Tuesday, February 22, 2011

40A - 13.8V Power Supply Switching

Here is a switched mode power supply circuit specifically designed to power a ham station. This switching power supply produces 13.8V regulated on a continuous load current up to 40A.

Very precise connected directly to a 12V battery backup because it has a current-limiting. If the potentiometer is rotated, while in a state regulated, switching power supply can deliver up to 60A on an intermittent basis. No minimum load is required.

With 88% efficiency there is the ripple on the output by about 20mV. The condition of the output voltage levels: normal, too high or too low is marked by a tricolor LED. A cooling fan operates depends on the average current drawn, and It does not produce any sound is detected at RF frequencies higher than the primary switching frequency 50kHz.

Switching Power Supply Schematic

Switching Power Supply Schematic

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 of switching power supply 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.

The control circuit is a 3524 IC (U1), powered from an auxiliary rectifier (D17). The IC contains a voltage reference, oscillator, pulse width modulator, error amplifier, current sense amplifier, flip-flop and driving circuitry. It senses the output voltage and the current level, and through transistors Q3 and Q4 controls the power oscillator. C37, C35 and R23 are used to implement a full PID (proportional-integral-derivative) response in the control loop.

A quadruple operational amplifier (U2) is used for two auxiliary purposes: To control the cooling fan according to the average current level, and to drive the voltage indicating tricolor LED: It will glow green if the voltage is OK, orange if the voltage is too low and red if it is too high.

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