Will this mosfet circuit work?

[liquibyte]

What is this, actually?  A load dump or something..??

What are the characteristics of this pulse?

I'm trying to delay the power into the power supply circuit past the filter cap by the time between power on and the filter caps being charged.  What's happening is that I'm getting about 63 volts at power on when the supply is set at 30 volts.  The surge at the output is actually dependent on the setting of the voltage pot.  I posted some of the numbers I'm seeing here.  I've tested fully loaded and unloaded and get basically the same results.  I don't have a DSO, just analog so I can't really get images of it.  The attached pic is the latest revision I've done of it with this mosfet circuit included.

Like I said, because your source is entirely constant current, the response is dominated by the bulk capacitors and load resistor, and the VPULSE does nothing at all.

Tim

I guess I'll have to live with the limitations of the simulator.  I have been doing more reading and have figured out variable loads etc.  I'll keep studying to learn the ins and outs of voltage and current sources.

[rs20]

Like I said, because your source is entirely constant current, the response is dominated by the bulk capacitors and load resistor, and the VPULSE does nothing at all.

Tim

I guess I'll have to live with the limitations of the simulator.  I have been doing more reading and have figured out variable loads etc.  I'll keep studying to learn the ins and outs of voltage and current sources.

To clarify, what T3sl4co1l is pointing out here is that if you put an ideal current source in series with anything (including, for instance, a pulse voltage source), then you just have a current source. In a similar way to how if you have an ideal voltage source in parallel with anything, you can just ignore the parallel components. This is not a limitation of simulation, this is just how it is.

[liquibyte]

I'm not sure if I understand completely but I've been playing around with a few things and I feel like I'm getting a more accurate representation of how I think the spike is behaving in the real world.  It seems like the circuit should work for the intended purpose so I'll be getting a couple of PFET's to test out with on my supplies and see how it goes compared to how I think it should.  As long as the startup delay is longer than the worse case transient scenario I should be good.  If everything works out, I'll call this project done and button up the case and start using it to help building the next one.

[T3sl4co1l]

And putting a current source in parallel with a voltage source gets you a voltage source again...

Just use a voltage source and series resistor.

So, you have a power supply, which exhibits nasty overshoot when turned on/off?  That sounds like an out of spec issue, or a bad design issue, or a compensation issue.  Fix that instead.

Tim

[liquibyte]

And putting a current source in parallel with a voltage source gets you a voltage source again...

Just use a voltage source and series resistor.

Heh, I just took out the current source and I'm getting the same neat results.  I feel dumb now.

So, you have a power supply, which exhibits nasty overshoot when turned on/off?  That sounds like an out of spec issue, or a bad design issue, or a compensation issue.  Fix that instead.

Tim

I don't even know where to begin to track the problem down at.  I always thought it was just because of the 15000uF cap being discharged at power up.  Looking back, I should have picked a different project but I've got so much into it now that I don't want to just give up.  Getting help with this or even what to look for has been difficult.  Q3 in the above schematic took care of the shutdown transient.  I got a little help with that here by David Hess mentioning an HP service manual that had that kind of scheme in it.  The part was actually in the original design but was redesigned without it.  To be honest, I'm not sure why it was taken out at all since it actually did something useful.

[T3sl4co1l]

Don't suppose you have a manual (with schematics) for the power supply in question?

[liquibyte]

Don't suppose you have a manual (with schematics) for the power supply in question?

Nope, this is an old hobby build that was redesigned using modern parts and values that didn't fail due to being overloaded.  Apparently the original had component failure issues but a couple of things were pulled out of it during the redesign that may have been needed.  I'm attaching the pics of the old and new version with part lists from each.  This is about all there is of this thing other than the work I've done with it in Eagle.

[T3sl4co1l]

Oh yeah, back in August, I think I remember that thing...

Try... different values for C6 and C9, and with resistors in series with them.  Like, 100p-1n and 10k-1M.  You probably don't need C6 at all, when you have a resistor in series with C9.

You have to do some sort of transient testing to prove out the selection -- if you have a DSO, you can do this by just shorting a load resistor across the output momentarily, and triggering on the little transient blip the output voltage goes through.  Use AC coupling so you can zoom in on the disturbance.  If you don't have a DSO, you can still do this, but it'll be kind of hard to see... instead, you can switch the resistor automatically, using something like a MOSFET driven by a 555 or signal generator (at a low enough frequency to see things -- figure it out based on how long the transient is... might be 10Hz to 10kHz).  Also, you can try with or without the final output capacitor -- it's not always the best thing for a power supply (they all do it, but that doesn't mean they have *reason* to do it!), and it can mask spooky behaviors like this, that need to be dealt with in a basic fashion.

