ECE1750, Spring dc-ac power conversion

Transcription

1 ECE1750, Spring 2018 dc-ac power conversion (inverters) 1

2 H-Bridge Inverter Basics Creating AC from DC Single-phase H-bridge bid (voltage Switching rules source) inverter topology: Either A+ or A is closed, Vdc but never at the same time * Either B+ or B is closed, but never at the same time * *same time closing would cause a A+ B+ short circuit from Vdc to ground (shoot-through) *To avoid dhoot-through when using real switches (i.e. there are turn-on Va Load Vb and turn-off delays) a dead-time or blanking time is implemented A B V load V A V B V Corresponding values of Va and Vb A+ closed, Va = Vdc A closed, Va = 0 B+ closed, Vb = Vdc B closed, Vb = 0 AB 2

3 H BRIDGE INVERTER Vdc Corresponding values of Vab A+ closed and B closed, Vab = Vdc A+ closed and B+ closed, Vab = 0 B+ closed and A closed, Vab = Vdc B closed and A closed, Vab = 0 A+ B+ Va + Vdc Load Vb A B The free wheeling diodes permit current to flow even if all switches are open These diodes d also permit lagging currents to flow in inductive loads V load V A V B V AB 3

4 H BRIDGE INVERTER Vdc Corresponding values of Vab A+ closed and B closed, Vab = Vdc A+ closed and B+ closed, Vab = 0 B+ closed and A closed, Vab = Vdc B closed and A closed, Vab = 0 A+ B+ Va + 0 Load Vb A B The free wheeling diodes permit current to flow even if all switches are open These diodes d also permit lagging currents to flow in inductive loads V load V A V B V AB 4

5 H BRIDGE INVERTER Vdc Corresponding values of Vab A+ closed and B closed, Vab = Vdc A+ closed and B+ closed, Vab = 0 B+ closed and A closed, Vab = Vdc B closed and A closed, Vab = 0 A+ B+ Va Vdc + Load Vb A B The free wheeling diodes permit current to flow even if all switches are open These diodes d also permit lagging currents to flow in inductive loads V load V A V B V AB 5

6 H BRIDGE INVERTER Vdc Corresponding values of Vab A+ closed and B closed, Vab = Vdc A+ closed and B+ closed, Vab = 0 B+ closed and A closed, Vab = Vdc B closed and A closed, Vab = 0 A+ B+ Va + 0 Load Vb A B The free wheeling diodes permit current to flow even if all switches are open These diodes d also permit lagging currents to flow in inductive loads V load V A V B V AB 6

7 Square wave modulation: E H-Bridge Inverter E 4E 1 4E 1 1 vt ( ) sin kot sin 1ot sin 3ot sin 5ot k 3 5 k1, k odd 7

8 H-Bridge Inverter Harmonics with square wave modulation (switching frequency = fundamental frequency). 8

9 Square wave modulation: H-Bridge Inverter 4E Considerable low order harmonics that are difficult to filter out k1, k odd E 1 4E 1 1 vt ( ) sin kot sin 1ot sin 3ot sin 5ot k 3 5 k V 1 4E Cannot be changed 9

10 Basic Square Wave Operation (sometimes used for 50 Hz or 60Hz applications) Vdc Corresponding values of Vab A+ closed and B closed, Vab = Vdc A+ closed and B+ closed, Vab = 0 B+ closed and A closed, Vab = Vdc V load B closed and da closed, Vab = 0 Vdc t The Vab = 0 time is not required but can be used to reduce the rms value of V load V 1 4V dc cos 2 10

11 Many Loads Have Lagging Current Consider an Inductor There must be a provision for voltage and current to have opposite signs with respect to each other Vdc V load Vdc I load I I 11

12 Load Current Can Always Flow, Regardless of Switching State Example - when current flows left to right through the load Vdc here A+ B+ or here Va Load Vb A B or here here 12

13 Load Current Can Always Flow, cont. Example - when current flows right to left through the load Vdc A+ B+ here here Va Load Vb or here A B or here 13

14 Load Current Can Always Flow, cont. H BRIDGE INVERTER Vdc A+ B+ Corresponding values of Vab A+ closed and B closed, Vab = Vdc A+ closed and B+ closed, Vab = 0 B+ closed and A closed, Vab = Vdc B closed and A closed, Vab = 0 Load consuming power Load generating power Va + Vdc Load Vb A B 14

15 Load Current Can Always Flow, cont. H BRIDGE INVERTER Vdc A+ B+ Corresponding values of Vab A+ closed and B closed, Vab = Vdc A+ closed and B+ closed, Vab = 0 B+ closed and A closed, Vab = Vdc B closed and A closed, Vab = 0 Load consuming power Load generating power Va + Vdc Load Vb A B 15

16 The four firing circuits do not have the same ground reference. Thus, the gate driving i circuits it require isolation. Vdc (source of power delivered to load) Local ground reference for A + firing circuit S A + B + Load S Local ground reference for B + firing circuit Local ground reference for A firing circuit S A B S Local ground reference for B firing circuit 16

17 Question - How can a sinusoidal (or other) input signal be amplified with low baseband distortion? Answer the switching can be controlled in a smart way so that the FFT of V load has a strong fundamental component, plus high- frequency switching harmonics that can be easily filtered out and thrown into the trash V load Vdc Progressively Progressively wider pulses narrower pulses at the center at the edges Unipolar Pulse-Width Modulation (PWM) Vdc 17

