® ®

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 1) Determine which graph (Fig. 2, 3, 4 or 5) is
select the proper TOPSwitch-II device from parameters that closest to your application.
are usually not available until much later in the design process.
Example: Use Figure 2 for Universal input,
The TOPSwitch-II Flyback Quick Selection Curves provide
12 V output.
the essential design guidance.

Efficiency and TOPSwitch-II power dissipation are two 2) Find your power requirement on the X- axis.
important performance parameters to the flyback power supply
designer. Both can be easily measured or accurately estimated 3) Move vertically from your power
after the power supply is designed. But what if the designer requirement until you intersect with a
must make project and resource decisions before actually TOPSwitch-II curve (solid line).
committing to and starting development? This application
note helps the designer quickly select the optimum 4) Read the associated efficiency on the
TOPSwitch-II device from simple curves of estimated efficiency Y- axis.
and TOPSwitch-II power dissipation.
5) Determine if this is the appropriate
Typical Power Supply Losses efficiency for your application. If not,
Power supplies have an input power which, because of internal continue to the next TOPSwitch-II curve.
dissipation, can be significantly higher than the output power.
Efficiency, defined as the ratio of output power to input power, 6) Read TOPSwitch-II power dissipation from
indicates how much power is dissipated in the power supply. the dashed contours to determine heatsink
In the typical TOPSwitch-II flyback power supply shown in requirements.
Figure 1, most of the power dissipation occurs in output
rectifier D2, Zener diode VR1 (or equivalent clamp circuit) 7) Start the design. Use the Transformer
and the TOPSwitch-II device. Other components, such as Design Spreadsheet from AN-17.
output filter inductor L1, input common mode inductor L2, and
bridge rectifier BR1, contribute lesser power dissipation terms. Note: See Selection Curve Assumptions for limits of use.

Overview of Quick Selection Curves

For higher nominal mains voltages, including 208, 220, 230,
The TOPSwitch-II Flyback Quick Selection Curves consider and 240 VAC, a low line AC input voltage of 195 VAC is used
these dissipation terms (and others as well) to provide a good to generate similar curves found in Figure 4 (for +12 V outputs)
estimate of expected efficiency for both Universal input and and Figure 5 (for +5 V outputs). For all curves, the maximum
230 VAC mains applications. Figure 2 (for +12 V outputs) and AC mains voltage is assumed to be 265 VAC.
Figure 3 (for +5 V outputs) show a set of curves for efficiency
and TOPSwitch-II power dissipation versus output power for For each TOPSwitch-II device, a family of efficiency curves
the entire family of TOP221-TOP227 devices. These curves (solid lines) is plotted on the Y-axis as a function of output
assume operation from a low line AC input voltage of 85 VAC, power on the X-axis. TOPSwitch-II power dissipation is
which is a suitable value for all Universal input applications. 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

C2 C3
VR1 1000 µF R2 120 µF
C1 P6KE200 35 V 200 Ω 25 V
47 µF 6, 7 1/2 W
400 V RTN
D1 D3
BYV26C U2
1N4148 NEC2501
BR1 2 4
400 V
L2
C4
33 mH
0.1 µF R4
5 49.9 kΩ
R1
C7 510 Ω
T1 1.0 nF
Y1
C9
C6
TOPSwitch-II 0.1 µF
0.1 µF D
CONTROL
F1 C U3
3.15 A TL431
L C5 R3
S U1 47 µF 6.2 Ω
TOP224Y R5
N 10 kΩ
J1

PI-2158-031698

Figure 1. Typical Flyback Power Supply Using TOP224.

Selecting the Right TOPSwitch-II Example 1: 30 W Universal Application

Using Figures 2, 3, 4 and 5 Assume a +5 V application requires 30 W of output power from

