An inverter controller may be used to control, among other things, a vehicle traction drive. In such cases, Discontinuous Pulse Width Modulation (DPWM) methods are commonly employed to control the fundamental output voltage component of voltage source inverters. For example, DPWM methods may be used to control the switching action of three-phase voltage source inverter. These three-phase voltage source inverters may in turn be used to control the phase currents of three-phase alternating current (AC) motors in electric drives.
Ideally, the switch pairs in each phase leg of the three-phase voltage source inverter each operate in a complimentary fashion such that one switch is always “on” and the other switch is always “off.” In practice, however, the switches are not ideal, and a blanking time, or dead-time, is typically inserted during each transition of a switching state of the voltage source inverter. The dead-time is a short interval during which both switches are gated “off.” This prevents both switches in a phase leg of the voltage source inverter from simultaneously being “on,” which could short-circuit the voltage source inverter.
Additionally, the gate drive circuitry may have limitations or the switches may impose limitations on the minimum “on” time duration that is commanded (e.g., directed by a control module, processor, or the like) to a switch in the voltage source inverter. The minimum pulse width limitations and dead-time limitations together result in finite minimum (e.g., non-zero) and maximum (e.g., non-unity) values of duty cycle which can be commanded by the controller (e.g., a DPWM modulator).
Conventional DPWM methods typically switch two phases of the inverter during any given switching cycle with the remaining phase having one switch continuously “on.” When the controlled output voltage is small, such as during low speed operation of the electric drive, the two phases that are switching are often required to produce output duty cycles that are very small or very large. As a result, the presence of maximum and minimum achievable output duty cycles impacts the accuracy of the output voltage.
Accordingly, it is desirable to provide a voltage source inverter controller that improves the accuracy of the output voltage. In addition, it is desirable to provide a method for controlling a voltage source inverter that improves the accuracy of the output voltage of the voltage source inverter. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.