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)： 8/20頁(yè)
文件大?。?/td> 177K
代理商： AN-19
AN-19
A
6/96
8
plus any leakage spikes. This voltage sets a practical limit to the
transformer turns ratio, and hence the maximum duty cycle for
a given
TOPSwitch
application. The limit can be expressed in
terms of a maximum recommended value of reflected secondary
voltage for a
TOPSwitch
application, and is used in the initial
stages of transformer design to determine transformer turns
ratio. For TOP100 series parts operating in the input voltage
range of 85-132 VAC, the maximum recommended value of
V
is 60 V. For TOP200 series parts operating in the input
voltage range of 195-265 VAC or 85-265 VAC, the maximum
recommended V
is 135 V. This sets a D
for 115 V input
supplies using TOP100 series parts of 40%. For 230 V input
supplies using TOP200 series parts, the D
limit set by
reflected voltage considerations is also 40%. For universal
input supplies (85-265 VAC) using TOP200 series parts, D
is 60%. The effect of D
and V
on power supply and
transformer design is discussed in application notes AN-16
and AN-17. The peak
TOPSwitch
drain voltage can be readily
controlled using a Zener clamp circuit. This circuit (VR1 and
D1) is shown in Figure 4. For 115 V input applications, the
recommended Zener voltage for VR1 is 90 V. For 230 V or
universal input, the recommended Zener voltage is 200 V.
Application Note AN-14 gives a table of Zener diode types
suitable for use with
TOPSwitch
.
The ST204A power supply design shown in Figure 4 is a
universal input design using a TOP204. The following
paragraphs discuss some of the trade-offs involved in varying
D
for this design from a minimum of 40% to the maximum
recommended value of 60%. Continuous mode operation was
assumed, with a primary inductance of 627 mH, 100 KHz
operating frequency, minimum primary DC bus voltage of
90 V, estimated efficiency of 80%, and 15 V, 30 W output
power. The minimum input voltage condition represents the
point of maximum current stress on
TOPSwitch
and other
components in the ST204A. Figures 8-11 show primary and
secondary peak and RMS currents and output capacitor RMS
ripple current as a function of D
. In Figure 11, the maximum
output diode peak inverse voltage (for V
= 265 VAC) is shown
as a function of D
. The actual ST204A operating point (D
MAX
= 0.57) is marked on each graph.
As shown in Figure 8, primary peak and RMS currents decrease
from 1.22A and 0.62A for D
= 40% to 1.1A and 0.53A for
D
= 60%.
TOPSwitch
conduction loss varies as the square of
RMS drain current, so the higher D
can cut conduction
losses by as much as 27%, increasing efficiency or allowing use
of a smaller
TOPSwitch.
Figures 9 and 10 show that although
the primary current levels are decreased by a higher value of
D
, the secondary peak and RMS currents and the output
capacitor ripple current increase with increasing D
. This
trade-off is characteristic of flyback power supplies in general.
The increase in secondary RMS current does not affect the
output rectifier to the same extent as the
TOPSwitch
, as the
output diode voltage drop as a function of forward current is
fairly constant. At higher D
, the current stress on the output
rectifier is increased, but the voltage stress is decreased, due to
the smaller ratio of secondary to primary transformer turns.
This can be shown by rearranging Equation (1) to obtain N
S
/N
P
as a function of D
MAX
.
N
N
D
D
V
V
V
S
P
MAX
O
D
MAX
MIN
=
×
×
+
(
)
(
)
1
(3)
For output voltages of 24 V or below, the lower secondary
voltage stress resulting from a higher D
may allow use of a
Schottky rectifier instead of an ultrafast rectifier. The lower
voltage drop of the Schottky rectifier reduces the output rectifier
power dissipation despite the increased peak and RMS secondary
currents resulting from the increase in D
, and can result in an
overall increase in supply efficiency. For the ST204A, the peak
inverse voltage across the output diode as a function of D
is
shown in Figure 11. This curve was calculated for the maximum
input voltage for ST204A (265 VAC) to show the maximum
peak inverse voltage on the output rectifier. For a D
of 0.5
or greater, the peak inverse voltage is low enough to allow use
of a Schottky output diode with adequate voltage margin.
Though the RMS secondary current is not a very strong function
of duty cycle, the peak secondary current rises dramatically
with D
. This high peak current can cause noise problems due
to interaction with the parasitic inductances of the secondary
circuit. Secondary circuit layout will be more critical for a
power supply designed for a high D
. The RMS ripple current
in the output filter capacitor also increases with increasing
D
, as shown in Figure 10. This will necessitate using a
capacitor with a higher ripple current rating for higher values of
D
MAX
. In general, using the highest recommended value of
D
for a given
TOPSwitch
and input voltage will maximize
the power capability and/or efficiency of a
TOPSwitch
flyback
supply by reducing the peak and RMS primary currents. When
using the maximum recommended D
, the output rectifier
and filter capacitor must be sized to accommodate the increased
secondary peak and RMS currents. The lower secondary
voltage stress resulting from a larger value of D
may allow
the use of a Schottky output rectifier, resulting in higher overall
supply efficiency. Setting the transformer turns ratio to the
recommended V
value for a given
TOPSwitch
application
will automatically set D
MAX
to the maximum recommended
value.
Operating Frequency
Operating frequency of a switching power supply should be
carefully considered, due to the trade-off between the size
advantage gained by high frequency operation and the decrease
in efficiency due to transformer core and copper losses, diode
and MOSFET switching losses, and snubber losses. 100 KHz
appears to be the optimum operating frequency for supplies
相關(guān)PDF資料
PDF描述
AN-20 Transient Suppression Techniques for TOPSwitch Power Supplies
AN-22 OBSOLETE when inventory is depleted. 10% tolerance no l
AN-23 TinySwitch Flyback Design Methodology
AN-29 TOPSwitch-GX Flyback Quick Selection Curves
AN-30 TOPSwitch-GX Forward Design Methodology
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