參數(shù)資料
型號(hào)： AN-16
英文描述： TOPSwitch Flyback Design Methodology
中文描述： TOPSwitch的反激式設(shè)計(jì)方法
文件頁(yè)數(shù)： 22/32頁(yè)
文件大?。?/td> 247K
代理商： AN-16
AN-16
A
6/96
22
Step 27. Select Output capacitor
ESR is the most important parameter for output filter capacitor
selection. Capacitor ESR directly determines the output ripple
voltage of the power supply and the ripple current rating of the
capacitor while the actual capacitance value only affects control
loop bandwidth. Below 35V, ESR is mainly determined by
capacitor case size. Consider two Nichicon PL series capacitors:
1500
μ
F/6.3V and 390
μ
F/35V. Both capacitors have a case size
of 10 mm diameter and 25 mm length, and both have the same
ESR of 55 m
. To keep control loop bandwidth high, the
smaller capacitance, higher voltage rating capacitor is preferred.
Ripple current is typically specified at 105
o
C ambient which is
much higher than the ambient temperature required in most
applications. Therefore, it is possible to operate the capacitor at
higher ripple currents determined by a multiplier factor from
the capacitor data sheet.
Actual ripple current of the output capacitor can be calculated
as follows:
I
I
I
RIPPLE
SRMS
O
=
2
2
where I
is the secondary winding RMS current and I
O
is the
DC output current.
Step 28 to Step 29. Select Output post filter L, C
If the measured switching ripple voltage at the output capacitor
is higher than the required specification, an LC post filter
consisting of a 2.2 to 4.7
μ
H inductor or ferrite bead (only for
power levels below 5W) with a 120 uF/35V, low ESR electrolytic
capacitor is recommended. This will provide a lower cost
solution compared to increasing the capacitance value and/or
lowering the ESR of the main output filter capacitor.
The output post filter, to a first order, is independent of output
power except that the DC voltage drop across the inductor may
be a concern at high currents. Inductors with larger gauge wire
and higher current rating solve this problem.
Step 30. Select bias rectifier
Bias rectifier selection is similar to output rectifier selection
with the exception that since the bias winding carries very little
current (typically less than 10 mA), the considerations for
current capability and very fast recovery no longer apply.
Step 31. Select bias capacitor
Because of the low voltage and the minimal power required at
the bias output, a 0.1 uF, 50V ceramic capacitor always meets
the requirement.
Step 32. Select Control pin capacitor and series
resistor
A 47
μ
F, 10V low cost standard grade electrolytic capacitor
across the Control pin and Source pin of the
TOPSwitch
takes
care of loop compensation for all types of feedback
configurations. Low ESR capacitors should not be used for this
purpose, as the ESR resistance of the standard grade capacitor
(2
typical) improves the loop stability by introducing a zero.
In fact, a 6.2
resistor in series with this capacitor is
recommended to improve phase margin in designs that either
have excessive gain in the secondary (such as the TL431 circuit
shown in Figure 6), or a K
RP
value of less than one (continuous
mode).
Step 33. Select feedback circuit components
Primary feedback: Refer to RD1
Opto/Zener feedback: Refer to ST202A
Opto/TL431: Refer to ST204A
Select opto-coupler with CTR between 50% and 200%
(Refer to AN-14, Table 3)
Step 34. Select bridge rectifier based on input voltage
V
ACMAX
and input RMS current I
ACRMS
Maximum operating current for the input bridge rectifier occurs
at low line:
I
P
V
PF
ACRMS
O
ACMIN
=
×
×
η
PF is the power factor of the power supply. Typically, for a
power supply with a capacitor input filter, PF is between 0.5 and
0.7. Use 0.5 if there is no better reference data available.
Select the bridge rectifier such that:
I
≥
2 x I
, where I
D
is the rated RMS current of the
bridge rectifier
V
≥
1.25 x 1.414 x V
; where V
R
is the rated reverse
voltage of the rectifier diode
Step 35. Design complete
Following the step-by-step procedure completes the design of
a basic
TOPSwitch
flyback converter. Once built, the power
supply should be fully functional and capable of delivering
maximum rated output power at minimum input line voltage,
while meeting all specifications. Minor adjustments may be
necessary to center the output voltage.
相關(guān)PDF資料
PDF描述
AN-17 Flyback Transformer Design for TOPSwitch Power Supplies
AN-18 TOPSwitch Flyback Transformer Construction Guide
AN-19 TOPSwitch Flyback Power Supply Efficiency
AN-20 Transient Suppression Techniques for TOPSwitch Power Supplies
AN-22 OBSOLETE when inventory is depleted. 10% tolerance no l
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