I am currently designing a compact uninterrupted power supply system for surveillance cameras. It is a standalone UPS intended for use with a CCTV camera to supply long amounts of backup time in case of an electrical outage. The CCTV (closed circuit television) camera requires a constant and clean supply of power to function effectively. Without a UPS (uninterrupted power supply) employed, a main failure or power outage would result in a loss of power supply to the camera causing it to stop working. Therefore it is of utmost importance to have a reliable backup power supply in place for ensuring the camera remains active 24×7.
You might think that there is enough detail in this post to represent everything that you need to know about the construction of a CCTV camera UPS. But there is not – this post is about Power Multiplexers!
The experiment is still in progress, so you can see it later as a fully-fledged construction project here. This is a blueprint of that design.
Power Multiplexer (Power MUX)
Electronic devices powered by multiple power sources are common. However, choosing the correct input supply to power the device’s electronics is not a trivial task, as an improper implementation oscillates between supplies, causes power brownouts, or damages the input supplies by allowing reverse current flow. This calls for an appropriate power multiplexer to simplify the task of dynamic power source selection.
Quite often you too must design a power multiplexer to select between two different power sources. For example, a battery-operated portable device that must select between the battery or wall adapter. Another example is a surveillance camera needs to be able to select between the main power source and the auxiliary solar power source.
A crude power-switching mechanism could be easily implemented with a pair of diodes wired together to perform a logic OR function. However, this approach has a severe impact on the system’s efficiency and heat generation.
On the surface, it seems a viable approach to increase the efficiency of the aforesaid “diode OR” idea is to use the body diodes of two p-channel MOSFETs as the diode OR function (see below).
Here, once the body diode is, its associated MOSFET can be turned on to provide a low imped path to effectively short the diode and remove the associated diode voltage drop. This method reduces lost power due to the diode and improves the overall efficiency.
You can go through this article to know more about this concept.
We know that a Power Multiplexer (Power MUX) is a set of electronic switches used to select and transition between two or more input power paths to a single output. In short, a basic power multiplexer switches between two (or more) input supplies to power a single output. Below is the block diagram of a basic power multiplexer.
Integrated Power MUX Solutions
Since most Power MUX solutions are used to manage input power paths, they are a good candidate to integrate additional protection features against overvoltage or overcurrent faults.
To give to an idea, the TPS2113A from Texas Instruments is a fully integrated Power MUX with auto switching feature. Note that the TPS211xA series of power multiplexers enables seamless transition between two power supplies, each operating at 2.8V to 5.5V and delivering up to 2A. The TPS211xA series includes extensive protection circuitry, including user programmable
current limiting, thermal protection, inrush current control, seamless supply transition, cross-conduction blocking, and reverse-conduction blocking.
TPS2113A Datasheet https://www.ti.com/lit/ds/symlink/tps2112a.pdf
You can see the typical application circuit of the TPS2113A Power MUX below (Thanks to TI).
The NCP3901 is another similar (dual input to single output) power source multiplexer, but from ON Semiconductor. It is optimized for multiplexing two different charging inputs to feed a single input battery charger. To address all types of applications, the device can support autonomous and slave modes of operation. Reverse USB on−the−go (USB OTG) is fully supported (https://www.onsemi.com/pdf/datasheet/ncp3901-d.pdf).
Shown below is a photograph of an NCP3901 0.4mm pitch BGA Breakout Board fabricated by Mark Osborne (https://twitter.com/becomingmaker/status/1036079421512278018).
And this is the datasheet of RICHTEK’s 80m 1A Power MUX – RT9705B https://www.richtek.com/assets/product_file/RT9705B/DS9705B-03.pdf
The design and assembly of fine-pitch circuit boards is not for the faint-hearted! Moreover, soldering complex ball grid array (BGA) packages having 0.4mm ball pitch require close and thoughtful attention to a number of crucial things (https://www.protoexpress.com/blog/bga-features-soldering-x-ray- inspection/).
I Love Soldering 💞 but…
So, what I ordered was a bunch of TPS2113A chips as my current project does not require complicated power multiplexers, and the TPS is easier to handle than the NCP. One more thing, TPS2113A is a bit old, and there are a lot of new chips in the TPS211xA family. But the only chip I could buy at the time was the TPS2113A.
