I have found these:
ON Semiconductor MJD44H11T4G.
MMBT3904
they are cheap and most importantly available at JLCPCB assembly service which is critical to me as mentioned. However the transistor is no -A but rather -G. the A is for automotive, does this really matter?
also the MJD44H11T4G has a junction-to-ambient temperature of 71.4 degrees per watt.. meaning if it has 2v of drop voltage at maximum current it will be 6 watts. 6 watts = around 450 degrees! but if we reduce the drop voltage to 1v this will be about 240 degrees which is still too much.
So if we wanted to use this approach then heatsink is a must?? this DPAK package can use heatsinks like this: https://www.fischerelektronik.de/fileadmin/fischertemplates/images/SMD_Bauteile/bild1.gif right?
if I wanted cheaper ones I could get a piece of Aluminum cut at a square shape then screw it to the pad. this could work but requires more money and labor. no way i can order these from outside! plus I don't think I can solder them with that thermal mass they have.
can we make this work without heatsinks? I am really ready to adjust the design altogether if we can do it without heatsink.
My assumption is this:
12v source -> AOZ1284 -> Cap. multiplier -> LM39302 for 3.3v and 5v.
how much ripple and noise are expected after this?
Do you have the Junction to ambient temperature of the 'LM39302'? If it is in the same package, it is not much different.
Also, because of those 0.5v spikes coming from the AOZ1284, and the regulator dropout of ~500mv at top load, with a little regulation safe zone, you would still be powering the LM39302 with ~+1.3v, 3.75 watts of heat.
The transistor derating you are reading is the transistor not even mounted on a PCB, complete open air. Even a PCB alone will drain away heat.
Why do you need the 'LM39302' when your cap multiplier becomes a linear regulator just by adding the right zener diode between the MMBT3904's base and GND in parallel with your say 10uf cap.
Ok, scrap the MMBT3904 and LM39302.
Just get the cheapest adjustable linear 100ma regulators (LM317 in SMD) which can go to at least 18v input and down to 3.3v out.
You will use the 100ma regulator's output to feed the base of the MJD44H11T4G, multiplying that supplied current by the transistor's current gain curve which could drive ~ 5amps, however, the sweet spot is at the 3amp mark where the transistor's gain is clearly above 100.
The trick to preventing your 12V supply's ripple from reaching the 2x 50ma linear regulators is use a 1/2watt 100 ohm resistor from 12v to the regulator's Vin, and at that Vin, have a good 10uf 25v cap to GND (The GND trace by the output connector). This 1 resistor and cap can power both regulators simultaneously, or, if you want super separation, use 1 resistor and cap for each LM317. (Remember, if the MJD44H11T4G is driving a full 3amp load, the regulator powering it's base is driving ~15ma + a minimum pulldown resistor = ~20ma total max.) While each AOZ1284, remember it makes spikes up to 0.5v, plus you want a little headroom, should power the collector with 1.3v more than the output voltage. ~4.6v for the 3.3v output and ~6.3v for the 5v output.
However, the 2 problems with this circuit is output regulation unless you try something I never had. Tune the LM317 resistors for the desired voltage, however, place the voltage output feedback divider resistor on the emitter output of the transistor instead of the output of the LM317 directly. This may regulate/correct the temperature drift and load change on the output of the transistor which may have introduced a ~0.3v variance over temp and load conditions if the LM317 took it's feedback from it's output pin. But you need to make sure that there is no power-up overshoot spike in this case. You may wire your PCB to operate in both modes since it will only be the placement of 1 resistor or the other.
The second problem is that there is no true over-current protection. You will be relying on the maximum current of the AOZ1284 to limit the output power.
With this, since the 2 linear regulators and transistors should be close and share the GND on your output power connectors, and you can move the switchers and V+ traces which feed the collectors further away from everything else, you would do fine.
This wiring configuration relies on the MJD44H11T4G collector absorbing and not passing through all the switching noise to it's emitter. For this, the output will need a minimum load with a small cap. Otherwise down at 0ma, some ripple may make it through as the transistor's internal capacitance will transmit some signal through as a slight DC error offset on the output. This shouldn't be bad as a single transistor like this can operate above 85MHz.
If you knew the individual currents for both 3.3v and 5v, a single switcher at 6.3v may be enough to feed both MJD44H11T4G, though, the 3.3v one may get really hot is all the current is on the 3.3v supply. Same with the 5v, if it is only an ~1 amp load, you may get away with feeding it more voltage on it's collector.
It's too bad the collector is the tab and not the emitter. The collector is where you are getting all the noise from the switching supply source and a hunk of metal heatsink and fat PCB power trace may act as an antenna.
As for your illustrated heatsink, for ~5watts of heat, that will be fine, though placing the heatsink under the PCB right under the transistor with stitched vias and a rectangular copper pour on both sides will get rid of the heat more effectively. Just gluing/taping that heatsink right ontop of the transistor's plastic case may be enough.
If you want to truly maximum protect your transistor, a TO-220 version vertically mounted with a screwed on heatsink would be best, however, for ~5 watts, I don't think you need to go that route.
More powerful transistors exist than the 'MJB44H11T4-A', I just picked the cheapest which would meet your needs.