Jonathan BruneauLately I've been looking for the right current sensor, or said differently, the best bang for the buck, while staying in budget. Taking into account the other components on the PCB, I figure I have around $1 to budget. I also prefer high side current sensors as they eliminate the possibility of accidentally tying the PVs GND to the output's GND, which would bypass a low-side current sensor.
High-side current sensors come in two flavors: those with an external VDD pin, and those who use the sensor input (common-mode voltage) to power the IC. The latter tend to be less costly but require a minimum common-mode voltage in order to properly operate. This means that said current sensor could not be tied to the output of the buck regulator since at lower voltages the sensor would not work (imagine charging a supercap at 0V). These sensors would need to be tied to the solar input. The two common-mode current sensors I'm currently eyeing are the TSM9938 and the TS1103.
On the pricier end, the MAX4428 is powered using a dedicated VDD pin. This sensor gives much better results and could be placed on the output of the regulator, but again, it cost more.
And so I'm in a pickle. I could either place a single high-side current sensor on the input or at the output, but not both, as I go over budget.
Having the current sensor on the input has certain advantages: I can easily track the actual MPP as I would have both the input voltage and current for the PV proper. However, this configuration prevents me from measuring the current on the output. I could play a game where I calculate the output current in function of the output voltage and assume that the power supply is 100% efficient (to be conservative), but doing so is unreliable (actual output current reading is still unknown) and the over current protection may be too slow to respond (could only respond as fast as I could run the control loop).
Conversely, having the current sensor on the buck regulator's output gives me an accurate over current protection but an inaccurate actual PV current. I could get away with estimating the PV's current assuming 100% efficiency, but this is not ideal for the same reasons listen above.
Maybe I'm causing myself entirely too many headaches. Maybe the real answer is placing a low-side current sensor (cheap) and somehow emphasize that the PV's GND should not be tied to the output's GND.
As usual, I welcome suggestions.
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simple current sense solution
https://hackaday.io/project/1936-1charger
And note you could easily have current sense on the input /output ..with the cost of just a 0.01-0.02 ohm resistor....
But I still think the easiest/cheapest way to Mppt is by hacking existing DC/DC. By controlling both voltage/current with the same fedback pin (diode oring). The current (Mppt ) could be controlled with a SPI - resistor ( thus leaving the processor to almost no work )
BUT why bother...if you have the option to select a solarpanel
Estimate min. Vin needed for: step-down = battery float voltage (Vout) + 1 V
Suggested open voltage: solar panels = 1.25xVin (MPPT)
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HI
I'm looking at an MPPT unit for our remote (and, boy, Siberia's remote!) data loggers.
Due to the current logger I designed around the Atmel Xmegas, I'm going the Xmega 32E5-AU. However current/voltage...I've found the Texas Instruments INA3221 - tiny, I2C, 3.3v working, yet measure high-side currents up to 26V.
It's a bit pricey at €5, BUT there's 3 channels on the chip - and it consumes about ½ a milliamp. The Xmega comes out at about €3,20 (oddly, the 16 K flash is cheaper than the 8 K version....I'll probably use P-channel to dispose of the charge pump, although there's some "user-defined logic" - a bag of gates, flip-flops etc. on the E5 version I could turn into a charge pump I guess.
Still, the initial component count looks low, probably whole thing inc. PCB leaa than €20. Sure, could add another solar panel, but even so at the moment our batteries are just connected to the panel via a diode....NOT good!
HTH.
-Andy
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