Power factor corrector
The power stage of the Power Factor Corrector is a boost converter represented by Block 4. The circuit operates with 100 kHz constant frequency in continuous inductor current mode. Because of the relatively high switching frequency, a loss-less snubber has been added to the basic boost converter to reduce switching losses and semiconductor stresses.
When the boost transistor conducts, the energy is stored in the boost inductor increases. During the off-state of the transistor, energy is transferred from the inductor to the output capacitor through the boost diode. The inductor current is measured with a sense resistor and it is forced to follow the input voltage waveform. The technical literature refers to this technique as the resistor emulation mode, which is the most preferred load by utility companies.
The output Capacitor of the boost converter is marked by number 5 in the block diagram. This capacitor is used for low-frequency energy storage as well. Due to the nature of ac sources, the energy absorbed at the input of the unit varies according to the mains cycle. In order to deliver constant power at the output energy must be stored inside the unit. Therefore, high voltage 450V electrolytic capacitors are used at the output of the boost converter to provide cost and volume effective energy storage.
Block 12 is the controller of the Power Factor Corrector. It uses the UC3854B integrated circuit which had been developed to control boost converters in power factor corrector applications. This integrated solution takes care of all sensing, controlling, and protection functions that are necessary to achieve proper input current waveform and to stabilize the output voltage of the power factor corrector circuit. The control principle implemented in the UC3854B is average current mode control.
The heart of the module is the dc/dc Converter shown in Blocks 6-9. Block 6 shows the primary arrangement of the full-bridge power converter employing a safety isolated high-frequency transformer. Because of its important role in providing safety isolation between the input and the output of the module, the transformer coupling is emphasized in Block 7. The secondary side of the dc/dc stage provides rectification (Block 8) and filtering (Block 9) functions which are realized using current-doubler topology. The particularity of the implemented solution is the integration of two inductors on a common ferrite core.
The full-bridge converter takes energy from its input when two diagonally located switches are turned on at the same time. This energy is transferred to the output through the transformer immediately. The energy will be stored in the output filter inductor showed in Block 9 and transferred to the output capacitor of the dc/dc converter during the passive interval when energy is not absorbed from the source. This sequence can be achieved by different ways depending on the implemented control strategy.
The dc/dc Controller, shown in Block 13, is using the phase-shift pulse width modulation technique which provides loss-less, zero voltage turn-on condition for the primary side semiconductors. A further benefit is the greatly reduced electromagnetic interference generated by the converter. The control principle is peak current mode control.
Like the power stage, the controller circuit of the dc/dc converter is also divided between the primary and the secondary side of the rectifier. Communication between the separated parts are realized using optical isolators marked by number 14.
The major part of the dc/dc controller is referred to as the Secondary Controller in Block 15. The secondary side controller is responsible for output voltage and current regulation functions.
Block 10 forms the physical Output Section of the sub-module. It is a shielded, common-mode, low-pass filter stage to reduce conducted electromagnetic interferences to the required level.
The name Housekeeping refers to the auxiliary power supply and to all internal primary side supervisory functions necessary for the operation of the unit. Besides the auxiliary power converter (current-mode controlled flyback converter), Block 11 also includes the master clock, under-, and over-voltage lock-out, and start-up sequence generator.
The power system controller can set the rectifier into the 3 modes of operation, i.e. ‘auto float’, ‘auto charge’ and ‘manual boost’.
The output characteristic is different for these 3 modes as shown below :