MPPT for Wind Generators and Solar-Panels with LTC1871 - Page 1
Forum section: electronics / projects

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 29, 2016, 12:34:15 am
content: Hi,My father and I are working on a MPPT for his boat and we hit a little bump:How do we add variable power capability to a LTC1871? The primary requirement here is efficiency - 80W Solar-Panel + 2x 60W Wind Generators against the interior lighting, navigation-equipment and comfort-devices like a refrigerator. The issue with the LTC1871 being asynchronous has been adressed by implementing an ideal diode that should work up to 2Mhz.The first idea (manipulating the Ith-Pin with a DAC) resulted in turning the whole thing into an oscillator and sent us back to the drawing-board.I then came up with the idea of using a very low Rds MOSFET capable of handling over 20A of continuous drain current with Vgs provided by a DAC instead of a sense-resistor but I'm not entirely sure that it would work too well. The tests I've done so far show that the Gate-Voltage difference between Rds being too high (DC/DC-Converter turns off) and too low (Converter is fully on) is rather limited and also moves around with increasing Vds. So I would like to get a second opinion on this.Schematic:Note: C20 and R44 are in there to remove noise picked up by the traces from the DAC to the MOSFET and are placed as closely as possible to the Gate-Pin.

author: moffy
author_rank: Super Contributor
timestamp: March 29, 2016, 01:30:00 am
content: MPPT requires maximising the power output of solar panels dynamically so that the maximum power is always output. To do that you have to actively monitor the current/voltage/temp to track where that point occurs. The MPPT for the solar cells will be very different for the wind generators. I assume you are charging batteries which then feed an AC inverter? Could you provide some more detail?

author: e97
author_rank: Newbie
timestamp: March 29, 2016, 01:33:37 am
content: Not sure if you want to leverage other open source work but it's a common problem other people have tried to solve:http://thesolarpowerexpert.com/introducing-the-v4-charge-controller/http://smartmppt.blogspot.com/http://opensourceecology.org/wiki/Charge_Controllerhttp://freechargecontroller.com/Hope it helps!

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 29, 2016, 01:56:57 am
content: Quote from: moffy on March 29, 2016, 01:30:00 amThe whole thing (actually 2 DC-DC Converter on one board to save cost on cases that cost 20€ each) will be controlled by a Atmega328 that measures Current and voltage and takes action to limit the current of the DC-DC Converter to achieve maximum power output from the generators.In the schematic I've posted, it is supposed to do so by varying the Gate-Voltage of Q6 with a DAC (to have 4096 steps instead of 256). But as I said, I'm not sure that it'll work or is the best solution.We specifically chose to do it this way (µC controlling a normal DC-DC Converter instead of letting the µC do all the work) to have redundancy in case there is a problem with the Software or the controller itself.The output of the MPPT is connected to a collector-rail and regulated to 24V by the DC-DC Converters. From that rail 3 Charge-Controllers, based on a Ti BQ24650, are being fed to charge 3 Lead Acid Batteries (these batteries can also be charged directly by the boat's Diesel-Engine - A Dummy-Load prevents the wind-generators from running without load if the batteries are full or the engine is running or if the voltage at the input gets too high, this can be disabled in case a solar-panel is connected).The voltage on board is 12V and there are 3 nets: Engine, Comfort and Navigation.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 30, 2016, 12:25:15 am
content: I've just changed the LTSpice-Model a bit (Result attached) and it is looking promising although it did that last time too.The constant resistane load at the output is dissipating more or less power depending on the voltage applied to the Gate of Q6.The area where something actually happens is starting at about 7,25V V2 and stops (no more changes because the chip goes into Voltage-Regulation) somewhere between 8.5 and 8.75V. Enough room for a 12bit DAC to have enough steps to implement a not too coarse regulation (output of the DAC has to be buffered of course).

author: moffy
author_rank: Super Contributor
timestamp: March 31, 2016, 12:20:49 am
content: The problem as I see it is that an MPPT controls its input voltage/power to maximise the output power of a solar panel. It cannot as a single controller, control both input power and output voltage because the 2 control loops will fight against one another, oscillate. You have a FB input and I assume that you use it to set the output voltage at the 24v you mentioned previously. I think you have a problem with the system design rather than the regulator.

author: johansen
author_rank: Super Contributor
timestamp: March 31, 2016, 01:30:29 am
content: For wind turbines I recommend you try and use the multiplier from a PFC chip to square the dc voltage available from the turbine. Use the output of the multiplier to set the duty cycle of the pwm converter. most pfc control chips keep the inductor current discontinuous, so the relationship between the current at turn off of the discontinuous triangular waveform, and average current is linear and no additional math is required.Given that the output voltage rises linearly with windspeed, and power available follows the cube of the windspeed, you can achieve nearly optimal match, provided there are no other non linear problems.Due to resistance in the generator and cabling losses, you may find the wind turbine "run away" at high windspeeds. you will have to figure out how to compensate for this. either that or set the system voltage to clamp the turbine at the battery voltage (given that its a boost converter, the turbine can't develop more volts(rpm) than the battery voltage.no uprocessors required....

