Announcement

Collapse
No announcement yet.

AT PSU mod - take two!

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • zaero
    replied
    Re: AT PSU mod - take two!

    Very interesting topic!!!

    Regarding current limiting, it is typical to use either the spare amplifier on the TL494 or the DTC pin (4). You can always add a primary current limit with a current transformer. The not-so-crappy AT PSU have one. They usually are configured as latching, to DTC pin.

    For sensing I have been looking at the ACS712 modules. Affordable ($3.20), isolated and very low resistance (it can even be ignored) makes it a pretty interesting candidate

    Leave a comment:


  • Pentium4
    replied
    Re: AT PSU mod - take two!

    Can't wait to see the improvements

    Leave a comment:


  • nickon51
    replied
    Re: AT PSU mod - take two!

    I don't think I explained that very well. I was looking at the start up resistor R2 on the Seven Team power supply. It goes from the non-isolated side to the switching transistors, which I thought were in the isolated section, but of course they are not.
    Cheers
    Greg

    Leave a comment:


  • Th3_uN1Qu3
    replied
    Re: AT PSU mod - take two!

    Dunno where you read that it goes to the cold side. The 220k is between base and collector of Q1, which is on the high side of the primary DC bus.

    For current limiting i will be using a shunt and an opamp to amplify the small voltage drop of the shunt to something the 494 can read. Something like this oughta do for the shunt resistor. I will be using 10mOhm. For the opamp, i believe i'll be using the MC33078.
    Last edited by Th3_uN1Qu3; 09-18-2012, 03:14 AM.

    Leave a comment:


  • nickon51
    replied
    Re: AT PSU mod - take two!

    This is an interesting project TH3, I want to do something similar, with about the same quality supply. Please keep us up to date.
    Tom and PeteS, I have to say that I'm a bit surprised to see the 220k from the mains live go to the startup cct for the supply. That breaches the "cold" zone by a mains lead into it. I don't think I've seen that before with the driver control in the "cold" zone.
    I wouldn't be surprised if that wire from the 220K was supposed to take a right turn to the collector of Q1. It has after all, got a winding on the driver transformer, that should provide the required startup pulse.
    I can see the 330uf that you refer to, I wonder if that would charge quickly enough to power the 494 rail for start up, I somehow doubt it.

    What are you going to do for current limiting? The supply that you are using has, like mine, no current limiting. I'd be interested to see.

    Cheers
    Greg
    Attached Files

    Leave a comment:


  • Behemot
    replied
    Re: AT PSU mod - take two!

    Originally posted by PeteS in CA View Post
    Besides the buzz Th3 pointed out during power limit protection, you may also see interesting stuff when turning off the P/S (the P/S "tries" to turn on during the I/P 'lytic voltage bleed-off, resulting in significant voltage pulse or train of pulses on the +5V - a Mitac ATX supply I saw would actually have a >6V pulse on the +5V).
    Yep, I see that in one Enlight Corporation AT supply. Didn't measure +5 V, but +12 V gets as high as over 13 V with this spike. You can clearly hear fans getting faster for a fraction of second.
    Last edited by Behemot; 09-16-2012, 09:50 AM.

    Leave a comment:


  • Th3_uN1Qu3
    replied
    Re: AT PSU mod - take two!

    Have successfully modified the feedback for 12v only, works fine. Don't have the file right now, as my dv9000 kicked the bucket for good last week, and i don't have a SATA to USB converter.

    Anyway. It's about time to start picking parts for the final configuration of this power supply. I want an adjustable split supply from +/-20v to +/-70v at a maximum power of 650W, adjustable current limit 0 to 5A, 50mVp-p maximum ripple.

    By my SMPS cheat sheet i am going to need a 2x250uH coupled inductor. I will be making this on a ferrite core, most likely an EI-33. The secondary caps don't need to be more than 1000uF, i will go with Rubycon ZL. I'll also cheat a little and use 63v caps, relying on the surge rating - 100v ones aren't cheap.

    Next, i have to pick my diodes. Because the peak voltage at the transformer is going to be a bit over 200v at high line and no load, i would need at least 250v diodes. Common anode diodes are less often found than common cathode, my search at Farnell revealed only one suitable candidate: the Fairchild FEP16JTA. Unfortunately they don't stock the 400v version in common anode. So, for the positive rail, i will be using its common cathode counterpart, the FEP16JT.

    This will take a little bite from the efficiency of the power supply as the 600v versions have 1.5v forward voltage as opposed to 1.3v for the 400v, but considering the currents i will be working with, this won't be a problem. 1,5v*5A*2 = 15 watts total dissipation on the secondary heatsink at maximum load, which is quite lax. The switching transistors will dissipate 60 to 80W, so clearly, the dissipation of the diodes is no issue.

    Leave a comment:


  • Th3_uN1Qu3
    replied
    Re: AT PSU mod - take two!

