The scheme is a classic flyback PSU on the basis of the UC3842 PWM. Since the basic scheme, the output parameters of PD can be easily restated as necessary. As an example for consideration selected BP laptop with power supply 20V 3A. If necessary, you can get several strains, independent or related.
Output power outdoor 60 watt (long-term). Depends mainly on the parameters of the power transformer. When change can be obtained output power up to 100W in this size of the core. Operating frequency of the selected block kg and can be rebuilt the capacitor C1. The supply is rated at the same or a little changing the load, hence the lack of stabilization of the output voltage, although it is stable with fluctuations network 190...240 volts. PSU works without load, there is a custom protection/z. The efficiency of block - 87%. External no control, but you can enter through the optocoupler or relay.
Power transformer (armature core), the output the throttle and choke network borrowed from a computer PSU. Primary winding of the power transformer contains 60 turns, the winding on the power circuit 10 turns. Both windings are wound with round wire 0.5 mm with a single interlayer insulation of PTFE tape. Primary and secondary windings are separated by several layers of insulation. Secondary winding is recalculated at the rate of 1.5 volts per coil. For example, Moldova winding will be 10 turns, 30 V - 20, etc., Because the voltage at one loop is large enough, with a low output voltages 'll need to fine tune the resistor R3 in the range of 15...30 ohms.
Setting
If you need to get a few stress can take advantage of the schemes (1), (2) or (3). The number of turns of considered separately for each winding in (1), (3), and (2) otherwise. Because a 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 - voltage one the crimp. The resistor R7 determines the threshold limit of the output current of the power supply, and the maximum current drain of the power transistor. It is recommended to choose the maximum drain current is not more than 1/3 of the passport on this transistor. Current can be calculated by the formula I(Amps)=1/R7(Ω).
Assembly
Power transistor and rectifier diode in the secondary circuit are mounted on the radiators. Their size is not given, because for each variant (in the case, without housing, high output voltage, low, etc.) the size will be different. Required the area of the radiator can be set experimentally, the temperature radiator during operation. The flanges of the parts should not be heated above 70. The power transistor is mounted through the insulating gasket, diode - without her.
ATTENTION!!!
!!! Observe the specified values voltage capacitors and power resistors, as well as the phasing of the windings of the transformer. At the incorrect phasing of the power supply will start but power will not give up.
!!! Do not touch the drain (flange) of the power transistor at working PSU!!! At the outlet there is an ejection voltage to Wat.
Replacement elements
Instead you can apply 3N80 BUZ90, IRFBC40 and others. Diode D3 is CD, CD, BYV28 for a voltage not less 3Uвых and the corresponding current.
Start
The block starts after 2-3 seconds after feed the mains voltage. To protect against burnout at incorrect elements installation first launch PSU is made through a high resistor 100 Ohm 50W, included before the network rectifier. It is also desirable before the first starting to replace the smoothing capacitor after the bridge on lower capacity (about 10...MF 400V). The unit is then turned on for a few seconds, then turn off heating and evaluate the power of the elements. Next time gradually increased, and in the case of a successful launches unit is turned on directly without resistor with the standard capacitor.
Well, the last
The described BP is assembled in the housing MasterCut BOX G-010. In it keeps 40W load, more power you must take care of the extra cooling. In the case of withdrawal from BP system crashes Q1, R7, 3842, R6, can burn C3 and R5.
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