Application Note AN-41
PeakSwitch
Design Guide
Introduction
®
The
PeakSwitch
family is a highly integrated, monolithic,
off-line switcher IC designed for use in power supplies that have
to deliver peak loads for short durations. Example applications
include inkjet printers, audio amplifiers and DVRs. When peak
power is required, the effective switching frequency can approach
277 kHz, allowing a transformer with a small core size to be used.
Innovative proprietary features, such as adaptive switching cycle
on-time control, adaptive current limit, AC line sense and fast
AC reset greatly simplify the design. This reduces engineering
design time and system cost while providing complete
system-level protection and robust functionality.
Each member of the family has a high-voltage power MOSFET
and its controller integrated onto the same die. Internal start-
up bias current is drawn from a high-voltage current source
connected to the DRAIN pin, eliminating the need for external
start-up components. The internal oscillator is frequency
modulated (jitter) to reduce EMI. In addition, the ICs have
integrated functions that provide system-level protection. The
auto-restart function limits the dissipation in the MOSFET, the
transformer and the output diode during overload, output short-
circuit and open-loop conditions, while the auto-recovering
hysteretic thermal shutdown function disables MOSFET
switching during a thermal fault. On-time extension enables
more power to be delivered at low line and extends hold-up
time. The smart AC line sense and undervoltage lockout
(UVLO) functions enable the IC to latch off whenever a fault
activates the auto-restart function, and to be reset quickly after
AC power is removed.
Power Integrations’
EcoSmart
®
technology enables supplies
designed around the
PeakSwitch
family members to consume
<300 mW of no-load power and to meet harmonized energy
efficiency standards such as the California Energy Commission
(CEC), EU and ENERGY STAR.
C10
1 nF
250 VAC
R8
68
Ω
C11
1/2 W 330 pF
D9
1N4148
R9
C13
47
µF
0.33
Ω
2W
16 V
Q1
2N3906
VR2
1N5255B
28 V
R11
3 kΩ
R10
1.5 kΩ
30 V @
C14
L2 220 nF
1.07 A Cont.
2.7 A Peak
5.3
µH
50 V
C17
4.7 nF
1 kV
D1-D4
1N4007
C4
150
µF
400 V
R15
2.2
Ω
C5
2.2 nF
1 kV
VR1
1N4764A
100 V
1
9,10
D8
STPS3150
C12
330
µF
50 V
7,8
R3
10 kΩ
1/2 W 3
4
R4
22
Ω
1/2 W
D6
FR106
C6
47
µF
35 V
5
T1
EE25
RTN
L1
5.3 mH
R2
R1
1.3 MΩ 1.3 MΩ
C3
680 nF
X1
D5
1N4007
t
RT1
10
Ω
O
2
R12
1 kΩ
R7
4.7 kΩ
C15
100 nF
50 V
D10
UF4003
VR3
1N5258B
36 V
Q2
FS202DA
R5
2.2 MΩ
PeakSwitch
D
U1
PKS606Y
C7
100 nF
400 V
S
R6
2.4 MΩ
D7
1N4148
C16
100 nF
R14
100
Ω
R13
1 kΩ
R16
2.7 MΩ
EN/UV
BP
GND
F1
3.15 A
J1
L
PE
N
C1-C2
100 pF
250 VAC
C8
220 nF
50 V
U2
PC817X4
RTN Connected to PE via Flying Lead
C19
1 nF, 250 VAC
J3
PCB Term 18 AWG
PI-4170-060706
Figure 1. PeakSwitch PKS606Y, 32 W Average, 81 W Peak, Universal Input Power Supply.
February 2007
AN-41
Scope
This application note is intended for engineers designing an
isolated AC-DC flyback power supply using the
PeakSwitch
family of devices. It provides guidelines to enable the engineer
to quickly select key components and also complete a suitable
transformer design. To simplify the task, this application note
refers directly to the
PI Xls
design spreadsheet that is part of
the
PI Expert™
power supply design software suite.
In addition to this application note, the reader may also find the
PeakSwitch
Reference Design Kit (RDK) (the RDK contains
an engineering prototype board, engineering report and device
sample) useful as an example of a working power supply. Further
details on downloading
PI Expert,
obtaining an RDK and updates
to this document can be found at www.powerint.com.
Quick Start
Readers can use the following information to quickly design a
transformer and select the components for a first prototype. Only
the information described below needs to be entered into the
PI Xls
spreadsheet; other parameters will be automatically
selected by the spreadsheet, based on a typical design. References
to spreadsheet cell locations are provided in square brackets
[cell reference].
