Rev. Q 06/23

5

LinkSwitch-TN2

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Applications Example

A 1.44 W Universal Input Buck Converter

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).

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 (t

RR) of approximately 75 ns,

acceptable for MDCM operation. For continuous conduction mode

(CCM) designs, a diode with a t

RR 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.

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.

Figure 8. Universal Input, 12 V, 120 mA Constant Voltage Power Supply using LinkSwitch-TN2.

Regulation is maintained by skipping switching cycles. As the output

voltage rises, the current into the FEEDBACK pin will rise. If this

exceeds I

FB then subsequent cycles will be skipped until the current

reduces below I

FB. 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.

Key Application Considerations

LinkSwitch-TN2 Design Considerations

Output Current Table

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 t

RR ≤75 ns is used for MDCM

operation and for CCM operation, a diode with t

RR ≤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.

*KRP is the ratio of ripple to peak inductor current.

RTN

12 V,

120 mA

85-265

VAC

PI-7857-120424

FB

BP/M

S

D

LinkSwitch-TN2

C4

4.7 µF

400 V

C1

100 nF

D4

1N4007

D3

1N4007

D1

UF4005

U1

LNK3204

D2

1N4005GP

C2

100 µF

16 V

RF1

8.2 Ω

2 W

R1

11.8 kΩ

1%

R5

26.7 kΩ

R3

2.49 kΩ

1%

L2

1 mH

L1

1 mH

280 mA

C5

4.7 µF

400 V

C3

10 µF

25 V

R4

3.3 kΩ

1%