ISL6731A Datasheet, PDF (12/20 Pages) Intersil Corporation – Power Factor Correction Controllers

ISL6731A, ISL6731B
Since the R_CS sees the average input current, high value R_CS
generates high power dissipation on the R_CS. Use a reasonable
R_CS according to the resistor power rating. The worst-case power
dissipation occurs at the input low line when input current is at
its maximum. Power dissipation by the resistor is:
P_RCS = I_RMSMAX²  R_CS
(EQ. 5)
where:
I_RMSMAX is the maximum input RMS current at the minimum
input line voltage, V_RMSmin.
Select the R_SEN according to the peak current limit requirement.
The resistor is sized for an overload current 25% more than the
peak inductor peak current.
Negative Input Capacitor Generation
(Patent Pending)
The patent pending negative capacitor generation capability of
ISL6731A and ISL6731B allow the capacitor C_F2 to be moved
from before the bridge rectifier (Figure 12) to after the bridge
rectifier (Figure 13). Thus, a smaller, lower cost C_F2 can be used.
The change in topology reduces the size of the EMI filter.
Furthermore, C_F1 can be increased thus decreasing the size of L_F
(Figure 13).
EMI CHOKE ^LF
BRIDGE RECFIFIER
V_LINE
C_F3
C_F2
C_F1
L_m
FIGURE 12. TYPICAL PFC INPUT FILTER CIRCUIT
V_LINE
BRIDGE RECFIFIER
EMI CHOKE
L_F
C_F3
L_m
C_F2
C_F1
FIGURE 13. LOW COST PFC INPUT FILTER CIRCUIT
For applications where the output power is above 500W, the
negative capacitance helps to improve the power factor
dramatically. Refer to Table 2 for the recommended filtering
capacitor to be placed after the bridge rectifier, C_F1.
TABLE 2. RECOMMENDED FILTERING CAPACITOR
C_F1
Typical
C(µF)/100W
P_O < 100W
0.68
100W < P_O < 500W P_O > 500W
0.33
0.22
Additional C_F1 may be used to accommodate the use of small
boost inductor or to eliminate the differential mode filter inductor
as long as the equipment meets the power factor or goal.
The equivalent negative capacitor is a function of the input
voltage divider ratio, K_BO, the current sensing gain and current
compensation error integration gain.
Adjusting the negative CEQ can be achieved by adjusting the
current compensation network.
Frequency Modulation
The ISL6731A and ISL6731B can further reduce EMI filter size by
lowering the differential noise power density. The reduction is
achieved by switching frequency modulation.
The frequency varies with the VIN pin. The switching frequency
reaches the peak value when the VIN pin voltage is 2V as shown
in Figure 6. The peak value of ISL6731A is 124kHz, and the
ISL6731B is 62kHz.
Output Voltage Regulation
The output voltage is sensed through a resistor divider. The
middle point of the resistor divider is fed to the FB pin. The
resistor divider ratio sets the output voltage. The
transconductance error amplifier generates a current in
proportion to the difference between the FB pin and the 2.5V
internal reference. The PFC is stabilized by the compensation
network that is connected from the COMP pin to the ground.
The voltage of the COMP sets the input average power by
determining the amplitude of the current reference. To keep the
harmonic distortion to a minimum, it is desirable to set the
control bandwidth much lower than twice of the line frequency.
The recommended voltage loop bandwidth is 10Hz.
During start-up, the compensation capacitors and the charging
current from the error amplifier sets the input power increase
rate. Thus, soft-start is achieved.
The COMP is discharged during shutdown and fault conditions.
Light Load Efficiency Enhancement
For PC, adaptor and TV applications, it is desirable to achieve
high efficiency at light load conditions and low standby current.
The ISL6731A and ISL6731B can enter light load skip mode
automatically. The skip mode trigger threshold is adjustable by
the SKIP pin. A 20µA current source out of the SKIP pin sets the
voltage on the pin via a resistor connected between the pin and
ground. Connecting this pin to ground disables the light load skip
function.
The voltage error amplifier output, COMP, is an indicator of the
average input power level. The controller compares the V(COMP)
and V(SKIP). If V(COMP)-1V is less than V(SKIP)*0.25, the PFC
controller stops gate switching and the COMP pin voltage is
clamped to V(SKIP)+0.6V.
The controller exits skip mode when V_FB drops to 88% (typical) of
the reference voltage or when the sensed returned current
exceeds 29µA.
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FN8582.1
February 13, 2015