Datasheet LNK3202, LNK3204-7, LNK3294, LNK3296 (Power Integrations) - 4

LinkSwitch-TN2 Hysteretic Output Overvoltage Protection Line Overvoltage Protection
The output overvoltage protection provided by the LinkSwitch-TN2 IC In a flyback converter LinkSwitch-TN2 can sense indirectly the DC bus uses auto-restart that is triggered by a current >I into the BYPASS overvoltage condition during the power MOSFET on-time by monitor- BPSD pin. In addition to an internal filter, the BYPASS pin capacitor forms ing the current flowing into the FEEDBACK pin depending on circuit an external filter providing noise immunity from inadvertent triggering. configuration. Figure 7 shows one possible circuit implementation. For the bypass capacitor to be effective as a high frequency filter, the During the MOSFET on-time, the voltage across the secondary capacitor should be located as close as possible to the SOURCE and winding is proportional to the voltage across the primary winding. BYPASS pins of the device. The current flowing through emitter and base of transistor Q3 is therefore representing V . Indirect line sensing minimizes power The OVP function can be realized in a flyback converter by connect- BUS dissipation and is used for line OV protection. The LinkSwitch-TN2 ing a Zener diode from the output supply to the BYPASS pin. The will go into auto- auto-restart mode if the FEEDBACK pin current circuit example shown in Figure 6 describes a simple method for exceeds the line overvoltage threshold current I for at least 2 implementing the output overvoltage protection. Adding additional FBSD consecutive switching cycles. 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 In order to have accurate line OV threshold voltage and also for good with the OVP Zener diode also limits the maximum current into the efficiency, regulation performance and stability, the transformer BYPASS pin. The current should be limited to less than 16 mA. leakage inductance should be minimized. Low leakage will minimize ringing on the secondary winding and provide accurate line OVP During a fault condition resulting from loss of feedback, the output sampling. In some designs, a RC snubber across the rectifier diode voltage will rapidly rise above the nominal voltage. A voltage at the may be needed to damp the ringing at the secondary winding when output that exceeds the sum of the voltage rating of the Zener diode line voltage is sampled. connected from the output to the BYPASS pin and bypass voltage, will cause a current in excess of I injected into the BYPASS pin, which BPSD will trigger the auto-restart and protect the power supply from overvoltage. T1 +
VO
+
VBUS D
VOV = VBP + VDOVP
FB LinkSwitch-TN2 BP
RBP
S
CBP DOVP PI-8024-092916 Figure 6. Non-Isolated Flyback Converter with Output Overvoltage Protection. T1 n:1 + R3*
VO
n:1 VR3 + D3 VOV = (VVR3 + VD3 + VBE(Q3) – VBP + VR3) × n + VDS(ON)
VBUS
*R3 limits the current into the FEEDBACK pin. R4** A maximum current of 120% of I Q3 FBSD is
D
recommended.
FB LinkSwitch-TN2
**R4 is a pull-down resistor for Q3 to avoid
BP
inadvertant triggering. 2 kΩ is a typical starting point
S
cBP PI-8025-092616 Figure 7. Line-Sensing for Overvoltage Protection by using FEEDBACK Pin.
4
Rev. M 10/20 www.power.com Document Outline Product Highlights Description Output Current Table Pin Functional Description LinkSwitch-TN2 Functional Description Applications Example Key Application Considerations Quick Design Checklist Absolute Maximum Ratings Thermal Resistance Key Electrical Characteristics Typical Performance Characteristics PDIP-8C (P Package) SMD-8C (G Package) SO-8C (D Package) PDIP-8C (P) and SMD-8C Package Marking SO-8C (D) Package Marking MSL Table ESD and Latch-Up Part Ordering Information