http://www.ti.com.cn/general/cn/docs/lit/getliterature.tsp?baseLiteratureNumber=tidu312&fileType=pdf
The PFC stage consists of two boost converters each operating alternately in boost mode during half of the input AC line cycle.
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During one half of the line cycle one converter converts the AC line voltage to the DC
bus voltage, while the other converter performs thesame action during the other half of
the line cycle. Inductor L1, MOSFET switch Q1, current sense transformer CT1 and
diode D1 together form one of the boost stages while, L2, Q2, CT2 and D2 form the
other boost stage. A capacitor Cb at the boost converter output acts as an energy
reservoir and provides regulated dc voltage to the PFC load denoted by RL.
Figure 1.1 indicates all the interface signals needed for full control of this bridgeless PFC
converter using a C2000 micro-controller (MCU). TheMCU controls the hardware using
five feedback signals, two PWM outputs and one GPIO output. The signals that are
sensed and fed back to the MCU include, the line and neutral voltages (Vin_L & Vin_N),
the two PFC switch currents (Isw1, Isw2), and the boost output voltage (Vbus). These
sensed signals are used to implement the voltage and current control loops for this BL
PFC converter.
The dc bus voltage Vbus, sensed through one of the ADC channels, is compared
against the reference bus voltage Vref. The resulting error signal Ev is then input the
voltage loop controller Gv which regulates the bus voltage at the reference level. The
voltage controller Gv has the form of a two pole two zero (2P2Z) compensator. The
output of Gv, denoted by the letter A in Figure 1.1, is proportional to the amount of power
transfer by the PFC converter. This output A is then multiplied by three parameters,
indicated by B, C and Km in Figure 1.1, in order toform the reference current command
Iref for the PFC current control loop. The signal indicated by B is the inverse of the square of the RMS input voltage which enables fast feed-forward control of the PFC
system. The signal C is proportional to the rectified input voltage, which modulates the
voltage controller output A such that the PFC inputcurrent has the same shape of the
PFC input voltage. The parameter Km is called the multiplier gain which is used to adjust
the range of Iref corresponding to the full input voltage range of the PFC converter. The
output of the multiplier provides the reference signal for control of average inductor
current IrefL. However, for BL PFC system the current feedback is from the PFC
switches (Q1 & Q2), and not from the boost inductors (L1 & L2). This means the
reference signal IrefL for average inductor current control has to be converted before it is
used for PFC switch current control. In Figure 1.1 this is indicated by the conversion
block between IrefL and Iref. This reference current command Iref for the PFC current
control loop is then compared against the PFC switch current Ipfc sensed through two
ADC channels. The resulting current error signal Ei is then input the current loop
controller Gc which generates the PFC duty ratio command d such that the PFC switch
current tracks the reference current Iref.
In addition to implementing the voltage and currentloop controllers, C2000 MCU also
uses the sensed line and neutral voltage signals todetermine the polarity of the input
voltage (+ve & –ve half cycle) and to calculate therectified input voltage, the RMS input
voltage and the input line frequency. Based on the polarity of the input voltage the
C2000 controller selects the appropriate PFC switchcurrent (Isw1 or Isw2) to be used as
PFC current feedback Ipfc. The polarity informationis also used to set the appropriate
PFC switch (Q1 & Q2) to either in PWM mode (boost PFC) or in forced ON mode. All
these time critical functions are implemented in a fast sampling loop enabled by the
C2000 Micro-controller high speed CPU, interrupts, on chip 12-bit ADC module and high
frequency PWM modules. A detailed description of the software algorithm is provided in
the following chapters.