Amateur question: how circuit analysis methods are applied in “complicated” circuits?

This will sound maybe stupid, but I'm amateur in electronics and I'm in process of learning.

I have previously analysed some basic circuits involving opamps, diodes etc.

Although some circuit looks simple (only few opamps, diodes, resistors, capacitors and sources), mathematics that describe that simple circuit can be very complicated, especially if we want to calculate full response of circuit (not steady state).

I was wondering, how then some complicated electronics schematics are analysed? I mean there are devices containing hundreds of components inside it, it would be impossible to apply some circuit analysis methods there (KCL, KVL etc), right?

How important are those network analysing methods we are studying in school in practice (in repairing electrical devices)?

To clarify what I mean, lets look at one switched-mode power supply schematic I found on the web:

http://320volt.com/wp-content/uploads/2010/12/ice3b0365-atx-smps-uc3843-wt7525-circuit-diagram.png

Probably, professionals doesn't see there something terrific complicated, but to me, it is very complicated. If one has to repair this device for example, which knowledge he need to have? If he start measuring some voltages, or tracing signals waveforms on oscilloscope, he must need to know firstly what he should expect in normal circumstances (when device is working), so he can detect exact problem. But how can he know that if he doesn't analyse circuit mathematical, which would be quite impossible in this case, what returns us to first lines of my question :)

2 Answers

Basically for pen-and-paper analysis you break down the circuit in "logical" blocks, like in a block diagram and you mostly analyze them separately. I put that "logical" in quotes because it's not in any formal sense.

There are however fast SPICE simulators that can simulate an entire chip and these obey KCL, KVL etc. but may use somewhat simplified component models.

I should also note that breaking down circuits into simpler sub-components (as we do on paper) is also done in some software tools like HSIM. Hierarchical simulation is one of the tricks in the arsenal of fast SPICE simulators.

Regarding that SMPS schematic that was added as example: for pen-and-paper analysis you need to be familiar with the kind of circuit in question (starting with SMPS topology) and the identify the "macroblocks" like input filter, rectifier, main switcher[s], transformer, main controller IC (usually includes error amplifier), etc.

• So every circuit, no matter how complex it is, is made of simpler "sub-circuits", and to know how whole complex circuit works, one should know how it's "sub-circuits" works solely? – hari Nov 18 '15 at 2:57
• regarding KCL and KVL, if the circuit contains linear elements (resistors, capacitors, inductors), the analysis will normally be the "node-voltage" or "loop-current" techniques. you can go directly from circuit to matrix with either. – robert bristow-johnson Nov 18 '15 at 2:58
• @hari: That's pretty much how our brains work. Computers are different though; can simulate a circuit like that without having a clue to its purpose. – Fizz Nov 18 '15 at 2:58
• Actually what I said in my previous comment about "how our brains work" is partially incorrect in the sense that if you had a lot of time and patience, you could [in theory at least] analyze an SPMS like that down to every node voltage and branch current... by solving large systems of equations, but it will take you a very long time. And computers run into the same problem (of precise circuit analysis not scaling up) but with much larger [chip-level] circuits, that's when fast-SPICE techniques become necessary... pretty much for the same reason: it takes too long to be very accurate. – Fizz Nov 18 '15 at 3:30
• @hari to paraphrase your first comment 'So every circuit, especially the complex ones, is made of simpler sub-circuits'. As a designer, I could not handle the complexity to put 3nF here, and 10$\Omega$ there, without an overall organising principle. So I'll have a power supply here, feeding an amplifier, that gets its signal from a tone control, and feeds its signal through loudspeaker protection. Then I specify the interface signals that connect the blocks, +/-25v from the supply, 1v audio etc, then design the individual blocks. The interface specs go in the service manual! – Neil_UK Nov 18 '15 at 6:26

Basically, what fluff said is correct - you break down complicated circuits into smaller blocks and by intuition you know how they're going to react. You can always apply KVL and KCL and fundamental rules like that to everything but you're going to go insane.