Antec SL-400 5vsb Mod with NCP1200

[LDSisHere]

Hello everyone,

This SL-400 is one that I recently resurrected in another thread. The entire reason I rebuilt it was to experiment with it in various ways. One of the things I wanted to do was to get rid of the two transistor 5vsb and replace it with something better. Although the Viper22A is an option it has several drawbacks especially in a surface mount package which I wanted to use for this project. Everell had found the NCP1200 driving the 5vsb FET on a power supply that had a 5A 5vsb circuit. If this chip would work it seemed like an ideal solution.

I removed all the components that would not be used with the NCP1200 circuit. The big difference in this modification and the Viper22A mod is that the 5vsb FET attached to the heat sink will be used. I am attaching pictures of the power supply before, during, and after modification. I will make another post with the load test results.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

I just got this working tonight so I only had time to do some quick initial testing. So far the results look favorable in my opinion. I want to run it under a heavy load for period time when I get an opportunity but for now this is what I got out of it:

No load -> Vdd = 12.55 <-> 5vsb = 5.143

10.0 ohm -> Vdd = 14.02 <-> 5vsb = 5.127

5.1 ohm -> Vdd = 14.39 <-> 5vsb = 5.111

3.4 ohm -> Vdd = 14.88 <-> 5vsb = 5.094

2.6 ohm -> Vdd = 14.88 <-> 5vsb = 5.078

2.1 ohm -> Vdd = 14.88 <-> 5vsb = 5.061

1.7 ohm -> Vdd = 14.89 <-> 5vsb = 5.045

The Vdd is capped with at 15V Zener diode, as the max Vdd for the NCP1200 is 16V. One odd thing was that under no load I could hear a slight high pitched ringing coming from the supply. The ringing disappeared as soon as the first load resistor (10 ohms) was added to the supply. My load tester is capable of adding even more load but I did not want to blow anything up, at least not yet, I will save that until after I do more testing at reasonable loads.

[everell]

Re: Antec SL-400 5vsb Mod with NCP1200

Wow,,,,that looks great!

What kind of transistor is the NCP1200 driving? Bipolar transistor or FET?

I'm also wondering how much the voltage is increasing to the VDD going to the NCP1200 and the VDD going to the main PWM chip (UC3843). If either of these voltages get too high, things start blowing up and a domino effect starts taking out everything until the fuse blows.

Fantastic job on fixing that old SL400 and that 5vsb mod looks great.....so far. I would think that using a FET (faster switching time) would keep the transformer from blowing up.

[ben7]

Re: Antec SL-400 5vsb Mod with NCP1200

Nice!

Uh, wait... is it me or are R2 and D1 not hooked up properly on the PCB layout?

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

What kind of transistor is the NCP1200 driving? Bipolar transistor or FET?

The transistor is a FET it is a Fairchild SSS4N60B. I am attaching a picture and datasheet for it.

I'm also wondering how much the voltage is increasing to the VDD going to the NCP1200 and the VDD going to the main PWM chip (UC3843). If either of these voltages get too high, things start blowing up and a domino effect starts taking out everything until the fuse blows.

I did some quick testing when I got home tonight. I tested it with no load and a 1.7 ohm load to compare the differences in the winding output. In the attached pictures the meter on the left is attached to D8 which is supplying the power to UC3844. The NCP1200 is getting its' voltage off of the side of the coil winding that used to go to L2.

With no load the D8 voltage is 19.33V but the NCP1200 is only seeing 12.58V for its' Vdd. When the load goes to 1.7 ohms, D8's voltage jumps to 23.35V while the NCP1200's Vdd is limited to 14.84V by the Zener diode.

Since the UC3844 can take up to 30V input that is not a problem, the only concern is how much power the Zener is having to burn off as it would be bad for the NCP1200 if it fried.

I will do some more testing when I get time.


On a side note, I do think the NCP1200 works much better with a FET than it does with a NPN transistor.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

Nice!

Uh, wait... is it me or are R2 and D1 not hooked up properly on the PCB layout?

I think the "silkscreen" is skewing your perspective, the copper is not connected. I am attaching a picture of a populated board that I was experimenting with on a different supply so the 110 ohm resistor is replaced with a wire. (Best picture I have on hand of an etched board.)

