The Design of a Bi-level Input DC/AC Automotive Inverter

- The Inverter -

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Conceptually, the inverter works by turning on switches in a specific order.  A low power circuit must be constructed to perform as the control circuitry that drives the high power stage.  One such control method is the Square-Wave Inverter.  In Figure 1, if switches SW1 and SW4 are turned on, the load is subjected to +Vs.  However, if SW2 and SW3 are turned on, the load is subjected to Vs.  Note that Vs is a constant DC voltage.  Assuming the switches are voltage controlled, Figure 2 shows possible input waveforms to the switches.  Figure 3 shows the corresponding output waveform.  As shown, the source DC voltage is converted to a square-wave AC voltage.   

H-Bridge Circuit Diagram

Figure 1:  Basic Concept Diagram Inverter 

Text Box: SW2, SW3
 SW1, SW4

 

 Switching Voltages

Figure 2:  Switch Waveforms 

Text Box: Load Voltage

 

Load Voltage (Square Wave)

Figure 3:  Output Waveform

Since a square wave is not suitable for most applications, significant filtering must be implemented to reduce the square wave to a sinusoidal wave.  This can be accomplished since a square wave can be represented as a Fourier series.  The Fourier series of a square wave is the sum of a sine wave of the fundamental frequency and its odd harmonics.  Another practical consideration is the fact that real switches can not change states instantaneously.  Therefore, there must be time when all switches are in the off state to avoid shorting the supply voltage.

Another technique to control the output of an inverter is pulse-width modulation (PWM).  PWM control compares the output waveform to a reference signal and adjusts the duty cycle of the switching mechanism.  Compared to the square-wave method of control, pulse-width modulation has two distinct advantages:  higher frequency harmonic content and amplitude control.  The filtering requirements for PWM generated outputs are less stringent since the non-fundamental components are of much higher frequency than that of the fundamental sine wave.  Also, by using PWM techniques, the output voltage of the inverter can be directly controlled.  Pulse-width modulators are available in IC packages and their exact method of control varies by manufacturer and by model.

For PWM control, two inputs are required:  a sinusoidal reference signal, also called a control signal or a modulating signal, and a carrier signal [5].  The triangle wave controls the switching frequency of the inverter.  Bipolar and unipolar switching are the two available methods of switching.  Both methods compare the reference signal and the carrier signal and cause switching conditions that correspond to the two signals.  For bipolar switching, the output is either +VDC or VDC corresponding to the reference voltage being greater than the carrier wave and the reference being less than the carrier, respectively.  The output is VDC when the sine wave is less than the carrier wave.  For unipolar switching, the output is switched between positive voltage and zero or negative voltage and zero.

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Last updated: 04/22/03.