Playing with the TPS2113A once it arrived proved to be extremely difficult as the minuscule package itself has tiny legs making it hard to access at ease. One quick solution then is to fabricate a breakout board to break out all of its features to 0.1”-spaced header pins, making it easy to use with standard breadboards (extra vias might also be needed in pads that channels power to increase heat dissipation) . Oh, at this age of poor dexterity it seems like a tough job for me 😥 A prewired power multiplexer module could be a lifesaver this time!
Luckily, I was able to find an online store very close by and they are authorized resellers of Pololu products. So, I bought a couple of Pololu TPS2113A modules from their limited stock in short order.
The Pololu TPS2113A breakout board/ module (https://www.pololu.com/product/2596) allows us to select which of two power sources is connected to a load while blocking reverse current into either of the sources. It has an adjustable current limit that can be set as high as 2A and an adjustable switching threshold. Each input power rail on the multiplexer can be supplied with 2.8V to 5.5V, and the board also breaks out a USB Micro-B connector that can be used to supply one of the rails.
This is the official schematic of the Pololu TPS2113A module (borrowed from their products page)!
I won’t dive into the full details of this module but want to say a few things about it.
According to Pololu documentation, the module by default operates in auto switching mode, which means that it will automatically select the source that has a higher voltage. But the VSNS pin can be used to maintain IN1 as the selected source until it falls below a configurable switching threshold, making IN1 the primary or preferred source.
The module ships with USB power connected to IN1, which enables the common application of having a device that can be powered by USB or an external power supply. This automatically chooses the appropriate source based on what is connected. The USB power can be switched from IN1 to IN2 by cutting a trace and readying an SMT jumper (see succeeding figure).
- IN1 of the module is connected to VBUS by default through a surface-mount jumper
- IN2 is the secondary power switch input
- VSNS of the module is pulled low through an on-board 20KΩ resistor (R4), configuring the auto switching mode
- EN of the module is pulled low through an on-board 100KΩ resistor (R3) so the TPS2113A chip onboard is enabled by default (HIGH = Disable).
- ILIM of the module is wired to GND through a 1KΩ resistor (R1) and this sets the default current limit to 500mA. An external resistor 330Ωcan be added in parallel with it get the maximum current (2A) – Formula: I limit (A) = 500/R1 (Ω)
- STAT is the open-drain output that indicates which input is currently connected to the output. This pin drives low when IN1 is selected and is pulled up to OUT through an on-board 100KΩ resistor (R2) when IN2 is selected.
Although the Pololu documentation serves us well, an in-depth reading of TI’s TPS2113A datasheet is highly recommended!
TheTPS211x EVM User Guide https://www.ti.com/lit/ug/sllu054/sllu054.pdf
At this moment, be mindful, an automatic power MUX does not require an external signal to switch between inputs. There is often a default priority (such as IN1). The device will determine under what conditions to switch to IN2. A manual power MUX is one in which each path is controlled by an external signal (logic or microcontroller). There could be one or two EN pins which need to be controlled.
However, there are some other Power MUX solutions which offer the flexibility to be used in an automatic configuration and be controlled by a manual control signal. The TPS212x MUX solutions can have a default (automatic) priority, but then be overridden by an external microcontroller if needed.
See the datasheet of TI’s Priority Power MUX TPS2121 https://www.ti.com/lit/ds/symlink/tps2121.pdf.
The last step before using any kind of prewired module/breakout board in a project is evaluating its basic functionality.
To aid in the pre-flight test of the Pololu TPS2113A Power Multiplexer module, a quick breadboard setup was created. Next, power sources were connected to the Pololu Power MUX module and a DVM and LED were added. Luckily, the overall performance was much closer to the desired level than expected. The only thing forgotten at that time was to measure the switching speed!
There are multiple ways to design a power multiplexer. However, for those that want to create a design that will not take a lot of time, have limited board space, or just want an easy solution with high power-to-space ratio without sacrificing performance, then a trusty power mux module may be the best and fast solution!
Disclaimer: The author has no commercial motives behind this article!