Good cross regulation with no linear regulators. The transformer turns ratio was optimized (including • Can deliver 20 W with universal input voltage range output diode forward drops) to minimize the output

Design Idea DI-9®                                                       POWER^®

TOPSwitch-FX                           INTEGRATIONS, INC.

Set-top Box (multiple output)

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

Set-top Box

TOP233Y

35 W

230 VAC

3.3 V / 5 V / 18 V / 30 V / -5 V

Flyback

Design Highlights                                                 output reflected or leakage inductance spike voltages)

allowing the AC rail to rise to 495 VAC (700 VDC

•  High efficiency (>77%)          BV_DSS rating) without damaging the TOPSwitch-FX.

•  Good cross regulation with no linear regulators • The transformer turns ratio was optimized (including • Can deliver 20 W with universal input voltage range output diode forward drops) to minimize the output

•  132 kHz allow smaller transformer size (EEL25)             voltage error between the 3.3 V and 5 V outputs.

•  Line under-voltage (UV) and power system surge • Feedback is taken from both the 3.3 V and 5 V outputs protection (OV) to the reference (U3) via R9, R10 and R11.  Other output

•  Meets CISPR22 B / EN55022B            voltages are set by the transformer turns ratio.  The low

•  ^{Surge requirements of IEC1000-4-5 (1.2/50} µs) to:          power –5 V output is shunt regulated by R5 and Zener

•  6 kV differential mode / 4 kV common mode    diode D13.  Pre-load resistor R12 is required to maintain

•  Ring wave immunity (IEEE C62.41) to 4 kV regulation of the 30 V output when lightly loaded. • A soft-finish characteristic is provided by C17,

Key Design Points                                                eliminating output overshoot.

•  R1 (2 MΩ, 1/2 W) sets UV at 100 VDC and OV at          •              Second stage LC post-filtering was used on the 3.3 V,

5 V and 18 V high power outputs for low output ripple

450 VDC.  A 1/2 W resistor is required to give a voltage

(L2, L3, L4 and C10, C12, C14).

rating ≥ 350 VDC.

•  Primary clamp components VR1 and D6 limit the

•  OV shut-down protects against long duration line voltage leakage inductance induced peak drain voltage to a safe surges (common in some countries).  During shut-down the value. A lower value of VR1 (170 V) improves cross

±15%                                                                                                                                                                                                                        PI-2642-061300

Figure 1. TOPSwitch-FX 35 W Set-top box supply schematic