Calculate temperature for low side switches?

Question

I would like to know how to calculate the temperature (to see if I am still in a working range) for a low side switch. Hope you can help me there.

I need to control 10 valves(12/24V - 500mA max. ea.) - First we used a bunch of MOSFETs and controlled them directly over the IO Ports of an ESP. That worked fairly good, but to free up some pins someone told me I could use low side switches. After looking at some datasheets I think I found some that could work but I am kinda worried that I may get super high heat because the voltage is fairly high with decent amount of current.

As an example I looked at a BD8LA700EFV-C or TLE8110ED as they have decent amount of outputs and allow high voltage with high current. But would they work for my task or should I keep my MOSFETS and just controll them over a driver/expander ?

Thanks !

Answer 1

The junction temp rise depends on a product of switch resistance, Ron times the total thermal resistance to the internal ambient where using a heat spreader of copper substrate or a clamped case heatsink can greatly reduce the case to ambient resistance but is bulky unless forced moving air velocity is high.

Assumptions and variables for each driver must be combined to determine the total power loss and heat rise.

Choices and calculations: for temp. of jcn (Tj), case (Tc), ambient (Ta), and interface thermal resistance between each, Rjc, Rca where Rca depends on air flow and if a heatsink is used otherwise none, just convection open air Rja.

1. Heat loss: \$P_d= I^2* R_{on @ T_{j ~max}}~*~N_{drivers}\$

2. Temp. Rise = \$P_d*R_{ja}\$ unless a heatsink is used,

   • then Rja=Rjc+Rca.

3. Absolute max energy clamp diode = \$E=1/2 ~LI^2\$.

4. Max Tj=85’C to 100’C depends on reliability not worst case, Ta (inside) , case design.

5. cost and size may be critical factors for getting lower Ron FET’s

Simulation example of transistor switch driven from an ESP at 3.3V using Ic/Ib=20

Lower Rce = Vce(sat)/Ic

Answer 2

Calculate the power dissipation by using the current and the on resistance. Add for all your outputs (assuming all can be on together). My quick calc suggests < 1.5W with the TLE8110. It will need a heatsink as the package is probably around 70C/W so it will get to 105C over ambient. Having contact with a metal enclosure would probably take care of that.

Finding discrete mosfets with much lower on resistances is not too hard so it becomes an engineering decision - chose your poison. You could also consider low side switches with lower on resistance and probably less outputs.

Answer 3

This is the 16DO /24V 0.5A output module Siemens. It is a high side switching P-MOSFET. With yours N-MOSFETs low side could outperform this.

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I do think, that PCB manufacturing cost are high for such compact solution. For cheaper modules, with increased space consumption the same manufacturer also has cheaper solutions.

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Still the same 24VDC/0.5A P-MOSFET. The transistor itself is rated for 15A continous, but derated to 0.5A due to small PCB area and the fact that these can output a PWM upto 30kHz, while the 1st compact module it can't.

My suggestion is to use discrete transistors if the space is not a constraint, it can tolerate many imperfections.

Answer 4

At its simplest, a SIPO shift register with a driving N-channel MOSFET on each output would do the job, so long as you're confident in your own thermal analysis skills (see Tony's answer for thermal resistance calculations).

Significantly more complex is the two load drivers you suggested or the similar OnSemi NCV7750. In a category like this, you want a serial interface, due to your limited I/O capacity; and you get a bunch of other stuff including short detection etc.

Critically, though you should still do thermal analysis for a device like this to get a sense for the bounds of your problem, it will shut off a channel that is overheating, and your discrete MOSFET solution will certainly not. So if you're not as confident in your thermal analysis, and/or if you want to be assured of a failsafe, robust system, you'll want a driver with thermal shutoff.

Adding to the complexity even more, you'll find that having one package per output channel with physically separate I/O expansion will get you better thermal characteristics, but in this case if you wanted diagnostic feedback through your single serial connection you'd need to get creative. For instance you could have a shift register connected to 10 single-channel load drivers with thermal shutoff, such as the NCV8402 or ZXMS6005. Both have automatic over-current, over-voltage and over-temperature safety mechanisms.