Antec SL-400 5vsb Mod with NCP1200

[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.

[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?

[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.

[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.

[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.