Also, does Q1 even do anything?  I guess it was added or changed during/after that discussion, but it seems to me, the -1.3V supply will come up as quickly as the main supply, so that Q1 is basically just asking the question: is the supply over 136V (give or take temperature)?  Which seems like a really odd thing to do, disabling the output (forcibly, not in a nice way -- it's shunting the op-amp output!) when the voltage is already burning up all the transistors and op-amps.  (That's 1.2V for the supply, 0.6V for the transistor Vbe, and 12k into 160 ohms is a 0.013 voltage divider ratio, or for 1.8V from the divider, an input of 136.8V.)

Another approach, which isn't applicable here, but can be used if you have OTAs (e.g., LM13700) or a discrete circuit: bring up bias slowly, so the amplifiers begin to amplify gradually.  While coming up, they'll settle on the correct output voltage, while still having too little gain to cause overshoot.

Tim

[liquibyte]

Oh yeah, back in August, I think I remember that thing...

Try... different values for C6 and C9, and with resistors in series with them.  Like, 100p-1n and 10k-1M.  You probably don't need C6 at all, when you have a resistor in series with C9.

You have to do some sort of transient testing to prove out the selection -- if you have a DSO, you can do this by just shorting a load resistor across the output momentarily, and triggering on the little transient blip the output voltage goes through.  Use AC coupling so you can zoom in on the disturbance.  If you don't have a DSO, you can still do this, but it'll be kind of hard to see... instead, you can switch the resistor automatically, using something like a MOSFET driven by a 555 or signal generator (at a low enough frequency to see things -- figure it out based on how long the transient is... might be 10Hz to 10kHz).  Also, you can try with or without the final output capacitor -- it's not always the best thing for a power supply (they all do it, but that doesn't mean they have *reason* to do it!), and it can mask spooky behaviors like this, that need to be dealt with in a basic fashion.

I've got pictures of some testing that the other guy did that's working on this too.  Just as an FYI here, his circuit with the TIP141 didn't do so great under load which is why I've kept pursuing this avenue.  He's new to scopes I think so I'm not sure if these will help or not.  No DSO here unfortunately, but I did just pick up a Tek 465B so I have two channels to muck about with.  I'll try the trick you suggested as well.  I have a few 555's but not a real sig gen, just one I built out of a 555 oddly enough.  It's a simple circuit but it's allowed me to verify that my scope does indeed work.

Also, does Q1 even do anything?  I guess it was added or changed during/after that discussion, but it seems to me, the -1.3V supply will come up as quickly as the main supply, so that Q1 is basically just asking the question: is the supply over 136V (give or take temperature)?  Which seems like a really odd thing to do, disabling the output (forcibly, not in a nice way -- it's shunting the op-amp output!) when the voltage is already burning up all the transistors and op-amps.  (That's 1.2V for the supply, 0.6V for the transistor Vbe, and 12k into 160 ohms is a 0.013 voltage divider ratio, or for 1.8V from the divider, an input of 136.8V.)

We had a discussion in the other thread about that.  Apparently it serves the same function that the Tek PS-503 uses it for.  It seems to work as you can see in the screenshots.  Of course it may be causing other issues besides but I'll jump that hurdle when I get there.  I think D10 might be needed too, it seemed to have helped with a noise issue or something I believe.

Another approach, which isn't applicable here, but can be used if you have OTAs (e.g., LM13700) or a discrete circuit: bring up bias slowly, so the amplifiers begin to amplify gradually.  While coming up, they'll settle on the correct output voltage, while still having too little gain to cause overshoot.

Tim

I had to look up OTA's (Operational Transconductance Amplifier if any other newbies are looking).  I'll keep trying to find the problem based on your advice but I thought what I was doing with this mosfet circuit was essentially a solution in the same ballpark.  Allow the transient to pass before applying power to the supply.  It's hackish but I guess it would work.  Like I said though, I'll keep working on it and see if I can figure out where the problem(s) is(are)

[T3sl4co1l]

100ns/div shots?  Those aren't startup transients, those are RFI from the switch contacts closing!  Absolutely nothing to do with the power supply itself.

If those are causing problems, try some RFI methods:

- C or R+C across the switch contact(s), usually 1n to 10n (must be X1/X2 "across the line" rated) and 0-100 ohms

Filtering applied after the switch:

- Y1 ("line to ground") caps from AC line hot/neutral to ground (chassis ground), typically 1-4.7n
- Ferrite bead (run both H/N together through a ferrite bead, or make multiple turns through), proceeded and/or followed by caps
- Common mode choke (in series with H/N, observe winding polarity -- take apart an SMPS to see how it's done, or just hack the entire filter circuit out for use), esp. in combination with the Y1 caps
- Or, same things on the output side (use caps rated for the isolation voltage -- if your output is grounded or SELV (safe, person-touchable), you don't *have* to use Y1 type here, unless you'll sometimes be connecting the output side to AC line stuff again).

- Or, if this is periodic, the rectifier diodes can be doing the same thing as the switch.  An R+C across each diode can help that (usually on the order of 0.01-0.1uF + 10 ohm).

