*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)

* Switching rules H-Bridge Inverter Basics Creating AC from DC VdcLoad A+ B+ A B Va Vb Can use identical isolated firing signals for A+, A, with inverting and non-inverting drivers to turn on, turn off simultaneouslyThe A+, A firing signal is a scaled version of VaThe B+, B firing signal is a scaled version of VbThe difference in the two firing signals is a scaled version of VabSame idea for B+, B

* VcontVtriVcontImplementation of Unipolar PWMVcont is usually a sinewave, but it can also be a music signal.Vtri is a triangle wave whose frequency is at least 30 times greater than Vcont.The implementation rules are:Vcont is the input signal we want to amplify at the output of the inverter.Ratio ma = peak of control signal divided by peak of triangle wave Ratio mf = frequency of triangle wave divided by frequency of control signal

*Progressivelywider pulses at the center (peak of sinusoid)Progressively narrower pulses at the edgesUnipolar Pulse-Width Modulation (PWM)Implementation of Unipolar PWM Modulation for H-Bridge Inverter

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

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*This years circuit

* 8 5 Comp 1 4 270k VtriVcontVcont270k1k1.5k1.5k12Vfrom DC-DC chip+12Vfrom DC-DC chipCommon (0V) from DC-DC chipOutput of the Comparator ChipSince the comparator compares signals that can be either positive or negative, the comparator must be powered by V supply

* 8 5 Comp 1 4 270k VtriVcontVcont270k1k1.5k1.5k12Vfrom DC-DC chip+12Vfrom DC-DC chipCommon (0V) from DC-DC chipOutput of the Comparator ChipSince the comparator compares signals that can be either positive or negative, the comparator must be powered by V supply

*This years circuit

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*Flat toping indicates the onset of overmodulation

*Approaching a square wave

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