參數(shù)資料
型號(hào)： AN-30
英文描述： TOPSwitch-GX Forward Design Methodology
中文描述： 的TOPSwitch - GX系列正向設(shè)計(jì)方法
文件頁(yè)數(shù)： 10/40頁(yè)
文件大?。?/td> 311K
代理商： AN-30
AN-30
10
B
12/02
The computation is simply the reflection of peak currents in the
secondary circuits by the ideal turns ratios of the transformer.
Using the principle that the sum of the ampere turns for an ideal
transformer is zero, the instantaneous primary current for a
transformer with W secondary windings is just
(14)
where i
is the current in the secondary winding with turns
ratio n
. Thus, for a transformer with three secondary windings,
the primary current would be the sum i
n
+i
n
+i
n
3
divided by the turns ratio of the primary. Note that since all
turns ratios are defined with respect to the main output winding,
the turns ratio of the main output winding is 1. Equation (14)
may also be used with the actual number of primary turns N
P
substituted for the turns ratio n
, and the actual secondary turns
N
j
substituted for the turns ratios n
j
.
This estimate does not include the effect of magnetizing
current in the transformer, which will be determined after the
transformer is designed. The magnetization current will raise
the peak value of this estimate by typically less than 10% worst
case.
The computation in
PI Expert
includes the ripple current in the
output inductors to find the peak primary current. Ripple
current is ignored to calculate the RMS value. The resulting
error in the RMS current is less than 1% for practical values of
inductance and current. The RMS current is computed at the
duty ratio that corresponds to V
because worst case steady-
state resistive losses occur at that operating point.
Step 5. Choose the appropriate
TOPSwitch-GX
device.
Select a
TOPSwitch-GX
according to the requirements for peak
primary current and acceptable power dissipation. For operation
of the converter in continuous conduction mode it is
recommended to operate the device at no more than 80% of its
current limit for ordinary thermal design. To reduce device
dissipation it is possible to use a
TOPSwitch-GX
device that has
a lower R
DS(ON)
when the current limit is adjusted accordingly.
Lowering I
externally (using a programming resistor to the
X pin), takes advantage of the lower R
of the larger device
while maintaining the same level of overload protection.
The external current limit reduction factor is
(15)
where 0.4
≤
K
≤
1.0, and is set by the value of a resistor
connected between the X pin and SOURCE pin. Refer to the
TOPSwitch-GX
data sheet for details.
With external current limit reduction, the actual (external)
current limit is
I
XLIMIT
=
(16)
PI-2822-121401
T
S
= 1
I
P
I
P
I
PP
I
PP
DT
S
(1-D) T
S
(a)
(b)
DT
S
D
MA
T
S
(1-D)T
S
0
0
t
t
f
S
T
S
= 1
f
S
Figure 7. Typical Primary Current Waveforms for a Converter Without Magnetic Amplifier (a) and with a Mag Amp (b).
K
I
=
External Current Limit
Data Sheet Current Limit
I
K
LIMIT
I
I
n
i n
P
P
j
W
=
=
∑
1
1
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