What is actually inside a phone charger

This is written to be used rather than admired. Each section below is a decision about switch-mode power supplies, and each one has a default.

Rectify, chop, transform, smooth

The charger first converts mains alternating current to a high direct voltage, then switches it on and off tens or hundreds of thousands of times per second. That high-frequency chopped waveform drives a small transformer, which steps the voltage down and provides isolation.

The output is rectified again and smoothed, and a feedback loop adjusts the switching to hold the output steady as load changes. The whole design exists to avoid a mains-frequency transformer, which for the same power would weigh a great deal more.

Frequency buys size

Transformer size for a given power falls as switching frequency rises, which is why every modern supply switches fast. Faster switching increases losses in the switching device itself, which sets the practical limit.

Wide-bandgap materials such as gallium nitride switch faster with lower losses, which is what allows the noticeably smaller high-power chargers now sold. The saving is real and comes from physics rather than from marketing.

Isolation is the safety-critical part

A barrier must separate the mains side from the output so that a fault cannot put mains voltage on a cable you are holding. That barrier is provided by the transformer construction, by physical spacing on the circuit board, and by the optical coupling used for feedback. Counterfeit and very cheap chargers cut exactly these costs, with spacing measured in fractions of a millimetre where standards require several.

In practice, teardowns of seized counterfeit supplies have repeatedly found inadequate isolation, and this is the mechanism by which they kill people.

Negotiation makes one charger fit many devices

Modern chargers advertise a set of voltage and current combinations, and the device requests one over a dedicated signalling channel. This is why the same charger delivers five volts to earphones and twenty volts to a laptop. Programmable modes allow fine voltage adjustment so the device can charge its cell directly with less internal conversion, which reduces heat inside the phone.

If negotiation fails, both ends fall back to a low default, which presents as very slow charging.

Efficiency and idle draw

Conversion efficiency is high at moderate load and worse at very light load, which is why a charger is warm when charging and cool when idle. Efficiency standards in several markets set minimum requirements at several load points and cap no-load consumption.

Under load, the warmth you feel is the wasted fraction, and a charger that is hot rather than warm is either heavily loaded or poorly designed. Enclosed spaces such as under a pillow prevent that heat escaping, which is a genuine fire risk rather than a caution notice.

Firmware updates change this behaviour more often than hardware does.

Buying without opening it

Look for certification marks appropriate to your market, a named manufacturer, and a stated output specification rather than a vague wattage claim. A supply that is suspiciously light for its rating is a warning, since transformers and heatsinks have mass. Buying from the device maker or a known accessory brand is the practical protection available to someone who cannot inspect the internals.

In the datasheet, damaged cables and cracked casings are worth replacing immediately rather than taping.

The takeaway

It is a high-frequency converter with a safety barrier inside. The barrier is the part cheap ones omit.

The constraint is almost always physical, and marketing rarely mentions which one.