Transformers are very versatile for electronics applications and can be used in many ways, explains Robin Evans, the Senior Design and Development Engineer at Wound Magnetics Ltd. The possible variations are endless so, other than for the most basic applications, custom design is usually necessary.
In any application there can be conflicting requirements and some compromise may be needed. It is very important that these compromises are fully investigated and understood if a design, viable for manufacture at acceptable cost, is to be achieved.
Defining parameters
An optimum design requires all the important parameters to be defined, and the designers need a full understanding of the application and an awareness of any test voltages. In addition to a complete performance specification, a circuit description and diagram is also very useful. The various trade-offs can then be fully investigated.
Pertinent parameters will depend on the application, but there will always be a need for details of the working environment, the available cooling and whether there is temperature rise limit. Any size and weight constraints will be considered together with preferred mounting and termination methods.
Devices will generally fall into one of four electrical/magnetic categories: power, signal, pulse and inductive.
Power category
Power transformers may be "mains" type, operating from the utility or from aircraft power supplies, for example. Voltages, currents and frequency should be stated together with the type of load (e.g. resistive, inductive, rectifier, rectifier with capacitor input filter, etc). Non linear loads presented by, for example, rectifier circuits can lead to poor power factor and large harmonics in the supply. With increased concern about this, consideration may need to be given to three phase circuits with multi-pulse rectification.
If the transformer is to be only intermittently loaded this should be mentioned because a size reduction may be possible. Other considerations could, for example, be magnetising current, regulation, efficiency and inrush current.
Power transformers also include high frequency SMPS (Switched Mode Power Supply) types and it is then essential that the circuit (e.g. Forward, Push/Pull, Bridge, Flyback) is known. Frequency range, input voltage range and maximum duty cycle are now important and, where appropriate, the minimum loading at which the circuit must stay in continuous mode.
The design must provide good coupling between the windings to control leakage inductance which if excessive can easily degrade the performance of a switching power supply. Careful winding design is also required to limit skin and proximity losses.
Losses result from eddy currents flowing within the conductors and can become significant at high frequency causing an increase in the effective winding resistances. In an inappropriate design the increase can be very considerable causing rapid overheating.
Signal category
Signal transformers may be analogue or digital. Important parameters are the source and load impedances, ratio, voltage or power levels, frequency response and wave shape, while DC bias, distortion, common mode rejection and return loss requirements may need to be included.
Pulse category
For pulse transformers the source and load impedances and the ratio are again required while other essential parameters are pulse height, pulse duration, PRF (pulse repetition frequency) and acceptable values of rise-time, droop, etc.
Inductive category
Inductive devices include smoothing chokes, energy storage inductors and components for EMC (electromagnetic compatibility) control. We will need to determine the required inductance together with any resistance limit and details of the DC current, AC ripple current and ripple frequency. Q and self resonant frequency limits may also be needed. Inductors do not necessarily have just one winding. Simple common mode chokes have two while integrated chokes for SMPS will have two or more. The so called Flyback transformer is actually an inductor with multiple windings.
Note that the parameters mentioned above for the different categories are not exhaustive and there may be some crossover between categories.
Robin Evans is the Senior Design Engineer at Wound Magnetics Ltd.
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