Understanding and specifying op-amp and transistor spec to deliver maximum current from input to output

Question

I have the circuit below working in LT Spice and on a breadboard. It appears to be working as expected - but I'm now at my theoretical limits so I need some help developing and testing it from here 1.

My question: When selecting components to develop this circuit - what are the most important op-amp and transistor specs I should be looking at to ensure it can get as much input current to the output as possible? You're welcome to specify some potential components.

I need a single-supply, non-inverting, summing op-amp config that outputs voltage at unity gain - but that is also capable of handling 'as much current as possible' (sorry I can't specify an exact figure here at this point).

It will combine the DC outputs of multiple renewable energy sources e.g. solar panels, wind turbines, etc. I'm currently testing it with two small solar panels that each output a maximum of approx. 12V and 7W but I need to significantly increase that.

It'll eventually run off a 24V supply. - I know the op-amp won't output 24V with that supply but I think that's fine for this application for now.

It will be connected to a MPPT charge controller which will be charging a 12V (or possibly 24V) battery - so it needs to provide enough current from input to output for that to happen 'optimally'.

The schematic below is where I'm at right now 1. It's based on a combination of a single-supply summing config 2 and a buffer that I also see described as a current booster 3. I understand the summing config is not technically 'unity gain' but V In == V Out.

For now - I'd be happy seeing this circuit getting 2x 12V inputs @ 50W ea. from input to output if that's possible?

1: Current Config: Single Supply, Non-Inverting, Unity Gain Summing Amp w/- Current Buffer/Booster

2: Design reference 1: Single Supply, Non-Inverting, Unity Gain Summing Amp

3: Design reference 2: Current Buffer/Booster

Microchip: Non-Inverting Summing Amplifier

Electronics Hub: Non-Inverting Summing Amplifier

Renesas: Boosting Op-Amp Output Current

All About Circuits: How to Buffer an Op-Amp Output for Higher Current

Answer 1

For handling high power, none of your designs are very good.

If you want to handle over 1 A, then with a BJT at Q2 your op-amp will need to supply at least 10 mA, and should be designed to handle 100 mA. Needing to supply 100 mA means you will need to use one of a limited number of high-power op-amp types.

It would be better to change the BJT over to a MOSFET. Then, at least at steady state, the op-amp won't need to supply any current at all. You will have to watch that as you choose higher-power MOSFETs, the gate capacitance tends to increase, and the circuit stability may suffer. You might end up wanting to have an op-amp, buffered by a BJT or a smaller MOSFET, to drive your big MOSFET.

You'll also need to carefully manage the thermal dissipation of the MOSFET as your output power increases. With 24 V input and 5 V output, the MOSFET will be consuming nearly 4x as much power as the load, and heating proportionally. Realistically, to get above a few watts of output power, you will want to change to a switching design rather than your linear design.

Answer 2

I agree that you do not have a summing amplifier, also you do not have a unity gain amplifier.

2. Did you look at the datasheet of the little 2N5550 transistor? Its maximum allowed output current is only 600mA (0.6A) and its maximum allowed heating is only 0.625W. Then your circuit powered from 24V and with a 10V output will overheat the transistor if its output current is over (0.625W)/(24V-10V)= 45mA (0.045A).