Why do we go for a Low side / High Side Switch ? Why do I find more of Low side switches than High Side in Automobile electronics ?

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    \$\begingroup\$ cost - amp for amp n channel devices are cheaper than p channel types \$\endgroup\$ – JIm Dearden Jul 12 '16 at 9:51


simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) NPN open collector switch. (b) A failed attempt to make a PNP version.

The low side switch is very simple as shown in Figure 1a. The low-voltage logic just has to feed sufficient current into the NPN transistor to turn it fully on.

There is a temptation to think that we could do the same trick with an PNP transistor as shown in Figure 1b. The problem is that the emitter-base junction is always forward biased. This will apply the 12 V to the chip output and destroy it or, if there are protection diodes on the output, the current will flow through the protection diodes into the micro-controller supply (shown as 5 V in this case). The effect of this current flow is to turn on Q3 and the load can not be switched off.

There are ways around the problem of Figure 1b but they are all more complex.


what the guys said is totally correct . but your initial statment is not accurate "Why do I find more of Low side switches than High Side in Automobile electronics ?"

Proof in this document: HIGH SIDE SWITCH Stmicroelectronics

I will QUOTE :

Almost every electronic switch used in a modern automobile application is a high side switch. This configuration is preferred for automotive use because:
a) - This configuration protects the load from continuous operation and resulting failure, if there is a short circuit to the ground. Since the body of a car is metal and 95% of the total car is ground, the short to ground is much more common than short to VCC
b) - High Side Drivers cause less problems with electro-chemical corrosion. It is of primary importance in automotive systems because the electrical components are in an adverse environment, specifically adverse temperatures and humidity and the presence of salt. For this reason the series switch is connected between the load and the positive power source. Therefore when the electrical component is not powered (that is for the greatest part of the lifetime of the car) it is at the lowest potential and electrochemical corrosion does not take place.

  • \$\begingroup\$ Thank you "ElectronS". that was useful. Now I have a new question :) that if a Low side switch can detect a short to GND, and if a High side switch can detect a short to BAT. and How ? I know how a short to BAT at Low side and short to GND at a high side can be detected. \$\endgroup\$ – Babu Jul 13 '16 at 11:07
  • \$\begingroup\$ did you think before asking the question ??? why would you need to detect short to GND on high side driver ?? the load in this case is connected between GND and the driver . how can the load be shorted to ground if it is connected to ground !!!! and the same goes for the low side driver but vis versa. \$\endgroup\$ – ElectronS Jul 14 '16 at 14:22

As Jim pointed out, cost is a driving factor, because n-channel MOSFETs are easier to produce with low on-resistance and high current capabilities.

Other than that, ease of construction: With a single MOSFET low-side, you can switch a (basically) unlimited voltage, because the voltage that decides about whether the device is on or off is the Gate-Drain voltage; Let drain sit on Ground, then you see that you can use a typical logical level 0 or 1 to switch the device on or off.

If you put the switch on the high side, you inevitably need to have some circuitry that translates between the logic levels, which are typically much closer to ground than to the voltages of the high side.


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