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參數(shù)資料
| 型號(hào)： | TK75002 |
| 廠商： | TOKO Inc. |
| 英文描述： | ERROR SIGNAL ISOLATOR |
| 中文描述： | 誤差信號(hào)隔離器 |
| 文件頁(yè)數(shù)： | 6/10頁(yè) |
| 文件大?。?/td> | 77K |
| 代理商： | TK75002 |
Page 6
January 1999 TOKO, Inc.
TK75002
Figure 3 shows the magnitude and phase diagram of the
transfer function. As can be seen, the feed-forward path
adds a left half-plane zero (sometimes called a "parasitic"
zero) to the transfer function of the ideal integrator. The
parasitic zero is undesirable because it increases the high
frequency loop gain of the system, potentially leading to
subharmonic instability or chaotic behavior. To avoid the
effect of the zero, either a low pass (decoupling) filter must
be added to the supply rail of the opto-coupler, or an
additional pole must be introduced around the frequency
of the zero, or the loop gain must be reduced. (Note that
reducing the loop gain slows down the transient response
of the system.)
The TK75002 eliminates the parasitic zero by changing
the reference point of the error signal from the GND pin to
the V
CC
pin. That is achieved by inserting a level shifter
circuit between the output of the error amplifier and the
buffer of the pulse-amplitude modulator (see Figure 4).
The output voltage of the level shifter is proportional to the
error signal (i.e., the voltage of the COMP pin) and is
conveyed relative to the supply voltage V
1
. The end result
is that the transfer function becomes:
H(s) = Z
FB
/ Z
IN
(4)
In the case of a capacitive feedback impedance and
resistive input impedance, the transfer function will be that
of an ideal integrator, without the parasitic zero.
FIGURE 3: MAGNITUDE AND PHASE VS.
FREQUENCY OF AN ERROR AMPLIFIER / OPTO-
COUPLER COMBINATION
H (dB)
-20 dB / dec
0
f
1 / (2
π
RC)
-90
°
0
°
f
φ
APPLICATION INFORMATION
THE PARASITIC ZERO AND HOW IT IS ELIMINATED IN
THE TK75002
In a feedback-regulated isolated power supply, isolation is
usually provided by an opto-coupler whose photodiode
section is connected between the output of the power
supply and the output of the error amplifier (in series with
a resistor that converts the voltage difference between the
two outputs into a current). Due to the feed-forward path
established by that connection, the transfer function of the
error amplifier is significantly modified (the modification in
the transfer function appears also if the opto-coupler is
replaced by the combination of a transformer and peak
detector driven by a pulse amplitude modulator).
Figure 2 shows the error amplifier/opto-coupler
combination. The output voltage V
2
(that is the voltage
measured across the photodiode and the series resistor)
can be written as:
V
2
= V
1
- [ - (Z
FB
/ Z
IN
) x V
1
]
(1)
From (1), the transfer function of the error amplifier (from
the supply voltage V
1
, to the voltage V
2
) is:
H(s) = V
2
(s) / V
1
(s) = 1 + (Z
FB
/ Z
IN
)
(2)
In the case when the feedback impedance is a capacitor C
and the input impedance is a resistor R, the transfer
function becomes:
H(s) = (1 + SRC) / SRC
(3)
FIGURE 2: ERROR AMPLIFIER / OPTO-COUPLER
COMBINATION
V 2
Vref
+
+
-
V 1
+
R B
+
-
ZFB
ZIN
-
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| TK75002D | ERROR SIGNAL ISOLATOR |
| TK75002DIMG | ERROR SIGNAL ISOLATOR |
| TK75003 | PWM CONTROLLER |
| TK75003D | PWM CONTROLLER |
| TK75003DIMG | PWM CONTROLLER |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| TK75002D | 制造商:TOKO 制造商全稱(chēng):TOKO, Inc 功能描述:ERROR SIGNAL ISOLATOR |
| TK75002DIMG | 制造商:TOKO 制造商全稱(chēng):TOKO, Inc 功能描述:ERROR SIGNAL ISOLATOR |
| TK75002DIMG/75002 | 制造商:TOKO 制造商全稱(chēng):TOKO, Inc 功能描述:ERROR SIGNAL ISOLATOR |
| TK75003 | 制造商:TOKO 制造商全稱(chēng):TOKO, Inc 功能描述:PWM CONTROLLER |
| TK75003D | 制造商:TOKO 制造商全稱(chēng):TOKO, Inc 功能描述:PWM CONTROLLER |
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