LNK501
Continuous mode designs can result in loop instability and
are therefore not recommended.
Core gaps should be uniform. Uneven core gapping, especially
with small gap sizes, may cause variation in the primary
inductance with flux density (partial saturation) and make the
constant current region non-linear. To verify uniform gapping
it is recommended that the primary current wave-shape be
examined while feeding the supply from a DC source. The
gradient is defined as di/dt = V/L and should remain constant
throughout the MOSFET on time. Any change in gradient of
the current ramp is an indication of uneven gapping.
3. A secondary output of 5 V with a Schottky rectifier diode.
4. Assumed efficiency of 70%
5. The part is board mounted with SOURCE pins soldered to
sufficient area of copper to keep the die temperature at or
below 100 °C.
In addition to the thermal environment (sealed enclosure,
ventilated, open frame, etc), the maximum power capability of
LinkSwitch in a given application depends on transformer core
size, efficiency, primary inductance tolerance, minimum
specifiedinputvoltage,inputstoragecapacitance,outputvoltage
output diode forward drop etc., and can be different from the
values shown in Table 1.
Measurements made using a LCR bridge should not be solely
reliedupon;typicallytheseinstrumentsonlymeasureatcurrents
of a few milliamps. This is insufficient to generate high enough
flux densities in the core to show uneven gapping.
ForatypicalEE13coreusingcenterleggapping,a0.08mmgap
(ALG of 190 nH/t2) allows a primary inductance tolerance of
±10% to be maintained in standard high volume production.
This allows the EE13 to be used in designs up to 2.75 W. If film
gapping is used then this increases to 3 W with less than
300 mW no-load consumption. Moving to a larger core, EE16
for example, allows a 3 W output with center leg gapping.
In designs not required to meet 300 mW no-load consumption,
the transformer can be designed with higher VOR to extend
power capability as noted in the following section.
Transformer Design
To provide an approximately CV/CC output, the transformer
should be designed to be discontinuous; all the energy stored in
the transformer is transferred to the secondary during the
MOSFET off time. Energy transfer in discontinuous mode is
independent of line voltage.
The transformer turns ratio should be selected to give a VOR
(output voltage reflected through secondary to primary turns
ratio) of 40 - 60 V. In designs not required to meet 300 mW no-
load consumption targets, the transformer can be designed with
higher VOR as long as discontinuous mode operation is
maintained. This increases the output power capability. For
example, a 230 VAC input design using an EE19 transformer
core with VOR >70 V, is capable of delivering up to 5 W typical
output power. Note: the linearity of the CC region of the power
supply output characteristic is influenced by VOR. If this is an
important aspect of the application, the output characteristic
should be checked before finalizing the design.
Thepeakpowerpointpriortoenteringconstantcurrentoperation
isdefinedbythemaximumpowertransferredbythetransformer.
The power transferred is given by the expression P = 0.5·L·I2·f,
whereListheprimaryinductance, I2 istheprimarypeakcurrent
squared and f is the switching frequency.
To simplify analysis, the data sheet parameter table specifies an
I2f coefficient. This is the product of current limit squared and
switching frequency normalized to the feedback parameter
IDCT. This provides a single term that specifies the variation of
the peak power point in the power supply due to LinkSwitch.
Output Characteristic Variation
Boththedevicetoleranceandexternalcircuitgoverntheoverall
tolerance of the LinkSwitch output characteristic. Estimated
peak power point tolerances for a 2.75 W design are ±10% for
voltage and ±20% for current limit for overall variation in high
volume manufacturing. This includes device and transformer
tolerances and line variation. Lower power designs may have
poorer constant current linearity.
As primary inductance tolerance is part of the expression that
determines the peak output power point (start of the CC
characteristic) this parameter should be well controlled. For an
estimated overall output peak power tolerance of ±20% the
primary inductance tolerance should be ±10% or better. This is
achievableusingstandardlowcost,centerleggappingtechniques
where the gap size is typically 0.08 mm or larger. Smaller gap
sizes are possible but require non standard, tighter ferrite AL
tolerances.
As the output load reduces from the peak power point, the
output voltage will tend to rise due to tracking errors compared
to the load terminals. Sources of these errors include the output
cabledrop,outputdiodeforwardvoltageandleakageinductance,
which is the dominant cause. As the load reduces, the primary
operating peak current reduces, together with the leakage
inductance energy, which reduces the peak charging of the
clamp capacitor. With a primary leakage inductance of 50 µH,
the output voltage typically rises 30% over a 100% to 5% load
change.
Other gapping techniques such as film gapping allow tighter
tolerances (±7% or better) with associated improvements in the
tolerance of the peak power point. Please consult your
transformer vendor for guidance.
F
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POWERINT [ Power Integrations ]
