Instek GPD-3303S Power Supply Overshoot - Opinion wanted

[Housedad]

I understand.   I'll try to find it and get a pic.

As I look at the photos I took and compare them to the Siglent unit from the other thread, I'll be danged if they don't come extremely close to the same.   they launched their psu within a year of Instek.  I think they bought the innards from Instek and added their own intelligent display board and front panel. 

All the points on the control board that they mention to add parts to in the Siglent thread are there on the instek.   Heck it looks like the control boards are better than 95% the same layout and parts.


I gotta find a schematic of the siglent.  That will tell the story for sure.   

[Housedad]

Apparently Siglent SPD3303() power supplies also have / used to have this problem, and some fixes are discussed here:

https://www.eevblog.com/forum/testgear/siglent-spd3303d-review/25/

Depending on how close the designs of these supplies are, it could be useful to read that thread.

Yes, thank you for finding that thread!!


I decided that I'm going to pull the control boards from both units and see if I can figure out the differences between the old and new and see just how close they seem to the siglent.

[bitseeker]

Thanks for all the photos. They pretty much confirm that the changes were to support the new four-channel model.

Looking forward to your findings on the comparison with the Siglent version. Hopefully, it really can be fixed without too much trouble.

[Housedad]

Ok.  looking closely at the cad drawings from the Siglent thread,  you can see That pin 1 of UC3 goes to RC5 and RC9.  RC5 then goes to pin 2 of Ic4.  UC3 and UC4 are TLO72 dual op amps.  This aligns perfectly with The Instek schematics.   Look on page 83 of the Instek service manual and you see U704A and U704B.  These are near the exact same position as the siglent board and the rest of the traces shown in the x ray view of the siglent line up perfectly with the schematics in the instek service manual.  RC5 on the Siglent thread corresponds to R715 (1.5K) on the Instek. 
It seems more and more that Siglent had the schematics and had the board updated and went more SMT.   Way too much is the same to be coincidence.
A telling thing is that after installing the transistor and the other components, the OP in the Siglent thread stated that he still had the overshoot.  It was resolved only after he changed RC5 to a higher resistance, Changing the voltage going into the other op amp.   In other words, they are changing the bias input of the second OP amp.  I wonder if this is actually a saturation problem of the second op amp.

Siglent UC3 = Instek U704A
Siglent UC4 -  Instek U704B

Well, I finally figured out that my Nikon pocket camera has a macro mode.   I have PSU parts all over my table, and I hope to get more comparisons and pics out sometime soon.

I also want to figure out on the schematics what they were trying to do with the other components

[Kleinstein]

Due to a request by PM, here are some explanations to proposed modification with an additional RC in parallel to R712 (for channel 3, circuit from linked service manual):

The circuit for channel 3 is a basic low drop regulator with a floating regulator circuit. However it only has basic compensation with a single dominant pole. This known to be problematic, not only for overshoot, but also for ringing in case of a highly capacitive load.  I did a simulation of a slightly simplified circuit to reproduce the problem.  Adding a RC combination in parallel to R712 is somewhat standard in similar circuits. It is similar to the function of C110 for channel 1, though the circuit is slightly different. One can see this a little like going from an PI regulator loop to an PID loop. It adds response to the rate of change in output voltage and is this way reducing overshoot. It has two purposes:
1) It adds some more phase reserve in the 100 Hz-1 kHz range and this way should reduce possible problems with a capacitive load. This showed up in simulation - no report of this from real life, as highly capacitive load with low ESR is not that common.
2) The point more related to the overshoot is that this change speeds up the control loop. The original loop is rather slow and thus needs relatively long to recover from saturation. The OP still goes deep into saturation, but with a faster loop there is less time to charge the output capacitor and thus a lower overshoot.

So the modification is not a cure to the root cause of the overshoot. It is more like reducing the symptoms with few added parts. Reducing the value of R715 as part of the Sigilent mod is also making the loop faster, though without the added advantage to the capacitive load case, and less effective on recovery.

Speeding up the loop could in principle cause instability at higher frequencies - but the loop is still not super fast and the simulation was still OK. Also to one actual change seems to work OK.

Excerpt from circuit:

[AviZiv]

Thanks @Kleinstein! Really appreciate the extra explanations even if I have to look up a handful of terms new to me LOL. Always an opportunity to learn something. No matter what I will probably try this mod and see if it helps in my case.

[rfdes]

I realize that this topic has been dormant for a while.  However, I am wondering whether the fix for the Siglent version (SPD-3303S) is the same as the Instek (GPD-3303S)?
My understanding is that Instek did fix the problem on the current systems but I could not find a more recent schematic for the Instek (GPD-3303S) to compare against my Instek unit.  Maybe I am incorrect in this understanding.