Tim

[liquibyte]

I just wanted to be able to give you what I thought were the relevant pics.  I'll agree that the first two are set at a really small V/div setting and I couldn't figure out why he was looking at things at that setting but the one that concerns me is the third pic.  I've measured this effect at 10V/div and am seeing an overshoot from the steady rectified and filtered voltage of 43.7V up to ~63V which is what I think is pulling the output at 30V up by 6 or 7 volts at turn on.  If this is a real situation and I'm looking at it right, it's going to eventually damage the op amps right?  This is why I thought that a slow start or soft start circuit might be needed.  I'm fairly sure it's not just RFI because I can see the trace rise by that amount and I can also see it with my multimeter so it must be happening over a longer time period.

I've got a shot from mine, but just at operating conditions and not power on.  Like I said, I only have a 465B but it's a real treat compared to the Conar 255 I was using.  This screenshot is 30V @ 3A fully loaded at 2mV/div @ 20ms/div at the output.

[T3sl4co1l]

Oh, the blue trace I guess?  Those waveforms are pretty crusty anyway, lots of ripple or HF interference or something.

6-7V out of 30V doesn't sound like a big deal.  A power supply, being what it is, shouldn't have any overshoot and should be tuned for overdamped response instead, but it's not terrible.

Why would it be bad for the op-amps?  They're the things making it in the first place...

Are you saying the overshoot is on the internal supply, not the output?  I'm really confused about what voltages and parts you're concerned about.

I remember from the thread, there was concern about op-amps rated for enough voltage, which you'll have a hard time finding regardless.  Your only major alternatives there are:
1. Replace the transformer so it runs at lower voltage (24 or even 18VAC?), adjust output voltage scale for consistency
2. Replace op-amps with discretes
3. Use op-amps, but make a low voltage supply for them to run on, and level translate with discretes

(1) changes the output range which might be undesirable (i.e., sometimes, you simply have to have 30V from it), and (2) and (3) basically mean ripping everything out and starting over.  Which I would kind of rather have, just because it's a somewhat janky design to begin with, but it's a lot of work.

Tim

[liquibyte]

Oh, the blue trace I guess?  Those waveforms are pretty crusty anyway, lots of ripple or HF interference or something.

6-7V out of 30V doesn't sound like a big deal.  A power supply, being what it is, shouldn't have any overshoot and should be tuned for overdamped response instead, but it's not terrible.

Why would it be bad for the op-amps?  They're the things making it in the first place...

Are you saying the overshoot is on the internal supply, not the output?  I'm really confused about what voltages and parts you're concerned about.

I'm getting 43.7 volts at pin 4 of U1 and U2 which is right at the upper limit of the 2141's tolerance.  If I get a +20 volt spike on top of that voltage at power on, I'm worried that I'm going to damage those two op amps.  U3 is behind the 10V zener due to the negative supply.  What I think is happening is that the 63 volt spike is pulling the output up by the 6-7 volts @ 30V I see there.

I remember from the thread, there was concern about op-amps rated for enough voltage, which you'll have a hard time finding regardless.  Your only major alternatives there are:
1. Replace the transformer so it runs at lower voltage (24 or even 18VAC?), adjust output voltage scale for consistency
2. Replace op-amps with discretes
3. Use op-amps, but make a low voltage supply for them to run on, and level translate with discretes

(1) changes the output range which might be undesirable (i.e., sometimes, you simply have to have 30V from it), and (2) and (3) basically mean ripping everything out and starting over.  Which I would kind of rather have, just because it's a somewhat janky design to begin with, but it's a lot of work.

Tim

If U3 is behind a 10V zener, why could't I do that for the other two so that I drop a few volts there?  One of the things suggested in the other thread was that we really need an easy to build supply for hobbyists.  I see discussions come up all the time about which supply to build and the two options most talked about are variable voltage regulator supplies and this one, or one if its variations.  Now that I'm this deep into it, I will have to agree that it's somewhat janky.  I think my best bet would be to use this mosfet circuit on what I have now and then work on a redesign that doesn't suffer from these issues.  I'm far from qualified for this but I think I may give it a go.

[liquibyte]

I asked redwire to get some screenshots of the voltages across pin 4 and pin 7 of U1 and U2.  He said that the "Second try was a bit shaky as it was just holding against pin 7 while trying to plug in the transformer" for the fourth picture.  I think this should clearly show what kind of voltages we're getting into the power pins of the op amps.

Edit:  I meant to add that this is on power up.

[T3sl4co1l]

I don't see overshoot?  Or at least not much.

[liquibyte]

Do you think that the 2N3055's could be at fault here?  2N3055's have a Vceo of 60V, will that break down and allow a voltage across it if it's a bit more than that?  If so, would a faster transistor help in this, say 4, 6 or 8MHz and perhaps a bigger Vceo like a MJ15022?

[T3sl4co1l]

I'd be more worried about the op-amps.  Though I'd never use 2N3055 out of principle, anyway...

Vceo only matters on supply to output voltage.

Tim

[liquibyte]

I think I'll go with this mosfet circuit then and see how it goes.  I'll try and get to the bottom of the issue and see if it can eventually be solved.