ISL6753_14 Datasheet, PDF (11/15 Pages) Intersil Corporation – ZVS Full-Bridge PWM Controller

ISL6753
For simplicity, idealized components have been used for this
discussion, but the effect of magnetizing inductance must be
considered when determining the amount of external ramp
to add. Magnetizing inductance provides a degree of slope
compensation to the current feedback signal and reduces
the amount of external ramp required. The magnetizing
inductance adds primary current in excess of what is
reflected from the inductor current in the secondary.
∆I_P
=
-^V----^I--^N-----^â‹---^D-----^T----^S----^W---
L_m
A
(EQ. 19)
where V_IN is the input voltage that corresponds to the duty
cycle D and Lm is the primary magnetizing inductance. The
effect of the magnetizing current at the current sense
resistor, R_CS, is
∆V_CS
=
∆_-----I_--P-----⋅_---R_----C----S--
N_CT
V
(EQ. 20)
If ∆V_CS is greater than or equal to Ve, then no additional
slope compensation is needed and R_CS becomes
R_CS
=
--------------------------------------------------------------^N----^C----^T---------------------------------------------------------------
N_-----S--
N_P
⋅

 ^{I O}
+
D_-----T_{----S----W---}
2L_O
⋅



V_I
N
⋅
N_-----S--
N_P
–

V_{O}


+
V_{-----I--N-----}⋅_---D_-----T_{----S---W----}
L_m
(EQ. 21)
If ∆V_CS is less than Ve, then Equation 18 is still valid for the
value of R_CS, but the amount of slope compensation added
by the external ramp must be reduced by ∆V_CS.
Adding slope compensation is accomplished in the ISL6753
using the CTBUF signal. CTBUF is an amplified
representation of the sawtooth signal that appears on the CT
pin. It is offset from ground by 0.4V and is 2x the peak-to-
peak amplitude of CT (0.4 - 4.4V). A typical application sums
this signal with the current sense feedback and applies the
result to the CS pin as shown in Figure 7.
R9
R6
R_CS
1
2 CTBUF
3
4
ISL6753
5
6
7
8 CS
C4
FIGURE 7. ADDING SLOPE COMPENSATION
Assuming the designer has selected values for the RC filter
placed on the CS pin, the value of R9 required to add the
appropriate external ramp can be found by superposition.
V_e – ∆V_CS
=
-⁽--^D-----⁽--^V----^C-----^T---^B----^U----^F-----^–-----⁰---^.--⁴---⁾----⁺-----⁰---^.--⁴---⁾----^â‹---^R-----⁶-
R6 + R9
V
(EQ. 22)
Rearranging to solve for R9 yields
R9 = ⁽---^D-----⁽--^V----^C-----^T---^B----^U----^F-----^–-----⁰---^.--⁴---⁾----^–----^V-----^e----⁺-----^∆----^V----^C----^S-----⁺-----⁰----^.-⁴----⁾----^â‹---^R----⁶--
V_e – ∆V_CS
Ω
(EQ. 23)
The value of R_CS determined in Equation 18 must be
rescaled so that the current sense signal presented at the
CS pin is that predicted by Equation 16. The divider created
by R6 and R9 makes this necessary.
R′_CS
=
R_-----6_-----+_-----R_-----9_--
R9
⋅
R_CS
(EQ. 24)
Example:
V_IN = 280V
V_O = 12V
L_O = 2.0µH
Np/Ns = 20
Lm = 2mH
I_O = 55A
Oscillator Frequency, Fsw = 400kHz
Duty Cycle, D = 85.7%
N_CT = 50
R6 = 499Ω
Solve for the current sense resistor, R_CS, using Equation 18.
R_CS = 15.1Ω.
Determine the amount of voltage, Ve, that must be added to
the current feedback signal using Equation 15.
Ve = 153mV
Next, determine the effect of the magnetizing current from
Equation 20.
∆V_CS = 91mV
Using Equation 23, solve for the summing resistor, R9, from
CTBUF to CS.
R9 = 30.1kΩ
Determine the new value of R_CS, R’_CS, using Equation 24.
R’_CS = 15.4Ω
The above discussion determines the minimum external
ramp that is required. Additional slope compensation may be
considered for design margin.
11
FN9182.2
April 4, 2006