Providing Port Protection in a Plug & Play World
By John Fang - Power Interface & Audio Business Unit Director
We live in a Plug & Play world; one in which electronic devices can be connected and disconnected seemingly at will, without worrying about what will happen when two devices come together. It’s a world that’s enabled through standards, of course; standards that allow manufacturers to create products that will work together when connected. But it’s also a world that could be open to abuse or inadvertent misuse.
Perhaps the most successful standard of all time is the Universal Serial Bus, or USB. What began life as a convenient solution for connecting peripherals and PCs has evolved to support an incredibly wide range of products, many of which are powered through the USB port and very often make no use of the communication capabilities of the USB bus.
When an electrical device, like a PC, is also a power source for peripherals it introduces additional design considerations. Designers need to think about how to deliver that power in a way that is both safe and reliable for the device itself and the peripherals. Because USB ports are now so common it has become popular to design products that make use of the 5V (and up to 5A) each port provides. As a ‘hot swappable’ technology, it isn’t necessary to isolate the USB port’s power before connecting or disconnecting a device, a convenience that has fuelled its popularity.
The fact that there are standards to which devices should conform doesn’t really provide protection against products that simply haven’t been designed well. Anything with the right connector could be plugged into a USB port and the device supplying the power could be susceptible to those poor design choices.
Part of the simplicity of the USB standard is it doesn’t require an exchange between devices before applying power; it is this feature that enables devices such as lights or fans to run from a USB port. The reason for this is simple; if handshaking were imposed before power is applied it would require another power source, one inside the peripheral, in order to support a communications exchange. Potentially, this could be addressed through a contactless communication standard like NFC or RFID, but realistically there is no chance such a radical change will be introduced.
The penalty for this simplicity is the potential for faults. For instance, a device may try to demand too much power, or introduce a short-circuit across the supply pins. It may even route power from an external source back into the USB port, potentially causing the device supplying power to be exposed to a reverse bias. The way manufacturers protect against this kind of fault is through a power switch specifically designed for the job.
The right solution needs to offer a number of features, including over current, short-circuit and over temperature protection with auto-recovery, along with protection against reverse current and/or voltage. Ideally, it would also offer a wide input voltage range and, because this is a power switch, an extremely low on-resistance to minimise power losses. A soft-start function would ensure the output voltage has a reasonable rise time to protect both source and load, while a feature that provides some protection against any current surges will minimise false fault conditions.
The AP22811/AP22804/AP22814 family of power switches from Diodes offers all these features and more. These devices represent the ideal solution for USB port protection, delivering 2.0, 2.5 or 3.0A with an RDS(ON) of just 50mΩ. They offer high efficiency and excellent protection, ensuring a long and happy Plug & Play future!
