DER-50
Set Top Box Design using TOP245R
April 20, 2005
4 Circuit Description
AC input power is rectified and filtered by D4, D5, D9, D10 and C23 to provide high
voltage DC bus, which is applied to the primary of transformer T1. TOP245R DRAIN pin
drives the other side of the transformer primary. Components C18, R20, R5 and D12
clamp the DRAIN voltage leakage inductance spike to below 700 V maximum rating of
the TOPSwitch. The additional RC Snubber R21 and C27 is used to get better EMI
results at higher frequencies.
Resistors R6 and R13 connected to the LINE SENSE pin (L) of the TOPSwitch-GX U1
are 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 450V, set by the value of R6 and R13. 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.
Resistor R18 connected to the EXTERNAL CURRENT LIMIT pin (X) of U1 is used to
externally program the device current limit to just above the peak primary current of the
supply required for 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.
D7 and C12 provide a DC voltage of approximately 15V to power the TOP245R. A
relatively large value of C12 (1uF) is used to provide bias voltage ride-through during
severe output load transients.
Capacitors C24 and C25 filters the internal bias supply of the TOPSwitch-GX, with C24
providing the necessary peak currents to drive the gate of its internal high voltage
MOSFET. The larger capacitor C25 also determines the TOPSwitch-GX auto-restart
frequency, and along with resistor R17, helps to compensate the power supply control
loop.
Fine tuning and centering of the output voltage levels can be achieved by appropriate
choice of diode (D2, D3, D6 and D8) for each output: by changing diode types, there is
the option to change the voltage drop across it, and therefore the correspondent output
voltage level.
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