EPR-000013 Datasheet, PDF (7/56 Pages) Power Integrations, Inc. – Engineering Prototype Report (EP13) 43 W / 57 W pk, 5 Output TOPSwitch팜-GX (TOP246Y) Power Supply

08-May-2001
EPR-000013 – 43 W Multiple Output TOP246 Power Supply
4 Circuit Description
The EP13 is a five-output flyback power supply using the TOP246Y integrated circuit.
The circuit shown in Figure 3 provides 43 W continuous power, with peak capability of
57 W (thermally limited). Input voltage range is 180-265 V_AC.
AC input power is rectified and filtered by D1-4 and C2 to provide a high voltage DC bus,
which is applied to the primary of transformer T1. The TOP246Y DRAIN pin drives the
other side of the transformer primary. Components D5, VR1, R2 and C5 clamp the
DRAIN voltage leakage inductance spike to below the 700 V maximum rating of the
TOPSwitch.
The TOPSwitch-GX family provides several new features, as well as extended
specifications. The EP13 power supply is designed to take advantage of several of these
features.
Resistor R1 connected to the LINE SENSE pin (L) of TOPSwitch-GX U1 is used to
implement the built-in line voltage feed forward and overvoltage protection features. The
line feed forward feature modulates the control circuit of the TOPSwitch-GX with the AC
line frequency ripple component of the input DC, reducing the line frequency ripple at the
output of the supply. This simplifies the design of the power supply control loop by
reducing the amount of control loop gain required at the line ripple frequency in order to
meet output ripple specifications.
The overvoltage feature shuts down the power supply if the rectified DC bus voltage
exceeds approximately 450 V, set by the value of R1. The supply resumes operation
when the bus voltage falls again below the overvoltage threshold value. This feature
allows the supply to withstand severe line transients or extended surge conditions without
damage. This is an attractive feature for products designed for markets with poor power
quality.
Resistor R4 connects to the EXTERNAL CURRENT LIMIT pin (X) of U1 and is used to
externally program the device current limit to just above the peak primary current of the
supply at maximum peak load, minimum line voltage. This allows the transformer to be
better optimized for the chosen operating conditions, while at the same time avoiding
transformer core saturation during start-up or overload conditions.
Transformer optimization choices can include using a smaller core (less expensive
transformer), fewer primary turns (less leakage inductance), or higher primary inductance
(more continuous operation, less TOPSwitch dissipation).
The EP13 transformer design does not take full advantage of the flexibility offered by the
TOPSwitch-GX due to secondary volts per turn required to minimize voltage error
between 3.3 V and 5 V outputs. The secondary turns were deliberately chosen to
optimize output voltage centering with the fewest possible number of turns. The reflected
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