Help figuring out playstation 1 psu (SCPH-9002) - Page 1
Forum section: electronics / beginners

author: godmodder
author_rank: Newbie
timestamp: September 02, 2020, 09:43:26 pm
content: Hi,I want to learn more about power supply design. I'm not a complete newbie at electronics. I have some technical background as a computer scientist. But most of my experience is in the digital domain, not analog power circuits. So after studying the usual topologies (boost/buck/...), I thought I'd try something more challenging: the playstation 1 psu.Here is the schematic for the exact model I have, for those "playing along at home"Power Block 1-468-303-11 in the attached manualI've also drawn the primary half of the design in the online Falstad simulatorhttps://www.falstad.com/circuit/circuitjs.html. You can open this file by loading it in the menu. ( attached psx power block.circuitjs.txt)These are the things I have figured out:- isolated flyback converter topology- Q001 is the switching element- feedback comes from a TL431 like chip (IC101) through the optocoupler to Q002- IC101 is provided with 2.5V reference input voltage through voltage divider (R106 & R107)- IC102 is a reset IC to protect the CPU by delaying power-on after the reset signal- C106 is a delay capacitor, it controls how much to delay CPU after reset(1uF => 300ms delay)These are the things I might have figured out, but I'm unsure of:- R103: to maintain minimal current for TL431?- R105 & C105: frequency compensation network, so the op-amp operates more stably?- I think the optocoupler runs in linear mode, feeding back an error voltage to the primary side- I think Q002 is controlled by the optocoupler, influencing the pace at which Q001 switches. Lower switching pace means longer energy accumulation time for C004, which in turn puts more energy through to the secondary winding afterwards.- This looks like one of those quasi-resonant designs with zero voltage switching on the MOSFET. I get this feeling from the capacitor C004 in parallel with the MOSFET after the upper winding.- I think R009 and D009 are responsible for current sensing and/or protection. Not quite sure how it would work though- I think C103 and C104 are smoothing capacitors to convert the pulses of DC across the transformer back to stable DC, right?- What is that "digital transistor" Q101 for? Why does it use the 8V rail to control an LED on the 3.5V rail? My hypothesis: to prevent the LED from flickering when the voltage is unstable and to protect the LED from leakage currentThese are the things I cannot figure out at all:- The current flow is weird in this design! How does current ever flow to the base of Q002? How does it get to the lower primary winding? I see two primary paths of current:1) if Q002 is open, (line)C003=>R003=>R018=>R009=>C003(gnd)2) if Q002 is closed, (line)C003=>upper winding=>Q001=>R009=>C003(gnd)Basically every other current path is blocked by some capacitors (at least in the simulator). Or are they supposed to let current through at high switching frequencies?Conclusion: I really don't understand how Q002 exactly influences the switching.- What's up with that lower primary transformer winding anyway? It hasn't got any opposing secondary side. Is it just used as a glorified inductor?- What's up with all those diodes (D010/D011/D007)?- No idea what D104 and R104 are for- I tried simulating in Falstad, because I didn't feel like probing this thing live and testing my body's ability to conduct current. Can I even simulate this circuit? Maybe the simulation is just all wonky and I'm getting the wrong ideas.- I simulated the choke L001 with a transformer. Is that okay, or is that a trap for young players as Dave would say?- I replaced Q002 with a manual switch for testing purposes and it does seem to control Q001 in a way.I would be so grateful if you could clear up even some of these issues I'm having understanding this thing!

author: ledtester
author_rank: Super Contributor
timestamp: September 02, 2020, 11:56:43 pm
content: (Attached below is the left side of the power supply schematic.)I would recommend watching some videos by DiodeGoneWild on youtube:https://www.youtube.com/channel/UCQak2_fXZ_9yXI5vB_Kd54g/videosThe channel contains a lot of power supply teardowns and explanations of operation. Here, for example, is one of a simple (and what he calls deadly) 5A 1A USB charger (starting at 7:10):https://youtu.be/OHf1U8etd6M?t=7m10sI choose this one because it is representative of his explanations and it is easy to explain how the optocoupler feedback affects drive of the switching transistor, and I think a similar mechanism is being used here: if Q002 is open it doesn't affect the gate drive of Q001; if it is on it will pull the gate to the MOSFET's source potential effectively shutting it off (note both the source pin of Q001 and emitter of Q002 are connected to B1.)My guess is that the current for the transistor side of PC001 comes from the B1-B2 transformer winding.DiodeGoneWild has done many, many switching power supply teardowns and schematic explanations, so I would just look at his video list to see what interests you. One series of videos that stands out is his three part series on "How Does a Switching Power Supply Work":- Part 1:https://youtu.be/cX4q0e124C4- Part 2:https://youtu.be/mNquVjDnpxU- Part 3:https://youtu.be/3ySlJ99OSN4And here is a teardown / explanation of a cheap Li-ion charger which doesn't use an optocoupler for voltage regulation:https://youtu.be/grRCx4lgHtg&t=4m4s

author: amyk
author_rank: Super Contributor
timestamp: September 03, 2020, 12:05:46 am
content: Research "ringing choke converter". This is just another variant of that. Yes, these circuits operate in the frequency domain, so you cannot analyse them as if they were DC.

author: godmodder
author_rank: Newbie
timestamp: September 03, 2020, 08:21:21 pm
content: Thank you for the videos you recommended. I knew the youtube channel, but that particular video gave me some more insight in the feedback mechanism.Hmm that ringing choke topology looks interesting. I always thought this was used in cheap cellphone chargers, but apparently this can give quite nice results as well.I'll check these resources out and see what I can learn from them.Just one more question: is it doable to simulate circuits like these in Falstad or LTspice or something? Or would you recommend against it?