5.0 V, 2.0 A Flyback Converter

AND8099/D
5.0 V, 2.0 A Flyback
Converter
Prepared by: Kristie Valdez
ON Semiconductor
http://onsemi.com
APPLICATION NOTE
Design Parameters
The first step in designing a power supply is to define and
predetermine the input and output parameters.
Universal Input Voltage Range:
The design of a switching power supply is an iterative
process which involves many variables that have to be
adjusted in order to obtain an optimized solution. However,
there are trade-offs which allow for a simple low cost, low
component, single sided board design method. This
application note provides a simple approach to designing
a converter utilizing the ON Semiconductor NCP1055
high voltage switch regulator. The easy-to-follow
step-by-step procedure guides the user into designing the
different blocks that constitute the power supply, mainly the
input block, the power stage, the magnetics, the snubber, the
output block, and the feedback loop. The circuit diagram,
bill of material, and PCB layout are also included at the end
of the application note. This power supply is specifically
designed for a 5.0 V, 2.0 A output and a maximum duty cycle
of 48%. It meets IEC and UL requirements. EMI is minimal
and a 70% achievable efficiency or greater is possible.
The NCP1055 is a family of monolithic high voltage
switching regulators designed to work in rectified AC line
sources and flyback converter applications. They are
capable of providing an output power ranging from 6.0 W to
40 W with a fixed AC input of 100 V, 115 V, or 230 V
and 3.0 W to 20 W with a variable AC input ranging from
85 V to 265 V. This device features an on-chip 700 V
SENSEFET power switch circuit, an active startup
regulator circuit which eliminates the need for an auxiliary
bias winding on the converter transformer, fault logic with
a programmable timer for converter overload protection.
Protective features provide power switch current limiting,
input under voltage lockout with hysteresis, thermal
shutdown, and restart fault detection. For more information,
please contact an ON Semiconductor sales representative or
log on to www.onsemi.com.
Vin(min) 85 VAC, Vin(max) 265 VAC
Output Specifications:
Vout 5.0 V 2%, Iout 2 A
Input Power:
Pin Pout
, an efficiency of 0.78 is a good starting
est.eff
point for a flyback converter using
MOSFET technology:
Pout 5 · 2.0 10 W
Pin 10 12.82 W
0.78
DC Rail Voltages at Low Line and High Line:
Vpeak(min) Vin(min) · 2 85 · 2 120.21 VDC
Vpeak(max) Vin(max) · 2 265 · 2 374.77 VDC
Average Input Current at Low Line:
Iin(avg) V
Pin
,
in(low)
where Vin(low) Vpeak(min) Vripple Vdiode;
Vripple 32% Vpeak(min)
Iin(avg) 12.82 0.160 A
80.2
Input Peak Current:
t
Ipeak 2 · Iin(avg) · sw
ton
10 s
Ipeak 2 · 0.160 ·
0.667 A
4.8 s
 Semiconductor Components Industries, LLC, 2003
February, 2003 - Rev. 2
1
Publication Order Number:
AND8099/D
AND8099/D
Input Bulk Capacitor
The component losses can be evaluated and budgetized with
the following formula:
Ploss = Pin (1 - eff) · P% where P% is the percentage loss of
the desired circuit section per total power
supply loss.
The purpose of the input bulk capacitor C2 is to hold up
the rectified line voltage and also to filter out common mode
noise. It is placed between the bridge rectifier output and
ground. The size of the bulk capacitor depends on peak
rectified input voltage and the ripple voltage magnitude. A
larger capacitor will lower the ripple voltage on the DC input
line, but will induce a larger surge current when the supply
is powered up. Assuming a ripple magnitude of about 32%
of the peak rectified voltage at low line, Cbulk can then be
calculated using:
Usually, 35% of the losses come from the power
MOSFET, 60% from the output rectifier, 5% from the
magnetics, and 5% from miscellaneous sources.
Estimated Power Loss = Pin - Pout = 12.82 - 10 = 2.82 W
MOSFET Power Losses = 2.82 · 35% = 0.987 W
Rectifier Power Losses = 2.82 · 60% = 1.692 W
Cbulk Circuit Description
Input Block
Pin
(
fac · Vpeak(min)2 Vin(low)2)
12.82
27 F
60 · (1202 80.22)
The input block of the power supply consists of a fuse, an
EMI filter, a diode bridge rectifier, and an input bulk
capacitor.
Select the closest standard capacitor of 33 F with low ESR.
