Title
Engineering Prototype Report for EP-16 -
2.75 W Charger/Adapter Using LNK501
(LinkSwitch)
Specification 85 VAC to 265 VAC Input, 5.5 V, 500 mA Output
Application Low Cost Charger/Adapter
Author PI Applications
Document
Number EPR-16
Date 17-May-04
Revision 1.6
Features
• Very low cost, low component count charger/adapter – replaces
linear transformer based solutions
• Extremely simple circuit configuration designed for high volume,
low cost manufacturing
– No surface mount components required
• Small EE13 transformer allows compact size
• Approximate constant voltage, constant current (CV/CC) primary
sensed output characteristic
– No optocoupler or sense resistors required
• Efficiency greater than 71%
• No-load power consumption <300 mW at 265 VAC
• No Y1 safety capacitor required
– Only transformer bridges primary-to-secondary safety barrier
– Ultra low leakage design (<5 µA)
The products and applications illustrated herein (including circuits external to the products and transformer
construction) 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
5245 Hellyer Avenue, San Jose, CA 95138 USA.
Applications Hotline: Tel: +1 408 414 9660 Fax: +1 408 414 9760
www.powerint.com
EPR-16 - LinkSwitch 2.75 W Charger/Adapter 17-May-04
Table Of Contents
1 Introduction.................................................................................................................4
2 Power Supply Specification ........................................................................................5
3 Schematic...................................................................................................................7
4 Circuit Description ......................................................................................................8
4.1 Input Stage ..........................................................................................................8
4.2 LinkSwitch Operation ..........................................................................................9
4.3 Transformer.......................................................................................................10
4.4 Clamp and Feedback Components ...................................................................10
4.5 Output Stage .....................................................................................................11
5 PCB Layout ..............................................................................................................12
6 Bill Of Materials ........................................................................................................13
7 Transformer ..............................................................................................................14
7.1 Transformer Winding.........................................................................................14
7.2 Electrical Specifications.....................................................................................14
7.3 Materials............................................................................................................15
7.4 Transformer Build Diagram ...............................................................................15
7.5 Transformer Construction..................................................................................16
8 Performance Data ....................................................................................................17
8.1 Line and Load Regulation..................................................................................17
8.2 Efficiency ...........................................................................................................18
8.3 No-Load Input Power ........................................................................................18
9 Waveforms ...............................................................................................................19
9.1 Drain Voltage and Current Waveforms..............................................................19
9.1.1 90 VAC, Normal Operation.........................................................................19
9.1.2 265 VAC, Normal Operation.......................................................................19
9.2 Output Voltage Start-up Profile..........................................................................20
9.3 Load Transient Response (0.25 A to 0.5 A Load Step) .....................................20
9.4 Output Ripple Measurements............................................................................21
9.4.1 Ripple Measurement Technique ................................................................21
9.4.2 Output Voltage Ripple ................................................................................22
9.5 Thermal Measurements ....................................................................................23
9.6 Conducted EMI..................................................................................................24
9.6.1 Optional Components With Artificial Hand .................................................25
9.6.2 Optional Components Without Artificial Hand ............................................25
9.6.3 Optional Components Removed With Artificial Hand .................................26
10 Appendix A – EP-16 Enclosure Opening Procedures ..............................................27
10.1 Method 1 - Non-destructive ...............................................................................27
10.2 Method 2 - Destructive ......................................................................................27
11 Appendix B – LNK520P in the High-Side Configuration ..........................................28
11.1 Introduction........................................................................................................28
11.2 Comparison of LNK501 and LNK520 ................................................................28
11.3 Circuit Changes.................................................................................................29
11.4 Performance Data .............................................................................................30
11.4.1 Line and Load Regulation ..........................................................................30
11.4.2 Efficiency....................................................................................................31
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17-May-04 EPR-16 – LinkSwitch 2.75 W Charger/Adapter
11.4.3 No-Load Input Power..................................................................................32
11.5 EMI Performance...............................................................................................33
12 Revision History........................................................................................................34
Important Note:
Although the EP-16 is designed to satisfy safety isolation requirements, this engineering
prototype has not been agency approved. Therefore, all testing should be performed
using an isolation transformer to provide the AC input to the prototype board.
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EPR-16 - LinkSwitch 2.75 W Charger/Adapter 17-May-04
1 Introduction
This document is an engineering report giving performance characteristics of a 5.5 V,
500 mA charger/adapter. The charger uses LinkSwitch – an integrated IC combining a
700 V high voltage MOSFET, PWM controller, start-up, thermal shut down and fault
protection circuitry. The controller provides both duty cycle and current limit control to
yield a constant voltage/constant current output characteristic without secondary-side
sensing. This power supply is designed to provide a cost effective replacement for linear
transformer based chargers and adapters while providing the additional benefits of
universal input range and high energy efficiency.
This document contains the power supply specification, schematic, bill of materials,
transformer documentation, printed circuit board layout, and performance data.
