ISL6730D_14 Datasheet, PDF (11/19 Pages) Intersil Corporation – Power Factor Correction Controllers

ISL6730A, ISL6730B, ISL6730C, ISL6730D
V_I
L
Q1
CF1
RCS
RSEN
VOUT
COUT
CURRENT
MIRROR
2:1
ICS
ISEN
ICS > 0.5 IOC
FIGURE 11. INDUCTOR CURRENT SENSING SCHEME
A high value R_CS renders more accurate current sensing. It is
recommended to use the R_CS to render 120mV peak voltage at
the maximum line voltage during full load condition.
R_C
S
>
1_----2_---0_----m_-----V_------⋅_---V_{----R----M-----S----M-----A----X-----}⋅_---η_--
2 ⋅ P_Omax
(EQ. 4)
Where η is the efficiency of the converter at the maximum line
input with full load.
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
the ISL6730A, ISL6730B, ISL6730C, ISL6730D allows 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 RECTIFIER
V_LINE
C_F3
C_F2
C_F1
L_m
FIGURE 12. TYPICAL PFC INPUT FILTER CIRCUIT
BRIDGE RECTIFIER
EMI CHOKE
L_F
V_LINE
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. Please refer to Table 2 for the recommended
filtering capacitor to be placed after the bridge rectifier, C_F1.
TABLE 2.
C_F1
Typical
C(µF)/100W
Po < 100W
0.68
100W < Po < 500W Po > 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 ISL6730A, ISL6730B, ISL6730C, ISL6730D 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 ISL6730A/C is 124kHz, and the
ISL6730B/D 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
11
FN8258.1
August 8, 2013