Electronic – Why is earth pin absent in phone chargers plugs but present in laptop ac plugs

Although the question has provided limited details, this answer presents a somewhat different hypothesis from the standard assumption that there's an inductive coil hidden in there somewhere.

The charger in question possibly uses a Piezoelectric Transformer instead of the magnetic (inductive) transformers usually seen for isolation.

Does the charger looks somewhat like this?

If yes, the designers have used a Piezo transformer instead of a conventional one. Interestingly, the source of this image is a paper in a Korean academic publication. This makes the hypothesis even more apt.

A piezoelectric transformer designed for Mhz operation, 500 mA secondary current with 5 Volt signals, using Polyvinylidene Fluoride (PVDF) as the piezoelectric medium, could be fabricated as a thick 1210 SMD part. Since the question mentions SMD parts up to 4516 metric i.e. 1806 imperial, one of the largest of those components is probably the piezoelectric transformer providing the isolation as per the question.

Some interesting information gleaned while investigating this mystery charger:

• Piezoelectric transformers deliver 80% to (recent experimental versions) 90% efficiency, impressive in transformer terms
• These transformers can provide galvanic isolation at multi-kV levels - of course, not in a SMD 1210 size, where the contacts would be too close together.
• PVDF exhibits piezoelectricity several times greater than quartz. Hence it is ideal for making Piezo transformers.
• Many LCD display CCFL backlights are made using Piezoelectric transformers instead of the inductive coil ballast used in earlier versions. So it isn't really new technology.
• Equipment used in magnetism-sensitive areas (e.g. MRI labs) are expected to transition to non-magnetic electronics, hence Piezo transformers where a transformer is needed. (n.b. Any current flow, however, will still generate some magnetism courtesy H fields)

Some articles of interest:

• Piezoelectric Transformer With Very High Power Density
• Panasonic's Piezo transformers for LCD backlights - Of course, much bigger than the dimensions usable in USB chargers.
• A comparison of magnetic and Piezo transformers by Texas Instruments

Full disclosure: I have never worked with, or even seen Piezoelectric transformers before today - the above information was a new learning for me, in the process of investigating the mystery charger.

Firstly, note that in some parts of the world, neutral and ground are NOT connected at the house, but back at the substation (transformer) so there may be a few volts on neutral in those systems.

But to the question : Consider if you did just connect chassis to neutral.

Then what happens if part of the house wiring fails open circuit?

1) Live (Hot) fails ... appliance stops working safely.
2) Neutral fails ... appliance stops working - with the chassis live!

This is not good.

Connect the chassis to earth and what happens when a wire fails?
1) Live (Hot) ... appliance stops working safely.
2) Neutral ... appliance stops working safely. If a current path develops between live and earth (perhaps you are trying to fix it) 10 or 20ma will trip a modern breaker disconnecting the supply.
3) Earth ... Nothing happens unless something else goes wrong. The earth failure will be caught at your next scheduled safety test. Right?