Pure sine-wave inverters (PSWIs) produce a clean AC output, for instance 230V/50Hz, when provided with a 12 or 24V DC source. The AC output from a PSWI meets or exceeds specifications of the power grid, eliminating spikes and brown-outs, and exhibiting very low total harmonic distortion. A majority of inverters offered in the marketplace are modified sine wave or quasi-square wave inverters. The AC power output generated by these inverters is unsuitable for many applications.
Figure 1 shows a new approach to designing PSWIs using new advancements in switched mode power supply (SMPS) digital signal controllers (DSCs). This digital power conversion approach to the design of PSWIs eliminates the need for bulky and expensive low-frequency transformers, thus reducing cost and improving performance.
INVERTER CORE
The inverter control circuitry (Figure 1) is powered by a Microchip dsPIC DSC tailored for SMPS and digital power conversion. The design uses on-chip peripherals, such as pulse width modulation (PWM), analog-to-digital converters (ADCs) and analog comparators. The PWM operates in push-pull mode producing signals at a 1.1 nanosecond duty-cycle resolution. The ADC is automatically synchronized with the PWM, enabling precise and simultaneous measurements relative to the edge of the PWM signal. The comparator allows detection of transient conditions, and is tightly coupled with the PWM and ADC to eliminate processor intervention. These features enable the controller’s processing MIPS to be used for faster processing of the control algorithm and servicing the user interface.
DC to AC conversion takes place in two stages. First, battery voltage is converted to 400VDC, using push-pull topology switching at 20kHz. Next, 400VDC is converted to 230V/50Hz AC, using Insulated Gate Bipolar Transistor H-bridge inverter topology switching at 20kHz. Proportional Integral Derivative control loops are used to maintain output wave shape and constant output RMS voltage.
A highlight of this design is that a single controller is used for both, the inverter, as well as the battery charger, thereby reducing system costs. AC input is rectified through an output diode bridge. The Inverter H-bridge and high-frequency push-pull transformer are used to generate a buck rectified voltage for charging the battery. The body diode of the push-pull MOSFET is used as a secondary rectifier while charging the battery.
MARKET REQUIREMENTS
PSWIs help applications run more reliably, faster, quieter and cooler. Most inverters sold in India are less than 1000 VA, using a single 12 or 24V battery system. This design (shown in Figure 2) produces 800VA-12V and 1500VA-24V. The control algorithm detects power failures, brown-outs and spikes, and switches to the battery in less than 10ms to prevent the PC’s SMPS from rebooting. This enables maximum back-up time for a given battery.
Author information
You can reach
K. Vinod Kumar at
vinod@qsaki.com
Caption: Figure 1: DSC-based high-frequency pure sine-wave inverters eliminate bulky transformers and auxiliary battery-charging circuits.
Figure 2: Compact size and low weight allow the inverter to be housed in plastic cabinets.
