AN-21 [ETC]
TOPSwitch-II Flyback Quick Selection Curves ; TOPSwitch-II的反激式快速选择曲线\n
| 型号： | AN-21 |
| 厂家： | 
ETC |
| 描述： | TOPSwitch-II Flyback Quick Selection Curves
|
| 文件： | 总8页 (文件大小：402K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
®
TOPSwitch®-II Flyback
Quick Selection Curves
Application Note AN-21
Introduction
QUICK START
This application note is for engineers starting a flyback power
supply design with TOPSwitch-II. It offers a quick method to
select the proper TOPSwitch-II device from parameters that
are usually not available until much later in the design process.
The TOPSwitch-II Flyback Quick Selection Curves provide
the essential design guidance.
1) Determine which graph (Fig. 2, 3, 4 or 5) is
closest to your application.
Example: Use Figure 2 for Universal input,
12 V output.
2) Find your power requirement on the X- axis.
Efficiency and TOPSwitch-II power dissipation are two
importantperformanceparameterstotheflybackpowersupply
designer. Both can be easily measured or accurately estimated
after the power supply is designed. But what if the designer
must make project and resource decisions before actually
committing to and starting development? This application
note helps the designer quickly select the optimum
TOPSwitch-IIdevicefromsimplecurvesofestimatedefficiency
and TOPSwitch-II power dissipation.
3) Move vertically from your power
requirement until you intersect with a
TOPSwitch-II curve (solid line).
4) Read the associated efficiency on the
Y- axis.
5) Determine if this is the appropriate
efficiency for your application. If not,
continue to the next TOPSwitch-II curve.
Typical Power Supply Losses
Power supplies have an input power which, because of internal
dissipation, can be significantly higher than the output power.
Efficiency, defined as the ratio of output power to input power,
indicates how much power is dissipated in the power supply.
In the typical TOPSwitch-II flyback power supply shown in
Figure 1, most of the power dissipation occurs in output
rectifier D2, Zener diode VR1 (or equivalent clamp circuit)
and the TOPSwitch-II device. Other components, such as
outputfilterinductorL1, inputcommonmodeinductorL2, and
bridgerectifierBR1,contributelesserpowerdissipationterms.
6) Read TOPSwitch-II power dissipation from
the dashed contours to determine heatsink
requirements.
7) Start the design. Use the Transformer
Design Spreadsheet from AN-17.
Note: See Selection Curve Assumptions for limits of use.
Overview of Quick Selection Curves
For higher nominal mains voltages, including 208, 220, 230,
and 240 VAC, a low line AC input voltage of 195 VAC is used
togeneratesimilarcurvesfoundinFigure4(for+12Voutputs)
and Figure 5 (for +5 V outputs). For all curves, the maximum
AC mains voltage is assumed to be 265 VAC.
The TOPSwitch-II Flyback Quick Selection Curves consider
these dissipation terms (and others as well) to provide a good
estimate of expected efficiency for both Universal input and
230 VAC mains applications. Figure 2 (for +12 V outputs) and
Figure 3 (for +5 V outputs) show a set of curves for efficiency
and TOPSwitch-II power dissipation versus output power for
the entire family of TOP221-TOP227 devices. These curves
assume operation from a low line AC input voltage of 85 VAC,
which is a suitable value for all Universal input applications.
For each TOPSwitch-II device, a family of efficiency curves
(solid lines) is plotted on the Y-axis as a function of output
power on the X-axis. TOPSwitch-II power dissipation is
plotted separately on the same graph as a family of constant
power dissipation contours (dashed lines).
April 1998
AN-21
D2
L1
MUR610CT
3.3 µH
1
9, 10
15 V
RTN
C2
1000 µF
35 V
C3
120 µF
25 V
VR1
P6KE200
R2
200 Ω
1/2 W
C1
47 µF
400 V
6, 7
D1
BYV26C
D3
1N4148
U2
NEC2501
4
5
BR1
2
400 V
L2
C4
0.1 µF
33 mH
R4
49.9 kΩ
R1
C7
1.0 nF
Y1
510 Ω
T1
C9
C6
0.1 µF
0.1 µF
TOPSwitch-II
D
S
CONTROL
C
F1
3.15 A
U3
TL431
C5
47 µF
R3
6.2 Ω
L
U1
TOP224Y
R5
N
10 kΩ
J1
PI-2158-031698
Figure 1. Typical Flyback Power Supply Using TOP224.
