likevvii:
Wow awesome.
I don't really understand the equation dQ = I dt = C dV comes from and how you used it to calculate charge per cycle and voltage ripple. Can you please provide me some direction/terminology to learn more about this?
The purpose of the 100uF electrolytic capacitor is to absorb the spike that is generated when the MOSFET is switched back on?
It comes from circuit theory.
Charge has been given a general symbol of Q or q etc. Eg...the charge of an electron..... has some kind of units....the units happen to be given a name 'Coulomb'. For an electron....its charge is 1.60218e-19 Coulomb.... or 1.60218e-19 C.
Current I is defined as an amount of charge flowing PER some quantity of time. This means current is a time RATE of charge flow. Mathematically... that means something like I = dQ/dt .... so when you re-arrange it... you get dQ = I.dt
Capacitors have a relation between the current I through it, and the rate-of-change of voltage dV/dt across it. The relating factor is 'C' the capacitance. They find the relation to be I = C dV/dt which is C times the rate-of-change of capacitor voltage. Rearrange it and you get Idt = C.dV
You then have two expressions for I.dt seen above. Equating them gives you dQ = C.dV = I.dt
Yes Q is charge, dQ is change in charge, these are pretty much the definitions of current and capacitance
put in the simplest equation form - current is rate of change (ie flow) of charge, capacitance is the charge
change per volt change.
The unit of charge is coulomb, not Coulumb (all units are lower case, like foot and inch and second, with the one
exception of "degrees Celcius" where Celcius is deemed to be the guy's name rather than a unit) Single
letter symbols however have a defined case which matters - the symbol for coulomb is C, not c.
Note that a mAh (milliampere-hour) is just a colloquial way to say 3.6C. You can call a coulomb an As
(ampere-second) if you want, no-one does though.
In analyzing the circuit I stripped out the DC values, leaving the AC component of 3A peak-to-peak
current ripple into the load, and then figured out suitable capacitances given that and the timescale
involved - a back-of-the-envelop calculation.
I put on the fast recovery diode (50ns) and everything was basically fixed. Even with a high input capacitance of the IRL540 (2200pF), the heat produced was about 5x less. As I lowered my input capacitance 1000pF, 500pF, and 250pF, my result was that 500pF worked best for my case. The temperature would never really reach too hot to touch before my solenoid did without any heatsinks! The bottleneck moved on to the solenoid which works best for me since I had orignally planned to put thermal fuses to a relay to my solenoids.
PWM: 100kHz @ 5V logic 24mAh gate current (220ohm resistor)
MOSFET: IRL520 500pF gate capacitance, 0.27ohms at 5V gate
Solenoid: 4.8ohms, about 80grams
This is what worked best for me! Hope this may help someone out.