Does anyone have any further knowledge to share regarding the fix to the CH3 overshoot?
Thanks
Jim

[AviZiv]

Thanks for bringing this up again. I'll be following with interest.

I realize that this topic has been dormant for a while.  However, I am wondering whether the fix for the Siglent version (SPD-3303S) is the same as the Instek (GPD-3303S)?
My understanding is that Instek did fix the problem on the current systems but I could not find a more recent schematic for the Instek (GPD-3303S) to compare against my Instek unit.  Maybe I am incorrect in this understanding.

Does anyone have any further knowledge to share regarding the fix to the CH3 overshoot?
Thanks
Jim

[Fgrir]

However, I am wondering whether the fix for the Siglent version (SPD-3303S) is the same as the Instek (GPD-3303S)?
My understanding is that Instek did fix the problem on the current systems but I could not find a more recent schematic for the Instek (GPD-3303S) to compare against my Instek unit.

I don't know anything about the Siglent side of things, but for GW-Instek I think the opposite of your statement is true.  I believe that old Instek GPD-3303S units are OK, and newer ones built after the release of the GPD-4303S will have the CH3 overshoot problems.  If you read through the second page of this thread there is some discussion on the different versions out there.

I haven't looked into this since all the activity back in February so I suppose it is possible that GW-Instek have cleaned things up in the meantime, but for some reason I highly doubt it.

[rfdes]

I followed the post:  https://www.eevblog.com/forum/testgear/siglent-spd3303d-review/msg299246/#msg299246 Reply 27 where Siglent addressed the issue and contributed a hw patch to those ill siglent power supplies.

Based on the above thread, the designs are identical or near identical between the Instek and Siglent, although I don't know how that came to be.  Was this stolen or purchased from one another.  In any event I found a copy of the Instek Service manual online and the schematic contains about 90% of the fixes that are recommended in the Siglent post.  The Instek service manual has no dates or revision information so I am confused whether it  applies to the newer units or not.  I need to attempt to compare my service manual schematic against my hardware which I haven't yet done.

I have sent a request to Instek technical support today regarding this issue.  So, we will see what they have to say, if anything.

So, to reiterate what I believe to be true(please correct me if incorrect):
There are two designs of the GDP-3303S power supplies
#1 - Early model - only 3 channel - 2 adj (Ch1/Ch2) - 1 (2.5/3.3/5.0)(Fixed)  - This version had no issues with overshoot on the Fixed channel.
#2 - Later model -  Redesigned support two versions:
       4Ch version with Ch3/4 5V adjustable
       3Ch version same as #1 above (only with overshoot issues)

rfdes

[bitseeker]

Also on the later three-channel model, all binding posts are evenly spaced (part of the redesign to support the new four-channel model).

[rfdes]

I heard back from the Instek Service manager in California in regards to any suggested mods, etc. to resolve the overshoot issue.  Their recommendation was to piggyback a 100k resistor across R727.  This was discussed earlier by several members if I remember.

This did fix the output enable spike but obviously does nothing for the spike incurred following a current clamp release.  I then performed a spice simulation to test out the recommended fixes that Siglent had recommended to their customers. 
https://www.eevblog.com/forum/testgear/siglent-spd3303d-review/msg299246/#msg299246


The results were not promising in my simulation model.  There was significant instability that I encountered.  I believe the prevailing belief is that the Siglent and Instek models are nearly the same. The circuit topolgies appear the same but I cannot confirm the parts values.

Does anyone have a schematic for the Siglent power supply just to see the differences?  The fellow that posted the Siglent fix claims it worked for both his output enable and current short release spikes.

Any further help is appreciated.
Jim

[Kleinstein]

The additional 100 K parallel to R727 adds some offset to the set voltage and thus makes the added transistor for the output enable effective. It might work without, depending on the OPs offset.

The other suggested changes (a 20 K resistor in series to C717 (in the Instek schematics)) or an RC (e.g. 1.5 K 50 nF) combination in parallel to R712 have a rather similar effect. They kind of speed up the response and thus reduce the peaks, but they don't help against the root problem.

The root problem is integral windup of the voltage loop (U705 A).  One could in principle add some windup limiting to this. One scheme uses a PNP transistor with the emitter at the output of U705A, a diode from the inverting input to the collector and a high value resistor (e.g. 100-1M K range) from the base to the output of U705 B or the anode side of D705/D704  (the diode side works with output enable too, the OPs side works better when recovering from current limit). However there is a slight chance for instability, especially if the base resistor is too small. At least in the simulation it works reasonably. A more accurate integrator limiting as difficult to add to this circuit as the input side voltage can be higher than the output side.

