Design requirements of single-chip switching power supply circuit
(1) The feedback circuit of Top Switch-II needs to be equipped with a photoelectric coupler to isolate it from the output circuit. When designing a precision switching power supply, a TL431 adjustable precision correlation regulator should be added to form an external error amplifier to replace the voltage regulator in the sampling circuit. The voltage regulation rate Sv and current regulation rate Sl of precision switching power supply can reach about 0.2%, which is close to the index of linear integrated regulated power supply.
(2) Optocouplers with linearly varying current transfer ratio (CTR) should be selected, such as pC817A, NEC2501, 6N137, etc. It is not recommended to adopt 4N25, 4N35 and other 4N×× ordinary optocouplers. The latter has poor linearity, which will cause distortion when transmitting analog signals and affect the voltage stabilization performance of switching power supply.
(3) The primary of the high-frequency transformer must be provided with a protection circuit to absorb the peak voltage caused by leakage inductance and ensure that the MOSFET is not damaged. This protection circuit should be connected in parallel to the primary, and there are four specific design schemes: ① a clamping circuit consisting of a transient voltage suppression diode (TVS) and an ultrafast recovery diode (SRD); (2) Clamping circuit composed of TVS and silicon rectifier (VD); (3) absorption circuit composed of resistance-capacitance element and SRD; (4) absorption circuit composed of resistance-capacitance component and VD. Among the above schemes, ① is the best, which can give full play to the advantages of TVS, such as extremely fast response speed and high-energy transient pulse. Scheme 2 is the second.
(4) When using the chip, a suitable radiator should be added. For TO-220 package, it can be directly mounted on a small board. For DIp-8 and SMD-8 packages, four sources can be welded on the copper-clad printed board with an area of 2.3 instead of the heat sink.
(5) In order to suppress the interference introduced from the power grid and prevent the interference generated by the switching power supply from being transmitted to the outside, it is necessary to add an EMI filter (also known as the power supply noise filter (pNF)) at the incoming terminal of the power supply.
(6) When using this kind of chip, the source lead should be as short as possible. In order to make the output voltage stable under no-load or light load, a resistor of several hundred ohms should be connected to the output terminal of the regulated power supply as the minimum load, or a voltage stabilizing tube can be connected in parallel.
