12V 40A Switching Power Supply

The present trend is to use ever higher frequencies. But by doing so it becomes more difficult to filter out the RF noise inevitably generated by the switching. So I decided to stay at a low switching frequency of only 25 kHz for the full cycle, which due to the frequency doubling effect of the rectifiers results in 50 kHz on the output filter. For the main switching elements, bipolar transistors or MOSFETs can be used. Bipolars have lower conduction losses, while MOSFETs switch faster. As in this design I wanted to keep the RF noise at an absolute minimum, very fast switching was not desired, so I used bipolar transistors. But these tend to become too slow if the driving is heavier than necessary.
12V 40A Switching Power Supply - schematic

The half bridge converter is best controlled by pulse width modulation. There are several ICs available for this exact purpose. I chose the 3524, which is very simple to use and easy to find. Any 3524 will do the job. It can be an LM3524, SG3524, etc. Line voltage enters through a CEE-22 connector with included fuse and EMI filtering (P1). It is then passed through a 2-pole power switch, and an additional common mode noise filter (C1, L1, C2). Two NTC resistors limit the inrush current. A bridge rectifier delivers the power to a big electrolytic capacitor (C3), which works at the 300VDC level. The power oscillator is formed by Q1, Q2, the components near them, and the feedback and control transformer (T3). T2 and the associated components act as a primary current sensor. T1 is the power transformer, delivering about 20 V square wave to the Schottky rectifiers (D6..9). A toroidal inductor (L2) and a six-pack of low equivalent series resistance electrolytic capacitors form the main filter, while L3 and C23..24 are just there for additional ripple reduction. The 13.8V is delivered to the output through a string of ferrite beads with some small decoupling capacitors mounted directly on the output terminals. During operation at medium to high load, the duty cycle is about 70%. That means that at the cathodes of the Schottky rectifiers you will have a square wave that stays at about 20V for some 14├Žs, and then slightly below...

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