[Eric-H]

I heard back from the Instek Service manager in California in regards to any suggested mods, etc. to resolve the overshoot issue.  Their recommendation was to piggyback a 100k resistor across R727.  This was discussed earlier by several members if I remember.

I've tried adding a 100k resistor parallel to R727 but it didn't remove the overshoot. The only effect I saw was that the 5V output voltage was lowered to 4.9 V.

[Kleinstein]

The added resistor in parallel to R727 only helps, with the mod / version that turn off the ref. voltage, when in output off mode. With a BJT used to turn off the set voltage, some offset is needed and the original 5 mV + OPs offset tend to be not enough. By itself it only adds more offset - thus the change in voltage.

[rfdes]

Yes, the R727 fix will only work if the CE saturation voltage on Q706 and Q707 is lower than the offset voltage.  I also tried it and it didn't work for me. 

Measuring the voltage at U705 pin 3 (pos inp) I was measuring around 30mV due to the sat voltage..  I changed the Q706/Q707 clamping bipolars to N chan mosfets.  The 2N7002 is a drop in pinout.   Now, I only see 1-2 mV on U705 pin 3.  The output of U705-pin 1 now will clamp at about -800mV.  The transient spike is now completely gone.  You will need to remove the front panel to do this but it was not bad at all.

• Unplug the one cable at the top of the front panel.
• Remove the ground quick connect attached to the bottom plate.
• Remove the top support bracket.
• Remove the two bottom screws holding the front panel.
• The front pane can now be swung back to gain access to the two transistors.

Give this a try and the Output enable transient will be gone.  However, this does nothing for the output short - release transient.
Jim

[rfdes]

I wanted to present my final fix for the GPD-3303S.  The fixes were recommended by several members and I simulated the fixes with Tina-Ti spice, made my modifications and am now quite happy with the results.

In a nutshell:
Replace Q706/Q707 with N-Chan Mosfet 2N7002.  These are footprint compatible and will drop right in.
Replace R727 with a 330K resistor.
Replace R717 with a 220 ohm resistor
Remove C717 and 'teepee' a 22K resistor in series,

The photos show the before and after transient responses using these modifications.
The changes were not difficult with the replacement of Q706 being the most challenging.
Good luck to others that decide to make the modifications.
Jim
rfdes

[AviZiv]

Looks really good. I'm going to try it on mine first chance I get.

Thank you so much!
Avi

[Rolo]

 I have done some testing on channel 3 with my 4 channel GPD-4303S power supply. I did nine measurements and wil post a screen of each measurment with a short decsription. Hope this info is usefull. I think the results are good, no overshoot found.
This is my unit :



GDP-4303S, CH3, Uset 3.3V, no load, switched from off to on


GDP-4303S, CH3, Uset 5.0V, no load, switched from off to on


GDP-4303S, CH3, Uset 9.0V, no load, switched from off to on


GDP-4303S, CH3, Uset 3.3V, 8.2 ohm load, CV, switched from off to on


GDP-4303S, CH3, Uset 5.0V, 8.2 ohm load, CV, switched from off to on


GDP-4303S, CH3, Uset 9.0V, 8.2 ohm load, CV, switched from off to on


GDP-4303S, CH3, Uset 3.3V, Iset 0.3A, 8.2 ohm load, CC, switched from off to on


GDP-4303S, CH3, Uset 5.0V, Iset 0.3A, 8.2 ohm load, CC, switched from off to on


GDP-4303S, CH3, Uset 9.0V, Iset 0.3A, 8.2 ohm load, CC, switched from off to on

[djnz]

I wanted to present my final fix for the GPD-3303S.  The fixes were recommended by several members and I simulated the fixes with Tina-Ti spice, made my modifications and am now quite happy with the results.

In a nutshell:
Replace Q706/Q707 with N-Chan Mosfet 2N7002.  These are footprint compatible and will drop right in.
Replace R727 with a 330K resistor.
Replace R717 with a 220 ohm resistor
Remove C717 and 'teepee' a 22K resistor in series,

The photos show the before and after transient responses using these modifications.
The changes were not difficult with the replacement of Q706 being the most challenging.
Good luck to others that decide to make the modifications.
Jim
rfdes

The FETs are SOT-23 and all other parts are 0603. They are on the control PCB in the following locations:

[AviZiv]

Thanks djnz! I'm planning on making this mod in the next few months.

[ArthurDent]

I've owned the older version of this GPD-3303S supply that doesn't have the overshoot problem and recently got a pretty good deal on the newer version with the problem. The overshoot on CH3 was up to 5.8V and on the 2.5V setting that could be disastrous. I just finished the modification and it works. CH3 turns on slower, in milliseconds  instead of microseconds, and there is no overshoot. Here is what the output looks like on the scope with no load.