Implementation of Unipolar PWM Modulation for H-Bridge Inverter EE462L, 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)

H-Bridge Inverter Basics – Creating AC from DC ! H-Bridge Inverter Basics – Creating AC from DC Switching rules Vdc Either A+ or A – is closed, but never at th e same time Either B+ or B – is closed, but never at the same time A+ B+ Can use identical isolated firing signals for A+, A–, with inverting and non-inverting drivers to turn on, turn off simultaneously Va Load Vb Same idea for B+, B– A – 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

! 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 Ratio mf = frequency of triangle wave divided by frequency of control signal

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

! 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 + B + Local ground Local ground reference for A + reference for B + firing circuit firing circuit S S Load A – B – Local ground Local ground reference for A − reference for B − firing circuit firing circuit S S

This year’s circuit

Comparator Gives V(A+,A–) wrt. Common (0V) Output of the Comparator Chip +12V –12V Comparator Gives V(A+,A–) wrt. Common (0V) Vcont > Vtri Vcont < Vtri V(A+,A–) +12V from DC-DC chip 1.5kΩ 1.5kΩ 270kΩ Since the comparator compares signals that can be either positive or negative, the comparator must be powered by ±V supply Vtri Vcont 1kΩ 8 5 Comp 1 4 +24V 0V Vcont > Vtri Vcont < Vtri Use V(A+,A–) wrt. –12V 270kΩ –Vcont –12V from DC-DC chip Common (0V) from DC-DC chip

Comparator Gives V(B+,B–) wrt. Common (0V) Output of the Comparator Chip +12V –12V Comparator Gives V(B+,B–) wrt. Common (0V) –Vcont > Vtri – Vcont < Vtri V(B+,B–) +12V from DC-DC chip 1.5kΩ 1.5kΩ Since the comparator compares signals that can be either positive or negative, the comparator must be powered by ±V supply 270kΩ Vtri Vcont 1kΩ 8 5 Comp 1 4 +24V 0V – Vcont > Vtri – Vcont < Vtri Use V(B+,B–) wrt. –12V 270kΩ –Vcont –12V from DC-DC chip Common (0V) from DC-DC chip

This year’s circuit

+4V 0V −4V

+24V 0V +24V 0V

+24V 0V +24V 0V +24V 0V +24V 0V

+24V 0V −24V

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

Approaching a square wave