PWM transistor heating - Rapberry

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I have a raspberry and an auxiliary PCB with transistors for driving some LED strips. The strips datasheets says 12V, 13.3W/m, i'll use 3 strips in parallel, 1.8m each, so 13.3*1.8*3 = 71,82W, with 12 V, almost 6A. I'm using an 8A transistor, E13007-2. In the project i have 5 channels of different LEDs: RGB and 2 types of white. R, G, B, W1 and W2 are directly connected in py pins. LED strips are connected with 12V and in CN3, CN4 for GND (by the transistor). Transistor schematic.

I know that that's a lot of current passing through the transistors, but, is there a way to reduce the heating? I think it's getting 70-100°C. I already had a problem with one raspberry, and i think it's getting dangerous for the application. I have some large traces in the PCB, that's not the problem. Some thoughts:

1 - Resistor driving the base of the transistor. Maybe it won't reduce heating, but i think it's advisable for short circuit protection, how can i calculate this?

2 - The PWM has a frequency of 100Hz, is there any difference if i reduce this frequency?

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raspberry-pi3pwm
1) yes, it's a BJT, suggest you add 100ohm~1k ohm at base and 10ohm~100ohm at collector to see if anything has been improved. 2) Unless you would like to do any fading effect, PWM is not suggested as it would affect the brightness. – CK Wong
Not fading, just controlling brightnes, depending on user input. I already solved this, shouldn't use BJT, as it depends on gain, and as i need more than 400 gain, based on max raspberry IO current, it would require two BJT in series (actual current passing trought LEDs was around 1A with my circuit) Solved with MOSFET, as it doesn't depends on gain, should observe Max Vgs (th), should be less than 3,3V. Bought one with 2.2mohm resistante and solved my heating problem – Rodrigo Vernini

2 Answers

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The BJT transistor you're using has current gain hFE of roughly 20. This means that the collector current is roughly 20 times the base current, or the base current needs to be 1/20 of the collector current, i.e. 6A/20=300mA.

Rasperry PI for sure can't supply 300mA current from the IO pins, so you're operating the transistor in linear region, which causes it to dissipate a lot of heat.

Change your transistors to MOSFETs with low enough threshold voltage (like 2.0V to have enough conduction at 3.3V IO voltage) to keep it simple.

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Using a N-Channel MOSFET will run much cooler if you get enough gate voltage to force to completely enhance. Since this is not a high volume item why not simply use a MOSFET gate driver chip. Then you can use a low RDS on device. Another device is the siemons BTS660 (S50085B BTS50085B TO-220). it is a high side driver that you will need to drive with an open collector or drain device. It will switch 5A at room temperature with no heat sink.It is rated for much more current and is available in a To220 type package. It is obsolete but available as is the replacement. MOSFETs are voltage controlled while transistors are current controlled.