Flyback Transformer Design Tips For Beginners

August 2, 2019 Cadence PCB Solutions

Flyback transformer used in a circuit board

 

Relationships are usually a flowery bed until you get into one of those petty arguments. For me, it is the question of whether to keep the toilet lid open or closed that often gets me into trouble with my other half. Nevertheless, I try to be accommodating as a matter of principle.  

While I can be a bit inconsistent in the small, daily habits that can be frustrating, I ensure my consistency in turning off the transistor of the primary inductor at the right time and with frequency when I’m designing with a flyback transformer. That’s because getting the timing wrong can produce a very different result from what you’ve expected.

What Is A Flyback Transformer

A flyback transformer is made up of a primary and secondary inductive winding on a core material. In some cases, a flyback transformer is also referred to as a coupled inductor. Like regular transformers, the flyback transformer converts primary voltage to secondary voltage through the ratio of the winding and the core material.

The basic principle of a flyback transformer is that the current flowing through the primary inductive coil resulted in the built up of energy stored in the magnetic field on the core. The energy will eventually be converted to voltage and current on the secondary winding, thus powering the load connected to it.

Once designed for CRT TVs, this highly efficient energy storage and voltage converter has made its way to high-demanding aeronautics application. You may have encountered flyback transformers in various products such as a switch-mode power supply and battery chargers. 

Using flyback transformers is a cost-effective option for low to medium power requirements, typically ranging below 150W. Having a flyback transformer also allows AC-DC and DC-DC conversion. It also allows the construction of more than one secondary winding to provide various secondary voltage sources.

How Does A Flyback Transformer Works 

A regular transformer uses inductive windings for stepping down voltages and so does the flyback transformer. But having inductive windings is where the similarity ends. Unlike a regular transformer, which continuously transfers the energy from the primary to secondary windings, the flyback transformer store the energy in its core prior to releasing it.

The process of storing and releasing the energy makes a flyback transformer different and hence the name it’s given. It also allows a flyback transformer to be used in producing high-frequency conversion. 

In a typical flyback transformer, you’ll find a switch connected to the primary winding. The switch is usually in the form of a power transistor. When the switch is turned on, the magnetic field is gradually built-up on the core. A diode is placed on the secondary part of the transformer so that no current goes through during the energy build up stage.

 

Magnetic field represented in a coil on a black background

Energy is stored as magnetic field in the core.

 

When the transistor is turned off, the current is cut off from the primary winding. As such, the voltage polarity changes to the opposite on the secondary winding and current is released on the circuit. 

Flyback Transformer Design and Simulation Tips 

It seems that the flyback transformer is a simple electronics part, but there are certain processes that you’ll want to get right for producing the required voltage output. This means going beyond calculating the ratio between primary and secondary winding. 

You ought to know that the flyback transformer can be used in a continuous or non-continuous mode. In the continuous mode (CCM), the energy is fully transferred to the secondary winding before the transistor is turned back on whereas, in discontinuous mode (DCM), the transistor is turned on before the energy is fully depleted.

As high-voltage flybacks are used frequently for energy-storage, accurate models and simulations for leakage inductance and stray capacitances will be design-saving capacities. Utilizing equivalent models of flyback transformers, simulation results for real converters through both continuous conduction mode (CCM) and discontinuous conduction mode(DCM) are given in the analysis. 

 

Pair of batteries on top of a circuit board

Add a capacitor to smooth out ripples.

 

Regardless of the operation, you’ll want to get a flyback transformer that has a sufficient peak primary current for your application. Also, you’ll want to be wary of ripples generated by the fast switching speed of the transistor. Placing a bypass capacitor after the secondary winding helps to smoothen the ripples generated. 

Some flyback transformers usually consist of an auxiliary winding which provides a low-voltage reference for control circuits. The winding may introduce noise caused by common mode current and affect the secondary circuit. A great way to mitigate the issue is to place a small capacitor between the ground of the auxiliary winding and the ground of the load on the secondary winding.

Many difficulties from flyback transformers arise from parasitics and the complications that are associated therein. Parasitics in transformers can limit a power-supply from being able to provide its intended output voltage, as well as overstress a component leading to system unreliability. Smart models and predictions for timings and current operations will take into account parasitics. 

To ensure you’re making no mistakes in flyback transformer design, you’ll need a simulation-friendly PCB software. The OrCAD PSpice Simulator, which is offered by Cadence, provides the necessary model for simulating transformer and coupled inductance. PSpice will also take into account all the parasitics of active and passive devices involved during the simulation. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

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