DER-50
Set Top Box Design using TOP245R
April 20, 2005
Inductors L2, L3, L4, L6 are used along with capacitors C5, C7, C10, C15 to provide high
frequency filtering for the outputs of the supply. These filters greatly reduce the switching
frequency ripple and high frequency spike noise at the outputs of the supply.
A voltage divider consisting of resistors R8, R9 and R15 monitors the voltage on the 5V
and 3.3V outputs. The resistor values are weighted so that the voltage feedback loop is
controlled mostly by the 5V output (2/3), with some contribution from the 3.3V output (1/3).
Sharing the voltage regulation control between the two outputs in this way improves the
cross regulation for the 3.3V output at the expense of a slight change in the regulation of
the 5V output. Resistor R11 is used to set the overall gain of the supply control loop, while
R7 provides bias current for U2. R14 and C20 provide frequency compensation for U2 to
help stabilize the power supply control loop. Capacitor C22 is used to provide open loop
feedback through optocoupler ISO1 during start-up, which in conjunction with the built-in
soft start-up feature of the TOPSwitch, completely controls the start-up drain current
profile, preventing transformer saturation and output overshoot.
The approach used for this prototype is a multiple output flyback converter using the
integrated functions of TOPSwitch-GX to minimize component count and system cost. In
contrast to typical supplies, the design specification includes some extra requirements: -
•
Regulation on the 18V and 23V rails must be maintained to within 10% from zero
load to full 0.6A load.
Due to leakage inductance, track resistance, winding resistance diode current/voltage
characteristics, the 18V rail will most probably experience peak charging under zero load
conditions. In order to maintain the 10% regulation requirements on this rail, there are
two possibilities:
1. Pre-load the 18V rail to such a level to keep within 10% when the current drawn from
this rail falls to zero.
2. Post-regulate the 18V rail to ensure tight regulation during the peak charging
conditions.
The supply itself uses a TOP245R and EF25 core. Although the TOP244R could be used,
the TOP245R was used to lower the system losses at low line input.
Front end protection is provided by F1, RT1 and RV1. C11 and L5 provide both common
mode and differential mode conducted EMI filtering. The frequency jittering feature of
TOPSwitch-GX has allowed for small EMI filter components.
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