Electronic – Why is this MOSFET burning

I see a couple of things here; none directly obvious as the culprit.

1. How much current are you drawing from the slot car? It's hard to say whether the MOSFET (and your wiring, for that matter) is going to handle it OK.

2. Does the Arduino output 5V logic levels or 3.3V? Both parts sold by Sparkfun are 5V logic level MOSFETS -- shame on Sparkfun for saying -

   This is a very common MOSFET with very low on-resistance and a control voltage (aka gate voltage) that is compatible with any 3-5V microcontroller or mechanical switch.

   because it's not compatible with circuits that output less than 5V. If you have a system that outputs 3.3V, you need a MOSFET with on-resistance specified at 3.3V.

3. The freewheeling diode between MOSFET drain and the power supply is to protect the MOSFET, not the motor, from inductive voltage "kickback".

4. Keep your wires short! They add unnecessary resistance and inductance.

5. Breadboards are questionable for currents over 100mA; I'd keep the power supply wires off the breadboard and solder directly to the MOSFETs.

6. Yes, a heatsink would be helpful; that may be your problem. If the tab of the FET is too hot to touch, you probably need a heatsink. There are some easy clipon heatsinks at Digikey that are < 50 cents. Here's one: 591202B00000G.

7. I always put a resistor between microcontroller output and the MOSFET gate; it prevents oscillation due to inductance, and it prevents damage from propagating back to the microcontroller. (see my blog entry for more info)

8. I always put a pulldown resistor to keep the MOSFET in a defined state during reset.

9. grounding: The connection between microcontroller ground and MOSFET ground is critical; if you add inductance here, that can cause poor turn-on or turn-off in the MOSFET. Keep this short, and if you can't, then you may want to use a MOSFET driver located right at the MOSFET.

D1 is in the wrong place (on the circuit diagram at least) - it should be on Q4's drain and then to ground/0V - sort this out, let us know what happens and consider using fewer words to get a better response - I read this a while back and lost the will to survive, entered a coma but recovered a few minutes ago!!

Honestly dude - stick to the business and you'll get better answers (and a bit quicker too).

Trying to get an N channel working can be tricky given the supply bootstrapping required. I think you should try something else first (and this will potentially teach you more about this type of circuit). This would be a natural progression in my book....

At the moment, I think your regulation technique works by pulse skipping i.e. when the output voltage is high enough (despite the diode in the wrong place), you kill pulses on their way to Q3 by shorting its gate with Q2. Instead, try setting up the 555 to have a variable duty cycle (instead of fixed) and avoid pulse skipping - variable duty cycle will give better ripple voltage and I think it's more important than trying to replace the P channel with an N channel - there are plenty of buck regulators using a P channel for Q4. OK maybe Linear tech tend to use N channels on a lot of devices (with bootstrapping) but it's no shame using a P channel and building from scratch.

There are of course, plenty of bucks that do use pulse skipping but better performance is from PWM/duty cycle modulation. If you get that going then try using an N channel FET to replace the flyback diode (D1, the one that appears to be in the wrong place on the circuit). That gives you a synchronous buck converter and better efficiency.

If I've misread your diagram, don't be shy, tell me (in few words but with accuracy).