ETC AN-23

®
TM
TinySwitch Flyback Design
Methodology
Application Note AN-23
Introduction
Design Flow
This document describes a simple Design Methodology for
flyback power supply design using the TinySwitch family of
integrated off-line switchers. The objective of this Design
Methodology is to provide power supply engineers a handy tool
that not only eases the design task but also delivers design
optimization in cost and efficiency for most applications.
Figures 2A, B and C present a design flow chart showing the
complete design procedure in 22 steps. The logic behind this
Design Methodology can be summarized as follows:
Basic Circuit Configuration
2. Design for discontinuous mode operation using this calculated
VOR. If necessary, increase VOR.
1. Calculate minimum reflected voltage, VOR, allowed by a
given output diode.
Because of the high level integration of TinySwitch, flyback
power supply design is greatly simplified. As a result, the basic
circuit configuration of TinySwitch flyback power supplies
remains unchanged from application to application. Application
specific issues outside this basic configuration such as constant
current, constant power outputs, etc. are beyond the scope of
this document.
3. At VOR = 150 V, select bigger TinySwitch to stay in
discontinuous mode or go to continuous mode design.
4. Design transformer using EE16 core.
5. Select feedback circuit and other components to complete
the design.
Figure 1 shows the basic circuit configuration in a typical
TinySwitch flyback design using TNY253.
Output capacitor
Output post filter L, C
+
VO
D
TNY253
EN
BP
Fusible
CIN
S
VAC
0.1 µF
Feedback Sense
Circuit
Snubber Circuit
Bypass pin capacitor
PI-2336-112098
Figure 1. Typical TinySwitch Flyback Power Supply.
July 1999
AN-23
1. System Requirements
VACMIN, VACMAX, fL, VO, PO, η
2. Select Output Diode Based on VO & η
Estimate Diode Loss
3. Select Clamp/Snubber Circuit
4. Estimate Efficiency η
5. Determine VMIN, VMAX & CIN
6. Determine PIV
Calculate VOR from VMAX, VO, VD, & PIV
N
VOR < 150 V
Y
7. Choose TinySwitch
Based on VMIN ,VMAX & PO
To Step 8
PI-2345-111398
Figure 2A. TinySwitch Flyback Design Flowchart Steps 1 to 7.
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AN-23
From Step 7
8. Calculate DMAX
from VMIN, PO & IP
9. Calculate KDP from
VMIN, VOR & DMAX
10. Fully Disc?
N
N
11A. Fully Disc
Required?
Y
12. Mostly Disc?
Y
Y
N
13. Continuous
OK?
N
11B. Set KDP = (1- DMAX)/(0.67 - DMAX),
13B. Set KDP = 1,
Recalculate VOR
Recalculate VOR
Y
14. Calculate DMAX
from VMIN, VOR
Y
15. Calculate KRP from
VMIN, PO, IP & DMAX
VOR < 150 V
16B. Set KRP = 0.6,
N
Recalculate DMAX
N
16A. KRP ≥ 0.6
Y
Back to Step 7
N
VOR < 150 V
16C. Recalculate VOR
from VMIN, DMAX
Y
Discontinuous
17A. Calculate LP
Continuous
17B. Calculate LP
To Step 18
To Step 18
PI-2351-111398
Figure 2B. TinySwitch Flyback Design Flowchart Steps 8 to 17A, B.
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AN-23
From Step 17A, B
18. Design Transformer NP, NS, Lg
Discontinuous?
N
Y
19A. Discontinuous
Calculate Currents IRMS, ISRMS
19B. Continuous
Calculate Currents IRMS, ISRMS
20. Determine Output Short
Circuit Current IOS
N
21. Determine Output Capacitor COUT
22. Determine Feedback Circuit
and Post Filter
Design
Complete
PI-2347-112098
Figure 2C. TinySwitch Flyback Design Flowchart Steps 18-22.
