DC-DC converter with fixed input voltage and "best effort" power output. - Page 1
Forum section: electronics / general

author: Freesurfer
author_rank: Regular Contributor
timestamp: May 07, 2023, 02:01:10 pm
content: I am trying to find a way to make use of the wasted power from my PV array in an off-grid setup.My system consists of 1800W of PV, which has an open voltage of about 120V. When the MPPT controller finds the maximum power point during the batteries' "Bulk charge" phase, the voltage og the PV drops to approximately 92V (depending on the sun). Then as the batteries reach the absorption phase and eventually the float phase where the controller no longer needs maximum power from the PV, the voltage gradually climbs back up to 120V.I was thinking of exploiting this fluctuating PV voltage as a way to connect a dump load. My thought is to have some DC-DC converter which can be set only turn itself on once the input voltage reaches 100V. But it must also have a variable output voltage/power where it will only output as much power as possible without causing the input voltage to drop below 100V. (On the output of the DC-DC device I will have a resistive load such as a hot water tank or space heater.)I wonder, does such a DC-DC device exist? I know that even cheap chinese ebay DC-DC converters have some kind of "MPPT point" potentiometer which can be adjusted to a certain input voltage, and work as a crude way of MPPT (though it is not "true" MPPT).Any suggestions?

author: Faringdon
author_rank: Super Contributor
timestamp: May 07, 2023, 04:12:41 pm
content: Here in LTspice is the sim that shows how to reg an smps to 100v input....whilst also making sure iout stays below 12a, and ensuring vout stays below 80v.(ok i accidentally set the divider for 100v, but you can change that)It shuts off when vin goes less than 100v......well it keeps some residual power flowing when vin <100v, ..just add a comparator to the COMP pin if you want to kill that.

author: Freesurfer
author_rank: Regular Contributor
timestamp: May 08, 2023, 09:56:30 pm
content: Thanks for your response. I guess the idea is to have the converter regulate on the input side rather than the output side, and with the feedback reversed.Still, I wonder if there may be any "off the shelf" DC-DC converters that support operating in this mode? That would be more convenient than having to make one from schematics

author: f4eru
author_rank: Super Contributor
timestamp: May 09, 2023, 05:30:22 am
content: IMHO that is the wrong approach.the input voltage does not give you the information you need.I would rather tie the operation of the resistive load to the battery voltage, for example :- charger voltage plateau : 52V- comparator for load : 50V hysteresis 2VThat will limit your battery SOC a little, but give a good balance on your system, but take care it does not oscillateThere are probably specific mppt that can handle both battery and water heater, search for those, because integrating this function into the MPPT makesalwaysmuch more sense.https://www.google.com/search?q=mppt+charger+water+heater

author: Seekonk
author_rank: Super Contributor
timestamp: May 10, 2023, 12:43:15 pm
content: Monitoring PV array voltage is a great way to divert energy. I do this to heat water by adiabatic charging of a capacitor bank. and pulsing that power into a heating element. Practical point is 5% delta of array voltage. It only draws enough power to bring voltage back to power point. I use a temperature sensor to track the natural power point of the panel. That doesn't have to be done as long as the set voltage is done seasonably. You can do a lite diversion or be as aggressive as you want. The power point control will create a new artificial power point. I have multiple hot water tanks and a priority can be set by setting different voltages. Circuit also includes arc interrupt so heating elements can be daisy chained as in a normal water heater without damaging mechanical thermostats. Absolutely simple and cheap controls to build. I have a 40 gallon tank just for laundry which is my lowest priority. Still enough to do all cycles of my laundry with hot water. Below is the control and a half hour of diversion on a cloudy day. I never waste a watt, diversion fro5 to 1200W. Heating water from batteries is nuts!I turn cheap regular switch mode power supplies into charge controllers with the same method of faking controller into thinking output voltage is too high.

author: Freesurfer
author_rank: Regular Contributor
timestamp: May 10, 2023, 09:12:09 pm
content: Seekonk, your setup looks interesting. What are you using the capacitors for? Are you essentially charging the caps directly from PV, and then PWMing the DC from the caps into a load, adjusting the duty cycle so that the input voltage sits at just the desired voltage?

author: Seekonk
author_rank: Super Contributor
timestamp: May 16, 2023, 07:31:48 pm
content: Basically that is the method. I use a relatively low frequency of 50Hz max on time before it resets. Shortest off time is 0.6ms which provides an arc interrupt. Off times can be relatively long in hundreds of ms. This is a total of five caps is 6,000uF and each cap is rated at 3.2A ripple. This can supply about a half ms of power. It has temperature tracking so it can track the natural power point of the array and allow it to be in parallel with a MPPT controller. Google Performance Analysis of Photovoltaic Water Heating System for basic theory.

author: David Hess
author_rank: Super Contributor
timestamp: May 17, 2023, 06:28:01 am
content: Quote from: Freesurfer on May 08, 2023, 09:56:30 pmThat sums it up. The switching regulator operates as a shunt regulator as seen from the input side, with excess energy dumped into the load. I have seen this done in some circuits but I am not aware of anything you can buy.

author: Seekonk
author_rank: Super Contributor
timestamp: May 19, 2023, 12:43:23 pm
content: It appears that the first Polish two module ACTii could do a fixed voltage as an option. The newer model does not. I think solar people were confused by this option. I don't know of any other of the dozens of manufacturers I looked at that offer this option.