LNK3202/3204-6
Applications Example
R5
26.7 kΩ
R1
11.8 kΩ
1%
C3
R3
D2
RF1
8.2 Ω
2 W
10 µF
25 V
2.49 kΩ
1N4005GP
L2
C1
12 V,
1%
FB
BP/M
S
1 mH
100 nF
120 mA
D
L1
D3
1 mH
LinkSwitch-TN2
1N4007
C4
4.7 µF
400 V
C5
4.7 µF
400 V
C2
100 µF
16 V
R4
3.3 kΩ
1%
280 mA
85-265
VAC
D1
UF4005
LNK3204
D4
1N4007
RTN
PI-7857-092616
Figure 8. Universal Input, 12 V, 120 mA Constant Voltage Power Supply using LinkSwitch-TN2.
A 1.44 W Universal Input Buck Converter
Regulation is maintained by skipping switching cycles. As the output
voltage rises, the current into the FEEDBACK pin will rise. If this
exceeds IFB then subsequent cycles will be skipped until the current
reduces below IFB. Thus, as the output load is reduced, more cycles
will be skipped and if the load increases, fewer cycles are skipped.
To provide overload protection if no cycles are skipped during a
50 ms period, LinkSwitch-TN2 will enter auto-restart, limiting the
average output power to approximately 3% of the maximum overload
power. Due to tracking errors between the output voltage and the
voltage across C3 at light load or no-load, a small pre-load may be
required (R4). For the design in Figure 8, if regulation to zero load is
required, then this value should be reduced to 2.4 kΩ.
The circuit shown in Figure 8 is a typical implementation of a 12 V,
120 mA non-isolated power supply used in appliance control such as
rice cookers, dishwashers or other white goods. This circuit may also
be applicable to other applications such as night-lights, LED drivers,
electricity meters, and residential heating controllers, where a
non-isolated supply is acceptable.
The input stage comprises fusible resistor RF1, diodes D3 and D4,
capacitors C4 and C5, and inductor L2. Resistor RF1 is a flame proof,
fusible, wire wound resistor. It accomplishes several functions:
A. Inrush current limitation to safe levels for rectifiers D3 and D4;
B. Differential mode noise attenuation;
C. Acts as an input fuse in the event any other component fails
short-circuit (component fails safely open-circuit without emitting
smoke, fire or incandescent material).
Key Application Considerations
LinkSwitch-TN2 Design Considerations
Output Current Table
The power processing stage is formed by the LinkSwitch-TN2,
freewheeling diode D1, output choke L1, and the output capacitor C2.
The LNK3204 was selected such that the power supply operates in
the mostly discontinuous-mode (MDCM). Diode D1 is an ultrafast
diode with a reverse recovery time (tRR) of approximately 75 ns,
acceptable for MDCM operation. For continuous conduction mode
(CCM) designs, a diode with a tRR of ≤35 ns is recommended.
Inductor L1 is a standard off-the-shelf inductor with appropriate RMS
current rating (and acceptable temperature rise). Capacitor C2 is the
output filter capacitor; its primary function is to limit the output
voltage ripple. The output voltage ripple is a stronger function of the
ESR of the output capacitor than the value of the capacitor itself.
Optional resistor R5 supplies the BYPASS pin externally for signifi-
cantly lower no-load input power and increased efficiency over all
load conditions.
Data sheet maximum output current table (Table 1) represents the
typical practical continuous output current for both mostly discontinu-
ous conduction mode (MDCM) and continuous conduction mode (CCM)
of operation that can be delivered from a given LinkSwitch-TN2
device under the following assumed conditions:
1. Buck converter topology.
2. The minimum DC input voltage is ≥70 V. The value of input
capacitance should be large enough to meet this criterion.
3. For CCM operation a KRP* of 0.4.
4. Output voltage of 12 VDC.
5. Efficiency of 75%.
6. A catch/freewheeling diode with tRR ≤75 ns is used for MDCM
operation and for CCM operation, a diode with tRR ≤35 ns is used.
7. The part is board mounted with SOURCE pins soldered to a
sufficient area of copper to keep the SOURCE pin temperature at
or below 100 °C.
To a first order, the forward voltage drops of D1 and D2 are identical.
Therefore, the voltage across C3 tracks the output voltage. The
voltage developed across C3 is sensed and regulated via the resistor
divider R1 and R3 connected to U1’s FEEDBACK pin. The values of R1
and R3 are selected such that, at the desired output voltage, the
voltage at the FEEDBACK pin is 2.00 V.
*KRP is the ratio of ripple to peak inductor current.
5
Rev. F 01/17
www.power.com
POWERINT [ Power Integrations ]