Figure 1 – EP-16 Populated Circuit Board.
Figure 2 – EP-16 Assembled into Case with Cable
(barrel −ve, tip +ve).
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17-May-04 EPR-16 – LinkSwitch 2.75 W Charger/Adapter
2 Power Supply Specification
Description Symbol Min Typ Max Units Comment
Input
Voltage VIN 85 265 VAC 2 Wire – no Protective Ground
Frequency fLINE 47 50/60 64 Hz
No-load Input Power (265 VAC) 0.3 W
Output
Output Voltage VOUT 5.0 5.5 6.0 V At peak output power point
Output Ripple Voltage (res. load) VRIPPLE R 300 mV Resistive load, peak power
Output Ripple Voltage (bat. load) VRIPPLE B 150 mV Battery load, peak power
Output Current 1 IOUT 400 500 600 mA
Total Output Power
Continuous Output Power POUT 2 2.75 3.6 W
Efficiency η 71 % Measured at output peak power point, 25 oC
Environmental
1.2/50 Surge 2 kV
1.2/50 µs surge, IEC 1000-4-5,
12 Ω series impedance,
differential and common mode
100 kHz Ring Wave Surge 2 kV
100 kHz ring wave, 500 A short
circuit current, differential and
common mode
Ambient Temperature TAMB 0 40 oC Free convection, sea level
Conducted EMI Meets CISPR22B / EN55022B & FCC B with artificial hand connected to output return
Safety Designed to meet IEC950, UL1950 Class II
Output VI Characteristic Specification
0
1
2
3
4
5
6
7
8
9
10
0 100 200 300 400 500 600 700
IOUT (mA)
V O
U
T (
D
C
)
Figure 3 – EP-16 Output Characteristic Envelope.
(Shading represents no-go areas.)
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EPR-16 - LinkSwitch 2.75 W Charger/Adapter 17-May-04
Figure 4 – Battery Model Used for Testing.
Note: EP-16 is designed for a battery load. If a resistive or electronic load is used the
supply may fail to start up at full load. This is normal. To ensure startup into a resistive
load, increase the value of C3 to 1 µF (see circuit description for more information).
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17-May-04 EPR-16 – LinkSwitch 2.75 W Charger/Adapter
3 Schematic
Figure 5 – EP-16 Low Cost, 2.75 W LinkSwitch Cell Phone Charger Schematic.
Optional components L2, C6 and R4 may be fitted to the board to improve radiated EMI.
The effect of these components is shown in the “Conducted EMI Results” section of this
document.
Optional component R3 may be fitted in place of L1 in applications where conducted EMI
is filtered externally (e.g. in an embedded system). In this case a 0 Ω resistor or jumper
should be used. In low power applications (less than approximately 1.5 W), a low value
resistor may be used to provide sufficient conducted EMI filtering. A value in the range of
0 Ω to 10 Ω may be used.
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EPR-16 - LinkSwitch 2.75 W Charger/Adapter 17-May-04
4 Circuit Description
The schematic shown in Figure 5 provides a CV/CC (constant voltage/constant current)
type output characteristic from a universal input voltage range of 85 VAC to 265 VAC.
The nominal peak power point at the transition from CC to CV is 5.5 V at 500 mA.
The precise output envelope specification is shown in Figure 3.
4.1 Input Stage
The incoming AC is rectified and filtered by D1-4, C1 and C2. Resistor RF1 is a
flameproof fusible type to protect against fault conditions and is a requirement to meet
safety agency fault testing. This component should be a wire wound type to withstand
input current surges while the input capacitors charge on application of power or during
withstand line-transient testing. Metal film type resistors are not recommended, they do
not have the transient dissipation capabilities required and may fail prematurely in the
field.
Lower values increase the resistor dissipation (V²/R power term) during transients,
increasing resistor stress, while higher values increase steady state dissipation (I²R
power term) and reduce efficiency.
If a suitable flame proof resistor cannot be found (during failure flame proof resistors do
not emit flames, smoke or incandescent material that may damage transformer
insulation), then a standard fusible type may be used as long as a protective heat shrink
sleeve is placed over the resistor. Please consult with a safety engineer or local safety
agency.
The value of C1 and C2 were selected to provide the smallest standard values to meet
3 µF/W, in this case two 4.7 µF, 400 V capacitors. Smaller values are possible (either
2.2 µF or 3.3 µF) however, the lower DC rail voltage will increase LinkSwitch dissipation,
lowering efficiency and increasing line frequency output ripple. Differential mode EMI
(<500 kHz) also typically increases.
The input capacitance is split between C1 and C2 to allow an input π filter to be formed
by L1. This filters noise associated with the supply to meet EN55022B /
CISPR 22 B and FCC B conducted EMC limits, even when no Y safety capacitor is used.
Ferrite bead L2 is optional, fitted to improve radiated EMI.
