voltage of the tvs diode on adsl modem

[raminrajabi]

Hi guys,
I have a dsl-2877al modem and I think in surge voltage the DSL chip called MT7555n and 4 TVS diodes failed, and after I've changed the IC and 4 TVS diode with 4.7 volts zener diode the DSL section of the modem worked again but after ringing the phone the IC blown up and failed again I don't know what exactly the TVS diode I have to put on the PCB to protect the surge voltage.
Please help.
Thanks.

[pcprogrammer]

Depends on where they are connected. Not up to speed on modern phone systems, but the line voltage used to be 50V or so and the ring voltage can be 90V AC.

Also a zener diode won't work for this because it can't take the energy a TVS diode can. So it just blows and the IC is subjected to the higher voltage anyway.

Edit found the ring voltage with a simple google search to range from 60-90V AC, but it depends on what is used in your country.

[raminrajabi]

Thanks for your good explaining.

[TheMG]

Where's the transformer? Did you remove it?

In all the ADSL modems I've seen there is a transformer that isolates the modem from the phone line. There is also a DC-blocking capacitor (still present in your picture) that prevents DC and the low frequency ring voltage from saturating the transformer.

Essentially, I can't see any way the ring voltage would blow the DSL chip unless that transformer has broken down isolation between primary and secondary or has been bypassed entirely.

[raminrajabi]

Yes, I removed it for testing, and it's okay and I put it back.

[fzabkar]

Just FYI, there is a pinout diagram of the VDSL2 transformer (T200) on page 9:

https://www.linkcom.com.tw/data/catalog/file/1649987169706387516.pdf

[fzabkar]

It's a different modem, but this TrendChip TCB3162L-E02 Reference Design shows some insight into the AFE section.

ADSL2+ LQFP-128 TC3162L Bridge/Router Processor:
https://xdoc.es/download/2-48-pdf-free.html

The protection components on the Tx/Rx line driver pins are 1N4148 / BAV99 clamping diodes.

Here is another design with bidirectional TVS diodes:
https://fccid.io/PJZ6738/Schematics/Schema-2106571.pdf

[raminrajabi]

Yes, I learned my lesson

[raminrajabi]

I will try to use 12V TVS according to this application. I hope it will work.

[fzabkar]

I wouldn't use that circuit as a direct guide. The supply voltage of its line driver IC is 12V, so a 12V TVS diode would seem OK to protect its inputs. The MT7555N line driver in your application may have a lower supply voltage, so a lower rated TVS diode would be required. The circuits I have provided are only useful insofar as they provide a general insight into what is going on at the AFE (analogue front end).

Here is a telephone DAA circuit which uses a bidirectional 5V TVS diode to protect the line driver:

http://locus-engineering.com/wp-content/uploads/2018/08/E2210-Telephone-Line-Interface-DataSheet.pdf

It seems to me that the current in the TVS diodes is limited only by the turns ratio of the transformer and the resistance in the local loop and C200 (?). Could C200 be shorted?

The ringer frequency is 20Hz, and C433 in the reference circuit is a 0.033uF 630V film capacitor. This has an impedance of 240 Kohm at 20Hz. Therefore the ringer current flowing in the TVS diodes would be about 0.4mA, assuming a transformer turns ratio of 1:1. That would result in a dissipation of less than 1mW.

Therefore it seems to me that C200 would be a prime suspect, assuming there is no primary-to-secondary breakdown in the transformer.

[raminrajabi]

Thanks sir for your time, yes I thought I should try using the smaller TVS like 4.7V  for first replacement and if it doesn't work then I try higher voltage.
The cap looks ok, but for sure I will replace it with new one.

[fzabkar]

About 20 years ago an Iranian colleague told me that the old Iranian telephone plugs were physically compatible with AC mains sockets. People would accidentally plug a telephone into a mains outlet, with the result that the phone would ring once and then die.

Is this still true?

[raminrajabi]

Yes, still same plug for both of them