Design of switch power supply for set-top box based on OB2354L

Monolithic switching power supply integrated circuits have the characteristics of high integration, high cost performance, simplest peripheral circuits, and optimal performance indicators. They can form high-efficiency isolated switching power supplies without industrial frequency transformers [1]. Compared with traditional linear regulated power supplies, switching power supplies have the advantages of low power consumption, small size, and wide voltage regulation range, and are increasingly widely used in electronic devices.

This paper designs a three-way output low-power switching power supply based on the OB2354L chip of Onbao Electronics Co., Ltd. , which is used in digital TV set-top boxes and other products.

OB2354L is a single- chip switching power supply integrated chip, which integrates PWM controller, power switch MOSFET, high-voltage startup circuit and various protection circuits. It has the characteristics of high integration, few peripheral devices, wide input voltage, perfect EMI characteristics, and low standby power consumption.

Three-way output Set-top box Power supply circuit design

The author has designed a three-way output Set-top box Switching power supply , which adopts a typical flyback topology structure, and the outputs are 3.3V, 6.8V, and 12V respectively. Among them, the 3.3V output requirement is relatively high, so the sampling feedback from this output is formed to form a closed-loop feedback, thereby achieving precise voltage regulation. Figure 1 shows the designed three-way output Set-top box Switching power supply Circuit. In the figure, RT901 is an NTC thermistor, which is used to reduce the surge current when starting up, RV901 is a varistor, which protects the subsequent circuit during thunderstorms, D901-D904 is a bridge rectifier circuit, and the π-type filter circuit composed of C919, L901, and C906 is used as both a rectifier filter and a common-mode interference filter. The diode D905, capacitor C905, and resistor R905 form a clamping protection circuit, which is used to absorb the spike pulse added to the drain when the power MOSFET is turned off, so that the drain voltage is limited within a safe range to protect the switch chip. The 3.3V output in the secondary output adopts feedback control. After rectification and filtering by diodes D909, C908, L904 and C918, an isolated feedback circuit is formed by using a TL431 adjustable precision regulator and a linear optocoupler PC817. Resistor R907 is used to provide bias current for TL431 to ensure that when the current flowing through the PC817 light-emitting diode is almost zero, TL431 still has a minimum operating current of 1mA. Resistor R908 is the control resistor of the DC gain of the feedback loop, which can generally be selected based on empirical values ​​and adjusted according to the actual circuit. The output voltage of the circuit is given by the following formula:

Determine, where Iref is the input current of the feedback terminal of TL431, and its value is very small, so the last term in the formula can be ignored.

Since the motherboard loads of 6.8V and 12V outputs are relatively stable, the 6.8V on the motherboard will also be converted into 5V through a linear regulator. Under the premise of meeting the usage requirements, for cost considerations, no voltage stabilization measures are added to these two outputs.

High frequency transformer design

There are many methods for designing high-frequency transformers. This paper adopts the formula calculation method to design the relevant parameters of the transformer, and then optimizes and improves the parameters according to the actual debugging results to obtain the final result.

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Determine the basic parameters of the switching power supply

Input voltage 90V _ac ~264V _ac ; grid frequency f _L : 50Hz; switching frequency f: 50KHz; maximum duty cycle D _max = 0.45; output: 3.3V/1A; 6.7V/0.5A; 12V/0.1A; total output power P _O : 8W; power efficiency: 75%;

Determine the input capacitance C _IN and the minimum input DC voltage U _Imin

For a wide range of input voltage, the input capacitance is selected to be 2~3μF/W. Here, the output power PO _is 8W, so the input capacitance C919 and C906 are respectively 10μF/400V; U _Imin = 90×1.414-30=100Vdc;

Determine primary waveform parameters

Select core and bobbin parameters

EE25 core can be selected. The effective core cross-sectional area Ae is about 51mm2 ^and the frame width is 15mm.

Determine the primary turns Np and secondary turns

According to the formula, the secondary reflected to primary voltage VRO _is 82V; Bsat is the saturation flux density, which is 0.3T here, and the minimum number of primary turns is NP, _{which is} 62 turns; where Vs1 _is the secondary reference output winding turns, which is 4 turns, V01 _is the output voltage 3.3V, VF1 _is the forward voltage drop of the reference output circuit diode, and Schottky diode SR240 is used, with a voltage drop of 0.6V. The calculated Np is 82 turns, which is greater than the minimum number of turns 62 turns obtained above, and meets the requirements. The number of turns of the other n-th secondary output is determined by the formula, and it can be obtained that the number of turns of the 6.8V output winding is 8 turns, the number of turns of the 12V output winding is 13 turns, and the number of turns of the chip power supply winding is 14 turns.

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Actual determined transformer parameters

To reduce leakage inductance, the transformer adopts sandwich winding method; the wire current density is 4~6A/mm²; according to the actual experimental debugging, the primary inductance of the transformer is finally selected as 2mH, and the wire diameter, number of turns and winding method are shown in Figure 3, where E1 and E2 are shielding layers, which can improve the EMI performance of the power supply.

Power performance test

Voltage Regulation

For different AC voltage inputs and when the power supply is fully loaded, the tested output voltage values ​​are shown in Table 1.

Load Regulation

When using 90V and 220V AC input respectively, for different output loads, the tested output voltage values ​​are shown in Table 2 and Table 3.

Conclusion

This article introduces a three-way output set-top box switching power supply designed with OB2354L , and gives detailed transformer parameters, and finally gives the test results. From the test results, it can be seen that the tested power supply can meet the actual use requirements regardless of voltage regulation or load regulation. The power supply board of this solution has been used in domestic cable TV set-top boxes .

References:

[1] Sha Zhanyou, Wang Yanpeng, Zhou Wanzhen, et al. The latest application technology of single-chip switching power supply [M]. Beijing: Machinery Industry Press, 2002

[2] On-Bright Electronics, OB2354lL Datasheet

[3] Zhang Zhansong, Yuan Caixuansan. Principle and Design of Switching Power Supply [M]. Beijing: Electronic Industry Press, 1999

[4] Maniktala S. Switching Power Supplies A to Z.

[5] Switching Power Supply Design [M]. Translated by Wang Zhiqiang et al. Second edition. Beijing: Electronic Industry Press, 2005