    Yes exactly. Because the core is the same, you have in effect a transformer, so when you are paralleling multiple windings with the same number of turns you get higher current handling while the inductance stays the same. Also note that you cannot parallel windings with different number of turns on a common core - else the one with less turns will short the other.

    Edit: Now let's get dirty and get into the compensation.

    First, you will notice that unlike textbook examples of type II and type III, the compensation most commonly found in AT and ATX supplies is simply a resistor with a cap in series. I bet most of you have wondered about that. The reason for this is that feedback on group regulated AT/ATX PSUs is always 5v dominant, and the Q (peak) of the LC filter is small, because we have a small inductor with a large capacitance. In addition to that, the ESR of the capacitor is chosen so that its zero compensates the pole of the inductor, making the overall phase shift 90 degrees. This is why it is not recommended to use ultra low ESR capacitors in a power supply.

    With just 90 degrees additional phase shift in the negative feedback loop, the system is stable by itself! The only reason the RC is there, is to limit the high frequency gain of the TL494, ensuring the right crossover frequency. It is not a "compensation" circuit per se.

    Attached is an adapted version of a LTSpice simulation i found sometime last year on diyaudio, with the opamp model tweaked to represent a TL494 error amp. The original simulation used a LM358 which wasn't accurate in some aspects. There you can see the frequency and phase response of the LC filter, Take note, to see the correct phase you must have the output trace inverted, as in, -V(EAout).

    As you can see, at light load, this thing doesn't even respect the Nyquist criteria! Yet, it works... At nominal load the input voltage to the LC filter will decrease since the primary voltage decreases as well, and the gain will drop, making the crossover frequency lower.

    In this power supply, at nominal line and minimum load, we had 12v peak input voltage to the 5v diodes, and 29v for the 12v diodes. So we'd have modulator gain of 4 for the 5v rail, and 9.7 for the 12v rail. Feedback was taken from 5v only. How this gain is calculated is explained in the simulation. Now, i will attempt to modify this for 12v-only feedback, as an exercise.
    Attached Files
    Last edited by Th3_uN1Qu3; 08-31-2012, 05:44 PM.

    Leave a comment:


  • tom66
    replied
    Re: AT PSU mod - take two!

    So if I wind two windings on the toroid at say 8µH very close to each other, I should measure 8µH on each.

    Then I put them in parallel, but I still get 8µH. Is it due to the core that the inductance doesn't halve?

    Leave a comment:


  • Th3_uN1Qu3
    replied
    Re: AT PSU mod - take two!

    Well, i'm back home, and with more details about this PSU (i brought it home with me).

    The common output inductor is wound on a T90-26 toroid as far as it seems, and it has the following values: ~8uH on 5v and -5v, ~28uH on 12v and -12v. My meter isn't very accurate in the 100uH range, so take those numbers as +/-5%. I noted the number of turns somewhere, but forgot and i'm too lazy to count them again. It's 10 turns for 5v/-5v and 24 turns for 12v/-12v IIRC. You know the deal - the common inductor also acts as a transformer, so to ensure group regulation, the turns ratio of the windings must be equal to the voltages ratio, and you can see that in the above numbers.

    To be noted is that on a common core, putting two windings in parallel will NOT decrease the inductance by the parallel inductor formula (same as for parallel resistors, 1/Ltotal = 1/L2 + 1/L3 + ... + 1/Ln), it will remain the same, only with higher current capability. Practically all computer PSUs use multiple paralleled windings on 5v, and the good ones do it on 12v too.
    Putting windings in series WILL increase the inductance as per the formula (Ltotal = L1+L2+...), while maximum current capability remains the same it would be for the thinnest wire of the series windings (in theory). In practice maximum current decreases because of the added resistance of the wire which causes higher copper loss.

    What i will do now is move the feedback on the 12v rail and recalculate the compensation components and see what i get, to check if the formulas i am using are correct. Best to start off with some assurance before i get into the crazy mods.
    Last edited by Th3_uN1Qu3; 08-31-2012, 03:51 PM.

    Leave a comment:


  • tom66
    replied
    Re: AT PSU mod - take two!

    Many ATX PSU's I have encountered will have a slight (~0.5V) voltage spike before they shut down.

    Leave a comment:


  • PeteS in CA
    replied
    Re: AT PSU mod - take two!

    The P/S I referred to was an engineering prototype built by Mitac for Sun Microsystems. We reported the problems (the +5V had at least one pulse high enough to possibly do damage, and one or two other pulses that could turn logic on in an uncontrolled fashion) to Mitac, and they fixed the problem without totally redesigning the P/S. So self-oscillation can happen at turn-off with a proportional drive base drive design, but it doesn't have to happen. I should add that the restart attempts would happen several tenths of seconds after turning off power, so unless one had a 'scope set to a slow sweep speed, the pulses might happen but not be detected.

    Leave a comment:


  • b700029
    replied
    Re: AT PSU mod - take two!

