Flyback power supply shown in Figure 1 uses a chip TNY278PN to convert input voltage of 90-265 over ouput voltage in 5V and 2 A. This voltage is applied to the MP-3 audio players, USB-chargers, etc.
To ensure tight tolerances of output parameters has been chosen feedback on optocouplers with a chip voltage reference. The feedback signal on pin EN / UV chip U 1. Integrated circuit varies the number of committed and missed cycles to adjust the output voltage. It also increases the efficiency of the converter as a function of load. Characteristics of efficiency shown in Fig. 2.
Surge protection on output by the function of protection circuit U1. Similarly, are protected from breaking the feedback loop. As soon as the operating system open circuit, the rise of output voltage, which in turn affects the growth of the bias voltage and hence the voltage on C20. In this regard, the rise of current output BP / M, when the current exceeds 6.5 mA, operates “snap” the protection, terminate the MOSFET switching until the voltage on the capacitor C4 drops below 4.8 volts after removing the AC input.
To the internal thermal protection circuit U1 (142 C) added to a separate external thermal protection. She carried the elements Q 1, C 5, RT 1, R 6. Once the temperature and therefore resistance RT1 increased and the gate voltage of Q 1 increases. Q 1 starts to conduct, reducing the potential withdrawal of BP, respectively, U 1 stops. In order to restart the converter, you need to remove the voltage from the input terminals.
Key points of design
- Choosing a diode D6 diodes class Fast and Ultra Fast, we increase efficiency power supply. If not available diode 1N4007GP, you can use the FR107
- To prevent the growth of consumption at idle or false tripping on excess maksialnogo output voltage, VR1 should be set so that it will begin to conduct only when the output voltage is outside acceptable limits.
- Choice of current constraints and the power of U1 allows you to accurately adapt to the heat mode. For example when working in an open form factor chip TNY277PN can be used with a capacity of C4 0.1 uF to 10 uF.