ISL6754 Datasheet, PDF (14/19 Pages) Intersil Corporation – ZVS Full-Bridge PWM Controller with Adjustable Synchronous Rectifier Control

ISL6754
If ΔV_CS is greater than or equal to V_e, 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-----^â‹---^D-----^T----^S----^W---
L_m
(EQ. 21)
If ΔV_CS is less than Ve, then Equation 16 is still valid for the
value of R_CS, but the amount of slope compensation added
by the external ramp must be reduced by ΔV_CS.
Adding slope compensation may be accomplished in the
ISL6754 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 (0.4V to 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 10.
R9
R6
RCS
1
20
2
19
3 CTBUF
18
4
17
5
16
6 ^ISL6754 15
7
14
8 RAMP
13
9 CS
12
10
GND 11
C4
FIGURE 10. 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 – ΔV_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----+-----}Δ_----V_{----C----S-----+-----}0_----._-4_----)_----⋅_---R_----6_--
V_e – ΔV_CS
Ω
(EQ. 23)
The value of R_CS determined in Equation 18 or 21 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_C
S
(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, V_e, 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.
ΔV_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.
If the application requires deadtime less than about 500ns,
the CTBUF signal may not perform adequately for slope
compensation. CTBUF lags the CT sawtooth waveform by
300ns to 400ns. This behavior results in a non-zero value of
CTBUF when the next half-cycle begins when the deadtime
is short.
Under these situations, slope compensation may be added
by externally buffering the CT signal as shown in Figure 11.
14
FN6754.1
September 29, 2008