UCC3806: Degranding and calculating the Cbias

Diego Souza50

Part Number: UCC3806
Other Parts Discussed in Thread: UCC2894

Hello!

I am supporting an older design that uses the UCC3806 (please, see the schematic bellow) and now is showing problems degrading the Cbias capacitor.

Four power supplies, that were a long time (a least one year) turned-on, stopped to working after the turned-off/turned-on event. Measuring the voltage circuits, I saw that Vin voltage was oscillating between the 7V and 8V, therefore the controller are in hiccup. The power supply works properly after changed the Cbias capacitor (C9) for a new one.

So, I remove the C9 (100uF, 25 V from Epcos/TDK - B41821-A5107-M) for all products and measuring it the capacitance:

A -> 5.540 uF / 0.97 Ohm

B -> 18.721 uF / 33.04 Ohm

C -> 10.938 uF / 43.14 Ohm

D -> 4.544 uF / 0.118 Ohm

I measured the C9 capacitance of another board (that is turned-on a few months) but without the problem: 74.346 uF/ 1.98 Ohm

Looking the datasheet of UCC3806 and the application notes (slua150, slua138), I am not found the equation to calculate the capacitance of Cbias, nether the others parameters of this capacitor (ESR, ESL, Irush, etc). The datasheet typical application just shows a 15uF capacitor.

Please, somebody could explicam me how I calculate this capacitor and its features? In addition, I would like to understand why the capacitor is degrading and how I can fix this problem?

Thanks for attention!

over 7 years ago

0 Brian Johnson96 over 7 years ago

TI__Expert 3200 points

Diego,

An AE will help you soon.

0 Oliver Pan91 over 7 years ago

TI__Intellectual 2230 points

Hi Diego,

The schematic is lost, could you attach it with the waveforms, thanks

BR
Oliver

0 Diego Souza50 over 7 years ago in reply to Oliver Pan91

Prodigy 130 points

Hi Oliver!

Please, let me know if you can find the schematic attached. I will pick up the waveform and attach in sequence.

0 Diego Souza50 over 7 years ago in reply to Diego Souza50

Prodigy 130 points

Please, find the waveforms attached.

Let me know if you need more information.

Regards,

Diego

0 Oliver Pan91 over 7 years ago in reply to Diego Souza50

TI__Intellectual 2230 points

Hi Diego,

Yes, I see the schematic as well as the waveforms, but these information may not enable me to inferred the root cause of failure. As you know, the capacitor degrading can be caused by a lot reasons inculding physical and chemical reaction such as Liquid electrolyte vent, high shock and vibration, Voltage/Current and Temp. I suggest to take a look this training sections which may help you out, it covers overview of different types capacitors, and tips for selecting the appropriate capacitors for switching power supply.

Thanks

Oliver

0 Diego Souza50 over 7 years ago in reply to Oliver Pan91

Prodigy 130 points

Hi Oliver,

Ok, I agree if you that are a lot of reasons, but I would like to know: how I can calculate the value of this capacitor in this application with UCC3806? What the value of ESR should be?

The datasheet and the applications notes not have these answers.

I saw in another controller (UCC2894) an equation to calculate this capacitor to meet the requirements of Cbias. Therefore, should be an equation to calculate the Cbias of UCC3806 too.

Regards,

Diego

0 Diego Souza50 over 7 years ago in reply to Oliver Pan91

Prodigy 130 points

Guys,

It is serious that you will left me without answers?

Please, I would like to know: how I can calculate the Cbias capacitor in to UCC3806? What the value of ESR should be?

Regards,

Diego

0 Oliver Pan91 over 7 years ago in reply to Diego Souza50

TI__Intellectual 2230 points

Hi Diego,

I apologize for the delay, I think the calculation for UCC3806 bias capacitor is no difference with the calculation in UCC2894 datasheet. You can use these equations (13)~(16) to calculate a minimum bias capacitance for UCC3806, just guarantee the voltage drop on the capacitor would not fall below the its UVLO threshold. A minimum 0.01-μF ceramic capacitor must be placed closed to the VIN pin to bypass the HF noise.

Thanks
Oliver

0 Diego Souza50 over 7 years ago in reply to Oliver Pan91

Prodigy 130 points

Hi Oliver!

I will try to use these equations. Thank you.

Regards,

Diego.

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UCC3806: Degranding and calculating the Cbias