Buck-boost transformer and PWM topology help OEMs operate equipment in countries with mains fluctuating outside design limits

High-speed VRp PWM AVR

Its basic topology is a buck-boost transformer with a primary to secondary ratio of 5:1 for a voltage correction of +-20%. The input phase is connected to one end of the secondary, while the critical load is connected to the other end of the secondary with the current returning to the source via the neutral wire. The control voltage is imposed on the primary winding by connecting one end of the primary to the incoming phase, while the other end of the primary is connected to the neutral—in this manner 20% of the input voltage will be added to the secondary winding. If the connections are reversed the voltage will drop 20%, as the magnetic flux imposed by the primary winding is bucking the secondary winding.



The basic idea of VRp is to accomplish this task electronically without the step changes in voltage that occur when the system regulates. This is accomplished through a feedback and control system implemented by using a microcontroller. The system uses internal gate bipolar transistor (IGBT) power switches to form two power stacks. Rectified voltage is supplied to the inverter, the microprocessor measures the system output voltage fed back from the system and compares this voltage against a reference. Corrections are made by varying the duty cycle of PWM pulses. The more voltage that needs to be added the longer the duty cycle. The system responds in 20ms to any changes in mains voltage.