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參數(shù)資料
| 型號： | TK75003 |
| 廠商： | TOKO Inc. |
| 英文描述： | PWM CONTROLLER |
| 中文描述： | 脈寬調(diào)制控制器 |
| 文件頁數(shù)： | 8/14頁 |
| 文件大?。?/td> | 108K |
| 代理商： | TK75003 |
Page 8
January 1999 TOKO, Inc.
TK75003
DESIGN CONSIDERATIONS
SELECTING A START-UP RESISTOR
Figure 1 shows the typical application of the TK75003 in an
off-line flyback power supply (input full-wave bridge and
capacitor not shown). The IC starts when the voltage
across the capacitor C
AUX
reaches the UVLO on Voltage
V
IN(ON)
of the IC. The starting resistor R
ST
can be designed
as follows:
R
ST(MAX)
= (V
IN(MIN)
- V
CC(ON,MAX)
- 2 V) / I
CC(START,MAX)
(1)
At 85 Vrms line voltage, and taking into account the
specified maximum values of the UVLO on voltage and the
start-up supply current I
CC(START)
, the maximum allowed
value of the starting resistor is:
R
ST(MAX)
= (85
2
- 16 - 2 ) / 1.0 mA = 102.2 k
(2)
A practical choice for the starting resistor is R
ST
= 100 k
.
The worst-case dissipation of the resistor appears at high
line and at the minimum V
CC
voltage. At 265 Vrms line
voltage and 9 V V
CC,
the dissipation is 2.2 W, so a 3 W
resistor should be used. Note that 1.0 mA reflects the worst
case I
CC(START)
at the edge of UVLO release.
SELECTING THE TRANSFORMER TURNS RATIO
During steady-state operations, the auxiliary supply voltage
is generated by the auxiliary winding n
3
and the rectifier
diode D
3
. In the flyback power supply, neglecting the effect
of the leakage inductance of the transformer, the number
of turns of the auxiliary winding can be calculated from the
following equation:
n
3
= n
2
[(V
AUX
+ V
D3
) / (V
OUT
+ V
D2
)]
(3)
where V
D2
and V
D3
are the forward voltage drops of the
output rectifier diode and the auxiliary rectifier diode. The
voltage V
AUX
should be selected such that it stays between
the specified worst-case upper and lower limits of the IC,
considering the component tolerances, ripple, and other
second-order effects. The upper limit for V
AUX
is the
minimum voltage of the built-in clamp (16 V). The lower
limit for V
AUX
is the maximum UVLO off voltage (12.0 V). It
is prudent to choose the mean value of those two voltages
(i.e., 14.0 V), as V
AUX
.
COMPENSATING FOR LEAKAGE INDUCTANCE
The leakage inductance of the flyback transformer causes
a voltage overshoot at turn-off of the MOSFET. The
magnitude and duration of the overshoot depends on the
leakage inductance, the peak current at turn-offs, and the
voltage-clamping circuit employed to limit the overshoot.
The overshoot tends to increase the auxiliary voltage. The
simplest solution to reduce that increase is to add a
resistor R
AUX
in series with the rectifier diode D
3
. The
optimal value of the resistor can be calculated from the
subcircuit shown in Figure 2.
The average current flowing in R
AUX
is equal to the current
I
AUX
drawn by the IC. The following equation can be written
from the equality:
I
AUX
= (1 / R
AUX
)
x ([(V
1
- V
D3
- V
AUX
) x (T
1
/ T)] + [(V
2
- V
D3
- V
AUX
)
x (T
2
/ T)])
(4)
The voltage V
1
can be calculated as follows:
V
1
= (V
OUT
+ V
D2
) x (n
1
/ n
2
) + [V
OVERSHOOT
x ( n
3
/ n
1
)]
(5)
where V
OVERSHOOT
is the additional voltage appearing
across the MOSFET due to the leakage inductance.
The voltage V
2
can be calculated as follows:
V
2
= (V
OUT
+ V
D2
) x ( n
3
/ n
2
)
(6)
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| TK75003D | PWM CONTROLLER |
| TK75003DIMG | PWM CONTROLLER |
| TK75005DCTL | LOW-COST FLEXIBLE PWM CONTROLLER |
| TK75005 | LOW-COST FLEXIBLE PWM CONTROLLER |
| TK75005DCMG | LOW-COST FLEXIBLE PWM CONTROLLER |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| TK75003D | 制造商:TOKO 制造商全稱:TOKO, Inc 功能描述:PWM CONTROLLER |
| TK75003DIMG | 制造商:TOKO 制造商全稱:TOKO, Inc 功能描述:PWM CONTROLLER |
| TK75003DIMG/75003 | 制造商:TOKO 制造商全稱:TOKO, Inc 功能描述:PWM CONTROLLER |
| TK75005 | 制造商:TOKO 制造商全稱:TOKO, Inc 功能描述:LOW-COST FLEXIBLE PWM CONTROLLER |
| TK75005DCMG | 制造商:TOKO 制造商全稱:TOKO, Inc 功能描述:LOW-COST FLEXIBLE PWM CONTROLLER |
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