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AN-23
Step by Step Design Procedure
Output Diode
Symbols and parameters used in this design procedure are
defined in Application Note “TOPSwitch Flyback Design
Methodology” (AN-16).
1N5819
1N5822
Schottky MBR745
MBR1045
MBR1645
UF4002
MUR110
MUR120
UF4003
BYV27-200
UF5401
UFR
UF5402
MUR410
MUR420
MUR810
MUR820
BYW29-200
BYV32-200
Step 1.
Determine system requirements: VACMIN, VACMAX, fL,
VO, PO, η
• Determine input voltage range from Table 1.
Input (VAC)
100/115
230
Universal
VACMIN (VAC)
85
VACMAX (VAC)
132
195
85
265
265
Table 1. Input Voltage Range.
Step 2.
Select output diode. Estimate associated efficiency loss.
VR (V) ID (A)
40
40
45
45
45
100
100
200
200
200
100
200
100
200
100
200
200
200
1.0
3.0
7.5
10.0
16.0
1.0
1.0
1.0
1.0
2.0
3.0
3.0
4.0
4.0
8.0
8.0
8.0
20.0
Manufacturer
Motorola
Motorola
Motorola
Motorola
Motorola
GI
Motorola
Motorola
GI
Philips, GI
GI
GI
Motorola
Motorola
Motorola
Motorola
Philips, GI
Philips
Table 3. Output diodes.
• The output diode can be selected based on expected
power supply efficiency and cost (see Table 2).
- Use a Schottky diode for highest efficiency for
output voltages up to 7.5 V.
- For output voltages beyond 7.5 V use an Ultra Fast PNdiode.
- If efficiency is not a concern (or cost is
paramount), use a Fast PN-diode.
- The Schottky and Ultrafast may be used with
continuous mode of operation. The Fast PN-diode should
be used only with discontinuous mode of operation.
- Choose output diode type. Table 2 shows approximate
forward voltage (VD) for types of output diode discussed
above.
• Output diode efficiency loss is the power supply efficiency
reduction (in percentage) caused by the diode.
Diode Type
VD (V)
Step 3.
Select clamp/snubber circuit and determine associated
efficiency loss.
• Clamp/snubber circuit is required at DRAIN to keep DRAIN
voltage below rated BV:
- A snubber alone may be used at low power (< 3 W
with Universal input) and will provide lower video noise
and superior EMI performance.
- An RCD clamp may be used for power levels < 3 W for
higher efficiency and is required at power levels > 3 W
with Universal input.
• Table 4 shows the approximate efficiency loss due to clamp/
snubber circuits.
Schottky
0.5
Efficiency Loss
(0.5/VO) × 100%
Ultrafast-PN
1.0
(1.0/VO) × 100%
Clamp/Snubber
RC Snubber
Fast-PN
1.0
(1.0/VO) × 100%
RCD clamp
Table 2. Diode forward voltage (VD) and efficiency loss.
• The estimated efficiency loss due to the output diode is also
shown in Table 2.
• Table 3 shows some commonly used output diodes. VR is the
diode reverse voltage rating. ID is the diode DC current
rating.
• The final diode current rating is to be determined in Step 20
to accommodate continuous short circuit current IOS.
PO
0 to 3 W
Efficiency Loss
20%
>3W
15%
Table 4. Clamp/Snubber efficiency loss.
Step 4.
Estimate power supply efficiency η.
• Total efficiency loss is the sum of the output diode efficiency
loss (from Step 2) and the clamp/snubber efficiency loss
(from Step 3).
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• Calculate overall power supply efficiency as: η = 100% total efficiency loss.
Step 5.
Determine maximum and minimum DC input voltages
VMAX, VMIN and input storage capacitance CIN. (see AN-16 for
more detail)
• If VOR > 150 V, go back to Step 2 and choose a different diode
for higher VR.
• Refer to Table 6 for approximate VR range for different types
of diodes.
• Calculate the maximum VMAX as:
100/115
230
1
2-3
Universal
> 200
VMIN (V)
≥ 90
≥ 240
Step 7.
