Most modern phone/tablet chargers are switched mode power supplies and can accept a huge frequency range of input power. They do not have any problem at all with 400 Hz and work just fine. However, they are not officially certified for that use, nor are the power outlets.
I've used many chargers on a 400Hz circuit, with no problems.
The chargers may/may not be officially certified. The use at 400Hz will shorten the battery life of the device significantly. With a iPad or other device without replaceable batteries, this is a significant issue.
In the case of Apple products, while they say it will work, and are certified for use on deck, they will not warrantee the use because of the battery issue.
There is a certified, approved, and warranted charger available.
Dutch50 and others, on inquires, tell them Brian sent you!
The GNS USB flight deck portable EFB charger is a simple and effective device born out of necessity for flight crews to charge their iPads in-flight after extended use. It uses the 28VDC bus, not the 115VAC/400 Hertz outlet that can be a potentially noisy EMI transmitter and a fire hazard for consumer grade chargers designed for 50/60Hertz. The internal DC-DC circuit is 85% efficient converting 28VDC to iPad voltage and supplying 10+ watts of power with overload and short circuit current limit protection. Each channel also has dual input fuse to protect the aircraft 28VDC supply bus at all cost providing fail-safe protection from an overload before, during and after the USB power conversion. See our product datasheet for additional specifications and lab reports. (hyperlink to product datasheet). Current models include 3 common aircraft plug configurations.
Products - Global Navigation Sciences