18 Implementation of Unipolar Pulse Width Modulation (PWM) Vcont is the input signal we want to amplify at the output of the inverter. Vcont is usually a sinewave, but it can also be a music signal. Vcont Vtri Vcont The implementation rules are: Vcont > Vtri, close switch A+, open switch A, so voltage Va = Vdc Vcont < Vtri, open switch A+, close switch A, so voltage Va = 0 Vcont > Vtri, close switch B+, open switch B, so voltage Vb = Vdc Vcont < Vtri, open switch B+, close switch B, so voltage Vb = 0 V tri is a triangle wave whose frequency is at least 30 times greater than Vcont. Ratio m a = peak of control signal divided by peak of triangle wave Ratio m f = frequency of triangle wave divided by frequency of control signal 18

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22 1.5 Ratio m a = peak of control signal (also 1 called modulation signal) 0.5 divided by peak of triangle wave Ratio m f = frequency of -1 triangle wave divided by frequency of control -1.5 signal Load voltage with m a = 0.5 (i.e., in the linear region)

23 Load voltage with m a = 1.5 (i.e., overmodulation)

24 Variation of RMS value of no-load fundamental inverter output voltage (V 1rms ) with m a For single-phase inverters m a also equals the ratio between the peak output voltage and the input V dc voltage. V 4 V 1rms asymptotic to dc 2 square wave value V dc 2 m a V1, 2 V rms dc m a is called the modulation index 0 1 m a linear overmodulation saturation The amplitude of the fundamental sinusoidal output signal can be controlled by changing ma; i.e., by changing the amplitude of the modulation signal with respect to the triangle waveform V 1, rms mv a 2 dc

25 RMS magnitudes of load voltage frequency components with respect to V dc for f tri >> f cont 2 Frequency m a = 0.2 m a = 0.4 m a = 0.6 m a = 0.8 m a = 1.0 f cont ftri ± fcont f tri ± 3f cont f tri ±5f cont f tri ± f cont ftri ± 3fcont f ti tri ±5f cont f tri ± 7f cont f tri cluster 4f tri cluster 25

26 Harmonic content in PWM signals 60Hz component 2ftri cluster (46kHz) 4ftri cluster (92kHz) Figure 19. FFT of idealized V load in the linear region with m a 1.0, where the frequency span and center frequency are set to 100kHz and 50kHz, respectively 26

27 Harmonic content in PWM signals 60Hz component 2ftri cluster (46kHz) 4ftri cluster (92kHz) Figure 19. FFT of idealized V load in the linear region with m a 1.0, where the frequency span and center frequency are set to 100kHz and 50kHz, respectively Contrary to square wave modulation, if the PWM switching frequency is high enough (mf>30 but usually mf>100) all harmonic content is relatively easy to filter out with a simple low pass filter 27

28 Loaded (with a resistor) and no output filter Load Voltage No Filter. m a 1. 28

29 With Filter Load Voltage With Filter. m a 1. Very Effective! 29

30 100Hz Signal as Input, Inverter Output Dead spots at zero crossings appear because real switches cannot switch fast enough to realize the very short pulses commanded near zero crossings. This effect is compounded by the need of adding a dead time or blanking time into the switches control circuit. 30

31 Discrete gate driving circuit for inverters Once the MOSFET is connected, this asymmetrical circuit will add blanking by making the turn-on slower than the turn-off. (blanking is the opposite of overlap) MOSFET G D S 100kΩ Switching diode g Overlap is the time that A + and A are simultaneously on, which should be avoided. Hence, some blanking (time between one turning off and the other turning on) is desirable. 1.2kΩ 10Ω 0.1µF 5 4 Di Driver 8 1 g g Grounds (isolated from control circuit) 10kΩ 5 4 Opto 8 1 Isolating barrier Powered by +12V that is isolated from the PWM control circuit From the rest of the control circuit where the modulation signal and triangle are compared 31

32 Dead-time or Blanking time Asymmetrical firing circuit produces slow turn on, fast turn off, to provide blanking Otherwise, due to turn-off delays, both switches on a same leg may end up to be on at the same time causing a short circuit Vdc blanking time to eliminate overlap actual MOSFET turn on Multimeter check of V GS for A + and A. Expect about 4.0Vdc. A+ B+ V GS of A + V GS of A Va Load Vb Save screen snapshot #1 A B A Off A + On You must avoid overlap in on times 32

33 Three-Phase Inverter (called a six-pack) dc-link Source Loads Three inverter legs; capacitor mid-point is fictitious Source: Ned Mohan s power electronics book

34 Three- Phase PWM Waveforms NOTE: Modulation signals for each of the three phases have a 120 degree phase difference NOTE: Modulation index is different from that on single-phase inverters. In threephase inverters: m Vph, rms 3 ph 2 Vdc 2 Source: Ned Mohan s power electronics book

35 Three-Phase Inverter Harmonics Compare with singlephase inverter Source: Ned Mohan s power electronics book

36 Three-Phase Inverter Output Linear and over-modulation ranges Source: Ned Mohan s power electronics book