EXA40 SERIES
Application Note 101 Rev. 01 - Feb. 1999
NEW
Product
• Ultra high efficiency topology, 91% typical at 5V
• Operating ambient temperature of -40°C to +70°C (natural
convection)
• Approved to EN60950, UL1950, CSA C22.2 No. 234/950
• Complies with ETS 300 019-1-3/2-3
• Complies with ETS 300 132-2 input voltage and current
requirements
• Fully compliant with ETS 300 386-1
1. Introduction
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . .1
Models and Features . . . . . . . . . . . . . .1
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
The EXA40 Series is a new generation of DC/DC converters which
were designed in response to the growing need for low operating
voltage and higher efficiencies. They offer unprecedented efficiency
figures and the greatest range of low output voltages on the market.
In addition the automated manufacture methods and use of planar
magnetics together with an extensive qualification program have
produced one of the most reliable range of converters.
General Description . . . . . . . . . . . . . . .1
Electrical Description . . . . . . . . . . . . . . . . . . . .1
Physical Construction . . . . . . . . . . . . . . . . . . .2
4.
Features and Functions . . . . . . . . . . . .2
Overvoltage Protection . . . . . . . . . . . . . . . . . .2
Over Temperature Protection . . . . . . . . . . . . . .3
Current Limit and Short Circuit . . . . . . . . . . . . .3
Remote ON/OFF . . . . . . . . . . . . . . . . . . . . . . .3
Output Voltage Adjustment . . . . . . . . . . . . . . .3
2. Models and Features
The EXA40 comprises eight separate models as shown in Table 1.
All popular integrated circuit operating voltages are covered by the
entire range.
Model
EXA40-24S05
EXA40-24S3V3
EXA40-24S2V75
EXA40-24S1V8
EXA40-48S05
EXA40-48S3V3
EXA40-48S2V75
EXA40-48S1V8
Input Voltage
18-36VDC
18-36VDC
18-36VDC
18-36VDC
36-75VDC
36-75VDC
36-75VDC
36-75VDC
Table 1 - EXA40 Models
Output Voltage
4.5 to 5.5V
3.0 to 3.6V
2.5 to 3.0V
1.5 to 2.0V
4.5 to 5.5V
3.0 to 3.6V
2.5 to 3.0V
1.5 to 2.0V
5.
Safety . . . . . . . . . . . . . . . . . . . . . . . . . .3
Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Input Fusing . . . . . . . . . . . . . . . . . . . . . . . . . . .3
6.
EMC . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Radiated . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Conducted . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
7.
Use in a Manufacturing Environment . .5
Resistance to Soldering Heat . . . . . . . . . . . . . .5
Water Washing . . . . . . . . . . . . . . . . . . . . . . . . .5
ESD Control . . . . . . . . . . . . . . . . . . . . . . . . . . .5
8.
Applications . . . . . . . . . . . . . . . . . . . . .6
Optimum PCB Layout . . . . . . .
Optimum Thermal Performance
Remote ON/OFF Control . . . . .
Output Voltage Adjustment . . .
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9.
Appendix 1 . . . . . . . . . . . . . . . . . . . . . .8
Output Voltage Trim Curves . . . . . . . . . . . . . . .9
10. Appendix 2 . . . . . . . . . . . . . . . . . . . . .11
Output TVS Rating . . . . . . . . . . . . . . . . . . . . .11
Features
• Overtemperature shutdown
• Optional latching OVP
• Primary remote On/Off
• Output voltage adjustability
• Continuous short circuit protection
• Overcurrent limiting
11. Appendix 3 . . . . . . . . . . . . . . . . . . . . .13
Recommended PCB Layouts . . . . . . . . . . . . .13
3. General Description
Electrical Description
The EXA40 is a resonant reset Forward converter with synchronous
rectification. A simplified schematic is shown in Figure 1. The
significant gain in efficiency has been achieved by optimum driving
of the synchronous rectifiers.
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EXA40 SERIES
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Application Note
Input LC
Filter
+
Synchronous
Rectifiers
OTP
PWM Control
OptoCoupler
Feedback &
Adjustability
OVP
Detection
Trim
A separate paper discussing the benefits of “open frame low to
medium DC/DC converters” Design Note 102 is available from
Artesyn Technologies. The effective elimination of potting and a case
has been made possible by the use of modern automated
manufacturing techniques and in particular the 100% use of SMT
components, the use of planar magnetics and the exceptionally high
efficiencies.
