How 2-pin AC phone charger drain ESD charge to Earth/Ground?

[Wilson__]

Human walking on carpret can generate up to 30 to 50 kV.

Phone USB pins are 2mm from touch of human finger.   

6 kilo voltage can fly across and the phone is electrostaticly charged. 

How the charge discharge back to Earth for double insulation cellular phone chargers that use 2 pin AC plug?

https://www.sciencing.com/calculate-voltage-spark-gaps-8776030/

[Zipdox]

They usually have class Y safety capacitors between the input and output. I assume they absorb a significant amount of the charge. Someone correct me if I'm wrong.

[Ian.M]

I've just checked a typical small SMPS board I had lying about and the Y capacitor (between output 0V and the negative of the primary side DC bus) is 2.2nF.  The HBM equivalent capacitance for ESD testing is 100pF.   Real life capacitance of your body to ground may well be lower - it depends on geometry (i.e. proximity of nearby objects at ground potential), thickness and material of your shoe soles thickness and type of flooring, etc.

Assuming 100pF and direct contact with the plug shell and thus secondary side 0V, the charge will equalise with the Y capacitor, reducing the voltage by a factor of over 20, reducing a 50 kV HBM charge to under 2.5 kV, which for better quality SMPSes is within their insulation withstand ratings.   The charge then slowly leaks away to the primary side via PCB surface leakage etc.

Of course if the charger is a 'CE - China Export' low grade piece of cr@p, it may not even have a proper Y capacitor and there is a high risk of a 50 kV HBM ESD event causing insulation breakdown leaving its output a mains live shock hazard!

[PGPG]

Of course if the charger is a 'CE - China Export' l

It happened me not long ago that I connected one such charger to charge small headlamp and accidentally that headlamp just touched the Protective Earth socket pin. Nothing should happen, I think, but it ended with sparks, stench and smoke and both devices being damaged.

[Ian.M]

You were lucky - the fake 'CE' charger is better dead, and the headlamp is an acceptable casualty vs it killing *YOU*!

DiodeGoneWild frequently does full teardowns of dodgy USB chargers, often including stripping down the switching transformer and commenting on the insulation and creepage distances.   https://www.youtube.com/playlist?list=PLfUGOPyKWwiriWhjmdtvjBw_J7X_X5OnU