npn device is shown p minority carrier device n- on-state: base-emitter and collector-base junctions are both n forward-biased on-state: substantial minority charge in p and n- regions, conductivity Collector modulation Fundamentals of Power Electronics 56 Chapter 4: Switch realization  Darlington-connected BJT Increased current gain, for high-voltage Q1 applications In a monolithic Darlington device, Q2 transistors Q1 and Q2 are integrated on the same silicon wafer Diode D1 speeds up the turn-off process, D1 by allowing the base driver to actively remove the stored charge of both Q1 and Q2 during the turn-off transition Fundamentals of Power Electronics 62 Chapter 4: Switch realization  Conclusions: BJT G BJT has been replaced by MOSFET in low-voltage (