guiDE TO SPECIFYING Toroidal Power Transformers

Plitron engineers are experienced and ready to design a custom toroidal transformer for OEM applications.

Following is a brief overview of default specs for custom transformer designs, including options, and general information needed to specify a custom design. (In the absence of other specifications, we will use the default criteria referenced in the Standard Line.)

How to Specify a Custom Toroidal Transformer

Insulation System – Our standard is UL approved Class A (105°C). Options include UL Class B (130°C), F (155°C), and Class H (180°C), or to suit.

Temperature Rise – Typically we design for 60°C rise over ambient of 30°C, but can be application dependant. This yields the size and regulation specs listed in the Standard Line.

Inputs – As these are application specific, Plitron has several traditional input configurations for both 50 Hz and 60 Hz, which are described in our standard line. Optionally, we can design for 10 – 400 Hz using GOSS cores, and will select appropriate materials, as higher F is needed.

Power – No factory limits are implied, but typically we can design and manufacture single-phase toroids up to 25 kVA,  from a switch mode power supply load.

Duty Cycle – Significant reductions in transformer size and weight may be realized in most cases where the transformer is loaded intermittently. Duty cycle is an important design consideration.


Output Voltage Parameters

Outputs (secondary windings) can be specified as AC RMS out, or loaded DC.

Output voltages and currents are assumed to be referenced to the nominal line voltage, unless otherwise specified. Regardless if the output is specified in AC or VDC after rectification, in situations where other components impose limits, three outputs can be specified follows, and all three conditions have to be met:

Maximum V under conditions can be specified at quiescent Power Conditioning load, at high line (typically +10% of nominal) conditions. Example – in an audio amplifier where the load is signal dependant, VDC must not exceed filter cap rating after rectification. Spec can be supplied in a form as ‘not to exceed 80 VDC after FWB when 132 VAC is applied to the 120 V Primary under conditions of no load.’

Minimum V out under conditions of full load, at low line (typically -10% of nominal). Example – To maintain regulator operation at low line under full load. Spec can be supplied in a form as ‘minimum ripple valley of VDC across regulator > 8 VDC when 98 V is applied to the 120 V primary under load of 4 A)

Nominal V under nominal load. Spec can be supplied in a form as simple as 27 V when 120 V is applied.



When output is specified in DC load parameters, PLITRON can engineer the optimum transformer from your specific DC load data. This data should include DC voltage, current, rectifier type (full wave, full wave bridge, etc.) and specification, capacitor type and value, regulator type and specifications, and special load characteristics, including duty cycle.

Please supply a simple schematic, if possible. With this information, we will determine the optimum secondary AC specifications for each output.

Isolation and step up / down transformers are usually specified in terms of AC RMS, even when used to supply equipment with rectifiers. A conversion factor is required between AC and DC voltages and currents.

Where possible try to specify the load signature, as we would need to factor rated power and efficiency into the design calculations. Designs for a linear load would differ from a switch mode power supply load.


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