Example 1: 30 W Universal Application
Selecting the Right TOPSwitch-II
Using Figures 2, 3, 4 and 5
Assumea+5Vapplicationrequires30Wofoutputpowerfrom
Universal input voltage. From the curves in Figure 3, the
TOP224 can deliver 30 W with an estimated Y-axis efficiency
of 71%. The projected TOPSwitch-II power dissipation is
approximately 2.5W. The TOP225 can also be used with an
expected efficiency of 75% and interpolated power dissipation
of approximately 1.7 W. With these curves, a heat sink can be
selected or evaluated immediately because an estimate for
TOPSwitch-II power dissipation is now available before the
design is even started!
First we use the Power versus Efficiency curves to find the
efficiency of the power supply for each TOPSwitch-II device
that will deliver the output power. Then we estimate the
TOPSwitch-II loss from the contours of constant power
dissipation.
Start with the output power of the application on the X-axis.
Move vertically to the intersection with the first TOPSwitch-II
curve and then read the efficiency directly from the Y-axis.
From the same intersection point on the TOPSwitch-II curve,
interpolate the TOPSwitch-II power dissipation from the
constant power dissipation contours.
Example 2: 30 W Application from 230 VAC
Consider a +12 V output at 30 W from 230 VAC input. Figure
4 shows the TOP223 is the optimum device with an expected
efficiency slightly over 85% and power dissipation of
approximately 0.75 W.
Some output powers can be delivered by more than one
TOPSwitch-II device. When moving vertically from the X-
axis, the first curve encountered will be for the smallest, lowest
cost TOPSwitch-II device, while the last curve will be for the
largest, most efficient TOPSwitch-II device suitable for the
desired output power.
Example 3: TOPSwitch-II Temperature
It is easy to estimate the junction temperature TJ of the
TOPSwitch-II from the ambient temperature TA and the
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AN-21
effective junction to ambient thermal impedance θJA. This
technique works for any TOPSwitch-II package as long as the
overall thermal impedance is known, which includes the
selected TOPSwitch-II thermal impedance, the thermal
interface to a heatsink, and the effective thermal impedance
of the heatsink itself. For example, with a TOP225 dissipation
PD of 1.7 W, ambient temperature T of 40 °C, and overall
thermal impedance θJA of 20 °AC/W, the maximum
TOPSwitch-II junction temperature TJ can be found as
follows:
Adjusting for Minimum Input Voltage
Using Figures 6 and 7
To use the power ratio curves, start on the X-axis with the
desired minimum AC input voltage. Move vertically to the
intersection with the curve. Read the value of the power ratio
from the Y-axis. The effective output power at the originally
assumed minimum mains voltage of 85 or 195 VAC is simply
the actual required output power divided by this ratio.
The effective output power at 85 or 195 VAC mains voltage is
used as the X-axis value for the curves given in Figures 2-5.
The effective output power at 85 or 195 VAC will generate the
same TOPSwitch-II loss (obtained from the curves in Figures
2-5) as the actual required output power at the modified AC
input voltage. This ratio also scales the primary inductance to
avalueappropriateforthedifferentinputvoltage. Theoriginal
curves are derived from the typical values in Table 3, which is
discussed later in this application note. In addition,
TOPSwitch-II duty cycle limitations require a linear reduction
in reflected voltage VOR for AC mains voltages below 85 VAC,
as shown in Figure 7.
TJ = TA + (P ×θJA )
D
= 40 °C + (1.7 W × 20 °C/W) = 74 °C
The design should limit TJ to less than 100 °C at the maximum
ambient temperature.
Available Power
The minimum AC input voltage has a strong influence on the
choice of TOPSwitch-II device for a given output power. If the
minimum voltage is increased above the values assumed for
the curves in Figures 2 through 5, then more power will be
available from each TOPSwitch-II device.
Example 4: Input Voltage Adjustment
We can use the Output Power Ratio Curves in Figures 6 and 7
together with the original curves of Figures 2 through 5 to
determine the available power for different input voltages.
Suppose an application for only the US market requires 35 W
of output power at +12 V. The lowest AC input voltage is
typically 90% of 115 VAC or 103.5 VAC. Find the power ratio
from Figure 7 to be 1.15. The effective output power, obtained
by dividing the actual output power by the power ratio, is
Figure 6 gives a ratio curve for 230 VAC mains at low line
while Figure 7 shows a similar curve for low line Universal
mains applications.