Aluminum electrolytics are preferred because of their
sturdiness and high reliability.
Fuse
Power Stage
At the heart of the power stage is the ON Semiconductor
NCP1055. The NCP1055 is a high voltage switching
regulator that uses a fixed-frequency, duty cycle controlled
oscillator. Rectified AC line voltage is applied to the startup
circuit Pin 5 through the primary winding of the transformer.
The circuit then routes current to the supply capacitor C5
which is typically connected to Pin 1. A switching cycle
begins when the oscillator charges and discharges an on chip
timing capacitor which generates a square wave signal used
to pulse width modulate the power switch circuit. The
control input pin is monitoring source or sink current drawn
by an optocoupler. When the power supply output is greater
than the reference voltage, the optocoupler begins to
conduct pulling on the control input. The output of the
control input is then sampled continuously during ton and
has the ability to either turn the power switch circuit on or
off at any time within ton.
The fuse F1 is protecting the circuit from current surges
occurring at turn on. In this application, F1 is rated for 2.0 A,
125 VAC.
EMI Filter
The EMI filter is suppressing common mode and
differential mode noise and is very dependent upon board
layout, component selection, etc. An X capacitor C1 and a
common mode choke L1 are placed across the AC lines to
attenuate differential mode noise, see Figure 1. The EMI
inductor is slowing down any transient voltage surge to
reduce high frequency noise. Both the capacitor and choke
should be placed before the diode bridge and as close to the
AC line input as possible to minimize RFI.
Diode Bridge Rectifier
In order to choose the right diode bridge rectifier, the
values of the forward and surge currents and DC blocking
voltage must be considered. The surge current can reach
values up to five times that of the average input rms current.
It is therefore necessary to select a rectifier capable of
handling such large currents.
DC Blocking Voltage is calculated at high line:
Magnetics Calculations
The next step is the design of the flyback transformer. The
design of the magnetics block is the most important and
delicate part of the whole design process because it will
determine how well the power supply will perform. The
flyback-mode transformer functions by first conducting
current in the primary winding, thus storing energy in the
core of the transformer. The core energy is then transferred
to the secondary winding when the primary side is turned
off. The core and bobbin are standard EFD20 sizes.
VR Vpeak(max) Vin(max) · 2 375 VDC
Forward Current:
IF 1.5 · Iin(avg) 1.5 · 0.160 0.240 A
Surge Current:
IFSM 5 · IF 5 · 0.240 1.2 A
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AND8099/D
In order for the regulator to operate in discontinuous mode
under worse case conditions and to maximize power, the
maximum on time is 48% of the full period, therefore the the
maximum primary inductance is calculated based on a
maximum duty cycle of 48%. Using a larger inductance than
calculated will cause the power supply output to fall out of
regulation.
Lpri Lpri Vin(low) · max
Ipeak · fop
to high frequency noise. Since i C · dv, increasing the
dt
capacitance will also reduce the magnitude of the voltage
ripple. The snubber and ringing damper act together to
protect the IC from voltage transients greater than 700 V and
reduce radiated noise.
Output Block
The output consists of a diode rectifier, a pi-filter, and a
voltage regulator. For a flyback converter with an output
voltage less than 7.5 V, a Schottky rectifier provides the
maximum efficiency and is therefore the best choice. The
Schottky rectifier used is the ON Semiconductor 1N5822, in
which VR = 40 V, IF = 3.0 A, and VF = 0.525 V. The main
purpose of this rectifier is to take the secondary voltage and
convert it to a DC voltage. The following equations are used
in selecting the Schottky rectifier:
Maximum reverse peak voltage (calculated at high line):
, where max is the maximum
duty cycle
80.2 · 0.48
0.577 mH
0.667 * 100 · 103
Primary flyback voltage:
VFB Vin(low) · ton
toff
-6
80.2 · 4.8 · 10
6
5.2 · 10
74.03 V, where
ton is 4.8 s
N
VRout Vout Vpeak(max) · sec
Npri
Primary to secondary turns ratio:
Npri
VFB
74.03 13.4 13 turns
5 0.525
N sec
Vout VF
VRout 5 375 · 1 33.85 V
13
By rearranging the above equation and solving for Nsec
yields ≈1 turn.