Universal input voltage. From the curves in Figure 3, the
First we use the Power versus Efficiency curves to find the TOP224 can deliver 30 W with an estimated Y-axis efficiency
efficiency of the power supply for each TOPSwitch-II device of 71%. The projected TOPSwitch-II power dissipation is
that will deliver the output power. Then we estimate the approximately 2.5W. The TOP225 can also be used with an
TOPSwitch-II loss from the contours of constant power expected efficiency of 75% and interpolated power dissipation
dissipation. of approximately 1.7 W. With these curves, a heat sink can be
selected or evaluated immediately because an estimate for
Start with the output power of the application on the X-axis. TOPSwitch-II power dissipation is now available before the
Move vertically to the intersection with the first TOPSwitch-II design is even started!
curve and then read the efficiency directly from the Y-axis.
From the same intersection point on the TOPSwitch-II curve, Example 2: 30 W Application from 230 VAC
interpolate the TOPSwitch-II power dissipation from the
constant power dissipation contours. Consider a +12 V output at 30 W from 230 VAC input. Figure
4 shows the TOP223 is the optimum device with an expected
Some output powers can be delivered by more than one efficiency slightly over 85% and power dissipation of
TOPSwitch-II device. When moving vertically from the X- approximately 0.75 W.
axis, the first curve encountered will be for the smallest, lowest
cost TOPSwitch-II device, while the last curve will be for the Example 3: TOPSwitch-II Temperature
largest, most efficient TOPSwitch-II device suitable for the
desired output power. 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 Adjusting for Minimum Input Voltage
technique works for any TOPSwitch-II package as long as the
overall thermal impedance is known, which includes the Using Figures 6 and 7
selected TOPSwitch-II thermal impedance, the thermal
interface to a heatsink, and the effective thermal impedance To use the power ratio curves, start on the X-axis with the
of the heatsink itself. For example, with a TOP225 dissipation desired minimum AC input voltage. Move vertically to the
PD of 1.7 W, ambient temperature TA of 40 °C, and overall intersection with the curve. Read the value of the power ratio
thermal impedance θ JA of 20 °C/W, the maximum from the Y-axis. The effective output power at the originally
TOPSwitch-II junction temperature TJ can be found as assumed minimum mains voltage of 85 or 195 VAC is simply
follows: the actual required output power divided by this ratio.

The effective output power at 85 or 195 VAC mains voltage is

TJ = TA + ( PD × θ JA ) 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
= 40 °C + (1.7 W × 20 °C/W ) = 74 °C same TOPSwitch-II loss (obtained from the curves in Figures
The design should limit TJ to less than 100 °C at the maximum 2-5) as the actual required output power at the modified AC
ambient temperature. input voltage. This ratio also scales the primary inductance to
a value appropriate for the different input voltage. The original
Available Power curves are derived from the typical values in Table 3, which is
discussed later in this application note. In addition,
The minimum AC input voltage has a strong influence on the TOPSwitch-II duty cycle limitations require a linear reduction
choice of TOPSwitch-II device for a given output power. If the in reflected voltage VOR for AC mains voltages below 85 VAC,
minimum voltage is increased above the values assumed for as shown in Figure 7.
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 Suppose an application for only the US market requires 35 W
together with the original curves of Figures 2 through 5 to of output power at +12 V. The lowest AC input voltage is
determine the available power for different input voltages. 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
Figure 6 gives a ratio curve for 230 VAC mains at low line by dividing the actual output power by the power ratio, is
while Figure 7 shows a similar curve for low line Universal
mains applications. 35 W
Effective Output Power = = 30.4 W
1.15

PARAMETER VALUE 85 VAC 195 VAC

Switching Frequency (fs) 100 kHz Optocoupler

3.5 mA 5.0 mA
LED Current
Transformer Reflected Voltage (VOR) 135 V
Optocoupler
3.5 mA 5.0 mA
Clamp Voltage (VCLAMP) 200 V Transistor Current

Output Schottky Rectifier Table 2. Typical Power Supply Parameters that Change with
0.4 V TOPSwitch-II Duty Cycle.
Forward Voltage (VD)

Primary Bias Voltage (VB) 16 V

Table 1. Power Supply Parameters Independent of Input Voltage

and Output Power.

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 AN-21

TYPICAL POWER SUPPLY COMPONENT PARAMETERS

PARAMETER UNITS TOP221 TOP222 TOP223 TOP224 TOP225 TOP226 TOP227

Transformer Primary Inductance µH 8650 4400 2200 1475 1100 880 740

Transformer Leakage Inductance

(referred to the primary) µH 175 90 45 30 22 18 15

Transformer Resonant Frequency

kHz 400 450 500 550 600 650 700
(measured with secondary open)

Transformer Primary
mΩ 5000 1800 650 350 250 175 140
Winding Resistance

Transformer Secondary Resistance mΩ 20 12 7 5 4 3.5 3

Output Capacitor Equivalent mΩ 30 24 18 15 13 11.5 10

Series Resistance

Output Inductor DC Resistance mΩ 40 32 25 20 16 13 10

Common Mode Inductor

mΩ 400 370 333 300 267 233 200
DC Resistance

Table 3. Typical Power Supply Component Parameters for TOPSwitch-II Flyback Power Supply.

