dc/dc converterelectronic-loadmosfet
Good evening!
I want to create an active load system where I can vary the PWM signal from the microcontroller to the control input of the MOSFET which is attached to a fixed resistive load.
I want to change the effective load value by changing the PWM. How can this be done? Is it wise choice to do something like this?
Purpose : I made a DC-DC converter of output wattage of 4W and voltage 3.3V, I want to dynamically change the load so see my DC-DC converter's performance.
A single transistor will not provide an interface with your FET AND do it without inversion. You can do the job with two transistors, though.
simulate this circuit – Schematic created using CircuitLab
This will provide a decent drive for your FET with about a 1 usec added to on-time. That is, if you put in a 10 usec pulse, the FET will be on for about 11 usec.
Also note that R5 needs to be a 1-watt resistor.
EDIT -
It turns out I was using the wrong FET model, and my value for gate capacitance was too low. For a "real" IRFZ44, the added pulse width is about 3 usec, not 1. The problem is that, when you go to turn off the FET, The charge stored in the gate can only discharge through R5, and the time constant is in the 3 usec range. You can, of course, decrease R5, but then the power it dissipates when the load is driven on goes up. You can get a 2 usec delay rather than 3 by decreasing R5 to 100 ohms, but then you need a 2-watt resistor. If you need very narrow PWM cycles (low effective motor current) you're probably better off with a more sophisticated driver. I personally tend to go with the Maxim MAX4426/4427 (less than $4 each, with 2 channels per IC), but that's just habit.
Lead Acid cells have a very low internal resistance. So a LA Battery discharged to 9 V will behave as a 9 V voltage source with a low value resistor in series. So that is why you measure 9V at the output when you connect the discharged battery.
What happens in a "proper" LA battery charger is that it enters current limiting mode which means that is will charge the battery with a predetermined current for example 1 A untill the battery is sufficiently charged. Then the voltage limiting charging will occur.
I see no current limiting measures in your schematic. This is BAD I would never design a battery charger without current limiting. Especially for a switching converter where the inductor can saturate and currents can become exceptionally large.
It can be that the current limiting is in the 18 V supply. Possible but not so elegant. There are plenty of designs that integrate voltage and current limiting in one solution.
Oh that IR2111 is only a driver ! Geez, I expected it to be a proper SMPS chip. So you're just PWM controlling the MOSFET from a microcontroller ? Please consider implementing a proper switched more converter with voltage feedback and all. This is not a design that I would build.
Some other remarks:
Usually temperature controlled charging is not needed unless the LA battery is in an environment which can be very hot or cold. For normal room temperature a maximum charging voltage of 13.6 - 13.8 V is usually all you need.
Discharging a 12 V LA battery to 9 V is not good for the battery, the lower the voltage the more chance you have to permanently damage the battery (sulfite on the plates). I would not go below 11 V to keep the battery in good shape.
Best Answer
Depending on what aspect(s) of performance you're looking to evaluate, I would suggest using low inductance fixed power resistors of various values to evaluate the step response. One resistor for the static load (if any) and a second for the dynamic load.
PWM is going to complicate things to the extent that you may not be able to depend on the results.
I like to use commercial 'electronic loads' that can simulate an constant (programmable) resistance, current or constant power load, and that can programmed to cycle the load so you can look at the response on an oscilloscope, but of course that's a bit more complex.
If constant current is all you need it would be pretty simple to use a DAC and a voltage-to-current converter, and you could program the static and dynamic loads digitally on your micro. If your DC-DC converter is supposed to be constant output voltage (as opposed to, say, a constant current type for, say, LED driving) then constant current should be pretty close to the effect of constant resistance if the output voltage doesn't change much.