OptoCoupler
On/Off
4. Features and Functions
Overvoltage Protection
A TVS is used across all models to clamp all transients of short
duration that may occur. The levels of these transients are given in
Table 2.
Output Voltage
5V
3.3V
TVS Value
6.8V
4.0V
4.0V
4.0V
Figure 1 - Simplified Schematic
The DC input is filtered by an LC filter before it reaches the main
transformer. A PWM controller is used to regulate the output. The
main power switch is a MOSFET running at a control frequency of
300KHz.
The output is sensed and compared with a secondary side
reference and an error signal is fed back via an optocoupler to the
PWM controller. The trim pin on the secondary side allows the
output to be adjusted by connecting a resistor between trim and
either the positive or negative output depending on whether you
wish to trim up or down.
The optional latching Over-Voltage Protection (OVP) circuit sends
back a signal to the PWM controller to turn off if the output rises
above a pre-determined limit for longer than 1ms. OVP transients
less than this are eliminated through the use of an output TVS. More
details on the OVP section can be found in the applications section.
The controller latches off until the power is re-cycled or the remote
pin is toggled.
An Over-Temperature Protection (OTP) circuit on the primary side
shuts down the PWM controller if the converter is in danger of being
damaged. Unlike the OVP circuit the OTP circuit does not latch off
the controller. There is however a considerable amount of thermal
hysteresis which is used to protect the unit.
The output synchronous rectifiers are controlled by circuitry on the
secondary side which optimise the driving scheme.
2.75V
1.8V
Table 2 - Output TVS Clamping Voltages
The maximum duration of these pulses and their peak power is
dependent on a number of factors. Appendix 2 contains details of
the output TVS ratings. For a single Pulse the maximum Peak Power
at 1ms is 600W at 25°C (see Figure A2-1). As the ambient
temperature increases so the Peak Pulse power derates as shown in
Figure A2-2.
Repetitive Pulses are not as straightforward and an extra derating
chart is supplied in Figure A2-3. The derating is expressed here as
a function of the pulse duty cycle and pulse width. Note that these
derating factors are quoted at 25°C and must be further derated for
temperature by referring to Figure A2-2.
The OVP function is an optional function that is used to protect the
users circuitry from damage should the converter fail or if an
externally induced transient occurs on the output. The trip points
are set quite accurately at 125% of Vout nominal and are given in
Table 3.
The OVP function has a discrimination circuit to prevent it tripping
due to small duration transients. This filter eliminates any pulses of
between 500µs and 1ms duration over the trip point level.
Output Voltage
5V
3.3V
2.75V
1.8V
TVS Value
6.2V
4.2V
3.6V
2.5V
Physical Construction
The EXA40 is constructed using a single multi-layer FR4 PCB. SMT
components are placed on both sides of the PCB and in general,
the heavier power components are mounted on the top side in order
to optimise heat dissipation.
The converter is sold as an open-frame product and no case is
required. The open frame design has several advantages over
encapsulated closed devices. Among these advantages are:
•
Cost:
No potting compound, case or associated Process costs
involved.
•
Thermals:
The heat is removed from the heat generating
components without heating more sensitive, less tolerant
components such as opto-couplers.
•
Environmental:
Some encapsulants are not kind to the
environment and create problems in incinerators. In addition open
frame converters are more easily re-cycled.
•
Reliability:
Open Frame modules are more reliable for a number
of reasons.
Table 3 - OVP Trip Point
When the unit trips the PWM controller shuts down the output
within 1ms.
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EXA40 SERIES
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Application Note
Over Temperature Protection
This feature is included as standard in order to protect the converter
and the circuitry it powers from overheating in the event of a
runaway thermal condition such as a fan failure at high
temperatures. or continuous operation above Tmax at full power.
The actual ambient temperature it trips at is dependent on quite a
number of factors The airflow over the unit is the most dominant
parameter. The trip point is also affected by the input voltage,
output trim voltage, user PCB layout, output load and model.
For all models under full load conditions the trip point will be at a
minimum of 75°C in still air using the recommended layout in the
Applications section. Still Air or natural convection is defined as
0.1m/s airflow.
As the load is decreased and the unit is operated at higher
temperatures, the trip point also rises. This trip point will at all times
protect the unit and will be a minimum of 5°C away from the safe
operating temperature of the device.
5. Safety
Isolation
The EXA40 has been submitted to independent safety agencies and
has EN60950 and UL1950 Safety approvals. Basic insulation is
provided and the unit is approved for use between the classes of
circuits listed in Table 4.
