Voltage Source Converter

[stevendhansen]

A question for the guru's out there....

I am trying to understand the basic control methodology for voltage source converters (AC to DC conversion). What puzzles me is this: how can the output voltage be DC when Vref is a sinusoid? Even if you put tons of capacitance on the output won't the Hbridge produce a sinusoidal output??

Secondly, how is the amplitude of the output voltage actually adjusted? From what I read, the amplitude of Vref is what controls the amplitude of the output voltage and the phase of Vref controls the power factor. Is this even remotely correct or am I backwards here?

I would really appreciate someone's input on this, thanks!

 

[RCinFLA]

There are a lot of basic components to your question.

First you rectify and filter the AC input to DC. There may be a low frequency transformer before the rectifier or in the case of high frequency switching power supply a small ferrite based transformer to reduce the rectified AC input line voltage.

A reference can be made from a zener diode with a dropping resistor from the higher voltage DC. There are integrated circuit reference voltage generators known as bandgap references that work on diode drop principles and have near zero change versus temperature.

There is a feedback control electronics that drive the regulation of the output voltage. Varying the output voltage is simply a resistor divider network that outputs a voltage near the Vref voltage. The difference between feedback voltage and reference voltage drives the control regulator to maintain the correct output voltage.