Design requirements for single-chip switching power supply circuits
(1) The feedback circuit of TopSwitch II needs to be equipped with a optocoupler and isolated from the output circuit. When designing a precision switching power supply, an adjustable TL431 precision correlation voltage regulator should also be added to form an external error amplifier to replace the voltage regulator in the sampling circuit. The voltage adjustment rate Sv and current adjustment rate Sl of precision switching power supply can both reach about ± 0.2%, which is close to the indicators of linear integrated voltage stabilizing power supply.
(2) Optocouplers with linear changes in current transfer ratio (CTR) should be selected, such as pC817A, NEC2501, 6N137, etc. It is not recommended to use 4N × × type ordinary optocouplers such as 4N25 and 4N35. The poor linearity of the latter can cause distortion when transmitting analog signals, affecting the voltage stability performance of the switching power supply.
(3) The primary of the high-frequency transformer must be equipped with a protective 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, with four specific design schemes: ① a clamp circuit composed of transient voltage suppression diodes (TVS) and ultra fast recovery diodes (SRD); ② A clamp circuit composed of TVS and silicon rectifier (VD); ③ An absorption circuit composed of resistive and capacitive components and SRD; ④ An absorption circuit composed of resistive and capacitive components and VD. Among the above schemes, the effect of ① is the best, which can fully leverage the advantages of TVS's extremely fast response speed and ability to withstand high-energy transient pulses. Plan 2 comes second.
(4) Suitable heat sinks need to be added when using chips. For TO-220 packaging, it can be directly installed on small loose boards. For DIp-8 and SMD-8 packaging, four source electrodes can be soldered onto copper foil on a printed circuit board with an area of 2.3 instead of heat sinks.
(5) To suppress interference introduced from the power grid and prevent interference generated by switching power supplies from being transmitted to the outside, it is necessary to add an electromagnetic interference filter (EMI filter), also known as a power noise filter (pNF), at the incoming end of the power supply.
(6) When using such chips, the source lead should be as short as possible. To ensure stable output voltage during no-load or light load, a resistor of several hundred ohms should be connected to the output terminal of the voltage regulator as the minimum load, or a voltage regulator can be connected in parallel.
