MOSFET to cut off power

Hi Cabe,

If you want to use N-Channel Mosfet's for controlling on/off you need to switch the cathode of the LED (drain fet is connected to LED, source is connected to ground). As you need to drive the FET the gate-source voltage should be higher than 10V.

If you place the fet in series with the anode you need an additional voltage higher than the supply voltage on the drain. If the voltage on the drain is 14.5max, you need 24.5 Volt to drive the gate. As you don't have this voltage present you can make it with a switching circuit (bootstrap circuit) but this is over the top.

Furthermore, don't you think that 1Mohm is series with the base of the transistors is too much? (only 11uA).

Question: is 14.5V really the maximum voltage? There is a fault mode in automotive that for example the car battery is low and another car is jump starting. If for some reason the battery of that car is not securely connected the voltage on your car rail can be very high (load dump). In automotive systems you see often transil's for this purpose.

Best regards, Enrico Migchels

Enrico,

The Mosfet is not directly connected to the LED. It is actually used to cut power to a LED driver chipLED driver chip. Which untimately steps down the voltage, 3V, to the LEDs.

And also, the Mosfet I am using is an Enhancement Type N-Channel.

14.5V is indeed the max I can expect from the car. Though, this is my first production automotive product I ever worked on. I may find out I can get more.

Cabe

Enrico,

The Mosfet is not directly connected to the LED. It is actually used to cut power to a LED driver chipLED driver chip. Which untimately steps down the voltage, 3V, to the LEDs.

And also, the Mosfet I am using is an Enhancement Type N-Channel.

14.5V is indeed the max I can expect from the car. Though, this is my first production automotive product I ever worked on. I may find out I can get more.

Cabe

Hi Cabe,

Derating aspects are 1 one thing. I would go for a 50% parameter reduction in any direction. Furthermore you need to select components which can handle high environment stress:

temperature -40C +125C and the design may suffer from high mechanical stress (bump, shock, vibration). Sometimes 30G can be encountered in automotive systems, so big components (like elcaps) must be glued to the PBC (double layer, though hole part).

How is that CAN bus coming along? Do you have to go for J1939?

Best regards, Enrico

Enrico,

I did reduce some of the components specs. And the temperature range was definitely taken into consideration when I was choosing. As for shock, it never really crossed my mind. I will ask.

Today I was asked how I am going to take care of any transient voltages in my circuit to protect the customer's, car company, parts. I never knew I had to protect the rest of the system from itself. Apparently, no one is taking this into consideration on the car company's end.

However, I decided I will go with some Transil DiodesTransil Diodes, Transient Suppression Diodes, on each one of the inputs to ground.

CAN bus is a little side project of mine. I plan on moving to that as soon as I learn Altium Designer.

Engineers are always learning!

Cabe

Hi Cabe,

Even the power rail in a car is nasty. Have a look at this document.

http://www.st.com/stonline/products/literature/ta/16794.pdf

The ESD transients on data wires can be as much as 25kV peak.

I guess you have to do it yourself because other devices are to far away (inductance of the wires).

Best regards, Enrico Migchels