Requirements for designing single-chip switching power supplies
(1) The feedback circuit of TOpSwitch-II needs to be equipped with a photocoupler to isolate it from the output circuit. When designing a precision switching power supply, a TL431 adjustable precision related voltage regulator should be added to form an external error amplifier to replace the voltage regulator tube in the sampling circuit. The voltage regulation rate Sv and current regulation rate Sl of the precision switching power supply can both reach about ±0.2%, which is close to the indicators of the linear integrated regulated power supply.
(2) Optocouplers whose current transfer ratio (CTR) can change linearly should be selected, such as pC817A, NEC2501, 6N137 and other models. 4N×× type ordinary optocouplers such as 4N25 and 4N35 are not recommended. The latter has poor linearity and will cause distortion when transmitting analog signals, affecting the voltage stabilization performance of the switching power supply.
(3) The primary side of the high-frequency transformer must be equipped with a protection circuit to absorb the peak voltage caused by leakage inductance 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 A clamping circuit composed of a transient voltage suppression diode (TVS) and an ultra-fast recovery diode (SRD); ② A clamping circuit composed of TVS and a silicon rectifier (VD) A clamping circuit composed of RC elements; ③ an absorption circuit composed of RC elements and SRD; ④ an absorption circuit composed of RC elements and VD. Among the above solutions, ① has the best effect and can give full play to the advantages of TVS with extremely fast response speed and ability to withstand high-energy transient pulses. Plan ② is second.
(4) A suitable heat sink is required when using the chip. For TO-220 package, it can be mounted directly on a small loose board. For DIp-8 and SMD-8 packages, the four sources can be soldered to the copper foil of the printed circuit 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 a first-level electromagnetic interference filter (EMIfilter) at the power incoming end, also known as a power supply noise filter (pNF).
(6) When using this type of chip, the source lead should be as short as possible. In order to stabilize the output voltage at no load or light load, a resistor of several hundred ohms should be connected to the output end of the regulated power supply as the minimum load, or a voltage regulator tube can be connected in parallel.
