Series Inverter Output Voltage Explained

A basic series inverter is a type of inverter circuit that converts direct current (DC) into alternating current (AC). These inverters are primarily used in high-frequency applications, like induction heating or fluorescent lighting, where the load forms part of a series resonant circuit.

Basic Series Inverter Operation

In a series inverter, the load is connected in series with the commutating inductance and capacitance, forming an RLC series circuit. The switching devices, often Silicon Controlled Rectifiers (SCRs), are turned ON and OFF to excite this resonant circuit. When the SCRs are triggered, the current flows through the resonant circuit, producing a resonant voltage across the capacitor or the load.

Key characteristics of a basic series inverter's operation:

• The load is inherently part of the resonant circuit.
• SCRs used in these inverters can achieve natural commutation, meaning they turn off automatically when the current flowing through them falls to zero.
• The operating frequency is typically controlled to be close to the resonant frequency of the LC circuit.

Discontinuous Sinewave Output Explanation

The output voltage wave shape of a basic series inverter is a discontinuous sinewave. This occurs because of the natural commutation property of the SCRs. As the current in the resonant circuit passes through zero, the conducting SCR turns off. This causes the output voltage to momentarily drop to zero or become inactive until the next SCR is triggered to initiate the next half-cycle of current flow.

Since the SCRs can only conduct in one direction and naturally turn off when the current reverses or reaches zero, there are inherent gaps or 'discontinuities' in the voltage waveform. Unlike a parallel inverter, which can often produce a more continuous sinusoidal or quasi-square wave, the series inverter's output is characterized by these segments of sinusoidal waves separated by periods of zero voltage.

Therefore, the voltage across the load or the output terminals is not a perfectly continuous sine wave, but rather segments of sine waves, making it a discontinuous sinewave. This is a fundamental characteristic that distinguishes it from other inverter types.