LTSPICE
Simulating our circuit in SPICE was one of the first steps to help us gain insight to how the circuit worked and information about optimizing it. Of course, to begin our simulation we needed to pick components, and we started with our diodes and FETs. The important characteristics to consider in our component selection were voltage ratings, because we needed components capable of handling our voltage rails; low reverse recovery time in the diode, to minimize switching losses; low gate capacitance in the FETS, again to minimize switching losses; and low on resistance in the FETs, to keep the conduction losses low. After picking our switching components, we made a first guess at capacitor values for the clamped point and calculated values we thought would be large enough to filter out the high frequencies in our PWM signal above the 60Hz sine wave we wanted, then laid out the circuit topology in SPICE.
SPICE Single-Phase Simulation
The greatest complication in our SPICE modeling was modelling the gate driving signals, a task that proved much more formidable than we had initially anticipated. We started by using complex PWR voltage sources to emulate the signals, then made analog comparators and Boolean logic gates in SPICE, then discovered SPICE’s ability to use Boolean logic to create our intended waveforms. After this discovery, a simple triangle-sine comparator was implemented to greate a varying-width pulse signal, which was anded with appropriate signals to generate our desired output. As a reference, in the picture below, Fig. 24A represents the gate driving waveform on our top fet, 24B corresponds to the second fet, 24C to the third fet, and 24D to the bottom fet. Our actual simulated SPICE gate drive signals are show in the following picture, as well as our final filtered sine wave.
Desired Gate Driver Wave Outputs
SPICE Triangle Sine Comparator
Conditional PWM Signals
SPICE Gate Driver Waveform Output
SPICE Filter Sine Wave Output
