It's important to understand these key points when using power transformers!

Article Overview

Power transformers are typically not isolated devices, but rather integral parts of large systems. Therefore, their selection and design must consider both the AC power supply (input side) and the load (output side) , while also taking into account the dynamic changes transformer capable of stably handling such rapidly changing scenarios is itself an engineering challenge.

The situation becomes more complex when a transformer is integrated into a power supply system containing rectification and capacitor filtering . The AC waveform is no longer an ideal sine wave, but rather a series of sharp current pulses after rectification . This means that traditional steady-state AC parameters can only be used as a reference and cannot be directly used to evaluate system performance.

Key knowledge points of power transformers

To correctly understand and apply power transformers , the following points are particularly important.

1. Rectification Applications: Nonlinear systems lead to more complex selection.

In a DC power supply system, the transformer is typically responsible for stepping down the mains voltage, and then...

• Half-bridge or full-bridge rectifier

• Large-capacity filter capacitors

This forms a low-pass filter to filter out the pulsation of the AC waveform.

However, because the rectified current presents a narrow and sharp pulse , its peak value may be several times higher than the average current, resulting in:

• Transformer Temperature rise prediction is more difficult

• The winding must withstand high peak current.

• Traditional "sine wave based" ratings are not applicable.

Engineering recommendation: Select transformers with greater power margin , and prioritize models with low winding resistance (DCR) to improve the ability to withstand peak currents and reduce the risk of overheating.

2. Dual primary windings: adaptable to different mains voltages.

Most power transformers adopt a dual primary winding design to facilitate adaptation to different regional power grid voltages.

For example, common models in North America have two 120V AC windings :

• In a 120V system , the two windings are used in parallel.

• In a 240 V system , the two windings are used in series.

Both methods can ensure that the winding current is within the rated range, avoiding overheating due to excessive I²R losses .

⚠️Important Technical Note: When connecting windings in parallel, the polarity must be strictly observed (as marked on the schematic diagram). Reversing the connection will cause the windings to short-circuit, leading to the instantaneous damage of the transformer .

3. Size and frequency: Weight is inversely proportional to operating frequency.

The size and weight of a transformer are highly dependent on its operating frequency.

50/60 Hz power grid Transformer → Large size and heavy weight

400 Hz Aviation Systems Transformers → Much Smaller

High-frequency transformers in modern switching power supplies → Significantly lighter and smaller for the same power output

This is because:

The higher the frequency, the smaller the required core cross-sectional area, which can reduce the overall volume.

Therefore, aviation and high-performance electronic systems have generally shifted towards high-frequency design to gain size advantages.

4. Overvoltage issue: Risk of core saturation

The structure of a transformer is essentially an optimized electromechanical design; manufacturers minimize the use of copper and core materials to control cost and size. This means:

If the winding voltage is too high, the magnetic core will quickly enter the saturation region.

Core saturation can cause:

The winding current rose sharply

Copper loss and heat power surge

This could ultimately lead to insulation breakdown or coil burnout.

5. VA and W: Why is VA used for the rated power of a transformer ?

The rated power of a transformer is expressed in volt-amperes (VA) rather than watts (W), primarily because:

For a purely resistive load ⇒ VA ≈ W

For inductive or capacitive loads ⇒ there is a power factor difference

Therefore, W (actual power) will be less than VA (apparent power).

When a transformer is used in a rectifier circuit, the situation becomes further complicated because the current is pulsed, and the analysis must consider the peak current rather than the RMS sine wave.

Engineering rule of thumb: In all applications involving rectification, electrolytic capacitors, or fast dynamic loads, choosing a higher VA rating is generally a wise choice.

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