Project update 2 of 3
Understanding the Power Path
by David SA big thank you to all the backers who have shown support for the project so far!
This first update is quite technical, but it covers one of the most important features of USB Insight HUB, a feature that sets it apart from other commercial solutions: the power path circuitry. As part of an open source hardware project, it’s important I disclose the rationale of “why” and “how” a certain solution was implemented and, in this way, improve, find weakness, or find inspiration for better alternatives. Feel free to ask any technical question through the Ask a Question link on our campaign page.
Why the Power Path is Important
Over many years working with embedded systems, I’ve learned to always account for the presence of USB devices: tools, products in production, development cards, etc. This versatility is incredible but not without risks: short circuits, surges, or overloads can damage a computer (I’ve seen some ports die along the way), especially when making connections with equipment with unknown faults or that are undergoing active development.
For these reasons, during the inception of the USB Insight Hub, one of the most important goals was to provide a robust power path within the system. But what exactly does “robust” mean anyway? I gave myself the following requirements based on a set of specific reasons:
| Requirement | Rationale |
|---|---|
| Overvoltage protection for the power inputs and outputs | Protection for the internal hub components and downstream devices. Surprisingly, several off-the-shelf hubs I inspected lacked this protection. Basically they trust the power provided through the port to be in a safe range and just pass the input voltage through to the downstream devices. |
| Reverse input voltage protection. | A reverse voltage in a circuit simply fries all the electronics. A common and effective solution is to place a diode in series. This protection is mostly found in an external power supply input (if present). |
| No output should be able to-feed power back to any input or any other output. | This is important, but often overlooked in simple hubs: imagine you connect a USB device that is self-powered, say by a battery or an external supply, to a downstream port of the USB hub. If the voltage of this device is higher than the voltage of the hub, the current will start to flow back from the device to the hub until the whole USB hub power rail is powered by the downstream device, thus back-feeding all the other devices connected to downstream ports and even the host (upstream). This can cause anything from minor inconveniences (the hub stays powered on when upstream power is removed) to the destruction of the host computer due to reverse current. |
| Provide the option to power the hub using host power (i.e., a computer) or from an external source via a USB-C connector. | This is a common and necessary feature desirable in a USB hub. USB downstream ports have a limit to the current they can supply: 500mA for USB 2.0 and 900mA for USB 3.0 (by spec, though in reality many hosts can supply more current). If you need to source more current, an external power supply is required. In most cases the connector used is a barrel jack (in various sizes) or more exotic methods like screw terminals. |
| Handle up to 5 A total current. 2 A for each downstream port. | This limit is a self-imposed value to match with higher wattage commercial power supplies with a USB-C connector that can supply up to 5 V (e.g., Raspberry Pi 27W USB-C). |
| Measure current and voltage of the downstream device. | Initially, this feature was not contemplated. But I noticed how frequently I use a USB power meter (great tool by the way) so incorporating one to the Insight Hub added a lot of value. Later, it proved invaluable for improving the over-current and reverse current functionality |
The simplest solution and Its Problems…
Looking at a simplified diagram, the solution seems simple, right?
- D1 and D2 protect from reverse polarity and feed current back to the Host and Auxiliary power, respectively. This type of arrangement is called an OR’ed configuration, power combiner, etc.
- C1 is a bulk capacitor to handle power surges from downstream ports.
- D3, D4 and D5 protect from back current from downstream ports.
- SW1, SW2, SW3 are switches for each port.
- F1, F2, F3 (resettable) protect against shorts circuits
- Rs1, Rs2, Rs3.
There are several drawbacks with this simple solution, though:
- The biggest one is that, from input to output, there are two diodes in series. Even using the best Schottky diodes means that, at 2 A current, there is a 0.5 – 0.6 V drop. As we are working with a 5 V system, this means a 10% voltage drop (with the subsequent power dissipation). And, this is only accounting for the diodes' dropout, but it must be considered that there are also losses from the electronic switches, fuses, current sensing resistor, PCB and cables.
- The USB standard limits the in-rush current in the first connection to an equivalent 4.7 uF capacitor, a value too low to be considered useful as a bulk capacitor. This means that for larger capacitance, an in-rush current control is needed.
- Resettable fuses are fixed to a current limit and can’t be changed once in operation.
- Manual switches remove the option to implement software control of the power outputs.
Upstream Section
Fortunately, ICs exist today that help solve these issues in an efficient way, albeit with the penalty of a higher cost. For the upstream section, the section that implements the power combiner (D1 and D2), I chose TI TPS259470ARPW, an advanced eFuse, as the central piece to solve many problems at once:
- Implements an ideal diode functionality using MOSFETS and, with only 28 m ohm resistance, significantly reduces the voltage drop.
- Provides reverse polarity protection. While using USB Type-C connectors should avoid a mechanically reversed connection, this feature is still welcome.
- Provides configurable over-voltage and under-voltage protection. This not only protects the internal circuitry of the USB Insight Hub, but also the downstream devices.
- Provides a configurable in-rush current control, meaning that a large capacitor can be placed downstream without overloading the upstream (host) device.
- Can work in tandem with other TPS259470 to form a priority power muxer. In the case of USB Insight Hub, if the auxiliary power is present, it replaces the host power input completely. Contrary to an arrangement with diodes (combiner) in which the higher voltage input sources most of the current, the muxer selects one or another, independently of voltage… sorry for the technicality, but a muxer allows more predictable behavior of which input provides power.
- Provides configurable over-current and short circuit protection at the output. Though the downstream ports have their own protections, this feature prevents surpassing the total input current budget and gently shuts down the power.
- Bonus feature: TPS259470 provides Enable control so it's possible to add a global power switch without the need that it is rated at 5 A. This makes the design more compact and sleeker.
Downstream Power Control and Protection
The DIODES AP22653, a current-limited power switch, is used in each downstream port. This IC is specific for this application and provides the following features to USB Insight Hub:
- Allows digital control for power through a digital pin. It has a low on resistance of 65 mΩ.
- Provides feedback to a microcontroller if a fault is present.
- The current limit can be configured by a resistor and has a 5 ms reaction time. USB Insight Hub configures this current limit by switching two resistors through digital pins and can set four values: 0.5 A, 1.0 A, 1.5 A, 2.0 A. A hard short protection is also included in the device.
- The reverse current protection is fixed at 320 mA.
- As you can see, there are limitations in the resolutions for the forward and backward current limits. To improve the precision on these protections, a power meter (PAC1943) is used. It generates an interruption when over- and under-current limits are surpassed with a 1mA resolution and a reaction time of up to 20ms including the round trip through software. The hardware current limit is always set a step higher than the software current limit, e.g., a 750mA current limit sets the hardware at 1000mA, in this way if the overcurrent event is particularly fast the hardware can catch the fault.
Power Path Losses
The overall power path design, though somewhat complicated, provides protection, control, and efficiency. At full load you can expect the voltage drop in any channel is at most 0.385 V, or an equivalent 192 mΩ. If only one channel is draining 2 A, the expected voltage drop is 0.238 V (120 mΩ).
I hope this explanation provides a good understanding of the power path and the thorough process we followed to implement safe and high performance power delivery to the devices you work with.
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