LNK3202/3204-6
Hysteretic Output Overvoltage Protection
Line Overvoltage Protection
The output overvoltage protection provided by the LinkSwitch-TN2 IC
uses auto-restart that is triggered by a current >IBPSD into the BYPASS
pin. In addition to an internal filter, the BYPASS pin capacitor forms
an external filter providing noise immunity from inadvertent triggering.
For the bypass capacitor to be effective as a high frequency filter, the
capacitor should be located as close as possible to the SOURCE and
BYPASS pins of the device.
In a flyback converter LinkSwitch-TN2 can sense indirectly the DC bus
overvoltage condition during the power MOSFET on-time by monitor-
ing the current flowing into the FEEDBACK pin depending on circuit
configuration. Figure 7 shows one possible circuit implementation.
During the MOSFET on-time, the voltage across the secondary
winding is proportional to the voltage across the primary winding.
The current flowing through emitter and base of transistor Q3 is
therefore representing VBUS. Indirect line sensing minimizes power
dissipation and is used for line OV protection. The LinkSwitch-TN2
will go into auto- auto-restart mode if the FEEDBACK pin current
exceeds the line overvoltage threshold current IFBSD for at least 2
consecutive switching cycles.
The OVP function can be realized in a flyback converter by connect-
ing a Zener diode from the output supply to the BYPASS pin. The
circuit example shown in Figure 6 describes a simple method for
implementing the output overvoltage protection. Adding additional
filtering can be achieved by inserting a low value (10 Ω to 47 Ω)
resistor in series with the OVP Zener diode. The resistor in series
with the OVP Zener diode also limits the maximum current into the
BYPASS pin. The current should be limited to less than 16 mA.
In order to have accurate line OV threshold voltage and also for good
efficiency, regulation performance and stability, the transformer
leakage inductance should be minimized. Low leakage will minimize
ringing on the secondary winding and provide accurate line OVP
sampling. In some designs, a RC snubber across the rectifier diode
may be needed to damp the ringing at the secondary winding when
line voltage is sampled.
During a fault condition resulting from loss of feedback, the output
voltage will rapidly rise above the nominal voltage. A voltage at the
output that exceeds the sum of the voltage rating of the Zener diode
connected from the output to the BYPASS pin and bypass voltage, will
cause a current in excess of IBPSD injected into the BYPASS pin, which
will trigger the auto-restart and protect the power supply from
overvoltage.
T1
+
VO
+
VBUS
VOV = VBP + VDOVP
D
FB
RBP
LinkSwitch-TN2
BP
S
CBP
DOVP
PI-8024-092916
Figure 6. Non-Isolated Flyback Converter with Output Overvoltage Protection.
T1
n:1
+
R3*
VO
n:1
Q3
VR3
D3
+
VOV = (VVR3 + VD3 + VBE(Q3) – VBP + VR3) × n + VDS(ON)
VBUS
*R3 limits the current into the FEEDBACK pin.
A maximum current of 120% of IFBSD is
recommended.
R4**
D
S
FB
BP
**R4 is a pull-down resister for Q3 to avoid
inadvertant triggering. 2 kΩ is a typical starting point
LinkSwitch-TN2
cBP
PI-8025-092616
Figure 7. Line-Sensing for Overvoltage Protection by using FEEDBACK Pin.
4
Rev. F 01/17
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