• Enter AC input voltage range VAC
MIN
, VAC
MAX
and
minimum line frequency f
L
[B3, B4, B5]
• Enter nominal output voltage V
O
[B6]
• Enter minimum output voltage at peak load assuming an
output drop is acceptable (if applicable) [B7]
• Enter maximum output current at peak load or maximum
continuous load as applicable [B5]
• Enter continuous (average) output power [B9]
• Enter efficiency estimate:
0.7 for universal input voltage (85-265 VAC) or single
100/115 VAC (85-132 VAC) line voltage, and 0.75 for
single 230 VAC (185-265 VAC) line voltage designs.
Adjust the efficiency estimate accordingly, after
measuring the efficiency of the first prototype-board at
peak load and VAC
MIN
. [B11]
• Enter loss allocation factor Z [B12]:
0.65 for typical application (adjust the number
accordingly after first proto-board evaluation)
• Enter C
IN
input capacitance [B14]:
Use 2
µF/W
PK
for universal (85-265 VAC) or single
(100/115 VAC) line voltage, if output voltage droop
is acceptable, or 3
µF/W
PK
if output voltage droop is
unacceptable.
Use 1
µF/W
PK
single 230 VAC for a single (185-
265 VAC) high-line voltage.
• Select
PeakSwitch
from drop down list or enter directly
[B17]:
Select the device in the table below according to output power
and line input voltage.
OUTPUT POWER TABLE
PRODUCT
3
PKS603 P
PKS604 P
PKS604 Y/F
PKS605 P
PKS605 Y/F
PKS606 P
PKS606 Y/F
PKS607 Y/F
230 VAC ±15%
85-265 VAC
Adapter
Adapter Adapter
Adapter
Cont.
1
Peak
2
Cont.
1
Peak
2
13 W
23 W
35 W
31 W
46 W
35 W
68 W
75 W
32 W
56 W
56 W
60 W
79 W
66 W
117 W
126 W
9W
16 W
23 W
21 W
30 W
25 W
45 W
50 W
25 W
44 W
44 W
44 W
58 W
46 W
86 W
93 W
Table 1. Output Power Table (See Data Sheet for Notes 1, 2 and 3).
• Enter V
D
– forward voltage drop of the output diode
[B25]:
0.5 V for Schottky diode
0.7 V for PN diode
• Enter core type (if desired) from drop down menu [B43]:
A suggested core size will be selected automatically by the
spreadsheet if none is entered.
• Build transformer
• Select key components (see Steps 5 through 10)
• Build prototype, test and iterate the design as necessary,
entering measured values into the spreadsheet where
estimates were initially used (e.g. efficiency, V
MIN
)
2
Rev. E 02/07
AN-41
Step-by-Step Transformer Design
Procedure
Introduction
PeakSwitch
devices have current limit values that allow the
supply to deliver the specified peak power given in the power
table. With sufficient heatsinking, these power levels could
be provided continuously. However,
PeakSwitch
is optimized
for use in applications that demand short duration, high peak
power, while delivering a significantly lower continuous power.
Typical peak-to-continuous ratios would be P
PEAK
≥ 2
×
P
AVE
.
The high switching frequency of
PeakSwitch
allows a small core
size to deliver the peak power but the short duration prevents the
transformer windings from overheating and reduces heatsinking
requirement for the device.
As the average power increases, based on the measured transformer
temperature, it may be necessary to select a larger transformer so
that the current density of its windings can be decreased.
The power table provides some guidance for peak and
continuous (average) power levels in sealed adapters, although
specific applications may vary. For example, if the peak power
condition is of very low duty cycle, such as a two-second peak
occurring at power up to accelerate a hard disk drive, then the
temperature rise of the transformer is a function of the continuous
power. However, if the peak power occurs every 200 ms for
50 ms, then peak power heating effects would need to be considered.
Figure 2 shows how to calculate the average power requirements
for a design with two different peak load conditions.
P
AVE
=
P
1
+
(
P
3
-
P
1
)
$
d
1
+
(
P
2
-
P
1
)
$
d
2
D
t
D
t
d
1
=
T
1
,
d
2
=
T
2
Power (W)
P
3
P
1
∆t
1
T
∆t
2
Figure 2. Continuous (Average) Output Power Calculation Example.
The design procedure requires both peak and continuous powers
to be specified. The peak power is used to select the
PeakSwitch
device and design the transformer for power delivery at minimum
input line voltage while continuous power (or average power if
the peak load is periodic) is used for thermal design and may
affect the size of the transformer and the heat sink.
Step 1. Enter Application Variables VAC
MIN
, VAC
MAX
,
f
L
, V
O
, I
O
, V
O
at Peak Load, h, Z, t
C
, C
IN
Determine the input voltage range from Table 2.
Nominal Input Voltage
(VAC)
100/115
230
Universal
VAC
MIN
85
195
85
VAC
MAX
132
265
265
Where P
x
represents the different output power conditions,
Dt
x
represent the durations of each peak power condition and T is
the period of one cycle of the pulsed load condition.
