ISL6755 Datasheet, PDF (13/17 Pages) Intersil Corporation – ZVS Full-Bridge PWM Controller with Average Current Limit

ISL6755
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 CTBUF
19
3
18
4
17
5
16
6 ^ISL6755 15
7
14
8 RAMP
13
9 CS
GND 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_---)_----⋅_---R_-----6_-
R6 + R9
V
(EQ. 23)
Rearranging to solve for R9 yields
R9 = ⁽---^D-----⁽--^V----^C-----^T---^B----^U----^F-----–-----⁰---^.-_V-⁴---_e⁾----––----^V-_Δ----^e_V----+_C----_S-^Δ----^V----^C----^S-----+-----⁰----^.-⁴----⁾----^â‹---^R----⁶--
Ω
(EQ. 24)
The value of R_CS determined in Equation 19 or 22 must be
rescaled so that the current sense signal presented at the
CS pin is that predicted by Equation 17. The divider created
by R6 and R9 makes this necessary.
R′_CS
=
R_-----6_-----+-----R_-----9_--
R9
⋅
R_C
S
(EQ. 25)
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 19.
R_CS = 15.1Ω.
Determine the amount of voltage, Ve, that must be added to
the current feedback signal using Equation 16.
Ve = 153mV
Next, determine the effect of the magnetizing current from
Equation 21.
ΔV_CS = 91mV
Using Equation 24, solve for the summing resistor, R9, from
CTBUF to CS.
R9 = 30.1kΩ
Determine the new value of R_CS, R’_CS, using Equation 25.
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 below.
1
VREF 20
2
19
ISL6755
3
18
4
17
5 CT
16
6
15
R9
7
14
8 RAMP
13
9 CS
GND 12
R6
10
GND 11
RCS
C4
CT
FIGURE 11. ADDING SLOPE COMPENSATION USING CT
Using CT to provide slope compensation instead of CTBUF
requires the same calculations, except that Equations 23
and 24 require modification. Equation 23 becomes:
V_e – ΔV_CS
=
-²----^D------^â‹---^R-----⁶---
R6 + R9
V
(EQ. 26)
13
FN6442.1
September 29, 2008