IR2166
When the switch MPFC is turned on, the inductor
LPFC is connected between the rectified line
input (+) and (-) causing the current in LPFC to
charge up linearly. When MPFC is turned off,
LPFC is connected between the rectified line
input (+) and the DC bus capacitor CBUS
(through diode DPFC) and the stored current in
LPFC flows into CBUS. As MPFC is turned on
and off at a high-frequency, the voltage on CBUS
charges up to a specified voltage. The feedback
loop of the IR2166 regulates this voltage to a
fixed value by continuously monitoring the DC
voltage and adjusting the on-time of MPFC
accordingly. For an increasing DC bus the on-
time is decreased, and for a decreasing DC bus
the on-time is increased. This negative feedback
control is performed with a slow loop speed and
a low loop gain such that the average inductor
current smoothly follows the low-frequency line
input voltage for high power factor and low THD.
The on-time of MPFC therefore appears to be
fixed (with an additional modulation to be
discussed later) over several cycles of the line
voltage. With a fixed on-time, and an off-time
determined by the inductor current discharging
to zero, the result is a system where the
switching frequency is free-running and
constantly changing from a high frequency near
the zero crossing of the AC input line voltage,
to a lower frequency at the peaks (Figure 7).
V, I
t
Figure 7: Sinusoidal line input voltage (solid line), triangular
PFC Inductor current and smoothed sinusoidal line input
current (dashed line) over one half-cycle of the line input
voltage.
When the line input voltage is low (near the zero
crossing), the inductor current will charge up to
a small amount and the discharge time will be
fast resulting in a high switching frequency.
When the input line voltage is high (near the
peak), the inductor current will charge up to a
higher amount and the discharge time will be
longer giving a lower switching frequency. The
triangular PFC inductor current is then smoothed
by the EMI filter to produce a sinusoidal line
input current.
The PFC control circuit of the IR2166 (Figure 8)
only requires four control pins: VBUS, COMP,
ZX and PFC. The VBUS pin is for sensing the
DC bus voltage (via an external resistor voltage
divider), the COMP pin programs the on-time of
MPFC and the speed of the feedback loop, the
ZX pin detects when the inductor current
discharges to zero (via a secondary winding
from the PFC inductor), and the PFC pin is the
low-side gate driver output for MPFC.
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INFINEON [ Infineon ]
