5V Super Capacitor UPS

![IMG_6852 (1).jpg] **Basic situation**
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Using supercapacitor solution, 5V input/output, priority when input is valid, seamless switching, short backup time.

**Design features**
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- **Only for applications with short backup time requirements. ** Equipment that requires temporary power plugging and unplugging for maintenance, and equipment that occasionally restarts due to unstable voltage.
- Using supercapacitors as energy storage for backup power is more maintenance-free than battery solutions.
- With fixed priority, priority is given when input is valid. Using the switch chip solution to achieve seamless switching, avoid current backflow, and low voltage drop.
- Backup power output with hysteresis window. Avoid voltage and current fluctuations generated by the boost chip when the supercapacitor voltage is low.
- No change to the existing external power supply module. Compared with other supercapacitor solutions, no external power supply with higher voltage is required.
- Contains supercapacitor voltage status indicator and power path selection indicator.

**Detailed introduction**
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- **Capacitor charging: CN3125. ** Supercapacitor charging chip with constant current/constant voltage function.
> Charge the supercapacitor at a constant current of 0.495A by adjusting R2 (2.4kΩ).
> Clamp the supercapacitor voltage to a maximum of 4.9405V by adjusting R1 (62kΩ) and R3 (20kΩ).

- **Discharge voltage regulation: AP2007. ** High-efficiency synchronous rectification boost converter.
> Rated output is 5V2A.

- **Hysteresis window protection: CN302. ** Low-power battery voltage detection chip with adjustable hysteresis.
> Control AP2007 to start working when the supercapacitor voltage exceeds 3.394V by adjusting R9 (49.9kΩ), R10 (15kΩ) and R11 (36kΩ), and control AP2007 to stop working when the voltage is lower than 2.396V.

- **Seamless switching with priority: TPS2116. ** Power multiplexer with manual and priority switching functions.
> Through R15 (100kΩ) and R16 (33kΩ), when the input voltage is greater than 4.03V, the input power supply is used first.

**Description of ideas**
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At first, I wanted to use two ss54 or ideal diodes in parallel as the core components of seamless switching. But there is a problem with this design, that is, the external power supply must be stable and higher than the internal boost circuit power supply, but in fact, the external power supply is more likely to have voltage fluctuations, which makes the output easy to rely on the internal boost circuit for a long time, and the capacitor cannot be fully charged, jumping back and forth near the working lower limit voltage of the boost chip.
Later, I tried to use a switching circuit design that cleverly combined pmos and nmos (["zero" voltage drop dual power supply automatic switching circuit](https://zhuanlan.zhihu.com/p/452775267)). During the verification process, it was still found that when the external power supply voltage was unstable, the expected switching effect could not be achieved, and the switching speed was slow (different resistance values ​​needed to be tried to slowly adjust).
As a beginner, I chose to use a professional power switching chip (TPS2116) when designing again, which was verified to meet the requirements.


**Notes⚠**
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- **Resistors drawn in reverse. ** The R10 and R11 resistors need to be swapped. The correct R10 resistance value is 36kΩ and the R11 resistance value is 15kΩ. (Too lazy to change the picture)
- **Wrong resistance value. ** The resistance value of R16 in the picture is 25.5kΩ. This resistance value will cause the priority to be unstable when the input fluctuates around 5v. It has been verified that 33kΩ is effective, and other resistors with a resistance value of about 30kΩ can also be selected. (Too lazy to change the picture)
- **Unmarked supercapacitor. ** Because it needs to be placed horizontally, it is not marked in the picture. The model used for verification is **HCCCap (Hezhong Huineng) 2.7V-10F (LiChuang Mall No.: C397150)**.
- Reserved hole position. C2 and C3 are reserved holes for aluminum electrolytic capacitors, which are used to smooth the instantaneous voltage fluctuations generated when the power is switched. Only one was soldered during the solution verification.
- Shell box. The attachment contains the 3D file of the shell box. Since the printing tolerance is not considered, it is estimated that the edge of the cover needs to be polished before assembly. The PCB board does not have reserved fixing holes, which can be fixed with thermal conductive glue, silicone rubber, etc. or magnets. A groove has been reserved at the bottom of the box for magnets.