Solution to instability problems of grid-connected PWM dc-ac inverters

The stability properties of the forward and the flyback inverter are compared. The former combines high efficiency, reliability, and power capacity. In order to obtain low distortion and to avoid problems with the firing and the quenching of thyristors, the switching frequency must be externally determined. Instabilities are avoided by means of a described feedback loop; increasing the low-frequency open circuit gain with the wrong circuit parameter decreases the output power. A free-running inverter is found not to be suitable for grid-connected applications. The forward inverter has 250 W capacity, its input voltage is 48 V and the efficiency is 75 percent at 100 W, a good result for a very low input voltage. The switching frequency is 50 kHz.