Choose TinySwitch based on input voltage range and output
power PO.
• Select appropriate TinySwitch according to Table 7 based on
output power PO and input voltage range (from Step 1).
≥ 90
Output Power
Capability (W)
Table 5. CIN Range.
• Set bridge rectifier conduction time tc = 3 ms.
• Derive minimum DC input voltage, VMIN
VMIN
40-45
100-200
Table 6. Diode reverse voltage range.
• Choose input storage capacitor, CIN per Table 5.
CIN (µF/Watt)
2-3
VR (V)
Schottky
UltraFast-PN
Fast-PN
VMAX = 2 × VACMAX
Input Voltage
Diode Type
 1

− tC 
2 × PO × 
 2 × fL

= (2 × VACMIN 2 ) −
η × CIN
Device
PO for
Single
PO for
Universal
Voltage* Voltage
5.0
TNY253
2.5
TNY254
8.0
5.0
10
TNY255
7.5
Recommended Power Range
for Lowest Cost** (W)
PO for
Single
PO for
Universal
Voltage*
0-2.5
Voltage
0-1.5
2.0-5.0
6.0-10
1.0-4.0
3.5-6.5
Table 7. TinySwitch output power (PO) capability
where CIN : input capacitance
fL : line frequency
tc : diode conduction time
Step 6.
Determine output diode peak inverse voltage PIV. Calculate
reflected output voltage VOR based on VMAX, VO, VD and PIV.
• Look up output diode reverse voltage VR from diode data
sheet or Table 3 in Step 2.
• Calculate maximum peak inverse voltage PIV. The maximum
recommended PIV is 80% of the reverse voltage rating VR.
PIV = 0.8 × VR
• Calculate reflected output voltage VOR:
VOR
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V × (VO + VD )
= MAX
PIV − VO
* Single Voltage 100/115 VAC with voltage doubler, or
Single Voltage 230 VAC without doubler
** Based on EE16 core transformer
• For Universal input voltage and an output power range of
1W to 1.5 W, TNY254 is usually a better choice than
TNY253 except for applications requiring low video noise.
• For Universal input voltage and an output power range of
3.5W to 4 W, TNY255 usually results in smaller transformer
size and higher efficiency than TNY254.
Step 8.
Determine primary peak current IP. Calculate maximum
duty cycle DMAX for discontinuous mode of operation based
on VMIN, PO and IP.
• Primary peak current is 90% of minimum ILIMIT from the data
sheet of the selected TinySwitch. 0.9 is the over temperature
derating factor for ILIMIT:
AN-23
IP = 0.9 × minimum I LIMIT
• Calculate maximum duty cycle DMAX for discontinuous
mode of operation as:
DMAX =
2 × PO
η × VMIN × IP
Step 9.
Calculate KDP from VMIN, VOR and DMAX.
• KDP is the ratio between the off-time of the switch and the
reset time of the core:
K DP =
VOR × (1 − DMAX )
VMIN × DMAX
Step 10, 11A, 11B, 12, 13A, 13B.
Check KDP to ensure discontinuous mode of operation.
Raise VOR if necessary.
The mode of operation can vary depending on power supply
requirements. However, discontinuous mode of operation is
always recommended wherever it is possible.
• With discontinuous mode of operation, generally, the output
filter is smaller, output rectifier is cheaper with PN junction
diode, EMI and video noise are lower.
• Fully discontinuous mode of operation (discontinuous under
all conditions) may be necessary in some applications to
meet specific requirements such as very low video noise,
very low output ripple voltage. Use of RC snubber, and/or
PN junction diode as output rectifier also demand fully
discontinuous mode of operation. This can be accomplished
by raising VOR higher if necessary until KDP ≥ (1- DMAX)/(0.67
- DMAX). To keep the worst case DRAIN voltage below the
recommended level of 650 V, VOR should be kept below
150V.
