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12V / 325V 150W DC/DC converter

    Introduction: The converter is suitable for battery powering of mains appliances with switched supply (SMPS). Because the switching supply is rectifying the input voltage at the very beginning, so there is no need to create AC voltage (it is complicated). Such an appliance is powered simply by DC voltage with amplitude corresponding to the mains voltage.
    Use: The DC/DC converter can be used to power appliances equipped with switched supply, which only is rectifying and smoothing the mains voltage. Not applicable to power inductive and capacitive loads, ie products with conventional transformer, induction motors, etc. It can not be used for switched power suppleis that have conventional auxiliary transformer or that are obtaining aux voltage resistive or capacitive way from the voltage before the rectifier. In a CRT PC monitor and TV a problem with degaussing can occur. For resistive appliances (light bulbs, heating appliances) and universal motors (ie commutator motors) this DC/DC converter can also be used, but it is necessary to adjust the voltage to 230 DC. Most switching power supply operate at 230V DC too, so they can be used along with resistive loads. For loads with higher ripple rectified voltage and without regulation (eg energy-saving lamps - CFLs) should be set slightly less than 325V, so that the voltage corresponds to an effective value of the rippling voltage (lets say 280V). The schematic of the the converter can be used as a basis for switching DC/AC converter (the addition of the inverter bridge).
    Circuit description: The circuit works as a flyback converter. Maximum output power is 150W. Driving circuit is an integrated circuit IO1 - UC3843. The switching element is a MOSFET transistor T1. Supply operates in discontinuous current mode (DCM), which reduces the reverse recovery loss of diodes D4 and D4'. These ultrafast diodes are used to rectify the secondary voltage. The current is sensed using a current transformer Tr2, because of direct sensing would cause excessive loss. The converter works with over 50% duty cycle. It is good for transformer converters with low input voltage (longer pulse of lower current causes less loss of the MOSFET than the shorter pulse of higher current). Because of duty cycle over 50% converter is equipped with a slope compensation with T2 and R1 (as in datasheet of UC3842-5). Spikes are dampened by the primary "lossless" snubber with D2, C7, D3, L1. The advantage over conventional RCD snubber is a much lower power loss (there's no dissipation resistor) and also reduction of dv/dt for MOSFET T1. As T1 you can use any sufficiently fast MOSFET N with UDS = 75V and more, with the conductive resistance below 5mR. Smaller capacity of gate is an advantage. I used IRFB3207Z. T1 is located at a sufficient heat sink. Diode D2 should have a little cooler, possibly with a common heat sink with T1 or be soldered by its pad on the PCB. Other components are without a heat sink. Ferrite core transformer Tr1 is the central column 12x15mm and the cross section is therefore 1.8 cm2. air gap is 0.8mm at sides and 1.6mm in center column, so it is 2.4mm total. First we wind firs half of the secondary winding, that is 40turns of two magnet copper wires of diameter from 0.35 to 0.4 mm (I have it in 2 layers). Then primary winding, that is 6 turns of fifteen wires of diameter 0.5 mm (3 layers, wound using five wires in each). Finally, the second half of the secondary winding as well as the first. Each Windings and winding layers are isolated. Halving the secondary reduces the leakage inductance. L1 is iron dust choke, inductance is not critical. You can also use the finished coil from switching power supplies or active PFC. Diameter of wire can be any of about 0.8 mm or more. Current transformer TR2 is on a small ferrite ring and has a 1 turn in primary and 60 turns secondary. Core dimensions are not critical. If the number of secondary turns was different, you can adjust the value of shunt resistor R2. The working frequency of the inverter is about 40 kHz. Efficiency is more than 90% at full power. Note: For 220V~ mains the amplitude is 310V, for 240V~ mains the amplitude is 340V.

    Warning! The converter must be protected with a suitable fuse. Although the input voltage of the converter is safe, its output voltage is mortally dangerous. The output capacitor can remain charged to dangerous voltages even after turning off. The drive is suitable only for certain appliances. Everything you do on your own risk. I do not take responsibility for any of your harm.

Schematic diagram of 12V / 325V, 150W DC/DC converter.

Testing the completed DC/DC converter using light bulbs.

completed DC/DC converter 12V - 325V. Two crossed transistors are not in use :). The big one is only to hold the heatsink :).

The experimental converter board while testing the new wound transformer (note: some components seen are not connected at all).

Transformer before rewinding

Rewinding the transformer

Ferrite core of Tr1

Rewound Tr1

Added: 28. 3. 2012