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參數(shù)資料
| 型號： | ISL6752 |
| 廠商： | Intersil Corporation |
| 英文描述： | Octal Buffers/Drivers With 3-State Outputs 20-PDIP -40 to 85 |
| 中文描述： | ZVS全橋電流模式PWM可調(diào)的同步整流控制 |
| 文件頁數(shù)： | 11/16頁 |
| 文件大?。?/td> | 436K |
| 代理商： | ISL6752 |
11
FN9181.1
March 10, 2005
The inductor current, when reflected through the isolation
transformer and the current sense transformer to obtain the
current feedback signal at the sense resistor yields
N
P
N
CT
2L
O
where V
CS
is the voltage across the current sense resistor
and I
O
is the output current at current limit.
Since the peak current limit threshold is 1.00V, the total
current feedback signal plus the external ramp voltage must
sum to this value.
Substituting Equations 13 and 14 into Equation 15 and
solving for R
CS
yields
N
N
S
I
O
O
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.
where V
IN
is the input voltage that corresponds to the duty
cycle D and L
m
is the primary magnetizing inductance. The
effect of the magnetizing current at the current sense
resistor, R
CS
, is
I
R
CT
If
V
CS
is greater than or equal to V
e
, then no additional
slope compensation is needed and R
CS
becomes
N
S
N
P
O
If
V
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
V
CS
.
Adding slope compensation may be accomplished in the
ISL6752 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 - 4.4 V). A typical application
sums this signal with the current sense feedback and applies
the result to the CS pin as shown in Figure 6.
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.
Rearranging to solve for R9 yields
The value of R
CS
determined in Equation 16 must be
rescaled so that the current sense signal presented at the
CS pin is that predicted by Equation 14. The divider created
by R6 and R9 makes this necessary.
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 = 499
V
CS
------------------------
I
O
--------------------
V
IN
N
S
N
P
-------
V
O
–
+
=
V
(EQ. 14)
V
e
V
CS
+
1
=
(EQ. 15)
R
CS
-----------------------
O
-------
T
SW
1
π
--
D
2
---
+
+
------------------------------------------------------
=
(EQ. 16)
I
P
V
DT
m
-------------------------------
=
A
(EQ. 17)
V
CS
-------------------------
=
V
(EQ. 18)
R
CS
-------
I
O
----------------
V
IN
S
N
P
-------
V
O
–
+
V
DT
m
-------------------------------
+
-------------------------------------------------------------------------------------------------------------------------------------
=
(EQ. 19)
FIGURE 6. ADDING SLOPE COMPENSATION
R6
C4
R9
CTBUF
CS
1
2
4
3
5
6
7
8
R
CS
ISL6752
V
e
V
CS
–
D V
--------------------------------------+
0.4
–
)
0.4
+
(
)
R6
=
V
(EQ. 20)
R9
D V
---------------------------------–
0.4
V
e
)
V
e
V
CS
V
CS
0.4
+
+
–
–
(
)
R6
=
(EQ. 21)
R
′
CS
--------+
R
CS
=
(EQ. 22)
ISL6752
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