EE 462 L Spring 2014 Implementation of Unipolar

EE 462 L, Spring 2014 Implementation of Unipolar PWM Modulation for H-Bridge Inverter (pre-fall 2009 - but discrete components provide a better sense of how this circuit operates) 1

H-Bridge Inverter Basics – Creating AC from DC Switching rules Either A+ or A– is closed, but never at the same time Either B+ or B– is closed, but never at the same time Vdc A+ Va B+ Load A– ! Vb B– Can use identical isolated firing signals for A+, A–, with inverting and noninverting drivers to turn on, turn off simultaneously Same idea for B+, B– The A+, A– firing signal is a scaled version of Va The B+, B– firing signal is a scaled version of Vb The difference in the two firing signals is a scaled version of Vab 2

! Implementation of Unipolar 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 Vtri is a triangle wave whose frequency is at least 30 times greater than Vcont. Ratio ma = peak of control signal divided by peak of triangle wave 3 Ratio mf = frequency of triangle wave divided by frequency of control signal

! Implementation of Unipolar PWM Modulation for H-Bridge Inverter Vload Progressively wider pulses at the center Progressively (peak of narrower pulses sinusoid) at the edges Vdc Unipolar Pulse-Width Modulation (PWM) −Vdc 4

The four firing circuits do not have the same ground reference. Thus, the firing circuits require isolation. ! Vdc (source of power delivered to load) A+ Local ground reference for A+ firing circuit S S S Load A– Local ground reference for A− firing circuit B+ Local ground reference for B+ firing circuit B– S Local ground reference for B− firing circuit 5

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This year’s circuit 7

Output of the Comparator Chip Comparator Gives V(A+, A–) wrt. Common (0 V) V(A+, A–) +12 V from DC-DC chip 1. 5 kΩ +12 V 1. 5 kΩ – 12 V 270 kΩ Vtri 8 1 Comp 5 1 kΩ Vcont 4 Vcont < Vtri Since the comparator compares signals that can be either positive or negative, the comparator must be powered by ±V supply Use V(A+, A–) wrt. – 12 V 270 kΩ –Vcont > Vtri +24 V Vcont > Vtri – 12 V from DC-DC chip 0 V Common (0 V) from DC-DC chip Vcont < Vtri 8

Comparator Gives V(B+, B–) wrt. Common (0 V) Output of the Comparator Chip V(B+, B–) +12 V from DC-DC chip –Vcont > Vtri +12 V 1. 5 kΩ – 12 V 1. 5 kΩ 270 kΩ Vtri 8 1 Comp 5 1 kΩ Vcont 4 Since the comparator compares signals that can be either positive or negative, the comparator must be powered by ±V supply Use V(B+, B–) wrt. – 12 V 270 kΩ –Vcont – Vcont < Vtri – 12 V from DC-DC chip Common (0 V) from DC-DC chip – Vcont > Vtri +24 V 0 V – Vcont < Vtri 9

This year’s circuit 10

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+4 V 0 V − 4 V 15

+24 V 0 V 16

+24 V 0 V 17

+24 V 0 V − 24 V 18

+24 V 0 V − 24 V Flat toping indicates the onset of overmodulation +24 V 0 V − 24 V 19

Approaching a square wave +24 V 0 V − 24 V 20

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