This effective output power is then used with the curves in Typical values are given in Table 2 for two parameters that
Figure 2 to select the TOPSwitch-II device and to estimate the depend only on input voltage. These parameters change with
TOPSwitch-II dissipation. Predictions of efficiency and power TOPSwitch-II duty cycle.
dissipation may be less accurate when the ratio is used. The
new value of primary inductance is the product of the power The remaining power supply parameters depend on the output
ratio and original inductance value in Table 3. The new power. Table 3 gives typical values for the power-dependent
inductance value for the TOP224 would be: parameters

LP = 1475 µH × 1.15 = 1696 µH Input Capacitance

Efficiency and output power are both strong functions of bulk
Selection Curve Assumptions energy storage capacitor C1. For the Universal AC Mains
Several physical power supply parameters must be calculated, curves, the numerical value of C1 in microfarads is assumed to
estimated, or measured to determine efficiency. Measured be at least three times the maximum output power in watts. For
values can differ significantly from the curves’ predictions if 230 VAC mains, the C1 value (µF) is assumed to be at least
the design parameters are not the same as the typical values equal to the maximum output power (watts).
used to generate the curves.
For example, for 30 W of output power, the bulk energy storage
Typical values are given in Table 1 for several parameters that capacitor C1 is expected to be at least 90 µF for Universal
are independent of power level and input voltage. These mains and 30 µF for 230 VAC mains applications. The design
parameters are defined and discussed in AN-16 and AN-17. must consider the tolerance of the capacitor to guarantee
expected performance from the power supply.

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 AN-21
Lower values of input capacitance will reduce the available • Use a DC voltage source to prevent AC ripple voltage
output power. Going from 3 to 2 µF per watt will decrease the from modulating the duty cycle. Efficiency depends
output power by as much as 15% for Universal input. The heavily on actual DC input voltage. A convincing
available power falls dramatically for values less than 2 µF per experiment is to vary the DC voltage ±15 V to see how
watt. 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 • Measure transformer leakage inductance accurately. Take
Figures 2 and 3 were generated with an average DC bus value into account inductance of external circuitry, which can
of 105 VDC while the 230 VAC Mains curves in Figures 4 and increase effective leakage inductance by 30% or more.
5 were generated with an average DC bus value of 265VDC.
• Measure switching frequency accurately for the individual
Other Considerations TOPSwitch-II in the circuit to account for component-to-
component variations.
Curves in this application note were generated from the typical
power supply parameters in Tables 1, 2 and 3. If measured • Verify actual clamp voltage. Effective clamp voltage can
efficiency in a particular TOPSwitch-II application does not be 230 VDC or higher, even though the clamp Zener diode
agree with the values predicted from the curves, it is likely the is specified to be 200 V. See AN-16 for details.
physical parameters of the measured power supply do not
match the tabular values. Use the guidelines below to get best Determine which physical power supply parameters do not
agreement between measurements and predictions. match the typical values in Table 3. Change (temporarily) to
components that match the parameters in the table until
• When measuring efficiency from an AC source, use an measured efficiency matches the predicted value.
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

PI-2160-031898
84

0.25 W
82
Efficiency (%) at 85 VAC

80 0.5 W

78 0.75 W
3W
1.0 W 2W 4W
76
2.5 W
5W
74 1.5 W
8W
10 W

W
TOP222 6W

12
72 TOP223
TOP221 TOP224 W
14
70 TOP225
TOP226
TOP227
68
4 6 8 10 15 20 30 40 60 80 100
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

PI-2162-040298
78
0.25 W
76

74 0.5 W
Efficiency (%) at 85 VAC

72 0.75 W

70 1.0 W
3W
2.0 W TOP227
68 1.5 W
2.5 W
4W
66
TOP221 6W
TOP222
64 5W
TOP223
62 8W 10 W
W

TOP224
12

60 W
TOP225 14
TOP226
58
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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 AN-21
SINGLE VOLTAGE INPUT (230 VAC ±15%) 12 V OUTPUT
87

PI-2164-040298
86

85
Efficiency (%) at 195 VAC

0.25 W

84

0.5 W 1.0 W 2.0 W

3W

4W
83
0.75 W 1.5 W
2.5 W

6W
5W
82 TOP224
TOP222 TOP223
TOP221 TOP225

TOP226
81 TOP227

80
7 8 9 10 15 20 30 40 60 80 100 150 200
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

PI-2166-040298
0.25 W

78

0.5 W
Efficiency (%) at 195 VAC

76
1.0 W
0.75 W
TOP221
74 1.5 W
TOP222 2.0 W

2.5 W 3W
72
TOP223
W
4

TOP224
70 W
5 TOP227
TOP225
6W

68 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: SINGLE VOLTAGE (230 VAC ±15%) POWER RATIO: UNIVERSAL INPUT (85 TO 265 VAC)
1.3

PI-2170-040498
1.15
Output Power Ratio (195 VAC)

PI-2168-040498

Output Power Ratio (85 VAC)

1.1 1.2

1.05 1.1

1 1

0.95 0.9

POUT
0.9 0.8
VOR
POUT
0.85 0.7
VOR
140 160 180 200 220 240
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 Figure 7. Power Ratio and VOR vs Low Line AC Input Voltage for
230 VAC. 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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