Requirements for the Design of a Monolithic Switching Power Supply Circuit
(1) The feedback circuit of TOpSwitch II requires an optocoupler to be isolated from the output circuit. When designing a precision switching power supply, an adjustable TL431 precision associated voltage regulator should also be added to form an external error amplifier, replacing the voltage regulator in the sampling circuit. The voltage regulation rate Sv and current regulation rate Sl of precision switching power supplies can both reach around ± 0.2%, which is close to the indicators of linear integrated voltage regulators.
(2) Optocouplers with linearly varying current transfer ratio (CTR) should be selected, such as pC817A, NEC2501, 6N137, etc. It is not recommended to use ordinary optocouplers such as 4N25, 4N35, etc. The latter has poor linearity, which can cause distortion when transmitting analog signals and affect the voltage regulation performance of the switching power supply.
(3) The primary of the high-frequency transformer must be equipped 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 clamp circuit composed of transient voltage suppression diode (TVS) and ultrafast recovery diode (SRD); ② Clamp circuit composed of TVS and silicon rectifier (VD); ③ Absorption circuit composed of resistive capacitive elements and SRD; ④ An absorption circuit composed of resistive capacitive components and VD. Among the above schemes, the effect of ① is the best, which can fully utilize the advantages of TVS's extremely fast response speed and ability to withstand high-energy transient pulses. Option 2 comes second.
(4) Suitable heat sinks should be added when using chips. For TO-220 packaging, it can be directly mounted on a small PCB. For DIp-8 and SMD-8 packages, four source electrodes can be soldered onto a printed circuit board with an area of 2.3 copper foil instead of a heat sink.
(5) 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 electromagnetic interference filter (EMIfilter), also known as a power noise filter (pNF), at the input end of the power supply.
(6) When using such chips, the source lead should be as short as possible. To ensure stable output voltage under no-load or light load, a few hundred ohm resistor should be connected to the output terminal of the voltage regulator as the minimum load, or a voltage regulator can be connected in parallel.
