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#1 Mar 14, 2017
P-Dizzle
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Joined: 2017-01-12 05:05

Circuit does not work when lifting the potential of it

Hello together,

does anyone have some hints on what is going wrong in the following circuit? I want to build up a boost converter that boosts my 4VDC to approximately 16VDC. To do so I use the TPS61040 from TI (http://www.ti.com/product/TPS61040).
As can be seen in the first two pictures, the circuit is working just fine as desired.

But if I now add an additional voltage source betwenn GND and the node OUT-, the circuit does not work anymore (see pictures 3 and 4). But I do not understand why... as all my signals of the boost circuit are referenced to the node OUT-, the boost circuit should not see a difference between the two scenarios? Or what am I getting wrong here?

I also simulated the same circuit using a similar LinearTechnology device (LT1615) on a different simulation software (you can guess which one) and there it works perfectly fine if I add additional sources of whatever value...

Thank you very much for your help!

With best regards
P-Dizzle

1. Circuit 

2. Results 

3. Circuit with additional voltage source 

4. Results with additional voltage source 

Tue, 2017-03-14 04:15
naveenatmit
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Joined: 2017-03-14 03:24

Vfb - Feedback trip point voltage is already 1V when you connect 1V source between GND and OUT-.

For 4V Vin, Vfb reaches 1.3V when Vout is 4V... Hence it is not working as you expect. 

 

Normally, Vfb is 0V when Vout is 0V. When you turn ON, Vout reaches 16V because only at that time Vfb reaches 1.23V.

In your situation, Vfb reaches 1.3 V when Vout is 4V itself.

Tue, 2017-03-14 05:04 (Reply to #2)
P-Dizzle
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Last seen: 2 weeks 6 days ago
Joined: 2017-01-12 05:05

Hi naveenatmit,
first of all thank you for your fast reply!

But why is this a problem? The GND-Pin of the TPS61040 is also connencted to OUT-. So the V(FB) it "sees" should not be influenced by the voltage source Vadd, right? As it is (probably?) just taking the difference betwenn it's FB- and GND-Pin?

I just tried around a little more. If I assign a value of 0.0249V to Vadd it works fine. If I use 0.025V for Vadd I have the same problem as stated in the first post.Does this maybe help?

Regards
P-Dizzle

Thu, 2017-03-16 06:03
Alicia
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Joined: 2016-06-07 06:43

You simulate a similar circuit, not the same because you used a different device. 

This kind of devices relies on feedback values and sensing signals in order to work. Adding a 1V from GND to OUT- the value the devide read at FB should be quite differente form the one it reads without the additional 1V.

If you want to add a DC shift to the entire circuit you should add enough 1V DC sources so all the pins of the devide continues to read the same FB and sense.

 

Regards,

Alicia

Fri, 2017-03-17 08:22 (Reply to #4)
Alicia
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Joined: 2016-06-07 06:43

Well, checked again the the circuit and in fact the 1V voltage source is added to te entire circuit. The problem should have o do with the model.

Regards,

A

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