Insulation
Between
TNV-1 Circuit
TNV-2 Circuit
TNV-3 Circuit
Earthed or Unearthed
Hazardous Voltage
Secondary Circuit
And
Earthed SELV Circuit
Unearthed SELV Circuit
Earthed SELV Circuit
Unearthed SELV Circuit or or
TNV-1 Circuit
Earthed SELV Circuit
ELV Circuit
Unearthed Hazardous
Voltage Secondary Circuit
TNV-1 Circuit
Current Limit and Short Circuit
All models of the EXA40 have a built in current limit function and full
continuous short circuit protection.
The current limit inception point is dependent on the input voltage,
ambient temperature and has a parametric spread also. For all
models the inception point is typically 140% or 11.2A. It may go as
high as 180% or as low as 100% over all operating conditions and
the lifetime of the product.
None of the specifications are guaranteed when the unit is operated
in an overcurrent condition. The unit will not be damaged in an
overcurrent condition as it will protect itself through the use of the
OTP function before any damage occurs. However the lifetime of
the unit will be reduced.
In short circuit the unit enters a ‘hiccup’ foldback current mode and
may be operated continuously in this condition. The duty cycle of
this hiccup is dependent on input voltage, temperature etc. The
RMS value of the short circuit current is guaranteed to be a
maximum of 12A RMS over all operating conditions and the lifetime
of the product. While the unit is specified to operate into a
continuous short circuit, extended or frequent short circuits will
reduce the lifetime of the converter.
A short circuit is defined as a resistance of 20mΩ or less.
Remote On/Off
The remote On/Off function allows the unit to be controlled by an
external signal which puts the module into a low power dissipating
sleep mode. Methods of applying are given in the applications
section.
Output Voltage Adjustment
The output voltage on all models except for the 1.8V output is
trimmable by ±10%. The 1.8V output is asymmetrically trimmable
by +13% and -18%. Details on how to do trim all models are
provided in the applications section.
Table 4 Insulation categories for Basic
The TNV or Telecommunication Voltage definitions are given in Table
V.1 of IEC950 from which EN60950 and UL1950 are derived.
The EXA40 has an approved insulation system that satisfies the
requirements of the safety standards.
In order for the user to maintain the insulation requirements of these
safety standards it is necessary for the required creepage and
clearance distances to be maintained between the input and output.
Creepage is the distance along a surface such as a PCB and for the
EXA40 the creepage requirement between primary and secondary is
1.4mm or 55 thou. Clearance is the distance through air and the
requirement is 0.7mm or 27 thou.
Input Fusing
In order to comply with safety requirements the user must provide a
fuse in the unearthed input line if an earthed input is used. The
reason for putting the fuse in the unearthed line is to avoid earth
being disconnected in the event of a failure. If an earthed input is
not being used then the fuse may be in either input line.
For the 48V input models a 2A Anti-Surge Fuse should be used and
for the 24V models a 3.15A Anti-Surge fuse is required. High
Rupture Capacity (HRC) fuses are recommended.
6. EMC
The EXA40 has been designed to comply with the EMC
requirements of ETSI 300-386-1. It meets the most stringent
requirements of Table 5; Public telecommunications equipment,
locations other than telecommunication centres, High Priority of
Service. Following is the list of standards which apply and which it
has complied with.
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EXA40 SERIES
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Application Note
Radiated emissions
The applicable standard is EN55022 Class B (FCC Part 15). Testing
DC/DC converters as a stand-alone component to the exact
requirements of EN55022 (FCC Part 15) is very difficult to do as the
standard calls for 1m leads to be attached to the input and output
ports and aligned such as to maximise the disturbance. In such a
set-up it is possible to from a perfect dipole antenna that very few
DC/DC converters could pass.
However the standard also states that ‘An attempt should be made
to maximise the disturbance consistent with the typical application
by varying the configuration of the test sample’. In addition ETS 300
386-1 states that the testing should be carried out on the enclosure.
The EXA40 is primarily intended for PCB mounting in
Telecommunication Rack systems.
For the purpose of the radiated test the unit was mounted on a 6U
high PCB with a 40W load on board and connections to the remote
on/off and trim pins. The recommended PCB layouts were used on
the test PCB. The unit was then mounted in an ETSI standard 19”
rack system and the position of the card was varied to achieve
maximum emissions.
