I'm a Computer Engineer OJT and a newbie in making a circuit. My boss ask me to do a charge controller for solar panel and a sealed lead acid battery 12V120AH. I've already made the circuit. It has a 2 MOSFET(IRFZ4N) for solar panel and load it also has a pull down resistor. The circuit is working/charging but the problem is when i plug the solar panel in my circuit the mosfet for solar panel is experiencing a heat right away though it has a heat sink. Will somebody help me please? Thanks a lot.

by the way here's the image of my circuit. *.*

 

I assume that the charging circuit is a voltage regulator that tapers off the current when the battery is fully charged. If this is true, the only time that the series transistors will dissipate significant power is when the current is starting to taper off--when the battery voltage is low, the transistors are fully on so there is minimal power dissipation--when the battery is fully charged, the transistors are almost fully off so the power dissipation is again low--but during the transition from one state to the other, the power is high--the power can be easily calculated by P = EI--the heatsink must have a sufficiently low thermal resistance to keep the transistor case at a safe temperature--add up all the thermal resistances and calculate the junction temperature.

 

Gee, you have a board layout, but do you have a schematic? We usually start with that.

I see you are using an IRFZ44 - but I also see a 5v regulator on there, and it looks like you are driving the gate directly from whatever RC1 is. The problem is you are using a standard-level N-ch MOSFET, but you really need to be using a logic-level N-ch MOSFET.

An IRLZ44 is basically the same as an IRFZ44, but it is rated for operation at logic levels.