LNK302/304-306
PI-3660-081303
600
500
V
DRAIN
400
300
200
100
0
68 kHz
64 kHz
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
fl
ame 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) Input fuse should any other component fail short-circuit
(component fails safely open-circuit without emitting smoke,
fi
re or incandescent material).
0
Time (μs)
20
Figure 4. Frequency Jitter.
power MOSFET is turned off for the remainder of that cycle.
The leading edge blanking circuit inhibits the current limit
comparator for a short time (t
LEB
) after the power MOSFET
is turned on. This leading edge blanking time has been set so
that current spikes caused by capacitance and rectifier reverse
recovery time will not cause premature termination of the
switching pulse.
Auto-Restart (LNK304-306 only)
In the event of a fault condition such as output overload, output
short, or an open loop condition,
LinkSwitch-TN
enters into auto-
restart operation. An internal counter clocked by the oscillator
gets reset every time the FB pin is pulled high. If the FB pin
is not pulled high for 50 ms, the power MOSFET switching is
disabled for 800 ms. The auto-restart alternately enables and
disables the switching of the power MOSFET until the fault
condition is removed.
The power processing stage is formed by the
LinkSwitch-TN,
freewheeling diode D1, output choke L1, and the output
capacitor C2. The LNK304 was selected such that the power
supply operates in the mostly discontinuous-mode (MDCM).
Diode D1 is an ultra-fast 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
fi
lter 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.
To a
fi
rst 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 FB pin.
The values of R1 and R3 are selected such that, at the desired
output voltage, the voltage at the FB pin is 1.65 V.
Regulation is maintained by skipping switching cycles. As the
output voltage rises, the current into the FB 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
Applications Example
A 1.44 W Universal Input Buck Converter
The circuit shown in Figure 5 is a typical implementation of a
R1
13.0 kΩ
1%
R3
2.05 kΩ
1%
C3
10
μF
35 V
L1
1 mH
280 mA
D1
UF4005
RF1
8.2
Ω
2W
D3
1N4007
D4
1N4007
L2
1 mH
FB
D
BP
S
C1
100 nF
D2
1N4005GP
12 V,
120 mA
C2
100
μF
16 V
R4
3.3 kΩ
RTN
85-265
VAC
C4
4.7
μF
400 V
C5
4.7
μF
400 V
LinkSwitch-TN
LNK304
PI-3757-112103
Figure 5. Universal Input, 12 V, 120 mA Constant Voltage Power Supply Using LinkSwitch-TN.
2-4
4
Rev. I 11/08
POWERINT [ POWER INTEGRATIONS, INC. ]