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17-May-04 EPR-16 – LinkSwitch 2.75 W Charger/Adapter
4.2 LinkSwitch Operation
When power is applied to the supply, high voltage DC appears at the DRAIN pin of
LinkSwitch (U1). The CONTROL pin capacitor C3 is then charged through a switched
high voltage current source connected internally between the DRAIN and CONTROL
pins. When the CONTROL pin voltage reaches approximately 5.7 V relative to the
SOURCE pin, the internal current source is turned off. The internal control circuitry is
activated and the high voltage internal MOSFET starts to switch, using the energy in C3
to power the IC.
As the current ramps in the primary of flyback transformer T1, energy is stored. This
energy is delivered to the output when the MOSFET turns off each cycle.
The secondary of the transformer is rectified and filtered by D6 and C5 to provide the DC
output to the load.
Control of the output characteristic is entirely sensed from the primary-side by monitoring
the primary-side VOR (voltage output reflected). While the output diode is conducting, the
voltage across the transformer primary is equal to the output voltage plus diode drop
multiplied by the turns ratio of the transformer. Since the LinkSwitch is connected on the
high side of the transformer, the VOR can be sensed directly.
Diode D5 and capacitor C4 form the primary clamp network. The voltage held across C4
is essentially the VOR with an error due to the parasitic leakage inductance.
The LinkSwitch has three operating modes determined by the current flowing into the
CONTROL pin.
During start-up, as the output voltage, and therefore the reflected voltage and voltage
across C4 increases, the feedback current increases from 0 to approximately 2 mA
through R1 into the CONTROL pin. The internal current limit is increased during this
period until reaching 100%, providing an approximately constant output current.
Once the output voltage reaches the regulated CV value, the output voltage is regulated
through control of the duty cycle. As the current into the CONTROL pin exceeds
approximately 2 mA, the duty cycle begins to reduce, reaching 30% at a CONTROL pin
current of 2.3 mA.
If the duty cycle reaches a 3% threshold, the switching frequency is reduced, which
reduces energy consumption under light or no load conditions.
As the output load increases beyond the peak power point (defined by ½·L·I²·f) and the
output voltage and VOR falls, the reduced CONTROL pin current will lower the internal
current providing an approximately constant current output characteristic. If the output
load is further increased and the output voltage falls further to below a CONTROL pin
current of 1 mA, the CONTROL pin capacitor C3 will discharge and the supply will enter
auto-restart.
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EPR-16 - LinkSwitch 2.75 W Charger/Adapter 17-May-04
4.3 Transformer
The transformer is designed to always be discontinuous; all the energy is transferred to
the load during the MOSFET off time. The energy stored in the transformer during
discontinuous mode operation is ½·L·I²·f, where L is the primary inductance, I² is the peak
primary current squared and f is the switching frequency.
Since the value of LinkSwitch current limit and frequency directly determines the peak
power or CV/CC transition point in the output characteristic, the parameter of current
squared times frequency is defined in the datasheet. This parameter, together with the
output power, is used to specify the transformer primary inductance. With a primary
inductance tolerance of ±10%, the EP-16 is designed to provide the output current
characteristic shown in Figure 3∗.
As LinkSwitch is powered by the energy stored in the leakage inductance of the
transformer, only a low cost two winding transformer is required. Leakage inductance
should be kept low, ideally at less than 2% of the primary inductance. High leakage
inductance will cause the CC characteristic to walk out as the output voltage decreases
and increases the no-load consumption of the supply.
With a figure of 50 µH for leakage, this design is able to meet a voltage tolerance of
±10% at the peak power point, including the effects of output cable drop. For tighter
voltage tolerance across the whole load range, a secondary optocoupler can be added.
For most battery charging applications, only the voltage at the peak power point is critical,
thus ensuring sufficient voltage for charging.
4.4 Clamp and Feedback Components
Diode D5 should either be a fast (trr <250 ns) or ultra-fast type to prevent the voltage
across LinkSwitch from reversing and ringing below ground. A fast diode is preferred,
being lower cost. Leakage inductance is filtered by R2, the optimum value providing the
straightest CC characteristic.
Capacitor C4 is typically fixed at 0.1 µF and should be rated above the VOR and be stable
with both temperature and applied voltage. Low-cost, metalized plastic film capacitors
are ideal; high value, low-cost ceramic capacitors are not recommended. Dielectrics
used for these capacitors such as Z5U and Y5U are not stable and can cause output
instability as their value changes with voltage and temperature. Stable dielectrics such
as COG/NPO are acceptable but are costly when compared to a metalized plastic film
capacitor.
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∗ This includes LinkSwitch tolerance and line variation.
17-May-04 EPR-16 – LinkSwitch 2.75 W Charger/Adapter
R1 was selected to program the peak power point to be 500 mA when a transformer with
a nominal LP value was used. Initial values are selected using the expression (from
Power Integrations Application note, AN-35):
R1 ≅ (VFB - VC (IDCT)) / IDCT
≅ (54.1 – 5.75) / 2.3
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