EPR-000008 – 1.2 W Universal Input Non-Isolated TNY254
23-April-2001
4 Circuit Description
The circuit is a fail-safe, non-isolated fly-back topology. Fail-safe means that the output is
not subjected to high voltage DC if the switch (U1) fails, since the diode D2 blocks the
voltage.
During the ON time (U1 conducting), L1 is charged up to ILIMIT of the TNY254 (0.25 A type.),
from the energy stored in C1. During the OFF time (U1 blocking), the energy stored in L1 is
transferred to C3 and the load via D2.
The device switching frequency is 44 kHz. The surge protection circuit has to prevent the
TinySwitch VDSMAX from exceeding 700 V. The surge protection for configuration ”1” (2 kV)
and configuration ”2” (6 kV) is illustrated in the schematics (pages 4, 5).
Configuration ”1” relies on the current/energy-limiting resistor R2 to keep the maximum
charging voltage of C1 during a +2 kV surge below 700 V. For the -2 kV surge the diodes
D1+D3 block the voltage.
Configuration ”2” relies on the current/energy-limiting resistor R1 to limit the current in the
MOV (RV1) to approximately 50 A peak, such that the RV1 clamping voltage is kept well
under 1 kV (~700 V). During the +6 kV surge, R2 limits C1 charging current so that the
maximum voltage does not exceed 700 V. During the -6 kV surge, D1 blocks the RV1
clamping voltage from reaching C1.
In both configurations, the fusible resistor R2 provides protection for component failure.
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