Switch-Mode DC-AC Converters

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1 Switch-Moe DC-AC Converters EE 442/

2 Some Applications: AC Motor Drives & P Inverters 8-2

3 Switch-Moe DC-AC Inverter Four quarants of operation. 8-3

4 Half-Brige Inverter: 1. Capacitors provie the mi-point. 2. The transistors TA+ an TA- are switche using pulse-with-moulation (PWM. 8-4

5 m a control tri Synthesis of a Sinusoial Output by PWM Amplitue an frequency moulation ratios:, For small values of m f ( e.g., 21, the two signals must be synchronize to avoi sub-harmonics. m Peak value of funamental voltage: f f s f 1 1 ( ˆ Ao 1 ma, for ma 1 2 The harmonics in the inverter output appear as siebans aroun m f, 2m f, 3m f,,km f, Only o harmonics are present in the output voltage waveform m f shoul be an o integer value., 8-5

6 Harmonics in the DC-AC Inverter Output oltage 1. The funamental voltage is proportional to the amplitue moulation inex. 2. Some harmonics can be larger than the funamental component. 8-6

7 Funamental oltage as a Function of m a 1. Note the linear an the over-moulation regions; with squarewave operation in the limit. 1 2 ( ˆ Ao 2 1, for ma 1 8-7

8 Harmonics in the Over-Moulation Region The sie bans start to sprea out to a point where all the integer harmonics appear in the frequency spectrum (incluing the low-orer harmonics which are har to filter. 8-8

9 Square-Wave Moe of Operation Funamental an harmonic voltages: ( ˆ Ao 1 ( ˆ Ao h 2, 2, h h 3,5,

10 Single-Phase Full-Brige DC-AC Inverter 1. No nee for capacitor mi-point. 2. The output voltage now switches between + an

11 PWM to Synthesize Sinusoial Output: Bipolar Switching Peak value of funamental voltage: m, for m 1 ˆ o 1 a ˆ o 1 4, for a m a

12 8-12 Analysis with Ieal Filters o o D o o o o o o o o I I I I where t I I t i t i t v t i t I t i t t v 2, cos( cos( (, ( ( ( sin( 2 (, sin( 2 ( * * 1 1

13 PWM Unipolar oltage Switching Legs A an B are controlle separately: ˆ o 1 ma, ˆ 4 o 1 for, for m a 1 m a 1 The harmonics in the inverter output appear as siebans aroun 2m f, 4m f, 6mf, Note the harmonics at an aroun m f, 3m f, 5m f, are absent lower harmonic content. Note also only o harmonics are present. 8-13

14 DC-Sie Current with PWM Unipolar Switching The ripple content is significantly less than when using bipolar switching. 8-14

15 Sinusoial Synthesis by oltage Shift (Moifie Square Wave 4 ( ˆ o h sin( h, h 1,3,5,7... h 8-15

16 Funamental an Ripple in Inverter Output δ Active an Reactive Power: P o 1 Eo 01Eo cos( sin(, Q L L 1 1 E 2 o 8-16

17 Square-Wave versus PWM Operation PWM results in much smaller ripple current. 8-17

18 Push-Pull Inverter (requires transformer with center tap ˆ o 1 m a / n, for m a 1 n ˆ o 1 4 n, for m a 1 1. v o switches between /n an /n where n is the transformer turn ratio. 2. Avantage: no more than one switch conucts at any time less voltage rop. Also the control rives have the same groun. 3. Difficulty: strong magnetic coupling between the two half winings is require to reuce the energy associate with the leakage inuctance. 8-18

19 Three-Phase Inverter 1. Three inverter legs; 2. No mi-capacitor point is require. 8-19

20 Three-Phase PWM Waveforms Legs A, B an C are controlle separately: ˆ LL ma 2 ˆ LL 0.612m a, for 6 1, for ma 1 m a 1 The frequency moulation inex, m f, shoul be an o number that is a multiple of 3 to cancel out the most ominant harmonics See harmonic content of line voltage uring linear moulation in the next slie. 8-20

21 Three-Phase Inverter Harmonics 8-21

22 Three-Phase Inverter Output 8-22

23 DC-Sie Current in a Three-Phase Inverter The current consists of a c component an the switching-frequency relate harmonics. 8-23

24 8-24 Three-Phase Inverter: Funamental Frequency, cos( 3... (, ( ( ( ( ( ( ( * * o o C Cn B Bn A An I I t i t i t v t i t v t i t v t i (DC quantity only

25 Three-Phase Inverter: Square-Wave Moe 8-25

26 Square-Wave Operation 8-26

27 Square-Wave an PWM Operation PWM results in much smaller ripple current 8-27

28 PWM Operation: Short-Circuit States 8-28

29 Blanking Time: Non-Ieal switches Instantaneous switching from ON to OFF an vice versa. In practice, the turn-on an turn-off times are finite (non-zero. Blanking Time is chosen to avoi crossconuction through the leg. Impact on output voltage: 8-29

30 Effect of Blanking Time on oltage (uring current zero crossing 8-30

31 Programme Harmonic Elimination The notch angles are base on the esire output. 8-31

32 Current Control: Tolerance-Ban Current Control ariable switching frequency which epens on the loa inuctance, motor back emf, an DC voltage. 8-32

33 Fixe-Frequency Operation 8-33

34 Transition from Inverter to Rectifier Moe 8-34

Lecture 8 - Effect of source inductance on rectifier operation

Lecture 8 - Effect of source inuctance on rectifier operation 8.1 Rectifier with input source inuctance The output DC voltage an current of rectifier circuits iscusse so far have been foun by assuming