Chapter 6. Converter Circuits Where do the boost, 6.1. Circuit - PowerPoint PPT Presentation

Chapter 6. Converter Circuits • Where do the boost, 6.1. Circuit manipulations buck-boost, and other converters originate? 6.2. A short list of • How can we obtain a converters converter having given desired properties? 6.3. Transformer isolation • What converters are possible? 6.4. Converter evaluation and design • How can we obtain transformer isolation in a 6.5. Summary of key converter? points • For a given application, which converter is best? 1 Fundamentals of Power Electronics Chapter 6: Converter circuits

6.1. Circuit manipulations L 1 + 2 + V g C R V – – Begin with buck converter: derived in chapter 1 from first principles • Switch changes dc component, low-pass filter removes switching harmonics • Conversion ratio is M = D 2 Fundamentals of Power Electronics Chapter 6: Converter circuits

6.1.1. Inversion of source and load Interchange power input and output ports of a converter Buck converter example V 2 = DV 1 port 1 port 2 L 1 + + 2 + V 1 V 2 – – – Power flow 3 Fundamentals of Power Electronics Chapter 6: Converter circuits

Inversion of source and load Interchange power source and load: port 1 port 2 L 1 + + 2 + V 1 V 2 – – – Power flow V 1 = 1 V 2 = DV 1 D V 2 4 Fundamentals of Power Electronics Chapter 6: Converter circuits

Realization of switches as in chapter 4 • Reversal of power port 1 port 2 L flow requires new realization of + + switches + V 1 V 2 – • Transistor conducts – – when switch is in position 2 Power flow • Interchange of D and D’ V 1 = 1 D ' V 2 Inversion of buck converter yields boost converter 5 Fundamentals of Power Electronics Chapter 6: Converter circuits

6.1.2. Cascade connection of converters converter 1 converter 2 + + + V 1 V V g – V V 1 V 1 = M 2 ( D ) V g = M 1 ( D ) – – D V 1 = M 1 ( D ) V g V V g = M ( D ) = M 1 ( D ) M 2 ( D ) V = M 2 ( D ) V 1 6 Fundamentals of Power Electronics Chapter 6: Converter circuits

Example: buck cascaded by boost L 1 L 2 2 1 + + 1 2 + Vg C 1 V 1 C 2 R V – – – { { Buck converter Boost converter V 1 V g = D V D V g = 1 – D V 1 V 1 = 1 – D 7 Fundamentals of Power Electronics Chapter 6: Converter circuits

Buck cascaded by boost: simplification of internal filter remove capacitor C 1 L 1 L 2 2 1 + 1 2 + Vg C 2 R V – – combine inductors L 1 and L 2 L i L 2 1 + Noninverting 1 2 + Vg V buck-boost – converter – 8 Fundamentals of Power Electronics Chapter 6: Converter circuits

Noninverting buck-boost converter L i L 2 1 + 1 2 + Vg V – – subinterval 1 subinterval 2 + + i L + + V g V V g V – – i L – – 9 Fundamentals of Power Electronics Chapter 6: Converter circuits

Reversal of output voltage polarity subinterval 1 subinterval 2 + + i L noninverting + + V g V V g V – – buck-boost i L – – + + i L i L inverting + + V g V V g V – – buck-boost – – 10 Fundamentals of Power Electronics Chapter 6: Converter circuits

Reduction of number of switches: inverting buck-boost subinterval 1 subinterval 2 + + i L i L + + V g V V g V – – – – One side of inductor always connected to ground — hence, only one SPDT switch needed: + 1 2 V D i L V g = – + V g V 1 – D – – 11 Fundamentals of Power Electronics Chapter 6: Converter circuits

Discussion: cascade connections • Properties of buck-boost converter follow from its derivation as buck cascaded by boost Equivalent circuit model: buck 1: D transformer cascaded by boost D’ :1 transformer Pulsating input current of buck converter Pulsating output current of boost converter • Other cascade connections are possible Cuk converter: boost cascaded by buck 12 Fundamentals of Power Electronics Chapter 6: Converter circuits

6.1.3. Rotation of three-terminal cell mi er n t - a e l Treat inductor and e c r e h l t l SPDT switch as three- A a b B 1 terminal cell: + 2 + V g v – c C – Three-terminal cell can be connected between source and load in three nontrivial distinct ways: a-A b-B c-C buck converter a-C b-A c-B boost converter a-A b-C c-B buck-boost converter 13 Fundamentals of Power Electronics Chapter 6: Converter circuits

