TIDA-060027: Questions about SIDAC and PTC

Hi Hao,

Glad to hear that you have found the document helpful! Let's see if I can clarify on some things to help with your questions.

1. Is there any theoretical calculation for SIDAC and PTC selection? Because the reference design only describe the overall design considerations and the test result, we want to know more about how to choose SIDAC and PTC.
The main concerns when selecting protection devices for systems like this is to ensure two things - (1) the protection devices engage before damage would occur to devices they are protecting and (2) protection devices do not hinder normal system operation. For SIDAC or any crowbar device, this means ensuring the "snapback" or clamping voltage is below the absolute maximum voltage of the protected device. Conversely, the clamping device should not be active (or minimally impactful) at normal operating voltages. Similar considerations are needed for the PTC or other clamping devices. The current needed to activate the blocking device should be no more than what the clamping device can handle. This ensures that the current through the clamping device (SIDAC in this case) is limited so overheating does not occur). Coversley, the blocking device should not activate for normal operating currents expected for the system when no fault is present.
All of these values should be listed on the protection devices' datasheets so calculations should be minimal if needed at all - it will mostly be comparing thresholds to maximum specifications and so on.  

2. SIDAC belongs to Thyristor Surge Protectors in Bourns website, are SIDAC and thyristor surge protector the same devices?
I don't know for sure if these classifications use different technologies, but the application for both is the same - to divert excess energy away from a protected device by clamping the voltage to some threshold or activation value.  

3. Can SIDAC be replaced by TVS diode or Zener diode? Why or why not?
TVS diodes provide the same function as above. Because these are different technologies, there are different behaviors for each that lend themselves differently to different systems. Activation voltage range and curves, activation time, maximum current/heat tolerance, and other specification will vary based on what technology is used. From a high-level though, all of these provide the same function in this setting. 

4. Can PTC replaced by PPTC? Why or why not?
Yes, see answer to question 3. 

I hope this information is helpful. Let me know if anything is unclear or if you have any other questions. 

Regards,
Eric Schott