Power Integrations reserves the right to make changes to its products at any time to improve reliability or
manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit
described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND SPECIFICALLY DISCLAIMS ALL
WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS
FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications
illustrated herein (transformer construction and circuits external to the products) may be covered by one or more U.S.
and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A
complete list of Power Integrations' patents may be found at www.powerint.com. Power Integrations grants its
customers a license under certain patent rights as set forth at http://www.powerint.com/ip.htm.
The PI logo, TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, PeakSwitch, EcoSmart,
Clampless, E-Shield, Filterfuse, StackFET, PI Expert and PI FACTS are trademarks of Power
Integrations, Inc. Other trademarks are property of their respective companies. ©Copyright 2007, Power Integrations,
For the latest updates, visit www.powerint.com
DI-89
DI-122
Rev. A 1/07
Power Integrations
5245 Hellyer Avenue
San Jose, CA 95138
Phone: 1-408-414-9200
Apps: 1-408-414-9660
Apps Fax: 1-408-414-9760
For a complete listing of worldwide
sales offices, please visit
www.powerint.com
Table 2. Transformer Construction Information.
Key Design Points
• The secondary side bias current, set by R7, should be kept
low to minimize light load and no-load power consumption.
• The output voltage that is reected across the transformer
(V
OR
) should be kept low to minimize the losses in the
primary side RCD clamp.
• Optimize the turns ratio of the two outputs for voltage
centering.
• For good output cross regulation, design for continuous
conduction mode (minimize the value of K
P
in the
PI Xls spreadsheet).
• Use a Schottky diode for D3 to limit EN/UV pin > -0.3 V.
• As the forward drop of D3 is a function of temperature, diode
type and current, use a schottky diode. Do not reduce the
value of R3, and verify EN/UV pin absolute maximum
ratings are observed at low temperatures (~ <-10 °C).
TRANSFORMER PARAMETERS
Core Material
EF20 TDK PC40, or equivalent
A
LG
of 142 nH/T
2
Bobbin EF20, 10 pin (5 + 5)
Winding Details
Shield: 23T, Primary: 95T
Shield: 9T (trilar), –5 V: 5T,
–12 V: 7T
Primary
Inductance
1.28 mH ±10%
Figure 3. Conducted EMI at 230 VAC, Full Load.
Figure 2. Full Load Efciency vs. Input Voltage.
-5 V -12 V
20% on -5 V,
100% on -12 V
-5.05 V -11.49 V
100% on -5 V,
20% on -12 V
-5.03 V -12.95 V
100% on -5 V,
5% on -12 V
-5.02 V -13.90 V
Table 1. Worst Case Cross Regulation at 85 VAC.
85 115 175145 235205 265
AC Input Voltage (V)
Efficiency (%)
82
80
78
76
74
72
70
PI-4499-090606
Nominal Load
PI-4500-090706
1.0
EN55022B Limits
0.15 10.0 70.0
-20
-10
0
10
20
30
40
50
60
80
70
MHz
dBµV
QP
AV
QP
AV
• The value of R4 can be used to make small output
voltage adjustments.
• The regulation point is the sum of the voltage rating of
VR1, V
BE(Q1)
and the voltage across R4 ((V
BE(Q1)
/R7) × R4).
• Biasing the Zener below its test current lowers its
voltage drop, in this case to ~4.3 V.
• Select transformer wire gauge sizes so that each winding
layer occupies the entire bobbin width (lowers leakage
inductance and improves output cross regulation).
• Use option to add E-Shield windings in PI Transformer
Designer software to reduce conducted EMI noise
generation.
• No-load power consumption can be lowered by adding a
bias winding to feed the supply current of U1 into the BP pin.