TPS61299: TPS61299QDRL

I need to correct the supercapacitor value. It is 1.5 F, not 1.5 mF connected in V5V. 

Hi Meiling,

I suppose the TPS61299 hasn't fully start up. Could you help to see the switching waveform and inductor current.

Thanks for response. Extra information:  VR is almost clamped 1V. VGDA is around 3.6-3.7 volt. EN-L is high. 

I have search website  on that a boost converter connected to a rectifier output, specifically while charging a big supercapacitor, can lead to a lower than expected rectifier.  do you think if it is too big supercapacitor connected?  the output voltage (1-2V instead of higher) points towards the interaction between the boost converter's behavior and the supercapacitor's charging characteristics.

Here's a breakdown of the contributing factors:
Boost Converter's Regulation and Voltage Drop:
A boost converter aims to maintain a higher output voltage (in this case, 5V for charging the supercapacitor) from a potentially lower input voltage.
While doing so, it acts as a load on the rectifier output.
The boost converter's internal circuitry and operating principles, involving an inductor storing and releasing energy, require a certain voltage drop across its components to function effectively and regulate the output.
If the rectifier's output voltage is initially higher, the boost converter actively works to pull it down to a level it can efficiently process and regulate up to the desired 5V.
Supercapacitor Charging Characteristics:
Supercapacitors, unlike batteries, have a linear charging voltage characteristic.
This means that as they charge, their voltage gradually increases, with the initial stages of charging drawing more current at lower voltages.
During the initial charge of an empty supercapacitor, it will try to draw as much current as possible, essentially acting like a short circuit to the rectifier output.
Combined Effect: Boost Converter Loading and Supercapacitor's Initial Current Draw
The boost converter, striving to achieve its 5V output, presents a demanding load to the rectifier output, especially during the initial charging of the 1.5F supercapacitor.
The supercapacitor's tendency to draw high initial current at low voltages combines with the boost converter's loading effect.
This combined demand for current can effectively "clamp" or pull down the rectifier's output voltage to a lower value (1-2V).
The rectifier, with its own internal resistance and characteristics, can only supply a finite amount of current.
If the load (boost converter + supercapacitor) tries to draw more current than the rectifier can comfortably provide, the rectifier's output voltage will drop.
This phenomenon is further influenced by the rectifier's internal impedance and the efficiency of the boost converter at different input voltage levels.
In essence, the 5V boost converter's operation, particularly its need to draw current to boost the voltage for the supercapacitor, combined with the supercapacitor's inherent high initial current draw at low voltages, creates a situation where the rectifier output is heavily loaded. This heavy loading, exceeding the rectifier's capabilities, can result in the observed lower output voltage of 1 or 2 volts. 

Do you think this is good explanation? can you suggest a  method to solve the problems? Many thanks!

Hi Meiling,

You are right that the large super cap may lead to this start up stuck. Could you try to start up without load? 

we have bought evaluation board ye, it is ok with small capacitor but we need to have 1.5F capacitor connected. Can you give us suggestion to solve the clamp issue with 1.5F capacitor connected?  

It's a little hard to start up with such large capacitor.

I am thinking to add a voltage supervisor at V5V, waiting for the output of the boost to reach 5v and then I connect with 1.5F supercapacitor. Or I add a timer introducing a time delay for the boost to reach 5v and then V5V connected with 1.5 F capacitor. the main purpose is let boost to start and then connect with the big supercapacitor. do you think if this is ok?

we disconnected with 1.5F from boost when the boost operating as normal. However, when we connected with boost again, the voltage  drops  again. any way you can suggest this mover forward? Many thanks!

Hi Meiling,

I will see it and respond you later.