Energy entering the core during on-time (when the power
switch is conducting):
For discontinuous mode, the maximum forward peak
current can be approximated using:
Estored IFout 4 · Iout 8 A
Diode D6 along with C7, C8, C9, L2, and C11 rectify the
transformer secondary and filter the output in order to
provide a tightly regulated DC output . Capacitor C7, C8 and
C9 are placed in parallel in order to reduce ESR. In addition,
the voltage rating of C7, C8 and C9 should be high enough
for them to withstand the voltage spikes and the output
voltage. L2 and C11 form a low pass filter that attenuates
high frequency noise.
Output filter capacitor:
Lpri · Ipeak2
, where the stored energy is
2
measured in Joules.
- 3 · 0.6672
2
1.28 10 4 Joules
Estored 0.577 · 10
One can double check if the power capability of the
transformer is large enough to supply enough power to the
output with the following equation:
Pin(core) Iout(max) · Toff(max)
, where Toff(max)
Vripple(desired)
min · 1
fop
Cout Lpri · Ipeak2
· fop Pout
2
Pin(core) 1.28 10- 4 · 100 kHz 12.8 W Pout
and Vripple(desired) 40 mV
10 W
Cout Input Snubber
Because of the high dv/dt characteristic of the power
transistor drain voltage and of the transformer leakage
inductance, voltage spikes and ringing occur at the drain
when the power switch is turned off. Resistor R1, C3, D5
compromise an RCD snubber. In parallel to the primary
winding are R2 and C4 which compromise an RC ringing
damper which slows down the dv/dt and reduces the
peak voltage therefore decreasing the ringing due
8 · 0.52 ·
1
100 · 10 3
0.040
1040 F
Output filter choke (designed for a break frequency of
4.0 kHz):
L
L
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3
1
2 · · f · C
2
, where C C11 330 F and f
is the corner frequency.
1
2 · · 4k · 330 F
2
4.8 H
AND8099/D
Feedback Loop
The feedback loop is composed of an optocoupler, a shunt
regulator, a compensation capacitor and a resistor divider.
The optocoupler isolates the AC input from the DC output.
As a shunt regulator, the ON Semiconductor TL431 is used
to regulate the output voltage. This monolithic voltage
reference is programmable from Vref to 36 V using two
external resistors. It exhibits a wide current range of 1.0 mA
to 100 mA and is an excellent replacement for Zener diodes.
The reference voltage of the TL431 is set at 2.5 V, for a 5.0 V
output voltage, by a resistor divider R5 and R6 (low
tolerance, 2.0 k resistors). The TL431 monitors the 5.0 V
output voltage and compares the divided down voltage to its
2.5 V internal reference. A small increase in the output
voltage will cause the shunt regulator to start conducting,
thus sinking current through the optocoupler’s LED. In turn,
the optocoupler transistor becomes forward biased and starts
driving current into the control input pin of the NCP1055.
The power switch duty cycle is then adjusted accordingly. A
compensation capacitor C10 of 0.1 F is placed between the
cathode and the reference pin of the TL431 for improved
stability. The resistor R3 limits the current going through the
optocoupler to a safe level and prevents damage to the
optocoupler.
5.0 V, 2.0 A
D1
T1
D2
+ C7 + C8 + C9
330
330
330
R1
91 k
L1
C1
0.1
L2
D6
5.0 H
C3
220 p
D3
10 mH
+
D4
D5
C2
33
R3
47
IC2
+
C10
0.1
R2
2.2 k
C6
100 p
5
6
7
8
IC3
C5
10
C13
1n
4
3
2
NCP1055P100
+
X
Figure 1. Circuit Diagram
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4
R6
2.0 k
R4
1.0 k
C4
47 p
1
Vin 85-265 VAC
F1
2.0 A
R5
2.0 k
C11
330
+
C12
1.0
AND8099/D
Table 1. Bill of Materials
Ref.
Component Value
Qty.
