DC to AC Converters Inverters - elg.uottawa.ca

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. IDC Iac + +. VDC Vac .. Harmonics Filtering DC SUPPLY INVERTER (LOW PASS) FILTER LOAD. L. + +. C. vO 1 vO 2.. BEFORE FILTERING AFTER FILTERING. vO 1 vO 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.

2 Good filtering is a must. In some applications such as AC motor drive, filtering may not required. Single Phase Half-Bridge Inverter S1 ON. Vdc S2 OFF. +. S1 2. VC1. - V +. Vdc o G 0. t RL. +. VC2 S2. - Vdc . 2 S1 OFF. S2 ON. 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. Q1 on, Q2 off, vo = Vs/2. Peak Reverse Voltage of Q2 = Vs Q1 off, Q2 on, vo = -Vs/2. Single Phase Full Bridge VRG. Vdc 2. LEG R LEG R' p 2p wt +.

3 Vdc Vdc S1 S3 . 2 VR 'G 2. + - Vdc + Vo - 2. Vdc R R'. G p 2p wt - + Vdc . 2. Vdc S4 S2 Vo 2. Vdc - p 2p wt Vo = V RG VR 'G. G is " virtual groumd". Vdc 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. Q1-Q2 on, Q3-Q4 off, vo = Vs + Vs - Q3-Q4 on, Q1-Q2 off, vo = -Vs - Vs +. Performance Parameters Harmonic factor of the nth harmonic (HFn). Von HFn = for n>1. Vo1. Von = rms value of the nth harmonic component V01 = rms value of the fundamental component Total Harmonic Distortion (THD): Measures the closeness in shape between a waveform and its fundamental component 1.

4 1. THD = ( Von2 ) 2. Vo1 n =2,3,.. Design Constraints of a Pure Sine wave Inverter Quantity Details Voltage Convert 12 VDC to 120 VAC. Power Provide 300 W continuous Efficiency > 90% efficiency Waveform Pure 60 Hz sinusoidal Total Harmonic < 5% THD. Distortion Physical Dimensions 8 x x . Cost $ Required Components for Design 12 V DC Input from vehicle battery). PWM Control Half-bridge Transformer Circuit converter Low-pass Full-bridge Sinusoidal PWM. Filter Inverter Controller 120 VAC, 60 Hz, 300 W. Output PWM Controller Produces two complementary pulses to control half-bridge transistors.

5 Problem: Voltage may drop when the input voltage is decreased. Solution: A feedback network may be added for voltage regulation. Pulse Width Modulation h( x) = if ( k ( x) c ( x) 1 if ( k ( x) c ( x) 1 0) ). 1. t 1 t2 Sinusoidal modulating Carrier, vc(t). waveform, vm(t). Modulating Waveform Carrier waveform +1. M1. 2p t 0 p 1. Regular sampling waveform, vs (t ). Vdc t'1 t'2. 2. 0 t0 t1 t 2 t3 t 4 t 5. v pwm t Vdc . 2. 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.

6 Basically an analogue method. Its digital version, known as REGULAR sampling is widely used in industry. Software Flow Diagram (Dr. Yaroslav Koshka). Initialize all variables no 300 duty cycle yes Count0 = 300 (300 duty cycles) values? Output 1 = high, Output 2 = low Output 1 = low, Output 2 = high duty cycle table (increment pointer). Decrement Count0 by 1. Duty cycle and sampling period timer no One Sampling ye no yes Period? s Has duty cycle been reached? Low-pass Filter 2nd order L-C filter Filters to retain a 60. Hz fundamental frequency Few components Handle current Wind inductor (fine tune).

7 PCB Layout Case Study: Solar System Using Inverters Stand Alone; Simple Grid Tied; Grid Tie with Battery Solar Schoolhouse and San Mateo College Simple Grid-Connected System Utility Solar Array Inverter Distribution Panel Subpanel Solar AC in from Inverter Lightning surge arrestor Stand Alone Residential System Solar Array Distribution Panel Charge AC. Controller DC to AC Inverter Battery Distribution Panel DC. Small Stand Solar Array Alone System (to power an Charge office) Control Storage: Battery Fuel Gauge . Inverter DC to AC.