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This switching power supply was built because I needed a powerfull adjustable bench power supply. Linear topology would be unusable for this power (2400W = 2.4 kilowatts!), so I chose a switching topology of a two switch forward (half-controlled bridge). In my article about SMPS's it is a topology II.D. The switching supply uses IGBT transistors and is controlled by UC3845 chip. The schematic of my switching power supply you can see below. Mains voltage goes first through an EMI interference filter. It is then rectified using a bridge rectifier and smoothed using capacitor C4. Due to high capacity, there's an inrush limiting circuit with Re1 relay contact and R2 resistor. Relay coil and fan (from an AT / ATX PC power supply) are powered from 12V, which is dropped from 17V aux supply using a resistor R1. Select R1 value so that the voltage at the relay coil and the fan was 12V. The auxiliary power supply circuit uses TNY267. It is similar to a power supply described here. R27 provides undervoltage protection of auxiliary power - it will not turn on at less than 230V DC. UC3845 control circuit has the output frequency of 50kHz and 47% maximum duty cycle. It is powered through the zener diode, which reduces the supply voltage by 5.6 V (ie to 11.4 V) and also shifts the UVLO thresholds of 7.9 V (lower) and 8.5 V (upper) to 13.5 V and 14.1 V. The UC3845 chip then begins to work at 14.1 V and never runs below 13.5 V, which protects the IGBT transistors from desaturation. The original UVLO thresholds of UC3845 are simply too low. The chip controls the MOSFET T2, which drives the gate-drive transformer Tr2. It provides galvanic isolation and floating drive for upper IGBT. Via shaping circuits with T3 and T4 it drives gates of IGBTs T5 and T6. They then switch rectified mains voltage (325V) to power transformer Tr1. Its output is then rectified and finally averaged by inductor L1 and smoothed by capacitor bank C17. Voltage feedback is connected from the output to pin 2 of IO1. The output voltage of the power supply can be set using the potentiometer P1. Galvanic isolation of the feedback is not necessary because the control circuit is connected to the secondary SMPS side and isolated from the mains. Current feedback is fed through the current transformer TR3 into pin 3 of the UC3845 chip. The threshold current of the overcurrent protection can be set by P2 potentiometer.
Transistors T5 and T6, diodes D5, D5', D6, D6', D7, D7', and the bridge should be placed on the heatsink. Diodes D7, capacitor bank C15 and protective RDC snubbers R22 + D8 + C14 should be placed as close as possible to the IGBTs. LED 1 indicates operation of the power supply, LED 2 indicates the current limiting mode (overload / short circuit) or error. It lights up when the supply is not working in voltage mode. In voltage mode, on the pin 1 of IO1 there's 2.5 V, otherwise it is about 6V. The LEDs can be omitted.
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