Requirements for the design of monolithic switching power supply circuits
(1) TOpSwitch-II feedback circuit needs to be equipped with photocoupler and output circuit isolation. Design of precision switching power supply, should also add a TL431-type adjustable precision correlation regulator, which constitutes an external error amplifier, to replace the sampling circuit in the regulator. Precision switching power supply voltage adjustment rate Sv, current adjustment rate Sl are up to ± 0.2% or so, close to the linear integrated voltage regulator indicators.
(2) should be used to select the current transfer ratio (CTR) can be a linear change in the photocoupler, such as pC817A, NEC2501, 6N137 and other models, do not recommend the use of 4N25, 4N35, such as 4N x × × type of ordinary photocoupler. The latter's linearity is poor, the transmission of analogue signals will cause distortion, affecting the voltage regulation performance of the switching power supply.
(3) The primary high-frequency transformer must be set up to protect the circuit to absorb the leakage inductance caused by the spike voltage, to ensure that the MOSFET is not damaged. This protection circuit should be connected in parallel on the primary, there are four specific design options: ① by the transient voltage suppression diode (TVS) and ultra-fast recovery diode (SRD) clamp circuit; ② by the TVS and silicon rectifier (VD) clamp circuit; ③ by the capacitive resistive components and SRD absorption circuit; ④ by the capacitive resistive components and the VD composition of the absorption circuit. Among the above schemes, ① is the most effective, which can give full play to the advantages of TVS with very fast response speed and high energy transient pulse. Programme ② is the second best.
(4) The use of chips need to add a suitable heat sink. For TO-220 package, can be mounted directly on a small scattering board. For DIp-8 and SMD-8 packages can be 4 source poles welded in the area of 2.3 of the printed circuit board laying copper foil instead of heat sinks.
(5) In order to suppress the interference introduced from the power grid, but also to prevent the interference generated by the switching power supply to the external transmission, the need to add a level of electromagnetic interference filter (EMIfilter) also known as the power supply noise filter (pNF) at the incoming end of the power supply.
(6) When using such chips, the source leads should be as short as possible. In order to make no-load or light load output voltage stability, should be connected to the output of the regulated power supply a few hundred ohms of resistance as a minimum load, can also be connected in parallel with a voltage regulator.
