Capacitors or diode!!!!!!???

[Zerox100]

Hi,

I have faced a very strange thing.

I am designing a FPGA board. I had +5v and +3.3 v power on board. i created +3.3v from +5 by two serial diode. When i placed two smd diode i saw the 3.3v voltage is actually about 2v. I checked the circuit. I saw every diode falls about 1.4v. I supposed that they are double diodes (two serial diodes in one package). then i removed one of them and replace the one of diodes by a zero R. the power started to works and every thing was OK.
After finalizing the board and finishing coding of FPGA, I checked the diode again. They were not diode!!!!! It was a mistake. they were polar capacitors (3.3uf). But they were working like diodes without any problem for a long time!!!!!

What do you think??? Are they capacitors made by reverse diodes???? Is it possible????!!!

 

[DeboraHarry]

A revered biased diode will act as a capacitor, but the capacitance will be sub uF, probably sub nF. There is no way it will be 3.3 uF.

 

[FvM]

Your supply method sounds like a promising method to ruin a FPGA. Even when using regular diodes, I doubt that you are able to actually keep the supply voltage specification. A 3.3V LDO would be a more reliable way.

The only capacitors that work diode-like at lower voltages are tantal capacitors. Of course they aren't designed for this kind of operation and likely to fail with reverse voltage.

 

[Zerox100]

We had done that several times. It is regular. we use XC95288XL in several design. We need to keep IO voltage as high as possible. When we use two serial diodes the IO voltage is about 3.5v. This circuit is working in several industrial boards. No problem reported.

The problem is about tantal caps that operates like diodes!!!

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A revered biased diode will act as a capacitor, but the capacitance will be sub uF, probably sub nF. There is no way it will be 3.3 uF.

You are right. I know that. But i have seen that strange thing!!!!! I want to know why???

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A revered biased diode will act as a capacitor, but the capacitance will be sub uF, probably sub nF. There is no way it will be 3.3 uF.

You are right. I know that. But i have seen that strange thing!!!!! I want to know why???

 

[mtwieg]

A reversed bias polar capacitor can definitely act like a diode; it will conduct DC when biased with enough voltage in the wrong direction, while blocking in the other direction. Obviously this is something you shouldn't be doing on purpose.

 

[FvM]

"Diode"-like behaviour of tantal capacitors is created by the semi-conducting MgO2 cathode layer. The problem is that the capacitor will easily fail with short circuit when higher reverse currents are caused.

XC95 series has a rather large 3.0 - 3.6 V supply voltage, it agree that it can work with the diode circuit - if the 5V supply has tight tolerances and the temperature range is restricted.

 

[pulse90]

Capacitor is a voltage storage device used for power factor correction in the supply while as Diode allowed only one direction flow of voltage ,this is no storage.
Thanks.