transistor diode clamp

[Stickshifted]

I am very new to multisim, and I am trying to model a portion of this circuit: http://www.ti.com/lit/an/snoa600a/snoa600a.pdf

Scroll down to figure 10, "Son of Godzilla Booster." The portion that I am trying to model is the transistor diode clamp at Q5 and the 1N4148 diode. This circuit is supposed to protect the compensation pin number 9 in the lm3524 switching regulator from receiving more than 4v. Taking a look at the datasheet for the LM3524DN, when pin number 9 sees 1v, it runs at 0% duty cycle, and when it sees 4V, it runs at maximum (45%) duty cycle. I am building this circuit in lab, and after building that part of the circuit, the clamp cuts off at about 5.4 volts instead of 4V as i scroll voltage up past 5V on my power supply. 

I have modeled this circuit in multisim, and I have V1 used to represent the 411 opamp voltage output into the 2K resistor, and I have a 15 volt DC source to represent the +15 supply to the 10K resistor. 

My multisim model complies quite well with reality and I am wondering if I need to select different resistor values so they will actually limit my voltage to only 4v, OR if there was a good reason for making the cutoff 5.5 volts.

[_user32]

Stickshifted,

Do you have a partial circuit you can share?  This will make it much easier to try out and assist compared with trying to recreate your work up to this point.

You should be able to post any Multisim file as an attachment.

Regards,

Pat Noonan

NI  

[Stickshifted]

Sorry about that, here is both a jpeg image of my circuit, as well as the multisim file

[_user32]

stickshifted,

Since the exact internal structure of the compensation pin (pin 9) is not known, it is likely that the circuit in the Figure takes this into account when calculating the bias point for Q1.   Have you checked this voltage at pin 9 in the lab with the LM3524 in the circuit?   If not, it may be that pin 9 has non-ideal input impedance (R << inf), therefore it is likely that this is basically acting like a parallel resistance with the 100K resistor.   If it does have a large impedance, you can adjust the bias point by changing R2 to something like 2.9k.

An example scenario is shown here (this is a snippet - just drag into Multisim v12 from your browser):

Although a general block diagram of the chip is given, it is very unlikely you'll be able to generate an equivalent circuit with this information only.  It is highly recommended to check with someone from TI on the exact determination of what exactly is the structure on this pin 9 (or ask for an estimate on the equivalent circuit model for the input impedance on pin 9)

- Pat

[Stickshifted]

hey thanks.

that is a pretty interesting thing to think about now that you mention it. And i also see that you checked out the datasheet for the LM3524. I guess i probably should posted its datasheet also. 

right now everything which i call the "low voltage side" is all on a proto- breadboard, and the high voltage side - the transformer,bridge rectifier, feedback with the 1meg/10k divider, is all soldered up in the case of an old CPU power supply - I have tested both sides separately, but not together. 

I do need to do more tests in the lab - when i took my measurments, this was while it was connected to the compensation pin, and not connected. I have reason to believe that the compensation pin is of reasonably high impedance. Though what was interesting, is that before connecting anything to the compensation pin, the low voltage circuitry was running at max 45% duty cycle, but the second i connected the 100k resistor, the duty cycle dropped to what looked like 10%. I dont think i measured the voltage on the pin during that time however. 

I might have a chance tommorow to take some more measurments.

Heres a link to the LM3524 datasheet if anyone else was interested. Seems like its been around since at least the late 70's.

http://www.ti.com/lit/ds/symlink/lm2524d.pdf

[_user32]

Stickshifted,

Yes - it does appear to be an older chip, however it appears they still currently sell a new version of that device.  

In any case, I definitely over simplified the pin 9 circuit, but through some experimentation and measurements you will likely find a useful equivalent circuit so that you can approximate its structure and get adequate results. 

Also note that from your original circuit, I replaced the volt meters with probes (its the yellow icon on right side of schematic sheet, and I formatted it to just display voltage) and I also replaced your static DC source with our new v12 interactive DC source to give you the ability to manually sweep the DC voltage with the slider control while simulating.

 - Pat

[Stickshifted]

man that whole drag and drop thing is pretty cool, and im definitely liking the interactive DC voltage source.

If i dont call Texas Instuments, when I get the chance, ill ask one of the electrical professors to see if I should reduce the clamping voltage or not. So far there is one actual mistake on the schematic, the TY-85 transformer is depicted as having pin number 5 as the centertap - but pin 6 is the actual centertap. pins 5 and 6 just need to be reversed. And this was also confirmed with Triad Magnetics. And when i finally found a place that was selling those ancient transformers, i saw that pins 5 and 7 had blue wires, and pin 6 had a yellow wire indicating that its the centertap. 

How do I create those probes that you put on my circuit? I didnt seem to be able to find the same ones when i went to Place Component>Indicators>Probes. Is this something other than a component?

-Robby