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 03, 2016, 01:52:22 pm
content: Well, I've built up the DC-DC Converter and... It works!By varying the voltage on the "Measurement"-Transistor (Q6 in the schematic), I can vary the amount of power that is drawn from the Power Supply and then dumped into a big Dummyload-Resistor.Unfortunately, the MOSFET-Ideal Diode doesn't work. It is either not fast enough or there was a mistake in assembling it - One of the transistors suddenly started to glow@ Moffy: The Device has 2 control-loops independent from another: The LTC1871 regulates the maximum output-voltage but turns off the Power-MOSFET Q4 as soon as the voltage across Q6 exceeds 120mV.How soon that is going to happen is determined by the second control loop, which is implemented in software on the AVR µController and tries to maximize Input-Voltage multiplied with Input-Current by adjusting the gate-voltage of Q6.Btw. there is a dummy-load on the board to prevent run away-situations of the wind-generator when the batteries are full.

author: moffy
author_rank: Super Contributor
timestamp: April 04, 2016, 12:03:28 am
content: "The Device has 2 control-loops independent from another: The LTC1871 regulates the maximum output-voltage but turns off the Power-MOSFET Q4 as soon as the voltage across Q6 exceeds 120mV.How soon that is going to happen is determined by the second control loop, which is implemented in software on the AVR µController and tries to maximize Input-Voltage multiplied with Input-Current by adjusting the gate-voltage of Q6.Btw. there is a dummy-load on the board to prevent run away-situations of the wind-generator when the batteries are full."Glad to hear that it is working. So are you saying that the output voltage of the LTC1871 can vary a bit, and that the dummy load absorbs excess energy?

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 04, 2016, 10:18:59 am
content: In essence, yes.From an overall systems-overview, the DC-DC Converter with Power Point Tracking provides a stable voltage (instead of a illumination-dependent / windspeed-dependent voltage) for the Chargers so they can be used with their MPP-SET voltage.If the voltage on the supply-rail drops below a certain voltage, because the Software has reduced the maximum allowed current for the DC-DC Converters, the chargers in turn also reduce their current draw, trying to keep the supply-rail above the threshold.If there's too much energy available, the dummy-load is turned on at the input of the converter and effectively short-circuits the generator.It's a bit complicated and there are probably more elegant solutions, but by doing it this way, we're able to later add batteries and/or generators to the system.One example could be a generator coupled to the propeller-shaft that generates electricity when the boat is in motion or at anchor in a river with a strong current such as the Elbe.When testing the circuit yesterday, I discovered however that it gets HOT (around 110°C @ 70W input-Power - 18V, 3.8A - Load: Variable Resistor). The efficiency is about 80 to 88%, depending on the current flowing through the system.I don't know if the thermal-connections to vias and pads have something to do with that, but I'm pretty sure that the fact that this is a SEPIC-Design contributes heavily to the fact that it gets so ridiculously hot. So I'm thinking of using the boost topology instead. It'll also be cheaper because I don't have to use coupled inductors and there's a lot less board-space required.

author: moffy
author_rank: Super Contributor
timestamp: April 04, 2016, 11:44:17 pm
content: It's a lot more involved than I realised! If you can get away with a boost converter(output voltage always > input voltage) it would make sense, and you could use a synchronous rectifier (FET). But how would you implement your dual loop control?

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 05, 2016, 12:28:38 am
content: Yes, a synchronous Converter might help with the efficiency. The LTC8710 is a chip I've used in a project before and found it to be reliable and easy to implement, as well as easy to solder.It comes with a problem though: It requires 2 sense-resistors, which would have to be implemented by using an N-Channel and a P-Channel MOSFET.I'll have to see how well it can be controlled or if control of both sense-resistors is necessary.An Alternative would be the LTC3786, which only requires one sense-resistor. Unfortunately, this resistor has to be on the high-side of the converter, requiring a P-Channel MOSFET, or a charge-pump plus Op-Amp to create the necessary Gate-Voltage for a N-Channel MOSFET.The big problem with all of this: I have to have the boards ready and working by early to mid May, so with 3 weeks for production and delivery of the boards, there isn't too much time left for testing and developing new ideas^^Edit: Another Alternative would be the LTC3814-5. It is essentially the LTC1871 in terms of features, but includes a driver for a synchronous Output-MOSFET. It is a bit more expensive though^^CORRECTION: The LTC3814-5 seems to be a much better part - It has a Vrng-Pin that can be used to set the maximum Sense-Voltage. I'll have to read the datasheet (I'll do that tomorrow), but I think that chip would fit the requirements quite nicely.

author: station240
author_rank: Unknown
timestamp: April 05, 2016, 03:52:34 am
content: Quote from: SaabFAN on April 04, 2016, 10:18:59 amIt's core saturation, you need more iron core to store the energy. Found this out when I bought a cheap SEPIC converter from China.This is why in some DC-DC converters you see widely spaced turns with half the core exposed.Unfortunately it's unlikely a suitable off the shelf coupled inductor in this config, so an overrated (current) part is more practical.If you decide to redesign it, I'd be interested in seeing the current design.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: April 06, 2016, 12:20:00 pm
content: This is the old design.As you can probably see, on top of the fact that I accidentally switched 2 pins of the coupled inductor, I also managed to connect the coils the wrong way, which means that the current flows in opposite directions inside the 2 coupled coils.This version also includes a digital potentiometer because I had the idea to change the switching frequency depending on the current. I've removed that from the newer version though.The new design is more or less the same, but with a boost-topology. If the efficiency of this design is not high enough, my dad and I will switch to a synchronous design next year. With the 3 weeks it takes for the boards to be delivered, there's just not enough time left to restart basically the entire design-process.

author: uski
author_rank: Frequent Contributor
timestamp: June 01, 2016, 10:05:10 pm
content: Hi,Anything new with this project ?I'm working on a small wind turbine too (but I need a buck converter due to the use of a very-low-Kv brushless motor).So I'm curious about any progressThanks