Circuit Design of 30W Set Top Box Power Supply Based on TOP257PN

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Summary:

working principle

The isolated flyback converter shown in Figure 1 is based on the TOP257PN (U2) in the TOPSwitch-HX IC family. Selecting a P-package (8-pin DIP package) eliminates the need for external heat sinks and reduces cost. The converter acts as an external adapter for the set-top box power supply (where subsequent multi-output conversions are performed directly by several DC/DCs internally), which typically requires lower standby and no-load power consumption.

The AC input is rectified by D4–D7, filtered by C6 and C11 and then connected to primary side power components T1 and U2. C5, C12 and common mode choke L2 provide EMI filtering. The thermistor RT1 can limit the inrush current flowing into the input AC circuit.

To optimize efficiency under various loads, U2 needs to work in multiple modes. From no load to full load, these four modes are: multi-cycle modulation, fixed-frequency PWM (30 kHz), multi-frequency PWM, and fixed-frequency PWM (66 kHz). In all modes, the controller maintains a linear relationship between duty cycle and control pin current for seamless switching between modes. Diodes D3, C2, R1, and VR1 form a primary clamp circuit that ensures that the maximum voltage across the U2 drain node is less than 700V.

The voltage generated on the secondary side of T1 is D1 rectified and C3 filtered and the output voltage is 12 V. Connecting the LC post filter (C4, L1) to this output reduces the switching frequency output ripple.

The output voltage is controlled using the TL431 reference voltage (U4). Resistor R4 provides a bias current for U4. The low frequency signal fed back to U4 comes from the R5 and R8 divider circuits. The center point of this circuit is connected to the 2.5 VREF pin of U4. Capacitor C8 and resistor R6 reduce the high frequency gain of U4. Resistor R9 sets the loop gain.

design feature

• Low cost, low component count, compact and lightweight power supply
• Extremely energy efficient
• Low load and standby power consumption
• No-load power consumption less than 250 mW at 265 V AC input
• Output power is 30 W when operating at 50 °C, no external heat sink is required
• Integrated safety and reliability:
• Precise automatic recovery and hysteretic shutdown with hysteresis for PCB board
• Temperature is maintained within safe limits under all conditions
• Enters the auto-restart protection state when the output short circuit and the feedback loop open loop
• Excellent input voltage regulation and load regulation
• EMI margins in accordance with EN55022 and CISPR-22 Class B

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