As USB ports are increasingly being used as a power and data transfer interface on high-voltage commercial and industrial equipment, an alternative approach to protection is needed

By Michael Jackson, senior technical writer, Maxim Integrated and Timothy
Leung, executive business manager, Industrial and Healthcare Business Unit,
Maxim Integrated

The universal serial bus
(USB) was originally designed as a means for transferring data and power to
external peripherals connected to computers. Today, it is also used to transfer
data and supply power to a multitude of low-power portable devices. The USB port
has quickly become a near-ubiquitous feature on most consumer electronic
devices and, more recently, has begun to appear on commercial and industrial
equipment operating in high-voltage (>20 V) environments (Fig. 1). However, the proximity of high
voltages to a low-voltage USB port presents a safety hazard to any device
connected to the port itself and, potentially, for its user. If, due to a fault
or other reason, both voltage domains were to come into contact, the result
could be dangerous

 

Fig. 1: Charging a mobile phone on an airplane.

In this
design solution, we will review the conventional approaches to protecting USB
ports from high voltages and the consequent limitations that these approaches
place on port performance. We then propose an alternative approach that does
not compromise the safety or speed of USB port functionality for either the
users or their connected devices.
 

USB review

USB is a
four-wire interface used to connect an upstream host charger to a downstream
device (Fig. 2). VBUS (+5
VDC) and GND (0 V) are the power pins while D+ and D– are used for either
differential or single-ended data transfer. The data is organized into data
frames or packets. Each frame can contain bits for clock synchronization, data
type identifier, device address, data payload, and an end-of-packet sequence.

 

 

Fig 2 Maxim block diagram USB Pin interface

Fig. 2: Block diagram of a USB pin interface.
 

Data
transfer takes place at speeds of either 1.5 Mbits/s (low speed), 12 Mbits/s
(full speed), or 480 Mbps (high speed) using a range of connectors with
different form factors. Types A, B, mini-B, and micro-B are the most common
connector types.

Approaches to USB protection

Control of
the USB data structure is handled at each end of the cable by a serial
interface engine (SIE). This specialized controller, or subset of a larger
controller, usually includes the USB transceiver hardware that handles the USB
protocol.

When a peripheral
is first connected to the cable, the SIE provides the host with the
configuration information and power requirements. During operation, the SIE
formats all data according to the required transfer speed and provides error
checking and fault handling. The SIE handles bus flow control and enables or
disables the line drivers and receivers as necessary. The host initiates all
transactions, which then follow a sequence of data exchanges between host and
peripheral. This includes provisions for when data is corrupted or other faults
that may occur.

Common IC
solutions for USB data channel protection use process isolation technology to
ensure that no current path can exist between the high-voltage equipment and
the low-voltage device. This involves using an isolation IC at a location
between the controllers (Fig. 3).

Figure 3-Maxim Isolation-Placed-Between-USB-Controllers

Fig. 3: Isolation placed between USB controllers.

While this
ensures that the USB port can continue to operate normally, even when exposed
to voltages up to 5 kV, this approach allows data transfer rates only up to 12
Mbits/s.

Some IC
solutions are limited in that they provide only upstream protection (i.e., the host
side is protected but not the downstream device). Also, some isolation ICs
protect the data transfer pins (D+/D–) but not the power pins (VBUS/GND).
It is possible to construct a smaller, custom USB protection circuit using
discrete components. However, because many components are required, this
solution is very complex while still allowing data transfer at only 12 Mbits/s.

Protection without isolation

An
alternative integrated solution to USB port protection for commercial and
industrial equipment is shown in Fig. 4.

 

 

Figure 4 Maxim MAX22505

Fig. 4: MAX22505 USB protection solution.
 

The MAX22505 solution protects a USB port from faults while
allowing data transfer rates up to 480 Mbits/s. The device ensures that the USB
port is fully protected if exposed to the voltage levels typically encountered
in industrial and commercial environments (±24 VAC or ±40 VDC). In the case of
a wiring fault, which brings the USB port into contact with a high voltage,
this IC responds by effectively “open-circuiting” the connection to the port,
thus protecting both the upstream host and any connected downstream device. No
data transfer can take place until the fault has been resolved.

Another benefit is board-space savings. This solution requires an area
of only 120 mm2, representing a savings between 20% and 50%
(compared to other solutions). In addition, all USB port pins are protected in
the event of a fault (±50 VDC for power and ±40.7 V for data), with no
configuration required.

Conclusion

While
originally a common feature present on low-voltage consumer electronic
products, the USB port is increasingly being used as a power and data transfer
interface on high-voltage commercial and industrial equipment. The potential
hazard that this presents to users of peripherals connected to such ports means
that some form of protection is essential.

Current approaches to USB port
protection have focused on port isolation. However, this comes with the caveat
of slowing USB performance to the extent that it can support data transfer
rates only up to 12 Mbits/s while offering only asymmetric protection. An
alternative approach to USB port protection allows high-speed data transfer
(480 Mbits/s) while also fully protecting both upstream equipment and
downstream connected devices. These benefits make it suitable for use in
equipment such as industrial PCs, PLCs, and for both diagnostic and
general-purpose USB ports on industrial and commercial equipment.

Note: An evaluation kit (MAX22505EVKIT) is available for the MAX22505.

About the authors:

Michael Jackson has over 20 years’
professional experience as an analog IC design engineer and holds the position
of senior technical writer at Maxim Integrated. He has a MSEE from Dublin City
University.

Timothy
Leung is executive business manager in the Industrial and
Healthcare Business Unit of with Maxim Integrated. He holds a B.A in Industrial
Engineering from Cal Poly San Luis Obispo