Power Electronics Lab1Lecture 10ECEN 4517/5517Experiment 5Inverter systemExp. 4 Exp. 5Power Electronics Lab2Due datesThis week in lab (Mar. 30 – Apr. 1):Exp. 3 Part 2 report dueNext week in lecture (Apr. 6):Exp. 5 prelab duePower Electronics Lab3Exp. 5H-bridge inverter, off grid• Filtering of ac output not explicitly shown• Need MOSFETs and half-bridge gate drivers• Grid-tied: control iac(t)• Off-grid: control vac(t)Exp. 5: off-grid inverter• Demonstrate modified sine-wave inverter (required)• Demonstrate PWM inverter (extra credit)+HVDCfrom Exp. 4–Power Electronics Lab4“Modified Sine-Wave” Invertervac(t) has a rectangular waveformInverter transistors switch at 60 Hz, T = 8.33 msecT/2DT/2+ VHVDC– VHVDCvac(t)RMS value of vac(t) is: • Choose VHVDC larger than desired Vac,RMS• Can regulate value of Vac,RMS by variation of D• Waveform is highly nonsinusoidal, with significant harmonicsPower Electronics Lab5PWM InverterAverage vac(t) has a sinusoidal waveformInverter transistors switch at frequency substantially higher than 60 Hz• Choose VHVDC larger than desired Vac,peak• Can regulate waveshape and value of Vac,RMS by variation of d(t) (programming inside microcontroller)• Can achieve sinusoidal waveform, with negligible harmonics• Higher switching frequency leads to more switching loss and need to filter high-frequency switching harmonics and common-mode currents• For the same Vac,RMS, need larger VHVDCtvac(t)Power Electronics Lab6Two ways to generate a PWM sinusoidtvac(t)(a) Operate left and right sides with same (complementary) gate drive signalsv(t) = (2d(t) – 1) Vg(b) PWM one side, while other side switches at 60 Hzv(t) = ± d(t) VgTwo-level waveformThree-level waveformPower Electronics Lab7Controlling the inverterMSP430 generates logic signals to control the four gate drivers• Control MSP430 timer (you could use A or B or simply use logic outputs) to generate MOSFET drive signalsYour goal: adjust Vref and inverter duty cycle to obtain Vac = 120 V rmsPower Electronics Lab8Gate drive timingQ1Q2Q3Q4T = 16.67 msecDTdeadtimeT/2• For modified sine wave inverter: switch once per ac half cycle. Adjust duty cycle to control rms voltage. • You decide how to do this / which timer to use • Require deadtime > (switching/delay times of MOSFETs plus gate drivers); otherwise, simultaneous conduction of Q1 and Q2 causes “shoot-through” current that can damage MOSFETs.Power Electronics Lab9Half-bridge Gate DriverFAN 73832Contains two MOSFET drivers:• Low side driver• High side driverHigh side driver includes• Level-shifting circuitry• Provisions for bootstrap power supplyUndervoltage lockout circuitry holds MOSFETs off when driver power supply is below threshholdPower Electronics Lab10Half bridge gate driver circuit exampleHigh side circuitry includes external diode and capacitor for bootstrap power supplyTo charge bootstrap capacitor, low side MOSFET must conductIn this example, VCC = 12 VPower Electronics Lab11Filtering the ac outputRemoving the high frequency differential and common mode components of the output waveformNote: the “Kill-a-Watt” power meters cannot tolerate high frequency components in the ac voltage waveform. Do not connect these meters to an unfiltered inverter
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