Rotation of a dual three-terminal network er mi t n - al A capacitor and SPDT e e r c h e l t switch as a three- 1 l + A a b B terminal cell: 2 + V g v – c – C Three-terminal cell can be connected between source and load in three nontrivial distinct ways: a-A b-B c-C buck converter with L-C input filter a-C b-A c-B boost converter with L-C output filter a-A b-C c-B Cuk converter 14 Fundamentals of Power Electronics Chapter 6: Converter circuits

6.1.4. Differential connection of load to obtain bipolar output voltage dc source load converter 1 + Differential load V 1 voltage is + V 1 = M ( D ) V g – V V = V 1 – V 2 – + V g D – The outputs V 1 and V 2 may both be positive, but the differential converter 2 + output voltage V can be positive or negative. V 2 V 2 = M ( D ') V g – D' 15 Fundamentals of Power Electronics Chapter 6: Converter circuits

Differential connection using two buck converters Buck converter 1 } 1 Converter #1 transistor + driven with duty cycle D 2 V 1 + Converter #2 transistor – driven with duty cycle V complement D’ – + V g – Differential load voltage 2 is + V = DV g – D ' V g 1 V 2 Simplify: – V = (2 D – 1) V g { Buck converter 2 16 Fundamentals of Power Electronics Chapter 6: Converter circuits

Conversion ratio M(D) , differentially-connected buck converters V = (2 D – 1) V g M(D) 1 0 0.5 1 D – 1 17 Fundamentals of Power Electronics Chapter 6: Converter circuits

Simplification of filter circuit, differentially-connected buck converters Original circuit Bypass load directly with capacitor Buck converter 1 } 1 1 + 2 2 V 1 + + – V V – – + + V g V g – – 2 2 + 1 1 V 2 – { Buck converter 2 18 Fundamentals of Power Electronics Chapter 6: Converter circuits

Simplification of filter circuit, differentially-connected buck converters Combine series-connected Re-draw for clarity inductors C 1 1 2 L + V g + V – – i L 2 2 1 R + V + V g – – H-bridge, or bridge inverter 2 Commonly used in single-phase 1 inverter applications and in servo amplifier applications 19 Fundamentals of Power Electronics Chapter 6: Converter circuits

Differential connection to obtain 3ø inverter With balanced 3ø load, dc source 3øac load neutral voltage is + converter 1 V 1 V n = 1 V 1 = M ( D 1 ) V g 3 ( V 1 + V 2 + V 3 ) – – v an + Phase voltages are D 1 + V g V an = V 1 – V n – V n + converter 2 V bn = V 2 – V n + v bn – – v cn + V 2 V cn = V 3 – V n V 2 = M ( D 2 ) V g – Control converters such that D 2 their output voltages contain the same dc biases. This dc + converter 3 bias will appear at the V 3 neutral point Vn. It then V 3 = M ( D 3 ) V g – cancels out, so phase voltages contain no dc bias. D 3 20 Fundamentals of Power Electronics Chapter 6: Converter circuits

3ø differential connection of three buck converters dc source 3øac load + V 1 – + an v – + V g – V n + + v bn – – V 2 v cn + – + V 3 – 21 Fundamentals of Power Electronics Chapter 6: Converter circuits

3ø differential connection of three buck converters Re-draw for clarity: 3øac load dc source – v an + + V n V g – + v bn – – v cn + “Voltage-source inverter” or buck-derived three-phase inverter 22 Fundamentals of Power Electronics Chapter 6: Converter circuits

6.2. A short list of converters An infinite number of converters are possible, which contain switches embedded in a network of inductors and capacitors Two simple classes of converters are listed here: • Single-input single-output converters containing a single inductor. The switching period is divided into two subintervals. This class contains eight converters. • Single-input single-output converters containing two inductors. The switching period is divided into two subintervals. Several of the more interesting members of this class are listed. 23 Fundamentals of Power Electronics Chapter 6: Converter circuits

Single-input single-output converters containing one inductor • Use switches to connect inductor between source and load, in one manner during first subinterval and in another during second subinterval • There are a limited number of ways to do this, so all possible combinations can be found • After elimination of degenerate and redundant cases, eight converters are found: dc-dc converters buck boost buck-boost noninverting buck-boost dc-ac converters bridge Watkins-Johnson ac-dc converters current-fed bridge inverse of Watkins-Johnson 24 Fundamentals of Power Electronics Chapter 6: Converter circuits

Converters producing a unipolar output voltage M ( D ) = D 1. Buck M(D) 1 1 + 2 + 0.5 V g V – – 0 0 0.5 1 D 1 M ( D ) = M(D) 2. Boost 1 – D 4 2 + 3 1 2 + V g V – 1 0 – 0 0.5 1 D 25 Fundamentals of Power Electronics Chapter 6: Converter circuits