ELG4139: DC to AC Converters

Transcription

1 ELG4139: DC to AC Converters Converts DC to AC power by switching the DC input voltage (or current) in a pre-determined sequence so as to generate AC voltage (or current) output. I DC I ac + + V DC V ac

2 Harmonics Filtering DC SUPPLY INVERTER (LOW PASS) FILTER LOAD L + v O 1 C + v O 2 BEFORE FILTERING AFTER FILTERING v O 1 v O 2 Output of the inverter is chopped AC voltage with zero DC component. It contain harmonics. An LC section low-pass filter is normally fitted at the inverter output to reduce the high frequency harmonics. In some applications such as UPS, high purity sine wave output is required. Good filtering is a must. In some applications such as AC motor drive, filtering may not required.

3 Single Phase Half-Bridge Inverter V dc + V C1 - V C2 + - G V o R L S 1 + S 2 Vdc 2 0 Vdc 2 S1 ON S2 OFF S1 OFF S2 ON t Also known as the Inverter Leg! Both capacitors have the same value. Thus the DC link is equally spilt into two. The top and bottom switch has to be complementary. Meaning, If the top switch is closed (ON), the bottom must be OFF, and vice-versa.

4 Q 1 on, Q 2 off, v o = V s /2 Peak Reverse Voltage of Q 2 = V s

5 Q 1 off, Q 2 on, v o = -V s /2

6 Single Phase Full Bridge V RG + LEG R LEG R' V dc 2 p 2p wt + V dc - V dc 2 - G S1 R + V o - S3 R' V R ' G V dc 2 V dc 2 p 2p wt + V dc 2 - S4 S2 V dc 2 V o V dc Vo = V V RG R ' G G is "virtual groumd" p 2p wt V dc Single phase full bridge is built from two half-bridge leg. The switching in the second leg is delayed by 180 degrees from the first leg.

7 Q 1 -Q 2 on, Q 3 -Q 4 off, v o = V s + V s -

8 Q 3 -Q 4 on, Q 1 -Q 2 off, v o = -V s - V s +

9 Performance Parameters Harmonic factor of the nth harmonic (HF n ) HF n V on = for n>1 V o1 V on = rms value of the nth harmonic component V 01 = rms value of the fundamental component Total Harmonic Distortion (THD): Measures the closeness in shape between a waveform and its fundamental component THD 1 = ( V ) V o1 n= 2,3, on

10 Design Constraints of a Pure Sine wave Inverter Quantity Voltage Details Convert 12VDC to 120 VAC Power Provide 300 W continuous Efficiency Waveform Total Harmonic Distortion > 90% efficiency Pure 60 Hz sinusoidal < 5% THD Physical Dimensions 8 x 4.75 x 2.5 Cost $175.00

11 Required Components for Design 12 V DC Input from vehicle battery) PWM Control Circuit Half-bridge Converter Transformer Low-pass Filter 120 VAC, 60 Hz, 300 W Output Full-bridge Inverter Sinusoidal PWM Controller

12 PWM Controller Produces two complementary pulses to control half-bridge transistors. Problem: Voltage may drop when the input voltage is decreased. Solution: A feedback network may be added for voltage regulation.

13 Pulse Width Modulation h( x) = if( k( x) c( x) 1 if( k( x) c( x) 1 0) ) 1 +1 M 1 0 Modulating Waveform Carrier waveform t 1 t 2 Sinusoidal modulating waveform, v m (t) p Carrier, v c (t) 2p t 1 V dc 2 t' 1 t' 2 Regular sampling waveform, v s (t) 0 V dc 2 t0 t1 t t 2 3 t t 4 5 v pwm t Regular sampling PWM Triangulation method (Natural sampling). Amplitudes of the triangular wave (carrier) and sine wave (modulating) are compared to obtain PWM waveform. Analogue comparator may be used. Basically an analogue method. Its digital version, known as REGULAR sampling is widely used in industry.

14 Software Flow Diagram (Dr. Yaroslav Koshka) Initialize all variables Count0 = 300 (300 duty cycles) no 300 duty cycle values? yes Output 1 = high, Output 2 = low duty cycle table (increment pointer) Duty cycle and sampling period timer Output 1 = low, Output 2 = high Decrement Count0 by 1 no Has duty cycle been reached? yes no One Sampling Period? ye s

15 Low-pass Filter 2 nd order L-C filter Filters to retain a 60 Hz fundamental frequency Few components Handle current Wind inductor (fine tune)

16 PCB Layout

17 Case Study: Solar System Using Inverters Stand Alone; Simple Grid Tied; Grid Tie with Battery Solar Schoolhouse and San Mateo College

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20 Simple Grid-Connected System Utility Solar Array Inverter Distribution Panel

21 Subpanel Solar AC in from Inverter Lightning surge arrestor

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23 Stand Alone Residential System Solar Array Charge Controller DC to AC Inverter Distribution Panel AC Battery Distribution Panel DC

24 Small Stand Alone System Solar Array (to power an office) Charge Control Storage: Battery Fuel Gauge Inverter DC to AC