Control of switching converters

Fig. 1 - Block scheme (click for larger image)



A technological development which was recently carried out was related to design of the modulation section of a switching converter.
The converter is part of a power supply system composed of 11 switching dc/dc converters rated for 600V/200A which has been designed, tested and it is now operating on RFX experiment. It an industrial product with the power section based on the full bridge topology, implemented using Insulated Gate Bipolar Transistors (IGBTS), controlled by double Pulse Width Modulation technique. As for the control section, it implements a full digital 10kHz DPWM modulator, realized with EPLD’s devices, which also include a new compensation technique of the converter non-linearities which was developed in RFX in collaboration with the industry.

The typical operation of a full bridge requires the presence of a lag time between the turn off and turn on commands of the same leg switches which causes an error on the output voltage waveform and therefore on the load current. A linearization tecnique was worked out to compensate for this error; it is based on a feedback technique which performs an estimate of the missing voltage area and on its compensation during the next PWM commutation. A digital design seemed to be the most suitable solution to implement the modulator with a high flexibility level; the block scheme is shown in Fig. 1.




Fig. 2 - Output current waveforms with and without linearization technique. Vdc = 500V, fPWM = 10kHz, Vref = 1V, fref = 10HzThe performance obtained in terms of linearity are very satisfactory; tests with sinusoidal references showed that the converter maintains a quasi-sinusoidal output current supplying output voltage values till about 2% of the maximum and reference frequency of 0.1Hz. In Fig. 2 a result with 10Hz, 1V sinusoidal reference is shown; the improvement of the output current waveform with the linearization technique can be observed.

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