• Mostly discontinuous mode of operation (KDP ≥ 1 ) refers to
a design operating in discontinuous mode under most
situations, but do have the possibility of operating in
continuous mode occasionally.
• Continuous mode operation (KDP < 1 ) provides higher
output power. In this mode a Schottky output diode should
be used to prevent long diode reverse recovery times that
could exceed leading edge blanking period (tLEB).
Step 10.
Check for fully discontinuous operation.
• KDP ≥ (1- DMAX)/(0.67 - DMAX): Fully discontinuous.
Go to Step 17A.
• KDP < (1- DMAX)/(0.67 - DMAX):
Go to Step 11.
• 0.67 is the reciprocal of the percentage of duty cycle relaxation
caused by various parameters such as the tolerance in
TinySwitch current limit and frequency.
Step 11A, B.
Determine if fully discontinuous is necessary.
• If yes, set KDP = (1- DMAX)/(0.67 - DMAX).
Recalculate VOR as
VOR =
K DP × VMIN × DMAX
1 − DMAX
- If VOR < 150 V, go to Step 17A.
- If VOR > 150 V, go back to Step 7 and select higher
current TinySwitch.
• If not, go to Step 12.
Step 12.
Check for mostly discontinuous.
• KDP ≥ 1. Operation is mostly discontinuous. Go to Step 17A.
• KDP < 1. Go to Step 13.
Step 13A, B.
Determine if continuous is acceptable for the application.
• If yes, go to Step 14.
• If not, set KDP = 1. Recalculate VOR as:
VOR =
K DP × VMIN × DMAX
1 − DMAX
- If VOR < 150 V, go to Step 17A.
- If VOR > 150 V, go back to Step 7 and select higher
current TinySwitch.
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Step 14.
Recalculate DMAX for continuous mode of operation from
VMIN and VOR.
• Start continuous mode design.
• Recalculate DMAX as:
DMAX =
VOR
VOR + VMIN
Step 15.
Calculate KRP from VMIN, PO, η, IP, and DMAX.
• KRP is the ratio between the primary ripple current IR and
primary peak current IP. And IP is 90% of minimum ILIMIT.
• From AN-16,
and
IP =
I AVG
K RP
(1 −
) × DMAX
2
I AVG =
PO
η × VMIN
• By combining the above equations, KRP can be expressed as:
K RP =
2 × ( IP × DMAX × η × VMIN − PO )
IP × DMAX × η × VMIN
Step 16A, B, C.
Check KRP against 0.6.
• KRP ≥ 0.6, go to Step 17B.
• KRP < 0.6, set KRP = 0.6.
-
Recalculate DMAX using Step 15 equation.
Recalculate VOR using Step 14 equation.
If VOR < 150 V, go to Step 17B.
If VOR > 150 V, go back to Step 7and select higher
current TinySwitch.
Step 17A, B.
Calculate primary inductance LP.
• Discontinuous mode:
LP =
Z × (1 − η) + η
10 6 × PO
×
1
1
2
η
×
× I P × fS
2 0.9
• Continuous mode:
10 6 × PO
LP =
1
K
K RP × (1 − RP ) ×
× I P 2 × fS
2
0.9
Z × (1 − η) + η
×
η
• IP is 90% of minimum ILIMIT from TinySwitch data sheet as
previously defined in Step 8.
• fS is minimum switching frequency from TinySwitch data
sheet.
• Please note the cancellation effect between the over
temperature variations of IP and fS resulting in the additional
1/0.9 term.
• Z is loss allocation factor. If Z = 0, all losses are on the
primary side. If Z = 1, all losses are on the secondary side.
• Since output diode loss and clamp/snubber loss are both
secondary losses, Z = 1 is a reasonable starting point.
Step 18.
Design Transformer.
• Calculate turns ratio NP/NS:
NP
VOR
=
NS VO + VD
• Selecting core and bobbin
- With triple insulated secondary wire and no margin
winding, EE16 core is suitable for most TinySwitch
applications.
- To accommodate margin winding, EEL16 core must be
used.
