Electronics > Power/Renewable Energy/EV's

Workaround for Cheap MPPT Controller?

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NiHaoMike:

--- Quote from: Red Squirrel on September 16, 2016, 02:06:30 am ---I've also never had good luck measuring current with shunts, I'm probably doing something wrong, but it's always super unstable. 

--- End quote ---
You need to use a low offset opamp, preferably one designed for that use. Ordinary opamps have too much offset voltage to get reliable readings at DC. If you're doing high side sensing, you'll also need very good CMRR.

Red Squirrel:

--- Quote from: NiHaoMike on September 16, 2016, 03:59:01 am ---
--- Quote from: Red Squirrel on September 16, 2016, 02:06:30 am ---I've also never had good luck measuring current with shunts, I'm probably doing something wrong, but it's always super unstable. 

--- End quote ---
You need to use a low offset opamp, preferably one designed for that use. Ordinary opamps have too much offset voltage to get reliable readings at DC. If you're doing high side sensing, you'll also need very good CMRR.

--- End quote ---

Yeah, I was just using a fairly general purpose instrumentation op amp. Was trying to low pass filter and all sorts of stuff and no go.   I also read more on it after the fact, and they also make op amps designed for high side current sensing which is probably what I'd use. (they can handle the higher voltages, like say, up 6o 60v).  Problem with low side is messing with the ground path. Even more problematic if you have two shunts in series. (Ex: one measuring current going into the SMPS and one measuring current coming out, or something similar)

David Hess:
The usual problem I see with constant current feedback loops in power supplies is a slow current sensing circuit in series with the error amplifier leading to poor phase margin causing oscillation.  My first bench power supply design was a real dog in constant current mode because of this.

Solutions include using differential video amplifiers for current sensing or a discrete transistor circuit with offset compensation provided by an external slow operational amplifier.  Better yet is to level shift the current reference (not the current sense!) so that the current mode error amplifier can directly see both signals and have it take over from the voltage error amplifier as required.

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