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Flyback Power Module Circuit Design using ATX Supply

The electronics sectors has been rapidly advancing in manufacturing industry. There are countless electronic products out in the market which are cheap and also good to use. Among them the most commonly and widely used electronic components  product is “Power Supplies”. These Power supplies are extensively used in many electronic devices, machines and instruments. The power supply can supply a limited amount of power to the electrical load as per its requirements.

Flyback Power Module Circuit Design using ATX Supply

The power supplies most commonly used are DC power supplies that range from 0.5V to 100VDC or more depending on the application of DC power supply. There are mainly two types of DC power supplies. The linear power supply and the switch mode power supplier. The linear power supply uses the linear transformer that is physically heavy and have primary and secondary turns to drop the AC voltage to desired level and then the bridge rectifier and linear regulator will convert the AC voltage to DC voltage required by load. On the other hand the switch mode power supplies (SMPS) have smaller size, can supply large current and are light weight and use switching regulator to control the output DC voltage to constant level.

The common types of SMPS are buck, boost, buck-boost, Flyback and forward converters. In this article we shall discuss about the Flyback converter circuit designing using ATX power supply transformer. There are commonly proper design formulas that derive various requirements of a Flyback SMPS design. The duty cycle, the ripple voltage, the inductor current, the peak inductor current, output voltage, input voltage max and min, inductor values and output capacitor calculation. As we know that the ATX power supply is a solid and powerful SMPS supply used in many gaming PCs for powering motherboards. It outputs multiple DC voltage levels like 3.3V, 5V, +12V, -12V and GND. The maximum output current that this ATX SMPS can supply is 8A more current can be withdrawn but will cause performance degradation. We shall use the transformer from this ATX power supply and design our own Flyback Converter.

What is Flyback Converter.?

Unlike other DC-DC converter like buck and boost that uses only single inductor or choke to store and release energy when the switch is closed and open (for boost converter) respectively, the Flyback converter design uses the transformer windings primary and secondary to step up or step down the input voltage and get the desired voltage.

The switch at the primary section of the Flyback when closed, it causes the primary winding to induce current on secondary winding, thus this negative current causes the diode to reverse bias hence causing the output capacitor to supply energy to load. When the switch is open the secondary winding will supply energy / positive current to forward bias the diode and current flows in the secondary of transformer thus recharging the output capacitor and also power up the load.

The Flyback Converter Topology:

The primary of transformer is driven by transistor switch Q1. This transistor is a high power high speed switch transistor whose gate is driven by PWM control. The secondary drives the rectifier diode and output capacitor will supply current to load.

The Flyback Converter Topology

Advantages of Flyback Converter:

The advantage of Flyback converter is the isolation provided by the transformer on the primary and secondary winding will ensure safety. Unlike the bulky and heavy winding transformers of linear power supply, the Flyback converter uses small, lightweight cores that generate output DC voltage while at higher currents. The transformer used in Flyback SMPS can have multiple secondary windings so as to generate more than one DC output voltages like 3.3V, 5V and 12V. Also negative output voltage can be generated. The input voltage range can be wide enough to maintain the output voltage within limits.

Circuit Diagram of Flyback SMPS:

Circuit Diagram of Flyback SMPS:

Software for Simulation:

There are lots of power electronics simulation tools available online and free to download. Some of them are PLECS from Plexim, Simulink of MATLAB and LTSpice from Linear Technologies, Multisim from National Instruments and Lab center electronics PROTEUS. The online calculators are also available to calculate the values of power components used on your Fly-back converter topology.

Design Considerations:

As we mentioned that the switch was driven by PWM control. So the PWM is generated by the current mode control IC name UC3842. This is low cost commonly used PWM integrated circuit that controls the gate PWM signal of power switch and maintains the output voltage at desired level.



The UC3842 is an 8 pin PDIP IC package, has the internal SR latch, TOTEM pole output, current sense comparator and error amplifier. The external resistor RT and CT are used to set the oscillator frequency or switching frequency of Flyback converter. RT is connected between pin 8 and 4 while CT is connected between 4 and GND.


The current mode control uses the feedback external components RF and CF. In this example the feedback capacitor is not connected but the feedback resistor with 150K ohm is connected between pin 1 and 2 of UC3842.


Working of Circuit:

The MOSFET is selected with VDS rating 450V. The primary coil current is set limit to 0.5A, so a low current MOSFET will also be good. The on resistance of mosfet is also not a problem.

The circuit starts with 320V being dropped across the 150k resistor and into the 10µF cap. Once it has charged to 16V the UC3842 fires up, and pulses the primary through the mosfet. This pulse of energy is sent to the output and rises the voltage, and at the same time to the AUX winding. After the UC3842 has fired once, the 10µF cap would take some time to recharge with just the 150k resistor. The UC3842 has an under-voltage lockout function, which turns it off once the voltage sinks below 12V. The voltage must then rise above 16V to turn the UC3842 on again. However the power from the AUX winding puts more power into the 10µF cap, keeping the voltage 16V up, and clamped to 18V by the Zener. When a load is applied to the secondary the aux voltage will rise to dangerous levels, which is the reason for the Zener and limiting resistor.

The TL431 is a programmable reference detector, and can be set to turn on the optocoupler at the desired voltage. Once on, the optocoupler produces a signal at VFB, keeping the voltage regulated. If no VFB is present, the power is limited only by the current sense resistor (2.2 ohms) to 0.5A. You get output voltage between 1 – 20V easily obtainable. This Flyback SMPS design is 15 watt.


The Flyback transformer based SMPS design are robust and can be used in many applications like computers, motherboards, server etc. They provide high isolation and between input and output, high current output and maximum power delivered to load.