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

I was able to do some 5vsb testing on this supply today and so far the results look good.

I ran it with a 2.1 ohm load for one hour taking temperatures with my IR temp gun so the true values my vary but at least it is an idea of what it is doing under load.

NCP1200 chip - Initial Temp 68F -> one hour later 78F
Heat Sink (top center)- Initial Temp 68F -> one hour later 80F
Transformer - Initial Temp 66F -> one hour later 119F

I then went directly from this test to a 1.7 ohm load for an hour so there was no cool down.

NCP1200 chip - one hour later 80F
Heat Sink (top center)- one hour later 83F
Transformer - one hour later 128F

This supply is only rated for 2A on the 5vsb so I am leery of pressing it farther. The FET on it is rated for 4A and I really do not care if I blow it or the NCP1200 but I would really be unhappy if I burned up the transformer so I am not sure how much more load I can safely push without risking damage to the transformer. However, the temptation to keep pushing the limits is very great.

[everell]

Re: Antec SL-400 5vsb Mod with NCP1200

However, the temptation to keep pushing the limits is very great.

Do it!!! If you can fix that ole SL400 once, you can always fix it again.

If you don't push it, you will never know what it could have done. That 5vsb transformer IS replaceable.

[ben7]

Re: Antec SL-400 5vsb Mod with NCP1200

You could do other things like play around with the operating frequency, and also try to increase efficiency; if you would like to.

[everell]

Re: Antec SL-400 5vsb Mod with NCP1200

The NCP1200 is fixed frequency.......three different frequencies available. Here is my Antec SmartPower 330 sporting the NCP1200 5vsb mod circuit using the 65 Khz chip. I think most of those two transistors circuits run somewhere close to that frequency.

Under no load it is putting out 5.18 volts. Under 2.5 ohm load it is putting out 4.96 volts. That is good for about 2 amps which is what the power supply spec says on the label.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

OK I was did some more testing and I was surprised by the results. This supplies 5vsb is rated for 2A and I ran it for an hour today pulling 3.33A. 5V on a 1.5 ohm load.

NCP1200 chip - Initial Temp 65F -> one hour later 83F
Heat Sink (top center)- Initial Temp 67F -> one hour later 87F
Transformer - Initial Temp 64F -> one hour later 143F

I took some pictures this time, so pay attention to the time on the clock. The 5vsb voltage actually rose some shortly after I turned it on which I attribute to the parts warming up but that is just conjecture based on the fact that it stabilized after a few minutes. I think I reached its' limit with my next test however, but I will put that in another post to keep the pictures separate.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

After the 1.5 ohm test, I paused long enough to take a picture of the next resistance reading which was 1.3 ohms. I then turned back on to see what would happen. The 5vsb voltage started dropping almost immediately. I only ran it for ten minutes before stopping the experiment since no good could come from continuing. Check out the pictures to see how the voltage dropped in the short amount of time also look at how much the temperatures jumped.

NCP1200 chip - ten minutes later 85F
Heat Sink (top center)- ten minutes later 93F
Transformer - ten minutes later 160F

[everell]

Re: Antec SL-400 5vsb Mod with NCP1200

The power FET has a low value resistor between source and ground. This resistor develops a voltage which the CS (current sense) on the NCP1200 looks at to determine overcurrent cutoff. It could be that you have arrived at this point. To determine if this circuit will do more, you will need to change this resistor to a lower value. Then retest and hope............



hope that this doesn't happen.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

The power FET has a low value resistor between source and ground. This resistor develops a voltage which the CS (current sense) on the NCP1200 looks at to determine overcurrent cutoff. It could be that you have arrived at this point. To determine if this circuit will do more, you will need to change this resistor to a lower value. Then retest and hope............



hope that this doesn't happen.

I think the resistor you are referring too measured 2 ohms when I was tracing out the circuit. I now have 1 ohm resistors with choice of 2 or 3 watt ratings to choose from that I could use. I am not sure if the 3 watt will fit but if it will that is what I will use. I have a few other tests that I want to try first.

