LNK584-586
comparator for a short time (tLEB) 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
MOSFET conduction.
accomplished by the π filter formed by C1, C2 and L3. The
proprietary frequency jitter feature of the LinkZero-AX eliminates
the need for any Y capacitor or common-mode inductor. Wire-
wound resistor RF1 is a fusible, flame proof resistor which is used
as a fuse as well as to limit inrush current. Wire wound types are
recommended for designs that operate >132 VAC to withstand
the instantaneous power dissipated when AC is first applied.
Auto Restart
In the event of a fault condition such as output short-circuit,
LinkZero-AX enters into auto-restart operation. An internal
counter clocked by the oscillator gets reset every time the
FEEDBACK pin voltage exceeds the FEEDBACK pin auto-restart
threshold voltage (VFB(AR) typical 0.9 V). If the FEEDBACK pin
voltage drops below VFB(AR) for more than 145 ms to 170 ms
depending on the line voltage, the power MOSFET switching is
disabled. The auto-restart alternately enables and disables the
switching of the power MOSFET at a duty cycle of typically 12%
until the fault condition is removed.
The output voltage is directly sensed through feedback resistors
R3 and R9, and regulated by LinkZero-AX (U1) via the FEEDBACK
pin. Capacitor C7 provides high frequency filtering on the
FEEDBACK pin to filter noise and to avoid switching cycle pulse
bunching. The controller in U1 receives feedback from the
output through feedback resistors R9 and R3. Based on that
feedback, it enables or disables the switching of its integrated
MOSFET to maintain output regulation. Switching cycles are
skipped once the FEEDBACK pin threshold voltage (1.70 V) is
exceeded. When the voltage on the FEEDBACK pin falls below
the disable threshold (1.70 V), switching cycles are re-enabled.
By adjusting the ratio of enabled to disabled switching cycles
the output voltage is regulated. At increased loads, beyond the
output peak power point, where all switching cycles are
enabled, the FEEDBACK pin voltage begins to reduce as the
power supply output voltage falls. Under this condition the
switching frequency is also reduced to limit the maximum output
overload power. When the FEEDBACK pin voltage drops below
the auto-restart threshold (typically 0.9 V on the FEEDBACK
pin), the power supply enters the auto-restart mode. In this
mode, the power supply will turn off for approximately 1.2 s and
then turn back on for approximately 145 ms. The auto-restart
function reduces the average output current during an output
short-circuit condition.
Open Loop Condition on the FEEDBACK Pin
When an open loop condition on the FEEDBACK pin is detected,
an internal current source pulls up the FEEDBACK pin to above
the VFB (1.70 V), the part stops switching and after 160 clock
cycles goes into latched Power-Down mode.
Applications Example
The circuit shown in Figure 4 is a typical non-isolated 5 V, 300 mA
output auxiliary power supply using LinkZero-AX. Isolated
configurations are also fully compatible with the LinkZero-AX
where the FEEDBACK pin receives a signal from a primary
feedback/bias winding or through an optocoupler. The circuit
of Figure 4 is typical of auxiliary supplies in white goods where
isolation is often not required. AC input differential filtering is
C4
220 pF
100 V
R8
5.1 Ω
C8
R13 56 µF
510 Ω 16 V
L4
1.8 µH
L3
T1
1 mH
EE16
5 V, 300 mA
3
8
C6
D6
SS15
R2
4.7 kΩ
220 µF
25 V
10
D1
D2
1
1N4007 1N4007
RTN
RF1
10 Ω
2 W
C9
330 nF
50 V
R9
1 kΩ
1%
LinkZero-AX
C1
3.3 µF
400 V
C2
3.3 µF
400 V
85 - 265
VAC
U1
LNK584DG
D
S
Q1
FB
MMBT3904
BP/M
R16
750 Ω
R12
R10
20 kΩ 20 kΩ
D3
D4
PD Set
1N4007 1N4007
R11
100 Ω
C5
150 nF
25 V
C10
47 µF
25 V
C7
1 nF
50 V
R3
511 Ω
1%
Q2
MMBT3904
PD Reset
RTN
SW1
R14
2 kΩ
R4
10 kΩ
PI-6121-101210
Figure 4. Schematic of Non-Isolated 1.5 W, 5 V, 300 mA, 0.00 W Standby Consumption Power Supply.
4
Rev. B 05/11
www.powerint.com
POWERINT [ Power Integrations ]
