LNK3202/3204-6
Inductor L1
LinkSwitch-TN2 Selection and Selection Between
MDCM and CCM Operation
Choose any standard off-the-shelf inductor that meets the design
requirements. A “drum” or “dog bone” “I” core inductor is recom-
mended with a single ferrite element due to its low-cost and very low
audible noise properties. However, the inductor should be selected
as varnished type in order to get low audible noise. The typical
inductance value and RMS current rating can be obtained from the
LinkSwitch-TN2 design spreadsheet available within the PI Expert
design suite from Power Integrations. Choose L1 greater than or
equal to the typical calculated inductance with RMS current rating
greater than or equal to calculated RMS inductor current. Care
should be taken to ensure that the inductor has sufficient voltage
rating as this is a high-voltage application.
Select the LinkSwitch-TN2 device, freewheeling diode and output
inductor that gives the lowest overall cost. In general, MDCM
provides the lowest cost and highest efficiency converter. CCM
designs require a larger inductor and ultrafast (tRR ≤35 ns) freewheel-
ing diode in all cases. It is lower cost to use a larger LinkSwitch-TN2
in MDCM than a smaller LinkSwitch-TN2 in CCM because of the
additional external component costs of a CCM design. However, if
the highest output current is required, CCM should be employed
following the guidelines below.
Topology Options
LinkSwitch-TN2 can be used in all common topologies, with or without
an optocoupler and reference to improve output voltage tolerance
and regulation. Table 3 provides a summary of these configurations.
For more information see the Application Note – LinkSwitch-TN2
Design Guide.
Capacitor C2
The primary function of capacitor C2 is to smooth the inductor current.
The actual output ripple voltage is a function of this capacitor’s ESR.
To a first order, the ESR of this capacitor should not exceed the rated
ripple voltage divided by the typical current limit of the chosen
LinkSwitch-TN2.
Component Selection
Feedback Resistors R1 and R3
Referring to Figure 8, the following considerations may be helpful in
selecting components for a LinkSwitch-TN2 design.
The values of the resistors in the resistor divider formed by R1 and
R3 are selected to maintain 2.00 V at the FEEDBACK pin. It is
recommended that R3 be chosen as a standard 1% resistor of 2.49 kΩ.
This ensures good noise immunity by biasing the feedback network
with a current of approximately 0.8 mA.
BYPASS Pin Capacitor C1
Capacitor connected from the BYAPSS pin provides decoupling for the
controller and also selects current limit. A 0.1 μF or 1 μF capacitor may
be used as indicated in the data sheet. Though electrolytic capaci-
tors can be used, often surface mount multi-layer ceramic capacitors
are preferred for use as they enable placement of capacitors close to
the IC and design of compact switching power supplies. 16 V, 25 V or
higher X7R dielectric capacitors are recommended to ensure
minimum capacitance change under DC bias and temperature.
External Bias Resistor R5
To reduce the no-load input power of the power supply, resistor R5,
connected from the feedback capacitor C3 to the BYPASS pin, is
recommended. This is applicable to the power supply whose output
voltage is higher than VBP(SHUNT). To achieve lowest no-load power
consumption, the current fed into the BYPASS pin should be slightly
higher than 120 μA. For the best full load efficiency and thermal
performance, the current fed into the BYPASS pin should be slightly
higher than the current values stated below.
Freewheeling Diode D1
Diode D1 should be an ultrafast type. For MDCM, reverse recovery
time tRR ≤75 ns should be used at a temperature of 70 °C or below.
Slower diodes are not acceptable, as continuous mode operation will
always occur during startup, causing high leading edge current
spikes, terminating the switching cycle prematurely, and preventing
the output from reaching regulation. If the ambient temperature is
above 70 °C then a diode with tRR ≤35 ns should be used.
Part Number
LNK3202
Bypass Current
195 μA
LNK3204
222 μA
LNK3205
269 μA
For CCM an ultrafast diode with reverse recovery time tRR ≤35 ns
should be used. A slower diode may cause excessive leading edge
current spikes, terminating the switching cycle prematurely and
preventing full power delivery.
LNK3206
290 μA
Table 2.
BYPASS Pin Current Recommendations.
Fast recovery and slow recovery diodes should never be used as the
large reverse recovery currents can cause excessive power dissipa-
tion in the diode and/or exceed the maximum drain current specifica-
tion of LinkSwitch-TN2.
Feedback Capacitor C3
Capacitor C3 can be a low cost general purpose capacitor. It provides
a “sample and hold” function, charging to the output voltage during
the off time of LinkSwitch-TN2. Its value should be 10 μF to 22 μF;
smaller values cause poorer regulation at light load conditions.
Feedback Diode D2
Diode D2 can be a low-cost slow diode such as the 1N400X series,
however it should be specified as a glass passivated type to guaran-
tee a specified reverse recovery time. To a first order, the forward
drops of D1 and D2 should match.
Pre-Load Resistor R4
In high-side, direct feedback designs where the minimum load is
<3 mA, a pre-load resistor is required to maintain output regulation.
This ensures sufficient inductor energy to pull the inductor side of the
feedback capacitor C3 to input return via D2. The value of R4 should
be selected to provide a minimum output load of 3 mA.
In designs with an optocoupler a Zener diode or reference bias
current provides a 1 mA to 2 mA minimum load, preventing “pulse
bunching” and increased output ripple at zero load.
6
Rev. F 01/17
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