Saturday, October 6, 2012

High-side Switching

A lot of people feel, howbeit superficially, that they "get" NPN operation - the NPN conducts when its Base is at a potential more positive than its Emitter. Understanding a "high-side switch" supposes knowledge of PNP transistor operation.
Comparatively, the PNP is a little understood device. Looking back at some textbooks, they're never presented inside the "+V and GND" paradigm. The PNP is presented with an electrode connected to a negative voltage (gasp!), which provokes apoplexy and panic, or it gets mentioned in passing ("..there are PNP transistors, too, but you won't see them much and I don't like them, so let's move on..") and so is left to languish in obscurity, something to be avoided (ley de hielo.)
The PNP conducts when its Base is at a potential less positive (more negative) than its Emitter. Equally important, but what's not grasped, is that it does not stop conducting till its Base is at or near its Emitter - just as an NPN does not stop conducting till its Base is at or near its Emitter. When the PNP's Emitter is at +12V, you cannot turn it off by presenting +5V to its Base - because 5V is still less positive than Emitter potential (in fact, 7 volts less positive).

Here our PNP is configured Common Emitter (and definitely not to be confused with an "Emitter Follower" - a/k/a Common Collector). It's a "high-side switch", it opens/closes the current path to +V.

The NPN switches the PNP's Base.  The output enable, the input to the NPN's Base, is a logic-level signal from a microcontroller.
The 22KΩ pull-up to +12 biases the PNP off.  When the NPN is biased on, its Collector goes to Ground, taking the PNP's base, via its RB, to Ground (a potential much less positive than +V) which turns it on.
The PNP's IB = (+12V - VBE) / 4K7.  The PNP IC, the current available to the load, is basically = hFE * IB
When little current is drawn, VCE is negligible.  As IC increases, so does VCE.  In my test circuit, using a 2N3904 (NPN) and a 2N3906 (PNP), VCE was 200mV when IC = 200mA.
Potential applications:

  • an "anode driver" in a LED matrix
  • a MOSFET driver 
  • in an H-bridge  

The advantage over grabbing something ULN/UCN/UDN is that you may likely have these components on hand. Hey, RadioShack still has packs of PNPs. And consider, if you get careless or push past the envelope, it's more expensive to replace a whole IC on account of one smoked section than for a single transistor.
And, you learn something.

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