POWER ELECTRONICS power delivery
Another option that is available will be to generate vSafe5V on board, as described in Figure 2. In this case vSafe5V can be generated while still fully adhering to USB PD specifications. The example shown here uses the laptop’s sink capabilities and identifies the optimal charging profile for the connected device. Then via port 2 that profile is negotiated and the voltage level set before the role being swapped over to port 1. Once the swap over to port 1 has been completed, it can set the higher profile requested by the connected hardware as the charging profile is already available on port 2. This design layout is advantageous,
because the behaviour of the vSafe5V can be predicted. The reason for this is that the vSafe5V is generated internally. Also negotiating the voltage does not have to go through re-negotiation on port 2, which will accelerate the turnaround time experienced. There is a significant drawback to this approach that needs to be recognised though. Designs of this kind cannot be used with certain hosts, namely those, which switch between multiple profiles. Based on various hosts and hubs that the FTDI engineering team tested, implementation differences in power negotiation have been observed from one manufacturer to another.
Other Considerations
In addition to the vSafe5V detailed here, there are other factors that need to be taken into account when designing a charging pass-through device. These include the following:
1. VBUS discharge - As a charging pass- through device will initially act as a sink, it is important that engineers are aware of the need for VBUS discharge. When the device is disconnected as a source, it needs to revert to its initial role as a sink. At the same time, the VBUS must be discharged across the entire pass-through path within the specified time period.
2. Voltage drops - These must be minimised across the entire pass-through path. To ensure that the power path in the PD system runs in an efficient manner, load switches are used that are controlled by the pass-through PD device. Such load switches will consist of pass transistors (usually MOSFETs with on/off control blocks). As the charging current of the PD can be up to 5A, it is important to keep in mind that the drain-source on resistance
(RdsON) in the load switch has to be low - so that the power losses involved are not too great and the voltage at port 1 remains within the PD specification.
3. Inrush current limit - Inrush currents can be generated if the capacitive load is switched onto the power rail when the charger is connected to port 2 and charging device to port 1. The magnitude of the inrush current will depend on the rise time of the voltage ramp up and the load capacitance. A steep voltage ramp will increase the inrush current and cause a transient dip in the VBUS. This may result in the connected hardware resetting itself, which needs to be avoided. As well as impacting on the function of the connected hardware, such situations can also damage or shorten the operational lifespan of the load switch components. Though a large load capacitance will reduce the transient voltage dip, it can increase the inrush current. It is therefore important to have a slew rate control on the load switch - in order to lengthen the rise time of the voltage ramp, as well as optimising load capacitance to limit the inrush current and dip in VBUS. Another benefit of having slew rate control on the load switch is to prevent the possibility of charger shutting down power to the pass- through device as an over-current protection measure (due to the presence of a high surge current), as this will clearly cause disruption to the PD charging.
4. Internal power consumption - A certain amount of power obviously needs to be deducted from the charging profile for internal consumption purposes. This needs to be taken into account when establishing the system’s operational parameters.
The overall design objective of a pass- through capable PD device is to ensure smooth connectivity and power delivery across a wide variety of USB PD hosts or hubs. If engineers ensure that they are fully aware of all the items discussed in this article, then they will be able to achieve this goal. FTDI offers the advanced dual-port IC technology needed to support the development of USB power delivery systems such as those outlined above. The company’s power delivery ICs allow hardware to switch from being a sink to a source without any interruption in data flow being witnessed.
FTDI Chip
www.ftdichip.com
Figure 2: Schematic of an on-board vSafe5V pass-through implementation JUNE 2021 | ELECTRONICS TODAY 23
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