Using a TVS diode instead of, or in addition to a MOV for AC line protection?
It comes downto what are you trying to protect against
There are four main types of transient suppression devices
Gas Tube
- Protection time: > 1us
- Protection Voltage: 60 - 100V
- PowerDissipation: Nil
- Reliable Performance: No
- Expected Life: Limited
- Other: Only 50-2500 surges, can short powerlines
MOV
- Protection time: 10 - 20ns
- Protection Voltage: > 300V
- PowerDissipation: Nil
- Reliable Performance: No
- Expected Life: Degrades
- Other: Fusing required. Degrades
Avalanche TVS
- Protection time: 50ps
- Protection Voltage: 3-400V
- PowerDissipation: low
- Reliable Performance: yes
- Expected Life: long
- Other: Low power dissipation. Bidirectional requires dual
Thyristor TVS
- Protection time: <3ns
- Protection Voltage: 30-400V
- PowerDissipation: Nil
- Reliable Performance: yes
- Expected Life: long
- Other: High Capacitance
http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF
( http://web.archive.org/web/20051001082352/http://www.onsemi.com/pub/Collateral/HBD854-D.PDF )
http://www.vishay.com/docs/88440/failurem.pdf
The overvoltage protection circuit subject is complex and there simply isn't a one size fits all remedy. Things to consider are: where will the circuit be located in relation to the service entrance, are you attempting to protect for short and/or long duration surges, and what type of surge are you planning on protecting against? From my research, the best attempt at a one size fits all overvoltage protection scheme would include line conditioning filtering (inductors/capacitors), (MOV) Metal Oxide Varistors with integral thermal disconnect fuses and Silicon Avalanche Suppressor Diodes. I would talk to the engineers at Littlefuse for advice. They have a large library of downloadable PDF's pertaining to Overvoltage Suppression.