TPS61200 supplies only half the input current

It is not completely clear what the current output of your solar panel is, as well as what you measured output current is.  You need to look at these signals (Vin, Iin, Vout, Iout) with a scope to see what is going on.  But I would direct you to figure 1 in the D/S which shows maximum output current versus input voltage.  For a boost converter, the output current is always less than the input current.

A second consideration is your solar panel's voltage.  If its MPP is at a voltage of around 4.85V, then its open circuit voltage will be higher.  I think you may be too close to the maximum input voltage of the TPS61200.  You should not exceed its rating.

Hi Chris

Thanks for your inputs. Well the solar panel's no-load (short-circuit) output is 4.5-4.8V, 600-900mA. The higher values during peak light availability. The actual panel rating is 9V, 500ma but i paralleled the cells internally hence this figure.

Yes in theory, the boost converter's output power + losses = input power. But this is a significant power loss considering the nearly same input vs output voltage range. (5v output vs 4.5v input). 

As per the datasheet, at 4.6V the output current is 1600mA and then there is no record beyond this voltage for the TPS61200 . All i want is at least 500mA output at 5V.

Would it help to clamp the input voltage at 4.6V with a zener ? What about having a higher value inductor - say 4.7uH ?

Thanks and regards

Soumitra

I assume your OCV is around 4.8V and short circuit current around 600 mA, then.  What is the voltage and current at its maximum power point?  This calculation will need to be done to be sure you can get 5V at 1/2A.

You don't need a zener or larger L.  You may need more Cin.  You need to look at your circuit with a scope to see what is going on.

I would suggest testing the EVM with a lab supply as the input source first, instead of the solar panel.

Hi Chris

Well i did have a 1000uF capacitor in the output of the panel (% input of the EVM) but are you referring to a larger ceramic/X7R capacitor ? Currently i have a 10uF ceramic at the input.

I will try out with a power supply first as suggested but are there any specific pointers that I should look for with a scope ?

Regards

Soumitra 

1000uF should be fine.  Its purpose is just to stabilize the input source.

You should look at Vin and Vout first for signs of oscillations.

You should definitely test with a lab supply first to make it simpler to debug.

Hi Chris

Well, i did what you said even though i felt a little silly checking for oscillations on a dc input/output but there were none to be seen. 

The power supply is a stable 900mA at 5V added as the EVM input. But I noticed a couple of things on power up.

a) The EVM output short-circuit current starts at around 750mA and then ramps upto a little more than 1.0 Amps and then falls back to around 250-300mA.

b) I added an extra 10uF/40V electrolytic capacitor at the output, which is heating up mildly. 

In general there is a noticeable discrepancy of 150-200mA between input and output currents.

Regards

Soumitra

Ok, i was trying to check for the output waveforms corresponding to the datasheet but i have run into a new problem. The 10uF output capacitor (an extra electrolytic cap) is now heating up quite a lot. Though i removed it, the IC and the inductor are also heating up and the output voltage is only 1.2V ramping upto 1.7V for an input voltage of 5V while the IC and inductor heat up.  

Perhaps I may have damaged the IC somehow or something else is wrong here.

Regards

Soumitra