35 W
Effective Output Power =
= 30.4 W
1.15
VALUE
100 kHz
135 V
PARAMETER
195 VAC
85 VAC
Optocoupler
LED Current
Switching Frequency (fs)
5.0 mA
3.5 mA
3.5 mA
Transformer Reflected Voltage (VOR
)
Optocoupler
Transistor Current
5.0 mA
200 V
Clamp Voltage (VCLAMP
)
Table 2. Typical Power Supply Parameters that Change with
TOPSwitch-II Duty Cycle.
Output Schottky Rectifier
Forward Voltage (VD)
0.4 V
16 V
Primary Bias Voltage (VB)
Table 1. Power Supply Parameters Independent of Input Voltage
and Output Power.
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TYPICAL POWER SUPPLY COMPONENT PARAMETERS
PARAMETER
UNITS TOP221 TOP222 TOP223 TOP224 TOP225 TOP226 TOP227
2200
45
4400
90
1475
30
1100
22
880
18
740
15
Transformer Primary Inductance
8650
175
µH
µH
Transformer Leakage Inductance
(referred to the primary)
Transformer Resonant Frequency
(measured with secondary open)
500
450
550
600
650
700
400
kHz
Transformer Primary
Winding Resistance
650
7
1800
12
350
5
250
4
175
3.5
140
3
5000
20
mΩ
mΩ
mΩ
mΩ
Transformer Secondary Resistance
Output Capacitor Equivalent
Series Resistance
18
25
24
15
20
13
16
11.5
13
10
10
30
32
40
Output Inductor DC Resistance
Common Mode Inductor
DC Resistance
333
370
300
267
233
200
400
mΩ
Table 3. Typical Power Supply Component Parameters for TOPSwitch-II Flyback Power Supply.
This effective output power is then used with the curves in
Figure 2 to select the TOPSwitch-II device and to estimate the
TOPSwitch-II dissipation. Predictions of efficiency and power
dissipation may be less accurate when the ratio is used. The
new value of primary inductance is the product of the power
ratio and original inductance value in Table 3. The new
inductance value for the TOP224 would be:
Typical values are given in Table 2 for two parameters that
depend only on input voltage. These parameters change with
TOPSwitch-II duty cycle.
The remaining power supply parameters depend on the output
power. Table 3 gives typical values for the power-dependent
parameters
LP = 1475 µH ×1.15 = 1696 µH
Input Capacitance
Efficiency and output power are both strong functions of bulk
energy storage capacitor C1. For the Universal AC Mains
curves, the numerical value of C1 in microfarads is assumed to
be at least three times the maximum output power in watts. For
230 VAC mains, the C1 value (µF) is assumed to be at least
equal to the maximum output power (watts).
Selection Curve Assumptions
Several physical power supply parameters must be calculated,
estimated, or measured to determine efficiency. Measured
values can differ significantly from the curves’ predictions if
the design parameters are not the same as the typical values
used to generate the curves.
Forexample, for30Wofoutputpower, thebulkenergystorage
capacitor C1 is expected to be at least 90 µF for Universal
mains and 30 µF for 230 VAC mains applications. The design
must consider the tolerance of the capacitor to guarantee
expected performance from the power supply.
Typical values are given in Table 1 for several parameters that
are independent of power level and input voltage. These
parameters are defined and discussed in AN-16 and AN-17.
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AN-21
Lower values of input capacitance will reduce the available
output power. Going from 3 to 2 µF per watt will decrease the
output power by as much as 15% for Universal input. The
available power falls dramatically for values less than 2 µF per
watt.
• Use a DC voltage source to prevent AC ripple voltage
from modulating the duty cycle. Efficiency depends
heavily on actual DC input voltage. A convincing
experiment is to vary the DC voltage 15 V to see how
efficiency varies over the range of expected AC ripple
voltage.
The value of capacitor C1 also determines the average value of
the DC bus voltage. The Universal VAC Mains curves in
Figures 2 and 3 were generated with an average DC bus value
of 105 VDC while the 230 VAC Mains curves in Figures 4 and
5 were generated with an average DC bus value of 265VDC.
• Measuretransformerleakageinductanceaccurately. Take
into account inductance of external circuitry, which can
increase effective leakage inductance by 30% or more.
• Measureswitchingfrequencyaccuratelyfortheindividual
TOPSwitch-II in the circuit to account for component-to-
component variations.