Part Number
Manufacturer
IC1
IC, 680 mA, 100 kHz, DIP8
1
NCP1055P100
ON Semiconductor
IC2
Optocoupler, Dip
1
SFH615A-4
Isocom
IC3
2.5 V Shunt Reg., TO-92
1
TL431AILP
ON Semiconductor
D1-4
1.0 A, 800 V, Gen. Purp.
4
1N4006
ON Semiconductor
D5
1.0 A, 600 V, Ultra Fast
1
MUR160TR
ON Semiconductor
D6
3.0 A, 40 V, Schottky
1
1N5822
ON Semiconductor
T1
Flyback Transformer
1
31592
Midcom
L1
Choke, Common Mode, 10 mH
1
40479
Midcom
L2
Choke, Power, 5.0 H
1
40480
Midcom
C1
0.1 F, Film, Radial
1
XSC275V104-M15S
Nissei
C2
33 F, 400 V, Radial
1
400V33-HS
United Chem-Con
C3
220 pF, 1.0 kV, 10%, Disc
1
NCD221K1KVY5F
NIC Components
C4
47 pF, 1.0 kV, 10%, Disc
1
NCD470K1KVSL
NIC Components
C5
10 F, 16 V, 20%, Radial
1
16V10
United Chem-Con
C6
100 pF, 1.0 kV, 10%, Disc
1
NCD101K1KVY5FAB2
NIC Components
C7-9, C11
330 F, 10 V, Radial
4
10V330-HOSS
United Chem-Con
C10
0.1 F, 50 V, Cer., Radial, 10%
1
SR215C104KAA
AVX
C12
1.0 F, 35 V, Tant., Radial, 10%
1
105X9050
AVX
C13
0.001 F, 50 V, Cer., Radial, 10%
1
SR211C102KAA
AVX
R1
91 k, 1.0 W
1
MO-1-91K-5TR
SEI
R2
2.2 k, 1/2 W, 5%
1
1/2W-2.2K-5B
SEI
R3
47 , 1/4 W, 5%
1
1/4W-47R-5
SEI
R4
1.0 k, 1/4 W, 5%
1
1/4W-1K-5B
SEI
R5, R6
2.0 k, 1/4 W, 1%
2
CCF-55-2001FTR
SEI
F1
2.0 A, axial
1
251002TR1
LittleFuse
1757242
J1 2
J1-2
Term Block,
Term.
Block 2 Pos.,
Pos Plug and Header
1
J3 4
J3-4
Term Block
Term.
Block, 2 Pos
Pos., Inv
Inv. Plug and Header
1
1757019
Phoenix Contact
1786404
1786174
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Phoenix Contact
AND8099/D
Table 2. Converter Test Data and Results
Test
Conditions
Line Regulation
Vin = 85 VAC; Iout = 2.0 A
Vin = 265 VAC; Iout = 2.0 A
Load Regulation
Vin = 85 VAC;
Iout = 0
= 0.200 A
= 1.0 A
= 2.0 A
Vin = 110 VAC;
Iout = 0
= 0.200 A
= 1.0 A
= 2.0 A
Vin = 230 VAC;
Iout = 0
= 0.200 A
= 1.0 A
= 2.0 A
Vin = 265 VAC;
Iout = 0
= 0.200 A
= 1.0 A
= 2.0 A
Efficiency
Output Ripple
Voltage
Data
Vin = 85 VAC
Results
Vout = 4.971 V
Vout = 4.982 V
11 mV
Vout =
=
=
=
4.994 V
4.991 V
4.985 V
4.971 V
23 mV
Vout =
=
=
=
4.993 V
4.991 V
4.986 V
4.976 V
17 mV
Vout =
=
=
=
4.993 V
4.991V
4.987V
4.981V
12 mV
Vout =
=
=
=
4.994 V
4.991 V
4.987 V
4.982 V
12 mV
Vout = 4.971 V; Iout = 2.0 A;
Pout = 9.942, Pin = 13.64 W
72.8 %
= 110 VAC
Vout = 4.976 V; Iout = 2.0 A;
Pout = 9.952, Pin = 13.41 W
74.2 %
= 230 VAC
Vout = 4.981 V; Iout = 2.0 A;
Pout = 9.942, Pin = 13.47 W
73.8 %
= 265 VAC
Vout = 4.982 V; Iout = 2.0 A;
Pout = 9.964, Pin = 13.67 W
72.8 %
Vin = 85 VAC; Iout = 2.0 A
100 mVp-p
Vin = 265 VAC; Iout = 2.0 A
230 mVp-p
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AND8099/D
Figure 2. PCB Metal Layer
Figure 3. PCB Silk Screen
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AND8099/D
References
1. Brown, Marty, Power Supply Cookbook,
Butterworth-Heinemann, 1994.
2. Pressman, Abraham I., Switching Power Supply
Design, Second Edition, McGraw-Hill, 1998.
3. Motorola, Inc., Handling EMI in Switch Mode
Power Supply Design, AN-SMPS-EMI, Motorola,
Inc., 1998.
SENSEFET is a trademark of Semiconductor Components Industries, LLC (SCILLC)
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make
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AND8099/D