Microchip Technology dsPICDEM MCHV-3, Руководство пользователя, Страница: 33 / 68

Hardware

2016 Microchip Technology Inc. DS50002505A-page 33
3.1.3 PFC Feedback Circuitry
In a digitally controlled PFC, the relevant analog parameters and control loops need to
be redefined and discretized. This enables changeover from existing hardware, to its
digital counterpart, easier and more logical.
The PFC is an AC-to-DC Converter, which converts the AC input voltage to a DC
voltage and maintains sinusoidal input current at a high input power factor. In a digitally
controlled PFC, three inputs signals are required to implement the current control.
The input rectifier (BR1) converts (shown in
Section 3.1.1 “AC Supply Input”
) the alter-
nating voltage at power frequency into a unidirectional voltage. This rectified voltage is
fed to the chopper circuit to produce a smooth and constant DC output voltage to the
load. The chopper circuit is controlled by the PWM switching pulses, generated by the
Microchip PIC
®
MCU/dsPIC
®
DSC devices, based on four measured feedback signals:
• Rectified input voltage
• AC input voltage zero-crossing event
• Rectified input current
• DC bus voltage
The rectified input voltage is measured in two stages:
• Signal conditioning: R22, R23, R24, R25, C24 and D7 convert and filter the AC
input voltage waveform, from 90-265 VAC to 0-3.3 VAC. D7 limits the maximum
voltage to 3.3V. C24, along with R22, R23 and R25, form a low-pass filter. The
circuit is shown in
Appendix A. “Board Layout and Schematics”
.
• Amplification: The operational amplifier, MCP6024 (U13B), amplifies the
conditioned AC input voltage; R45, R46, R47, R48, R49 and R51 set the gain.
The MC6024 also shifts the conditioned AC input voltage to a 1.65V DC level.
Therefore, the voltage applied to the dsPIC
®
DSC ADC channel varies within 0V
to 3.3V. The offset is controlled by R50, R53 and R54. R39 and C39 filter out the
high-frequency noise.
An AC input voltage, zero-crossing event is sensed using a voltage divider (R2-R5) and
two optocouplers (U1 and U2). The circuit is shown in
Appendix A. “Board Layout
and Schematics”
.
Rectified input current is measured using the shunt resistor, R34, and the operational
amplifier, MCP6024 (U13A); R38, R39, R40, R41, R42 and R43 set the gain. R43 shifts
the voltage present at the shunt resistor to a 1.65V DC level. Therefore, the voltage
applied to the dsPIC
®
DSC ADC channel varies within 0V to 3.3V. The offset is con-
trolled by R43, R53 and R54. R44 and C37 filter out the high-frequency noise. The
circuit is shown in
Appendix A. “Board Layout and Schematics”
.
The DC bus voltage is sensed at the power module stage. Please refer to
Section 3.2.4 “Feedback Circuitry”
for more information.