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This is the schematic diagram of pa-12 family laptop adapter. I think it is based on flyback topolgy but i have a problem understanding many things in it. Actually, in many circuits in general electronics i find many things like that and I can't figure out why these components are here? i understand the main concept of the circuit but the details are big mysteries for me. Anyways, I have pointed out to some of them in this circuit.

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1- Are these resistors for filtering ? there are 2x1M ohm series resistors, why don't they just put one 2M ohm resistor?

2- The same, there are 2x750k ohm series resistors, why don't they just use one 1.5M ohm resistor? (Is that about standard values?)

3- What is that?

4- the same as question 1 and 2.

5- why don't they just use 1 equivalent capacitor ? .

6- What is that?

7- Is this a feedback network ? why don't they just use Ic ?

8- What is the purpose of this cap ?

9- What is the purpose of this resistor ?

  • 1
    \$\begingroup\$ this question is too broad as it would require a few pages of information and answers are limited in size. It may be best to restrict your question to one or two questions. A good way to learn about the circuit is to 1) build it yourself 2) get a book like art of electronics and study it 3) study power supply toplologies on the internet 4) simulate it with lt spice \$\endgroup\$ – Voltage Spike Aug 31 '16 at 16:02
  • \$\begingroup\$ 1) 2) Either for spacing regulations or for power dissipation 3) Maybe to reduce harmonics 4) Filter \$\endgroup\$ – Voltage Spike Aug 31 '16 at 16:05
  • \$\begingroup\$ @laptop2d Actually, I am doing all of this but I am advancing so slowly and I don't know why. Another something I always wanted to ask, Isn't "the art of electronics" a little harder than other electronic books ? or it's just me ? \$\endgroup\$ – iMohaned Sep 1 '16 at 12:29

Resistors have voltage ratings, as well as power ratings, and in SMT parts these can be surprisingly low.

Part of the reason for series chains is UL requirements, part is just seeking a high enough voltage rating.

Network 3 is a rather over complicated snubber to limit primary side voltage excursion as the mosfet turns off (Transformer leakage inductance and output diode recovery time means that you can get a big transient here even in a flyback).

Network 4 is an interesting special case, as it crosses the isolation barrier and is thus extremely safety critical, hence the 4 resistors. The resistor chain just bleeds off any static charge, the cap (Which is a special safety rated one) exists to shunt interference coupled across the transformer by its inter winding capacitance as otherwise this current will find an alternative path probably causing an EMC fail.

7 is actually two separate feedback circuits, the bit connected to PH1 which is the normal feedback arrangement for voltage control under normal operation, and the stuff around the dual comparator and PH2 that provides over voltage and over temperature shutdown.

9 This resistor controls the turn on speed of the mosfet, the diode ensures the gate discharges quickly to switch the mosfet off rapidly.

Hope that helps.

  • \$\begingroup\$ On 7 there is an overvoltage comparison (IC2A), overcurrent comparison (Imax ~= 0.108/R40) and a temperature comparison using Q3's Vbe, all wired-or'd to shut down the controller chip via PH2 optocoupler. \$\endgroup\$ – Spehro Pefhany Aug 31 '16 at 11:07

This question is likely to be closed as it's very broad, but I'll have a go at a couple of points.

The first one is that UL certification (UL508C I think) requires consideration of all components that could fail short. That causes the doubled-up resistors: them failing short will not be a disaster. The capacitors used are special "X" and "Y" rated ones.

1, 3 and 4 are indeed filters, including the chokes NF1 and NF2. The resistors will be "bleed" resistors, so that the capacitor does not retain charge for long after the device is unplugged.

5: The usual use of capacitors in parallel is to increase the capacitance while decreasing ESR (effective series resistance, because the resistance of a capacitor is not zero).

6 is a small identification IC; google "DS2501".

  • \$\begingroup\$ It's not broad, I just needed a brief explanation like you did. \$\endgroup\$ – iMohaned Aug 31 '16 at 10:13
  • \$\begingroup\$ 5. In addition certain parts may be cheaper in more common values, some types like solid polymer for example don't tend to have as good availability in large values, several smaller parts may fit better on the PCB layout than one large capacitor, they may have a large quantity of that value or use it in other PCBs so it is easier to manage the one product line and potentially get better price breaks also. \$\endgroup\$ – D-on Aug 31 '16 at 10:23
  • \$\begingroup\$ As for 4., it can't be "specail X and Y". Only Y are allowed from primary to secondary. \$\endgroup\$ – winny Aug 31 '16 at 10:38
  • \$\begingroup\$ I wasn't terribly clear about X and Y - as you say, CX1 must be X and CY1 must be Y. \$\endgroup\$ – pjc50 Aug 31 '16 at 10:41
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    \$\begingroup\$ @MohannadMaklad It certainly is broad as it's nine separate sub-questions and 7 is quite hard to answer without reverse-engineering that whole area of the circuit. It's just that I'm less pedantic about broadness than the average voter. \$\endgroup\$ – pjc50 Aug 31 '16 at 10:41

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