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參數(shù)資料
| 型號： | AN-19 |
| 英文描述： | TOPSwitch Flyback Power Supply Efficiency |
| 中文描述： | TOPSwitch的反激式電源供應(yīng)器效率 |
| 文件頁數(shù)： | 12/20頁 |
| 文件大?。?/td> | 177K |
| 代理商： | AN-19 |
AN-19
A
6/96
12
inrush limiter. The thermistor presents a large impedance to the
AC line upon initial startup, but self-heats and turns to a low
resistance after a short period of time. The current rating of this
thermistor should be carefully chosen. If a device with too high
a current rating is chosen for a given application, it will not self-
heat sufficiently, and will act as a considerable impedance
between the supply and the AC line, wasting power. Care
should be taken that the lead temperature of the thermistor
where it enters the PC board does not exceed the safety rating
of the board material at maximum ambient temperature. Many
manufacturers make their inrush limiting thermistors with steel
leads instead of copper to provide some thermal isolation from
the PC board.
Input Diode Bridge
The input diode bridge on a switching power supply should
have a rating at least equal to the RMS input current of the power
supply at the lowest limit of the AC line voltage. This current
can be estimated by:
I
P
V
PF
RMS
out
ACMIN
≈
×
×
η
(5)
I
is the RMS input current,
η
is the estimated efficiency of
the supply, V
is the minimum RMS AC input voltage, and
PF is an estimate of the input power factor. Practical values of
PF range from 0.6 to 0.8, depending on line voltage and the
effective impedance of the AC line feeding the supply. A small
efficiency increase can be obtained by increasing the current
rating of the input rectifier bridge, so that it operates at a lower
current density. This results in a lower forward voltage drop,
reducing power loss in the input rectifier
Input RFI Filter
As shown in the power loss tabulation of Figure 6, the losses in
the common mode choke L2 can be significant at low input
voltages, where the RMS input current is highest. In order to
increase the efficiency at low line, the common mode choke can
be changed to a physically larger size unit of the same inductance
with lower internal resistance. The necessary current rating for
the common mode choke can be estimated using Equation (5).
The choice of common mode choke size will be a trade-off
between supply size, cost, and efficiency.
Input Capacitor Selection
The choice of an input filter capacitor can have a bearing on the
efficiency of a power supply, especially at low input AC
voltage. If the input capacitor is too small, there will be a large
ripple component on the capacitor at low line, resulting in lower
average voltage available for the
TOPSwitch
, higher average
operating current through all components in the power path, and
lower efficiency. A rough guideline is to size the total input
capacitance at 1 microfarad per output watt for a 230 VAC-only
supply or 2 microfarads per watt for a 115/230 VAC dual range
supply or a 115 VAC single range supply. For a universal input
supply or a 100 VAC supply with a full wave bridge rectifier,
use 3 microfarads per output watt.
Snubber and Clipper Networks
Snubbers and clippers are used in a power supply circuit in order
to limit voltage swing and reduce EMI. A typical use of a
clipper circuit can be seen in the ST204A circuit in Figure 4. D1
and VR1 act as a clipper to limit the peak value of the leakage
spike generated when
TOPSwitch
turns off. In order to
minimize the power loss in snubbers and clippers, design the
transformer for low leakage inductance, and plan the circuit
layout for low stray inductance. Techniques for circuit layout
design can be found in AN-14. Low leakage construction
techniques for transformers are shown in AN-18. In some cases
a RC snubber circuit may be necessary on the output rectifier to
reduce EMI. In such cases, use the minimum amount of
capacitance in the snubber necessary for RFI reduction. Too
large a value of snubber capacitor will reduce power supply
efficiency, especially at high input line voltage.
TOPSwitch
Dissipation
Dissipation in the
TOPSwitch
is an important factor in
determining the efficiency of the power supply. The techniques
outlined in the preceding paragraphs for reducing primary RMS
current will help to reduce dissipation due to conduction losses
in the
TOPSwitch
. The conduction losses for
TOPSwitch
are
greatest at minimum input voltage, so reducing conduction
losses will have the most effect on efficiency at low input
voltage. If cost allows, choosing a
TOPSwitch
with lower
R
will help to reduce conduction losses. Generous heat
sinking of the
TOPSwitch
will also help to reduce conduction
losses, as the R
of
TOPSwitch
(or any other MOSFET)
increases with increasing junction temperature. Reducing the
parasitic capacitance of the transformer will help reduce
TOPSwitch
switching losses, which become more important at
high input voltages.
Secondary Components
Output Rectifiers
Output rectifiers are a major source of inefficiency in a power
supply, and should be carefully chosen. As shown by the
ST204A power dissipation budget of Figure 6, losses generated
by the output rectifier are about one fourth to one fifth of the
total system loss. Rectifiers have two important loss mechanisms:
forward conduction loss, and reverse recovery loss. Both loss
mechanisms can have a significant effect on power supply
efficiency.
Both the choice of output rectifier and operating parameters can
affect the efficiency of the power supply. A supply designed to
run in the continuous mode will have lower secondary RMS and
peak current than a supply designed for discontinuous operation,
resulting in lower forward conduction losses. However, the
output rectifier will be forced to recover while there is a
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