ENTER APPLICATION VARIABLES
VACMIN
VACMAX
fL
Nominal Output Voltage (VO)
Maximum Output Current (IO)
Minimum Output Voltage at Peak Load
Continuous Power
Peak Power
n
Z
tC Estimate
CIN
85
265
50
24.00
0.75
6.00
0.70
0.60
3.00
47.00
Table 2. Standard Worldwide Input Line Voltage Ranges.
AN41 Example
Volts
Volts
Hertz
Volts
Amps
24.00
Volts
6.00
Watts
18.00
Watts
Minimum AC Input Voltage
Maximum AC Input Voltage
AC Mains Frequency
Nominal Output Voltage (at continuous power)
Power Supply Output Current (corresponding to peak power)
Minimum Output Voltage at Peak Power (Assuming output droop
during peak load)
Continuous Output Power
Peak Output Power
Efficiency Estimate at output terminals and at peak load. Enter
0.7 if no better data available
Loss Allocation Factor (Z = Secondary side losses / Total losses)
mSeconds
Bridge Rectifier Conduction Time Estimate
Input Capacitance
47
uFarads
Figure 3. Application Variable Section of PeakSwitch Design Spreadsheet.
Rev. E 02/07
PI-4329-030906
P
2
Time (t)
3
AN-41
Line Frequency, f
L
47 Hz for universal or 100/115 VAC input. 47 Hz for single
230 VAC input. For half-wave rectification use f
L
/2. For DC
input enter the voltage directly into Cells B55 and B56.
Nominal Output Voltage, V
O
(V)
Enter the nominal output voltage of the main output during the
continuous load condition. Generally, the main output is the
output from which feedback is derived.
Output Current, I
O
(A)
Enter the maximum output current under peak load conditions.
If the design does not have a peak load condition, then enter
the maximum continuous output current. In multiple output
designs, the output current of the main output (typically, the
output from which feedback is taken) should be increased
such that the peak power (or maximum continuous power as
applicable) matches the sum of the output powers from all of the
supply’s outputs. The individual output voltages and currents
should then be entered at the bottom of the spreadsheet [cells
B98 to B131].
Minimum Output Voltage at Peak Load (V)
The output voltage may be specified in
PeakSwitch
designs based
on whether or not the output voltage is allowed to droop during
peak loads. If the application requires the output to remain the
same under continuous and peak load conditions, leave this cell
empty. The spreadsheet then assumes that the output voltage
under peak load conditions is equal to the nominal output voltage,
i.e. the output is not allowed to droop under peak load.
If the application allows the output voltage to droop under peak
load conditions, enter the minimum acceptable voltage at peak
load. The peak power is then calculated based on the output
current and the minimum acceptable output voltage. In multiple
output designs, if the main output is allowed to droop then all
the other output voltages will also droop proportionally under
peak load conditions.
Continuous Output Power (W)
Enter the continuous output power. If this entry is left blank the
design spreadsheet assumes that the continuous power is equal
to the peak output power. This value is used by the spreadsheet
to suggest a core size.
Peak Power (W)
This is a calculated value based on the minimum output voltage
at peak load, and maximum output current. It is used to calculate
the required value of the primary inductance.
Power Supply Efficiency,
h
Enter the estimated efficiency of the complete power supply,
measured at the output terminals under peak load conditions
and worst-case line (generally lowest input voltage). Start
with a value of 0.7 (typical) for a design where the majority
of the output power is drawn from an output voltage of 12 V
or greater, and no current sensing is present on the secondary.
Once a prototype has been constructed, the measured efficiency
should be entered and the design of the transformer should be
iterated.
Power Supply Loss Allocation Factor, Z
This factor represents the proportion of losses between the
primary and the secondary of the power supply. Z factor is
used together with the efficiency number, to determine the
actual power that must be delivered by the power stage. For
example, losses in the input stage (EMI filter, rectification, etc)
are not processed by the power stage (transferred through the
transformer), and therefore, although they reduce efficiency,
the transformer design is not impacted.
For designs that do not have a secondary current sense circuit,
enter 0.65. For those designs that do have a secondary current
sense circuit, use a value of 0.7 until measurements can be made
on a prototype. The higher number indicates larger secondary
side losses associated with the secondary side current sense
resistor.
Bridge Diode Conduction Time, t
C
(ms)
Enter a bridge diode conduction time of 3.75 ms, if there is no
better data available.
Total Input Capacitance, C
IN
(µF)
Enter the total input capacitance, using Table 3 for guidance.
Total Input Capacitance per
Watt Output Power (µF/W)
AC Input Voltage
(VAC)
100/115
230
85-265
Full Wave
Rectification
3
1
3
Table 3. Suggested Total Input Capacitance for Different Input
Voltage Ranges.
The capacitance is used to calculate the minimum DC voltage
and should be selected to keep the minimum DC input voltage
(V
MIN
) >70 V.
