Monolithic Power Circuit Design Requirements
(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 associated voltage regulator should be added to form an external error amplifier to replace the regulator tube in the sampling circuit. The voltage adjustment rate Sv and current adjustment 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 linear change in current transfer ratio (CTR) should be selected, such as pC817A, NEC2501, 6N137 and other models, and 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, which will affect the voltage regulation 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 the leakage inductance and ensure that the MOSFET is not damaged. This protection circuit should be connected in parallel on the primary, and there are 4 specific design schemes:
①The clamping circuit is composed of a transient voltage suppression diode (TVS) and a super fast recovery diode (SRD);
②Clamping circuit composed of TVS and silicon rectifier (VD);
③A snubber circuit composed of resistance-capacitance elements and SRD;
④A snubber circuit composed of RC components and VD.
In the above scheme, the
The effect of ① is the best, and it can give full play to the advantages of TVS with extremely fast response speed and high-energy transient pulse. Option ② is next.
(4) When using the chip, a suitable heat sink is required. For the TO-220 package, it can be directly mounted on a small bulk board. For DIp-8 and SMD-8 packages, 4 sources can be soldered on the printed circuit board copper foil with an area of 2.3 to replace 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-stage electromagnetic interference filter (EMI filter) at the power inlet end, also known as power 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 terminal of the regulated power supply as the minimum load, or a zener tube can be connected in parallel.
