ISL6731A Datasheet, PDF (15/20 Pages) Intersil Corporation – Power Factor Correction Controllers

ISL6731A, ISL6731B
THE TOTAL LOSS ON THE MOSFET
P_COND + P_SW + P_OSS = 2.71W + 2.09W + 1.28W = 6.08W (EQ. 33)
OUTPUT CAPACITOR SELECTION
The output capacitor, C_O, is required to hold the output above
300V during one line cycle. For capacitors with 20% tolerance,
the tolerance should be taken into consideration. Thus, the
output capacitance should be greater than:
C_O  ²-----^-_V--^T--_O2--^H--_U--^O---_T--^L---–^D----_V--^--_H-₂^P---_O-^O----_LM---_D--^A----^X--  ₁-----–---¹--₀---_.--₂--
(EQ. 34)
C_O  _--²-₃---₉-^---₀₂---⁰_---₂-^m---–--^s---_--^₃---³⁰---⁰-₀--⁰-_V---^W--_--₂-  1.25 = 242F
(EQ. 35)
Calculate the ripple RMS current through the capacitor:
I_{CORMSmax} = I_{OUTmax} ⁸---₃---_--²--  _V-----_R-^V---_M-^O----_SU----_m-^T---_i--_n- – 1
(EQ. 36)
I_{CORMSmax} = 0.77A ⁸---₃---_--²--  ³--₉--⁹-₀--⁰--_V--^V-- – 1 = 1.577A
(EQ. 37)
Select the proper capacitor according to the hold time and ripple
RMS current requirement. The actual capacitance is 270µF.
It is important to make sure the output peak-to-peak ripple is
less than the minimum OVP threshold as specified in the table
on “Electrical Specifications” on page 7. The ESR of the capacitor
at 2 times of line frequency is found in the capacitor datasheet.
The ESR is 737mΩ at 100Hz.
V_Opp = I_{OUTmax}  -----^--⁴----^-_--₂-^f--^l-_ⁱ--ⁿ--_f-^el--_i--_n-^--_e^C---_--^O--_---_C-^---^E_O----^S--_---^R0---_.-^-₈--²----+-----¹--
(EQ. 38)
The maximum power dissipation on the R_CS occurs at low line and
full load condition. The maximum power dissipation is calculated:
P_RCSMAX
=
I_I
N
2
MAX

R_C
S
(EQ. 42)
P_RCSMAX = 3.623A²  0.073 = 0.963W
(EQ. 43)
The resistor, R_SEN sets the overcurrent protection limit. From
Equation 3, R_SEN should be greater than:
R_SEN  ^R-----^C----^S-----^----₂-^I-^L--_--^P---₀-^e--_.-^a-₅--^k---_I-^-_O----^_C--¹-----+-----⁰---^.--²----^
(EQ. 44)
Where |x| stands for the ABS(x) function, I_OC is the overcurrent
threshold.
R_SEN  ⁰----^.-⁰----⁷---³----^--₂----^-_---⁶-₉---^.-₀₀---_-¹---^7A----^A-----^-----¹---^.--²- = 2.9k
(EQ. 45)
Select R_SEN from available standard value resistors, the selected
R_SEN is 3kΩ
CURRENT LOOP COMPENSATION
The input current shaping is achieved by comparing the sensed
current signal to the sensed input voltage signal. The current error
amplifier (Gmi), together with the current compensation network,
adjusts the duty cycle so that the inductor current traces the
sensed rectified voltage. Thus, unity power factor is achieved.
The compensation network consists of the Trans-Conductance
error amplifier (Gmi) and the impedance network (Z_ICOMP). The
goal of the compensation network is to provide a closed loop
transfer function with the sufficient 0dB crossing frequency
(f_0dB) and adequate phase margin. Phase margin is the
difference between the open loop phase at f_0dB and 180°. The
following equations relate the compensation network’s poles,
zeros and gain to the components (R_ic, C_ic and C_ip) in Figure 15.
V_Opp = 0.77A  -----^--⁴----^--_---₂^---⁵-_--⁰--_--^H-₅----_0z---_H-^---²-_z---⁷-_--⁰-_---^₂----_7F---₀--^--_-⁰---_F.--⁷---_⁷---₀-^--_.--₈-^--²-----+-----¹- = 9.6V
(EQ. 39)
The minimum OVP threshold is 103% of the nominal output
value. The maximum output peak-to-peak ripple should be less
than 6% of the nominal value, which is 23.4V_P-P.
CURRENT SENSING RESISTORS
Please refer to Equation 4 for calculation of the current sensing
resistor R_CS.
R_C
S

-¹---²---⁰----^m-----^V------^---²---⁶----⁵---^V------^---⁰---^.--⁹---²--
2  300W
=
0.069 
(EQ. 40)
V_I
L
Q₁
C_F1
R_CS
R_SEN
ISEN
V_OUT
C_OUT
CURRENT
MIRROR
2:1 ICS
While a large R_CS renders better current sensing accuracy, larger
R_CS also incurs higher power dissipation. Select three 0.22Ω
resistors in parallel as R_CS.
R_CS = 0.073
(EQ. 41)
ICOMP
R_ic
C_ic
C_ip
Gmi
I_REF
R_IS
FIGURE 15. INDUCTOR CURRENT SENSING SCHEME
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FN8582.1
February 13, 2015