There was a 4µF capacitor connected across the input and the
ground plane was connected to the output (pin 7). However the
difference between the ground plane being connected to the input
or output is minimal. Details of the capacitor and optimum
groundplane can be found in later sections.
The test results for the 48S05 and 24S05 models are shown in
tables 5 and 6 below as these are the models with the highest
switching voltages and currents. The testing was carried out by an
independent test house and a copy of the report is available on
request.
Frequency (MHz)
30.00
102.53
108.50
109.09
110.67
Response (dBµV/m)
14.80
23.75
21.80
22.85
23.00
Conducted emissions
The required standard for conducted is EN55022 Class A (FCC Part
15). The EXA40 has quite a substantial LC filter on board to enable
it to meet this standard with just the addition of one external
component for the 48V models and two for some of the 24V
models.
The conducted noise graphs for the EXA40-48S05 are given in
Figure 2 & Figure 3 . The graphs of all other models are available on
request.
Figure 2 - EXA40-48S05 Class A Conducted Noise
Table 5 - Radiated Emissions on EXA40-48S05
Frequency (MHz)
30.00
108.56
108.81
110.62
111.50
Response (dBµV/m)
14.80
15.80
16.85
19.00
21.20
Figure 3 - EXA40-48S05 Class B Conducted Noise
Table 6 - Radiated Emissions on EXA40-24S05
In both cases the unit passed the Class B limit which is 30 dBV/m
with a significant margin.
The required Filters to meet Class A & Class B for all models shown
in Figure 4 to Figure 7. Table 7 is a cross-reference which indicates
which filter is required for which model.
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EXA40 SERIES
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Application Note
Model
48S1V8
48S2V75
48S3V3
48S05
24S1V8
24S2V75
24S3V3
24S05
Class A
Filter A
Filter A
Filter A
Filter A
Filter A
Filter B
Filter B
Filter B
Class B
Filter C
Filter C
Filter C
Filter B
Filter C
Filter C
Filter C
Filter D
The part numbers of the parts used in each case are
given below.
Film Capacitor ITW Paktron part number 405K100CS4G.
Surface Mount Ferrite Bead part no; MMG DS52-9K3F-R1ES2.
24Vin models use 10µH TDK inductor part no; SLF10145T-
100M2R5.
48Vin models use 47µH TDK inductor part no; SLF10145T-
470M1R4.
Table 7 - Model/Filter cross-reference
7. Use in a Manufacturing Environment
Resistance to Soldering Heat
The EXA40 is intended for PCB mounting. Artesyn has determined
how well it can resist the temperatures associated with the soldering
of PTH components without affecting its performance or reliability.
The method used to verify this is MIL-STD-202 method 210D.
Within this method two test conditions were specified, Soldering
Iron condition A and Wave Solder condition C.
For the soldering iron test the UUT was placed on a PCB with the
recommended PCB layout pattern shown in the applications
section. A soldering iron set to 350°C ± 10°C was applied to each
terminal for 5 seconds. The UUT was then removed from the test
PCB and was examined under a microscope for any reflow of the
pin solder or physical change to the terminations. None was found.
+ Vin
E.U.T.
- Vin
2 x 4µF Film Caps
Figure 4 - Filter A
Inductor Dependent on Input Voltage
+ Vin
E.U.T.
- Vin
2 x 4µF Film Caps
For the wave soldering test the UUT was again mounted on a test
PCB. The unit was wave soldered using the conditions shown in
Table 8.
Temperature
Time
10s±1
Temperature Ramp
Preheat 4°C/s to 160°C.
25mm/s rate
Figure 5 - Filter B
260°C ±5°C
SM Ferrite
Inductor dependent on Input Voltage
Table 8 Wave Solder Test Conditions
+ Vin
E.U.T.
- Vin
2 x 4µF Film Caps
The UUT was inspected after soldering and no physical change on
pin terminations was found.
Figure 6 - Filter C
SM Ferrite
Inductor dependent on Input Voltage
Water Washing
The EXA40 is suitable for water washing as it doesn’t have any
pockets where water may congregate long-term. The user should
ensure that a sufficient drying process and period is available to
remove the water from the unit after washing
+ Vin
2 x 4µF Film Caps
ESD Control
The EXA40’s are manufactured in an ESD controlled environment
and supplied in conductive packaging to prevent ESD damage
occurring before or during shipping. It is essential that they are
unpacked and handled using an approved ESD control procedures.
Failure to do so could affect the lifetime of the converter.
- Vin
Figure 7 - Filter D
PAGE 5
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