    Originally posted by PeteS in CA View Post
    Besides the buzz Th3 pointed out during power limit protection, you may also see interesting stuff when turning off the P/S (the P/S "tries" to turn on during the I/P 'lytic voltage bleed-off, resulting in significant voltage pulse or train of pulses on the +5V - a Mitac ATX supply I saw would actually have a >6V pulse on the +5V).
    I guess that's the reason for the fan twitching when turning off from protection mode, but all the AT supplies I've seen have the output just die off otherwise when turning off normally.

    Leave a comment:


  • Th3_uN1Qu3
    replied
    Re: AT PSU mod - take two!

    Originally posted by momaka View Post
    I also have another half-bridge ATX PSU that I think is oscillating. Do you think that if I do the 10k resistor mod it will stop oscillating? It has a SG6105 PWM controller, though.
    SG6105 is TL494 + 2x TL431 + some comparators, so it's pretty much the same thing. What makes you think it's oscillating? Does it whine to various degrees, depending on the load? Can you draw a schematic of the components around the SG6105? You can find a reference design in its datasheet, i doubt it'll vary much from that.

    Putting a resistor between the reference divider and the inverting input of the error amp is to stop the interference from your hand getting into the controller when you are using a pot to vary the output voltage, not in any other case.

    @ PeteS in CA: I've seen this behavior (output voltage spike when unplugged from mains) in all AT and ATX PSUs. So it does not appear to depend on the self-oscillating startup of AT PSUs.
    Last edited by Th3_uN1Qu3; 08-26-2012, 10:16 AM.

    Leave a comment:


  • PeteS in CA
    replied
    Re: AT PSU mod - take two!

    They could be primary side controlled for low loads, e.g. by UC3842.
    The start-up components - a high-value resistor and a small lytic capacitor - are usual suspects in 384x-based P/Ss that won't power on. The resistor opens up or the cap won't hold/source enough charge, and the 384x can't start-up. Besides being able to work on the primary side, a UC384x is less ancient and can have a faster, better controlled loop than a TL494 or SG3524. The SG3524 and TL494 were voltage-mode PWMs designed in the 70s; the UC384x PWM family is current mode and produced in the 80s.

    Leave a comment:


  • PeteS in CA
    replied
    Re: AT PSU mod - take two!

    tom and Th3 called it. The start-up comes with the 220K turning on the first transistor, the main transformer current going through a winding on the base drive transformer creating positive feedback ("proportional drive") and the magnetic field flipping at core saturation, turning off the first while turning on the second transistor, self-oscillating until the TL494 and driver transistors have the Vcc to be able to take control, switching at a much higher frequency than the natural self-oscillating frequency. Besides the buzz Th3 pointed out during power limit protection, you may also see interesting stuff when turning off the P/S (the P/S "tries" to turn on during the I/P 'lytic voltage bleed-off, resulting in significant voltage pulse or train of pulses on the +5V - a Mitac ATX supply I saw would actually have a >6V pulse on the +5V).

    Leave a comment:


  • momaka
    replied
    Re: AT PSU mod - take two!

    Originally posted by tom66 View Post
    They could be primary side controlled for low loads, e.g. by UC3842. I think I have an old ~60W AT PSU somewhere, maybe I should open it (not sure if it went in the bin though.)
    Indeed.
    I have a 75W Mitac AT PSU like that. At first, I thought it was dead since it wouldn't start. Turns out it just needed a large load on the 5V rail (and when I say large, two 5V, 5W light bulbs weren't enough). When I connected it to an AT motherboard, it worked great.

    Cool mod with the AT PSU. I think I'll try the voltage mod with one of my crappy half-bridge ATX PSUs. I have a feeling the results will be interesting

    I also have another half-bridge ATX PSU that I think is oscillating. Do you think that if I do the 10k resistor mod it will stop oscillating? It has a SG6105 PWM controller, though.

    Leave a comment:


  • tom66
    replied
    Re: AT PSU mod - take two!

    They could be primary side controlled for low loads, e.g. by UC3842. I think I have an old ~60W AT PSU somewhere, maybe I should open it (not sure if it went in the bin though.)

    Leave a comment:


  • Th3_uN1Qu3
    replied
    Re: AT PSU mod - take two!

    Originally posted by b700029 View Post
    Would you say that almost all AT supplies use the same design or are nearly identical?
    Yes. There's no other way to do it anyway, at least not with a secondary side controller like the TL494 is, and i've yet to see an AT PSU based on another chip.

    Leave a comment:


  • b700029
    replied
    Re: AT PSU mod - take two!

    Originally posted by Th3_uN1Qu3 View Post
    This is what causes the characteristic buzz when an AT supply is in protection mode. Even at this low frequency, the voltage produced is enough to keep the TL494 running, so if a protection was triggered, it will not resume normal operation until it is unplugged and the primary caps have drained.
    Would you say that almost all AT supplies use the same design or are nearly identical? All the ones I've had the misfortune of shorting would buzz like that and twitch the fan until powered off manually.

    Leave a comment:

Working...
X