POWER ELECTRONICS Improving USB
Arun Pappan and Martin Tan, staff engineers at FTDI Chip discuss key considerations for implementing a USB type-C pass-through devices with power delivery function
T
he introduction of USB power delivery (PD) has enhanced the scope of the USB inter- face protocol by scaling up its power capa- bilities significantly - with the potential for systems requiring as much as 100W to be supported. Along with the transport of data, much greater power can now be provided to items of electronic/electrical equipment, while only requiring use of a single cable. Consequently, the space utilised and the cost of installation can both be lowered. USB PD means that the power direction is no longer fixed. Equipment can therefore act as either a sink (drawing power from the VBUS) or a source (providing power via the VBUS), depending on the particular situation. De- vices can leverage the PD specifications to implement high power applications, under- taking enumeration of an item of connected hardware. After that has been done, they can deliver the necessary power. A charging pass-through device is an example of this.
There are many potential ways in which a charging pass-through device can be designed. The idea is that the power from the charger is passed through port 2 to port 1 to charge the item of connected hardware (e.g. a laptop). However, this can be achieved in a multitude of different ways and each has its own particular pros and cons.
Attaining vSafe5V
In this section we will look at two different design approaches that can be taken for vSafe5V implementation. In one design the vSafe5V is also passed through from port 2, whereas in the other design vSafe5V is internally generated.
vSafe5V pass-through from port 2 is described in Figure 1. When a higher power profile is requested by the connected hardware, the same profile is negotiated by the port 2. Once port 2 has completed the negotiation process, port 1 will inform the
laptop that the power supply is ready. The main advantage of this approach is that it keeps the design simple, with the need to use an internal 5V generator being negated. However, there are certain disadvantages associated with this route too. Since the vSafe5V is pass-through, the voltage level and current capability will depend directly on port 2 power source. If the input voltage is towards the lower end of the PD specification (4.75V to 5.5V), then it is possible that vSafe5V will fall short of the specification on the port 1 side. This will be due to voltage drops being experienced across the pass-through circuit. Another disadvantage that needs to be noted is that the higher voltage negotiation process has to go through an additional negotiation step on port 2 side. This can be somewhat time consuming, and there is consequently a risk that the PD timing specification might be exceeded.
Figure 1: Schematic describing a vSafe5V charging pass-through implementation via port 2 22 JUNE 2021 | ELECTRONICS TODAY
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