Diodes AP1662 - Manual (Page 14)

Data Sheet
HIGH PERFORMANCE POWER FACTOR CORRECTOR AP1662
Jul. 2012 Rev. 1. 3 BCD Semiconductor Manufacturing Limited
14
Functional Block Description
AP1662 is a high performance power factor
correction controller which operates in DCM
boundary conduction mode. The PFC converter's
switch will be turned on when the inductor current
reduces to zero and turned off when the sensed
inductor current reaches the required reference which
is decided by the output of multiplier.
Error Amplifier and Over-Voltage
Protection
The error amplifier regulates the PFC output voltage.
The internal reference on the non-inverting input of
the error amplifier is 2.5V. The error amplifier's
inverting input (INV) is connected to an external
resistor divider which senses the output voltage. The
output of error amplifier is one of the two inputs of
multiplier. A compensation loop is connected outside
between INV and the error amplifier output.
Normally, the compensation loop bandwidth is set
very low to realize high power factor for PFC
converter.
To make the over voltage protection fast, the internal
OVP function is added. If the output over voltage
happens, excess current will flow into the output pin
of the error amplifier through the feedback
compensation capacitor. (see Figure 22) The AP1662
monitors the current flowing into the error amplifier
output pin. When the detected current is higher than
40µA, the dynamic OVP is triggered. The IC will be
Figure 22. Error Amplifier and OVP Block
disabled and the drive signal is stopped. If the output
over voltage lasts so long that the output of error
amplifier goes below 2.25V, static OVP will take
place. Also the IC will be disabled until the output of
error amplifier goes back to its linear region. R1 and
R2 (see Fig. 22) will be selected as below:
1
5 . 2 2
1
− =
V
V
R
R
O
A
V
R OVP
µ 40
1
∆
=
Multiplier
The multiplier has two inputs. One (Pin 3) is the
divided AC sinusoidal voltage which makes the
current sense comparator threshold voltage vary from
zero to peak value. The other input is the output of
error amplifier (Pin 2). In this way, the input average
current wave will be sinusoidal as well as reflects the
load status. Accordingly a high power factor and
good THD are achieved. The multiplier transfer
character is designed to be linear over a wide
dynamic range, namely, 0V to 3V for Pin 3 and 2V to
5.8V for Pin 2. The relationship between the
multiplier output and inputs is described as below
equation：
MULT COMP CS
V V k V
Ч − Ч = ) 5 . 2 (
where V
CS
(Multiplier output) is the reference for the
current sense, k is the multiplier gain, V
COMP
is the
voltage on pin 2 (error amplifier output) and V
MULT
is
the voltage on pin 3.
Current Sense/Current Sense
Comparator
The PFC switch's turn-on current is sensed through
an external resistor in series with the switch. When
the sensed voltage exceeds the threshold voltage (the
multiplier output), the current sense comparator will
become low and the external MOSFET will be turned
off. This insures a cycle-by-cycle current mode
control operation. The maximum current sense
reference is 1.8V. The max value usually happens at
startup process or abnormal conditions such as short
load.