Other Considerations
Curves in this application note were generated from the typical
power supply parameters in Tables 1, 2 and 3. If measured
efficiency in a particular TOPSwitch-II application does not
agree with the values predicted from the curves, it is likely the
physical parameters of the measured power supply do not
match the tabular values. Use the guidelines below to get best
agreement between measurements and predictions.
• Verify actual clamp voltage. Effective clamp voltage can
be 230 VDC or higher, even though the clamp Zener diode
is specified to be 200 V. See AN-16 for details.
Determine which physical power supply parameters do not
match the typical values in Table 3. Change (temporarily) to
components that match the parameters in the table until
measured efficiency matches the predicted value.
• When measuring efficiency from an AC source, use an
electronic wattmeter designed for average input power
measurements with high-crest factor current waveforms.
Do not simply measure RMS input voltage and RMS input
current. The product of these two measurements is input
volt-amperes or input burden (VA), not the real input
power in watts.
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AN-21
UNIVERSAL INPUT (85 VAC TO 265 VAC) 12 V OUTPUT
84
0.25 W
82
0.5 W
80
0.75 W
1.0 W
78
76
74
3 W
2 W
4 W
5 W
2.5 W
1.5 W
8 W
10 W
6 W
TOP224
TOP222
TOP223
72
TOP221
70
68
TOP225
TOP226
60
TOP227
80 100
4
6
8
10
15
20
30
40
Output Power (W) at 85 VAC
Figure 2. Typical Efficiency vs Output Power with Contours of Constant TOPSwitch-II Power Loss for Universal Input and 12 V Output.
UNIVERSAL INPUT (85 VAC TO 265 VAC) 5 V OUTPUT
80
78
0.25 W
76
0.5 W
74
0.75 W
72
1.0 W
70
3 W
2.0 W
2.5 W
TOP227
68
1.5 W
4 W
66
64
TOP221
6 W
TOP222
5 W
TOP223
62
8 W
10 W
TOP224
12 W
60
58
TOP225
14 W
TOP226
4
6
8
10
15
20
30
40
60
80
100
Output Power (W) at 85 VAC
Figure 3. Typical Efficiency vs Output Power with Contours of Constant TOPSwitch-II Power Loss for Universal Input and 5 V Output.
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SINGLE VOLTAGE INPUT (230 VAC 15%) 12 V OUTPUT
87
86
85
0.25 W
84
0.5 W
1.0 W
2.0 W
3 W
83
82
0.75 W
1.5 W
2.5 W
TOP224
TOP225
TOP222
TOP223
TOP221
TOP226
81
80
TOP227
7
8
9
10
15
20
30
40
60
80
100
200
150
Output Power (W) at 195 VAC
Figure 4. Typical Efficiency vs Output Power with Contours of Constant TOPSwitch-II Power Loss for Single Voltage Application and
12 V Output.
SINGLE VOLTAGE INPUT (230 VAC 15%) 5 V OUTPUT
80
0.25 W
78
0.5 W
76
1.0 W
0.75 W
TOP221
1.5 W
74
TOP222
2.0 W
2.5 W
3 W
72
70
68
TOP223
TOP224
TOP227
5 W
TOP225
6 W
TOP226
7
8
9
10
15
20
30
40
60
80
100
150
200
Output Power (W) at 195 VAC
Figure 5. Typical Efficiency vs Output Power with Contours of Constant TOPSwitch-II Power Loss for Single Voltage Application and
5 V Output.
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AN-21
POWER RATIO: UNIVERSAL INPUT (85 TO 265 VAC)
POWER RATIO: SINGLE VOLTAGE (230 VAC 15%)
1.3
1.15
1.2
1.1
1.1
1.05
1
1
0.9
0.95
P
V
OUT
OR
0.8
0.9
P
V
OUT
OR
0.7
0.85
140
160
180
220
240
200
0.6
Low Line AC Input Voltage (VAC)
60
70
80
90
100
110
Low Line AC Input Voltage (VAC)
Figure 6. Power Ratio vs Low Line AC Input Voltage of Nominal
230 VAC.
Figure 7. Power Ratio and VOR vs Low Line AC Input Voltage for
Universal Input.
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Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it
convey any license under its patent rights or the rights of others.
The PI Logo, TOPSwitch, TinySwitch and EcoSmart are registered trademarks of Power Integrations, Inc.
©Copyright 2001, Power Integrations, Inc.
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