Switching power supply based on PWM UC3842 60 watts. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Switching power supply based on PWM UC3842 60 watts. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The circuit is a classic flyback power supply based on PWM UC3842. Since the circuit is basic, the output parameters of the PSU can be easily recalculated to the required ones. As an example, a power supply unit for a laptop with a power supply of 20V 3A was chosen for consideration. If necessary, you can get several voltages, independent or coupled.

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Outdoor power output 60W (continuous). Depends mainly on the parameters of the power transformer. By changing them, you can get an output power of up to 100W in this core size. The operating frequency of the block is 29kHz and can be tuned by capacitor C1. The power supply is designed for an unchanging or little changing load, hence the lack of stabilization of the output voltage, although it is stable with network fluctuations of 190 ... 240 volts. The PSU works without load, there is a configurable short circuit protection. Block efficiency - 87%. There is no external control, but can be entered using an optocoupler or relay.

The power transformer (core frame), output inductor and network inductor are borrowed from a computer PSU. The primary winding of the power transformer contains 60 turns, the winding for powering the microcircuit - 10 turns. Both windings are wound turn to turn with a 0,5 mm wire with a single interlayer insulation of fluoroplastic tape. The primary and secondary windings are separated by several layers of insulation. The secondary winding is recalculated at the rate of 1,5 volts per turn. For example, a 15 volt winding will have 10 turns, a 30 volt winding will have 20, etc. Since the voltage of one turn is quite large, at low output voltages, fine tuning of the resistor R3 within 15 ... 30 kOhm will be required.

Setting

If you need to get several voltages, you can use the schemes (1), (2) or (3). The number of turns is calculated separately for each winding in (1), (3), and (2) otherwise. Since the second winding is a continuation of the first, the number of turns of the second winding is defined as W2=(U2-U1)/1.5, where 1.5 is the voltage of one turn. Resistor R7 determines the threshold for limiting the output current of the PSU, as well as the maximum drain current of the power transistor. It is recommended to choose the maximum drain current no more than 1/3 of the nameplate for this transistor. The current can be calculated using the formula I (Amps) \u1d 7 / RXNUMX (Ohm).

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Assembly

The power transistor and rectifier diode in the secondary circuit are mounted on radiators. Their area is not given, because for each version (with case, without case, high output voltage, low voltage, etc.) the area will be different. The required area of the radiator can be set experimentally, according to the temperature of the radiator during operation. Flanges of parts should not be heated above 70 degrees. The power transistor is installed through an insulating gasket, the diode - without it.

ATTENTION!!!

!!! Observe the specified voltages of capacitors and powers of resistors, as well as the phasing of the transformer windings. If the phasing is incorrect, the power supply will start, but will not give power.

!!! Do not touch the drain (flange) of the power transistor while the PSU is running!!! There is a surge of voltage up to 500 volts on the drain.

Replacing items

Instead of 3N80, BUZ90, IRFBC40 and others can be used. Diode D3 - KD636, KD213, BYV28 for a voltage of at least 3Uout and for the corresponding current.

Release

The unit starts up 2-3 seconds after the mains voltage is applied. To protect against burnout of elements in case of incorrect installation, the first start-up of the power supply unit is carried out through a powerful 100 Ohm 50W resistor connected in front of the mains rectifier. It is also advisable to replace the smoothing capacitor after the bridge with a smaller capacitance (about 10 ... 22 uF 400V) before the first start. The unit is turned on for a few seconds, then turned off and the heating of the power elements is evaluated. Further, the operating time is gradually increased, and in case of successful launches, the unit is switched on directly without a resistor with a standard capacitor.

Well, the last

The described PSU is assembled in the MasterKit BOX G-010 case. It holds a load of 40W, at higher power it is necessary to take care of additional cooling. In the event of a PSU failure, Q1, R7, 3842, R6 crashes, C3 and R5 may burn out.

Publication: radiokot.ru

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