Transistor Low-side VS High-side Switching Consideration

Thread Starter

iimagine

Joined Dec 20, 2010
388
Why is low-side switching always recommended? It is not always needed to be. For simple load such as LEDs, consider using high-side switching to reduces power consumption. How? Because bias current is not wasted, instead it helps source power to the LED.

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As you can see, both circuits provide about 18mA to light up the LED, but on the low side switching, 4.3mA is being wasted! Just something to consider when you design something that runs on battery
 

bertus

Joined Apr 5, 2008
20,040
Hello,

You can also switch the high side with a PNP transistor and have the base resistor to ground.

Bertus
 

Tonyr1084

Joined Sep 24, 2015
3,556
Once I built a chase light circuit and used high side switching (HSS). The problem I encountered (design wise) is that I wanted to strobe the LED's while they were chasing. Having it HSS caused the LED's to glow dimly when I wanted them fully off. So HSS is fine if you don't need the LED to be completely off in certain circumstances. In my case, HSS didn't work the way I wanted it to. LED's being powered from a CD4017 (decade counter) through a transistor while strobing the power to the LED's while HSS meant that at high enough sweep (clock signals) the LED's all appeared to be on at the same time.

What I was attempting to accomplish was depending on the strobe and sweep, the LED's could appear to be a single dot moving one way or the other - OR having multiple dots moving - OR having them moving in opposite directions at the same time. Some dots would appear as single LED's while other "DOTS" would appear as dashes, lighting up two or three LED's at once. The strobe source was from an audio signal. Complex audio signals would produce light moving in both directions at varying rates and of varying lengths. Low notes (frequencies) would produce long dashes while high notes would produce a high number of dots, and they could move in two directions at once. HSS made that impossible. But when I switched to LSS (Low SS) everything worked beautifully.

Incidentally, 10 LED's is not a long enough string to produce satisfactory results. Using a 20 string was better. 30 even better yet. Using 100 produced spectacular results. However, there was always the space between the LED's. I now accomplish the same thing using a laser reflected off a rotating mirror. The motor speed is variable, so I control the sweep. The audio signal (when it goes positive) sends the laser beam onto a screen. Depending on the frequency - I get the same result as using two 4017's and 11 transistors or MOSFETS. Again, LSS works far better than HSS.

You ask why? Well, in some cases LSS provides the results the designer is after. In other cases, HSS simply provides an indicator lamp. And the loss of a few microamps isn't going to be noticed. An unintended side affect is the LED will last a little longer. But that's also going to be hardly noticed as well. Nevertheless, a cooler running LED will light longer than a hotter LED.
 

Tonyr1084

Joined Sep 24, 2015
3,556
N-channel MOSFET's are recommended LSS. P-channel MOSFET's can be used HSS. They CAN be forced to work contrary to their design but it typically means you need a higher or lower signal to switch them. Please don't ask me to explain that - I'm pretty new to FET's.

N-FET LSS, the gate needs to be lower than the drain.
P-FET HSS, the gate needs to be higher than the drain.

I think. I probably have that wrong - someone correct me please.
 

ElectricSpidey

Joined Dec 2, 2017
684
It’s always been my understanding that when using a transistor as a saturated switch, the NPN or N-Channel devices just do a better job due to their inherent lower voltage drops then their p type counterparts.

Of course I learned that a long time ago and things may have changed, thus low side switching just became a convention.

Also I don’t believe a transistor in follower configuration can fully saturate. (your high side example)

If you want to save power in a battery powered device just use MOSFETs instead of BJTs.
 
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OBW0549

Joined Mar 2, 2015
2,977
N-FET LSS, the gate needs to be lower than the drain.
P-FET HSS, the gate needs to be higher than the drain.
Both of these statements are wrong.

In neither case is there any requirement relating gate voltage and drain voltage; the ON or OFF state of the MOSFET is controlled by the gate voltage relative to the source, not relative to the drain.
 

Tonyr1084

Joined Sep 24, 2015
3,556
Thanks @OBW0549. I always get that wrong. Even when I KNOW I'm going to get it wrong and do it the other way around - - - it's wrong.
 

crutschow

Joined Mar 14, 2008
23,291
Why is low-side switching always recommended? It is not always needed to be. For simple load such as LEDs, consider using high-side switching to reduces power consumption. How? Because bias current is not wasted
True. But you lose voltage with the NPN high-side configuration.
Using an NPN as a high-side switch means it's operating as an emitter-follower.
Instead of a saturated on-voltage of perhaps 100mV or so for the low-side common-emitter circuit, you will get about a 1 volt drop with the high-side emitter-follower circuit.

And you are using too small a resistor for the base (why did you use 1kΩ?).
Typically you need a base current of no more than 1/10th of collector current, giving a base current of 1.8mA for 18mA of LED current, so the base resistor need be no smaller than 2.4kΩ.

So it's a tradeoff between saving some current or reducing the voltage drop.
Since you already use a resistor to drop voltage for the LED, the voltage-drop of the high-side switch may not be an issue here (you can reduce the LED resistor value to compensate), so saving 10% of the current might be preferable.
 
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crutschow

Joined Mar 14, 2008
23,291
Just something to consider when you design something that runs on battery
Note that for battery operation you will save the most current with a low-side N-MOSFET or a high side P-MOSFET switch since they require no DC gate current to operate.
 
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Thread Starter

iimagine

Joined Dec 20, 2010
388
Note that for battery operation you will save the most current with a low-side N-MOSFET or a high side P-MOSFET switch since they require no DC gate current to opera
Right, what i am trying to say is that, if you are using NPN BJT, consider High-side switching as it does not consumes any bias current. Unless very high speed switching is needed as Tonyr has notice, slow turns off is not desired. But for application such as LED dimming this would be a bonus.
 
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