- In below 2 W and/or space constrained applications,
EE13 or EF13 cores with special bobbin meeting safety
requirements may be used.
• Calculate primary and secondary number of turns for peak
flux density (BP) not to exceed 3000 gauss. Limit BP
to 2500 gauss for low audio noise designs. Use the
lowest practical value of BP for the greatest reduction in
auido noise. See AN-24 for additional information.
• Calculate primary number of turns (NP)
N P = 100 × IP′ ×
LP
BP × Ae
where I’P equals to maximum ILIMIT
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AN-23
• Calculate secondary number of turns NS:
NS =
N P × (VO + VD )
VOR
• Calculate gap length (Lg). Gap length should be larger than
0.1 mm to ensure manufacturability.
 NP 2
1
Lg = 40 × π × Ae 
− 
 1000 × LP AL 
• Please refer to Power Integrations Web site
www.powerint.com for audio noise suppression techniques
applicable to transformer design.
Step 19A, B. Calculate primary RMS current I
RMS
and secondary RMS current I
.
SRMS
- Calculate primary RMS current IRMS
I RMS = DMAX ×
IP′ 2
3
where I’P equals to maximum ILIMIT
- Calculate secondary RMS current ISRMS
1 − DMAX
= ISP ×
3 × K DP
where I = I ′ ×
SP
P
ISRMS = ISP ×
 K RP 2

−
D
×
− K RP + 1
1
( MAX ) 

3
where ISP = I P′ ×
NP
NS
and I’P equals to maximum ILIMIT
• Choose wire gauge for primary and secondary windings
based on IRMS and ISRMS.
• In some designs, a lower guage (larger diameter) wire may
be necessary to maintain transformer temperature within
acceptable limits during continuous short circuit conditions.
• Do not use wire thinner than 36 AWG to prevent excessive
winding capacitance and to improve manufacturability.
Step 20.
Determine output short circuit current IOS.
• Discontinuous mode:
ISRMS
- Calculate secondary RMS current ISRMS
NP
NS
and I’P equals to maximum ILIMIT
• Continuous mode:
• Calculate maximum output short circuit current IOS from I’P
and NP/NS, where I’P is the maximum ILIMIT from TinySwitch
data sheet and NP/NS is the turns ratio from Step 18:
IOS = IP′ ×
NP
×k
NS
where k is the peak to RMS current conversion factor
• The value of k is determined based on empirical
measurements: k = 0.9 for Schottky diode and k = 0.8 for PN
junction diode.
• Check IOS against diode DC current rating ID. If necessary,
choose higher current diode (see Table 3).
Step 21.
Determine Output Capacitor COUT.
• Calculate output ripple current:
I RIPPLE = ISRMS 2 − IO 2
- Calculate primary RMS current IRMS
K 2

I RMS = IP′ × DMAX ×  RP − K RP + 1
 3

where I’P equals to maximum ILIMIT
• Choose output capacitor with RMS current rating equal to or
larger than output ripple current.
• Use low ESR electrolytic capacitor rated for switching
power supply use.
• Examples are LXF series from UCC, PL series from Nichicon,
and HFQ series from Panasonic.
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AN-23
Step 22.
Determine feedback circuit and output post filter.
• The output voltage of the TinySwitch flyback power supply
should be sensed at the first output capacitor, which is before
the output post LC filer. This way the output post LC filter
is outside the feedback control loop and the resonant
frequency of the output post LC filter can be as low as
required to meet the output ripple specification requirement.
• Use Zener diode in series with the optocoupler LED.
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• Output voltage VO is determined by
VO = VZ + VLED
where VLED ≈ 1V
• Replace the Zener with a TL431 for better output accuracy.
• In non-isolated design, use a bipolar NPN transistor in place
of the optocoupler. Replace the LED with the base emitter
junction and connect the collector to the ENABLE pin of the
TinySwitch.
AN-23
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AN-23
For the latest updates, visit our Web site: www.powerint.com
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, 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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