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Low-side vs. high-side switches

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carbonzit

Active Member
I think I'm finally able to wrap my head around the difference betwixt low-side and high-side switches: the terms simply refer to the location of the switch in relation to the load:

High-side vs. low-side.gif


My first problem with this is my usual question: why? What difference does it make whether one uses one or the other arrangement?

Remember, we're talking only about switches here, not amplifiers. What difference does it make (let's say from the load's point of view) whether it's on top of an NPN or under a PNP? Doesn't current flow through it just the same?

Now, I do understand that there are situations where one or the other arrangement is needed, say if the load is one of several common-cathode or common-anode units, in which case we'd use a low-side or high-side switch, respectively. (Or if the load needs be referenced to ground or to V+.) But otherwise, assuming the load doesn't care where it is with respect to the supply rails, what difference does it make?

To illustrate my puzzlement, let me replace the transistors with switches:

6-one-half-dozen-other.gif


Obviously it makes no difference to the load which way it's connected (apart from the cases mentioned above).

So would the following be wrong? would it not work correctly? If not, why not?

Low-side wrong.gif


I often see this duality stated along the lines of "PNPs source current, while NPNs sink current". But in this situation, what does that really mean? If the device is simply operating as a switch, then it's hard to see what difference it makes where the load is w/respect to the transistor, since current will flow through the circuit in any case.
 

audioguru

Well-Known Member
Most Helpful Member
The PNP transistor needs to have the input signal at almost to the positive supply voltage to turn off the load. Maybe the input voltage does not go that high.
The NPN transistor needs only a few volts on its input to turn it on and needs close to 0V to turn it off. almost any signal can drive it.
 

moffy

Member
Low side switches as audioguru states are generally easier to drive, no level shifting required, but as you also state there is no fundamental difference for the load. It gets a bit more interesting for NMOS/PMOS Fets. NMOS for the same die size and geometry carry more current than the equivalent PMOS device, so they are cheaper and easier to obtain. Hence FET H-Bridges use 4 NMOS devices which means you need specialty high side drivers for the high FETS.e.g. ir2110.
 

ronv

Well-Known Member
Most Helpful Member
Often high side is used in cars where the frame is used as ground.
 

carbonzit

Active Member
I think I'm beginning to get it: the reason for these standard topologies is to keep the base drive voltage close to the closest corresponding rail (V+ for high-side, ground for low-side), without the load "getting in the way". Is that correct?

So for a high-side switch, you need a base drive of, what, Vcc - 0.7V? something like that?
 

MikeMl

Well-Known Member
Most Helpful Member
HS.gif
 

MikeMl

Well-Known Member
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So Q2 is just a level-shifter, right?
By picking R3, the battery voltage can be quite high, even approaching hundreds of V, assuming the PNP and NPN are up to it...
 
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