Tonight, I finished populating a board with the 100K version of the NCP1200, but I only had time to do a quick check of the voltages. I also changed resistor R2 (Vdd) from 110 ohms to 200 ohms to reduce the load the zener has to burn off. This may have not been the best idea for comparison purposes so I will have to make a 60K board with a 200 ohm resistor for a more direct comparison. On my Ultra the 100K chip worked better but for this Antec that may not be the case. I just ran the supply for a few seconds at each setting, I will have to wait until I have more time to do longer tests.


No load -> Vdd = 10.33 <-> 5vsb = 5.140

2.1 ohm -> Vdd = 12.84 <-> 5vsb = 5.055

1.7 ohm -> Vdd = 12.87 <-> 5vsb = 5.037

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

I setup a 60k NCP1200 with a 200 ohm Vdd resistor to match the 100k NCP1200 I had setup previously so that the test results would have less variables involved.

100k chip - NCP1200 - 200 ohm vdd resistor (R2 on my schematics)


No load -> Vdd = 10.33 <-> 5vsb = 5.140

2.1 ohm -> Vdd = 12.84 <-> 5vsb = 5.055

1.7 ohm -> Vdd = 12.99 <-> 5vsb = 5.037

After 1 hour

1.7 ohm -> Vdd = 13.04 <-> 5vsb = 5.038



60k NCP1200 - 200 ohm vdd resistor (R2 on my schematics)


No load -> Vdd = 10.27 <-> 5vsb = 5.141

2.1 ohm -> Vdd = 14.07 <-> 5vsb = 5.056

1.7 ohm -> Vdd = 14.07 <-> 5vsb = 5.038

After 1 hour

1.7 ohm -> Vdd = 14.07 <-> 5vsb = 5.039


The 5vsb output difference between the two is trivial but there is quite a difference on the Vdd. In either case the increased resistor value prevents the Vdd zener's 15V threshold from being reached which I would think is a good for a longer service life of the zener itself.

Below are the results I got with the original 110 ohm Vdd resistor to make it easier to compare the difference between them.



60k NCP1200 - 110 ohm vdd resistor (R2 on my schematics)

No load -> Vdd = 12.55 <-> 5vsb = 5.143

2.1 ohm -> Vdd = 14.88 <-> 5vsb = 5.061

1.7 ohm -> Vdd = 14.89 <-> 5vsb = 5.045

[everell]

Re: Antec SL-400 5vsb Mod with NCP1200

With the 100k NCP1200 chip the Vdd went down a little, but did the temperature of the transformer go up? That might be the tradeoff. As you increase the 5vsb loading even more, you might be looking at transformer "melt down" much sooner than with the 60k chip. We MIGHT even find that the transformer is optimized for 60 khz, that the primary winding and 5vsb winding are right, but the Vdd winding simply has too many windings.

The Vdd winding and its DC voltage going to the 5vsb chip is why I had to be selective as to which chip I used and which chip worked best.

I am looking forward to seeing results when you go to 1.0 ohm loading on the 5vsb output.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

You have a good point about the transformer temperature, that is something that I need to check along with the voltage going to the UC3844 PWM chip to see how it is affected by the 100k chip. I was using my third meter at the time for other tasks while these tests were running off to the side. I think it is interesting to see the difference the frequency makes on the circuit.

I will hopefully have time to change out the resistor in the near future. As far as pushing the 5vsb transformer, what temperature should be the cutoff to make sure it is not pushed to the meltdown point? Keep in mind the accuracy of the IR thermometer I am using is questionable.

[everell]

Re: Antec SL-400 5vsb Mod with NCP1200

I am not sure what the meltdown temperature of the transformer would be. I am not even sure if we will be able to get 5 amps from the 5vsb using a one ohm resistor load. If we can, maybe some manufacturers will be watching and maybe we will start seeing some major improvements in 5vsb technology.

[ben7]

Re: Antec SL-400 5vsb Mod with NCP1200

With the 100k NCP1200 chip the Vdd went down a little, but did the temperature of the transformer go up? That might be the tradeoff.

Also an interesting thing to know would be the efficiency.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

What kind of setup would I need to measure the efficiency? If I knew how to test it I would try it as long as I already have the tools on hand.