參數(shù)資料
型號(hào)： 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(yè)數(shù)： 13/20頁(yè)
文件大?。?/td> 177K
代理商： AN-19
A
6/96
AN-19
13
Figure 18. Efficiency vs. Input Voltage for Schottky and Fast
Epitaxial Output Rectifier.
P
Schottky Rectifier
Fast Epitaxial Rectifier
Input AC Voltage
E
0.9
0.85
0.8
0.75
0.7
80
100
120
140
160
180
200
220
240
260
efficiency at 115 VAC. using the FMB-29L Schottky rectifier
is 84.05%. When the BYW29-200 is used, the efficiency is
82.53%. This efficiency difference is due mainly to the lower
forward voltage drop of the Schottky rectifier.
Output Rectifier Current Rating
A general design rule for high efficiency is to choose the output
rectifier current rating to be at least 3 times the rated DC output
current of the supply. This reduces the power dissipation in the
output rectifier by allowing it to run at a lower current density.
The actual RMS and peak operating current of the output
rectifier can be several times the DC output current of the power
supply, as shown in Figure 9. This figure shows the secondary
peak and RMS currents (identical to the output rectifier current)
as a function of D
for the ST204A, which is rated for an
output currents of 2A. At D
= 40%, RMS and peak secondary
currents are 2.6A, and 4.2A, respectively, while at D
= 60%,
they rise to 3.3A and 7.8A. The voltage drop and power
dissipation across the output diode is a function of the peak
current through the diode. Rating the output diode current at 3
times the DC output current is consistent with the actual
operating conditions. The output diode chosen for the ST204A
(Sanken FMB-29L) is a 90 V Schottky diode rated at 8 amperes.
Output Capacitors
In flyback power supplies, the output filter capacitors carry a
large amount of ripple current, as they must completely support
the output current of the supply while the primary switch is on.
For a supply operating in continuous mode, the ripple current in
the output filter capacitor can be estimated as
I
I
D
D
RIPPLE
O
MAX
MAX
×
1
(6)
Operating in the continuous mode reduces the peak and RMS
ripple current in the output capacitors, and consequently the
peak and output ripple voltage at the switching frequency.
Dissipation in the output capacitors can be calculated as:
P
I
ESR
RIPPLE
=
×
2
(
)
(7)
where I
is the RMS capacitor ripple current in amperes, and
ESR is the equivalent series resistance of the capacitor in ohms.
For long life, under continuous duty conditions the capacitor
current rating should be sized at 1.5 to 2 times the ripple current.
The temperature and frequency derating factors published in
the capacitor specifications should be taken into account when
sizing the output capacitors, as well as the duty factor of the
supply (continuous or intermittent operation). For intermittent
operation, a less stringent derating factor can be used.
significant amount of current flowing through it, resulting in
higher reverse losses in the continuous mode. Careful choice of
the output rectifier can help to reduce reverse recovery losses.
Where expense and voltage permit, Schottky rectifiers should
be used if efficiency is of great importance. Schottky rectifiers
have significantly lower forward losses than PN junction
rectifiers. Since Schottky rectifiers are majority carrier devices,
they do not have the reverse recovery losses exhibited by
conventional fast recovery devices. However, Schottky rectifiers
do have a comparatively large junction capacitance, which
results in reverse current spikes similar to those generated by
conventional fast recovery diodes, but without the internal
losses of conventional rectifiers. The use of Schottky diodes
is limited by their relatively low breakdown voltage ratings
(
≤
100 V). If Schottky rectifiers cannot be used due to breakdown
voltage considerations, the next best choice is a fast epitaxial
rectifier with moderate forward losses and very fast reverse
recovery times (< 50 nsec). These rectifiers are available with
voltage ratings ranging from 50 V to 1000 V. Standard fast
recovery rectifiers, with reverse recovery times in excess of
150 nsec, should not be used in
TOPSwitch
supplies unless
there is no reverse recovery current drawn during diode turnoff.
Under no circumstances should normal diffused-junction
rectifiers be used except as AC input rectifiers.
The efficiency of the ST204A example circuit was measured
using a Schottky output rectifier vs. a fast epitaxial rectifier to
determine the difference in total power supply efficiency at an
output power of 30 W. The two diodes compared were a Sanken
FMB-29L Schottky rectifier and a Philips BYW29-200 fast
epitaxial rectifier. The FMB-29L Schottky rectifier is rated at
90 V, 8A. The BYW29-200 is rated at 200 V, 8A, with a reverse
recovery time of less than 25 nsec. Efficiency vs. AC input
voltage for the two devices is shown in Figure 18. The supply
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