For designs that have a DC rather than an AC input, the value
of the minimum and maximum DC input voltages, V
MIN
and
V
MAX
, may be entered directly into the override cells on the
design spreadsheet shown below.
4
Rev. E 02/07
AN-41
DC INPUT VOLTAGE PARAMETERS
VMIN
VMAX
80 Volts
375 Volts
Minimum DC Input Voltage
Maximum DC Input Voltage
Figure 4. DC Input Voltage Parameters Showing Grey Override Cells for DC Input Designs.
Step 2 – Enter PeakSwitch Variables: PeakSwitch
Device, V
OR
, V
DS
, V
D
, V
DB
, V
CLO
, K
P(STEADY STATE)
, K
P(TRANSIENT)
Select the correct
PeakSwitch
device
Refer to Table 1 and first select a device based on the peak
output power of the design. Then compare the continuous
power rating to the continuous numbers in the power table.
If the continuous power exceeds the value given in the power
table, then the next largest device should be selected. Similarly,
if the continuous power is close to the power table’s power
levels, then it may be necessary to switch to a larger device
based on the measured thermal performance of the prototype.
ENTER PeakSwitch VARIABLES
PeakSwitch
Chosen Device
ILIMITMIN
ILIMITMAX
fSmin
I^2fmin
VOR
VDS
VD
VDB
VCLO
KP (STEADY STATE)
KP (TRANSIENT)
PKS603P
PKS603P
PKS603P
3. Higher V
OR
increases the leakage inductance of the transformer,
which reduces efficiency of the power supply.
4. Higher V
OR
increases the peak and RMS currents on the
secondary side, which may increase secondary side copper
and diode losses.
Optimal selection of the V
OR
value should be based on a
reasonable engineering compromise of the factors mentioned
above.
PeakSwitch
On-State Drain to Source Voltage, V
DS
(V)
This parameter is the average on-state voltage developed across
PeakSwitch device
Minimum Current Limit
Maximum Current Limit
Minimum Device Switching Frequency
I^2f (product of current limit squared and frequency is trimmed
for tighter tolerance)
Reflected Output Voltage (VOR <= 135 V Recommended)
PeakSwitch on-state Drain to Source Voltage
Output Winding Diode Forward Voltage Drop
Bias Winding Diode Forward Voltage Drop
Nominal Clamp Voltage
Ripple to Peak Current Ratio (KP < 6)
Ripple to Peak Current Ratio under worst case at peak load
(0.25 < KP < 6)
0.750 Amps
0.870 Amps
250000 Hertz
164
110
10
0.7
0.7
200
0.60
0.38
A^2kHz
Volts
Volts
Volts
Volts
Volts
Figure 5. PeakSwitch Section of Design Spreadsheet.
Peak Load Switching Frequency, f
s(min)
(Hz)
This parameter is the worst-case minimum switching
frequency based on the minimum data sheet value of I
2
f (not
adjustable).
Reflected Output Voltage, V
OR
(V)
This parameter is the secondary winding voltage during the
diode conduction time, which is reflected back to the primary
through the turns ratio of the transformer. The default value
is 110 V, however the acceptable range for V
OR
is between
80 V and 135 V, providing that no warnings are produced by the
spreadsheet. For design optimization purposes, the following
should be kept in mind:
1. Higher V
OR
allows increased power delivery at V
MIN
, which
minimizes the value of the input capacitor and the droop of
the output voltage when the on-time extension feature is used,
and maximizes the power delivery from a given
PeakSwitch
device.
2. Higher V
OR
reduces the voltage stress on the output diodes,
which in some cases may allow a Schottky diode to be used,
and will thus give higher efficiency.
the DRAIN and SOURCE pins of the
PeakSwitch
device. By
default, if the grey override cell is left empty, a value of 10 V
is assumed for Y/F package devices, and 5 V for P package
devices. Use the default value if no better data is available.
Output Diode Forward Voltage Drop, V
D
(V)
Enter the average forward voltage drop of the (main) output
diode. Use 0.5 V for a Schottky diode or 0.7 V for a PN diode,
if no better data is available. The spreadsheet uses a default
value of 0.7 V.
Nominal Clamp Voltage, V
CLO
(V)
Enter the nominal clamp voltage. The clamp is used to ensure
that maximum voltage developed across the DRAIN and
SOURCE pins of the internal MOSFET remains below the
BV
DSS
specification (700 V) limit, with sufficient margin. It is
recommended that a Zener diode with a value of 200 V be used
in the clamp circuit. Even if an RCD clamp is used, a Zener
should be placed in parallel with the RCD circuit to provide
hard clamping during fault conditions. By default, if the grey
override cell is left empty, a value of 200 V is assumed, which
is also the maximum value recommended. Lower values can
5
Rev. E 02/07
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