A simple switching power supply on a UC3842 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Any designer may face the challenge of creating a simple and reliable power source for the device he is designing. Currently, there are quite simple circuit solutions and their corresponding element base, which allow creating switching power supplies with a minimum number of elements.

Your attention is invited to a description of one of the options for a simple network switching power supply. The power supply is based on the UC3842 chip. This chip has been widely used since the second half of the 90s. It implements many different power sources for TVs, fax machines, VCRs and other equipment. The UC3842 gained such popularity due to its low cost, high reliability, simplicity of circuitry and the minimum required piping.

At the input of the power supply (Fig. 5.34), there is a mains voltage rectifier, including a fuse FU1 for a current of 5 A, a varistor P1 of 275 V to protect the power supply from overvoltage in the network, a capacitor C1, a thermistor R1 of 4,7 Ohm, diode bridge VD1 ... VD4 on FR157 diodes (2 A, 600 V) and filter capacitor C2 (220 uF at 400 V). Thermistor R1 in the cold state has a resistance of 4,7 ohms, and when the power is turned on, the charge current of capacitor C2 is limited by this resistance. Further, the resistor is heated due to the current passing through it, and its resistance drops to tenths of an ohm. However, it practically does not affect the further operation of the circuit.

Resistor R7 provides power to the IC during the startup of the power supply. Winding II of transformer T1, diode VD6, capacitor C8, resistor R6 and diode VD5 form the so-called feedback loop (Loop Feedback), which provides power to the IC in operating mode, and due to which the output voltages are stabilized. Capacitor C7 is the IC power supply filter. Elements R4, C5 constitute a timing chain for the internal IC pulse generator.

Resistive divider R2, R3 sets the voltage generated by the feedback loop at the input of the error amplifier, in other words, determines the stabilization voltage. Elements R5, C6 are necessary for compensation. frequency response of the error amplifier. Resistor R9 is current-limiting, resistor R13 protects the field-effect transistor VT1 in the event of a break in resistor R9. Resistor R11 is a measuring resistor for determining the current through the transistor VT1. Elements R10, C10 form an integrating circuit through which the voltage from the resistor R11, which is the equivalent of the current through the transistor VT1, is fed to the second IC comparator. Elements VD7, R8, C9, VD8, C11 and R12 form the required pulse shape, eliminate parasitic edge generation and protect the transistor from powerful voltage pulses.

The converter transformer is wound on a ferrite core with an ETD39 frame from Siemens + Matsushita. This set features a round center ferrite core and plenty of room for thick wires. The plastic frame has leads for eight windings.

The transformer is assembled using special mounting springs. Particular attention should be paid to the thoroughness of the insulation of each layer of windings with the help of varnished cloth, and several layers of varnished cloth should be laid between the windings I, II and the rest of the windings, ensuring reliable isolation of the output part of the circuit from the mains. The windings should be wound in a "coil-to-coil" manner, without twisting the wires. Naturally, the wires of adjacent turns and loops should not be allowed to overlap. The winding data of the transformer are given in Table. 5.5.

The output part of the power supply is shown in fig. 5.35. It is galvanically isolated from the input part and includes three functionally identical blocks consisting of a rectifier, an LC filter and a linear stabilizer. The first block - a stabilizer for 5 V (5 A) - is made on the IC of the linear stabilizer A2 SD1083 / 84 (DV, LT). This microcircuit has a switching circuit, package and parameters similar to the MC KR142EN12, however, the operating current is 7,5 A for SD1083 and 5 A for SD1084.

The second block - the stabilizer +12/15 V (1 A) - is made on the IC of the linear stabilizer A3 7812 (12 V) or 7815 (15 V). Domestic analogues of these ICs are KR142EN8 with the corresponding letters (B, C), as well as K1157EN12 / 15. The third block - stabilizer -12/15 V (1 A) - is made on the IC of a linear stabilizer. A4 7912 (12V) or 7915 (15V). Domestic analogues of these ICs are K1162EN12D5.

Resistors R14, R17, R18 are needed to dampen excess voltage at idle. Capacitors C12, C20, C25 are selected with a voltage margin due to a possible increase in voltage at idle. It is recommended to use capacitors C17, C18, C23, C28 type K53-1A or K53-4A. All ICs are installed on individual plate radiators with an area of ​​at least 5 cm2.

Structurally, the power supply is made in the form of one single-sided printed circuit board installed in the case from the power supply of a personal computer. The fan and network input connectors are used as intended. The fan is connected to a +12/15V stabilizer, although it is possible to make an additional +12V rectifier or regulator without much filtering.

All radiators are installed vertically, perpendicular to the air flow exiting through the fan. Four wires 30...45 mm long are connected to the outputs of the stabilizers, each set of output wires is crimped with special plastic strap clips into a separate bundle and equipped with a connector of the same type that is used in a personal computer for connecting various peripheral devices. The stabilization parameters are determined by the parameters of the IC stabilizers. Ripple voltages are determined by the parameters of the converter itself and are approximately 0,05% for each stabilizer.

Author: Semyan A.P.

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