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Q&A Power switches and automotive load dump, unclamped

Power switches used in automotive are usually rated to withstand clamped load dump events, of the 35 - 40 V peak variety. AFAICT there are two main clamping mechanisms - first one for the logic par...

2 answers  ·  posted 1y ago by kellogs‭  ·  last activity 1y ago by TonyStewart‭

Question transistor-switch load overvoltage power TVS
#4: Post edited by user avatar kellogs‭ · 2023-08-24T16:09:53Z (over 1 year ago)
  • Power switches used in automotive are usually rated to withstand clamped load dump events, of the 35 - 40 V peak variety. AFAICT there are two main clamping mechanisms - first one for the logic part and second one for the MOSFET DS channel within. These power switches will generally (re)open the DS channel once the second clamping mechanism has engaged.
  • Since manufacturers do not boast about their power switches being able to withstand full (unclamped) load dumps it must be the case that the FETs within won't take that kind of energy / peak power.
  • Worst case as per ISO 16750-2: `Ri = 0.5 ohm, Vs = 79 V, tr = 5 ms, td = 400 ms`. Could I use some TVS diodes effectively for protecting against this ?
  • My targeted power switches `Rds(on) = 30 - 40 mohm` and for a led light of 200 mA normal draw @ 12V I would throw in `60 ohm` of dynamic reistance even for voltage spikes (?).
  • My targeted TVS dynamic resistance `Rd = 1/Pdc +/- 50% ~= 1/8 = 125 mohm` (formula pricked up from [here](https://electronics.stackexchange.com/questions/299373/determining-dynamic-resistance-of-tvs-diode-from-datasheet)). Multiply that by 3 for a stack of 3 TVS diodes -> `375 mohm`.
  • It appears at a first glance that the TVS stack is sufficient to help out a power switch make it through load dump events unharmed. Of course, the logic part must also tolerate the clamping voltage of these 3 series TVS diodes. Does anything escape my thinking ?
  • **edit** Diagram:
  • ![Image_alt_text](https://electrical.codidact.com/uploads/igcdobagisowbwy04xvhhzjc41x0)
  • Power switches used in automotive are usually rated to withstand clamped load dump events, of the 35 - 40 V peak variety. AFAICT there are two main clamping mechanisms - first one for the logic part and second one for the MOSFET DS channel within. These power switches will generally (re)open the DS channel once the second clamping mechanism has engaged.
  • Since manufacturers do not boast about their power switches being able to withstand full (unclamped) load dumps it must be the case that the FETs within won't take that kind of energy / peak power.
  • Worst case as per ISO 16750-2: `Ri = 0.5 ohm, Vs = 79 V, tr = 5 ms, td = 400 ms`. Could I use some TVS diodes effectively for protecting against this ?
  • My targeted power switches `Rds(on) = 30 - 40 mohm` and for a led light of 200 mA normal draw @ 12V I would throw in `60 ohm` of dynamic reistance even for voltage spikes (?).
  • My targeted TVS dynamic resistance `Rd = 1/Pdc +/- 50% ~= 1/8 = 125 mohm` (formula pricked up from [here](https://electronics.stackexchange.com/questions/299373/determining-dynamic-resistance-of-tvs-diode-from-datasheet)). Multiply that by 3 for a stack of 3 TVS diodes -> `375 mohm`.
  • It appears at a first glance that the TVS stack is sufficient to help out a power switch make it through load dump events unharmed. Of course, the logic part must also tolerate the clamping voltage of these 3 series TVS diodes. Does anything escape my thinking ?
  • **edit**
  • Diagram:
  • ![Image_alt_text](https://electrical.codidact.com/uploads/kd4zxotfs0cv7sfyxcp1bdaevtx2)
#3: Post edited by user avatar kellogs‭ · 2023-08-24T16:08:17Z (over 1 year ago)
  • Power switches used in automotive are usually rated to withstand clamped load dump events, of the 35 - 40 V peak variety. AFAICT there are two main clamping mechanisms - first one for the logic part and second one for the MOSFET DS channel within. These power switches will generally (re)open the DS channel once the second clamping mechanism has engaged.
  • Since manufacturers do not boast about their power switches being able to withstand full (unclamped) load dumps it must be the case that the FETs within won't take that kind of energy / peak power.
  • Worst case as per ISO 16750-2: `Ri = 0.5 ohm, Vs = 79 V, tr = 5 ms, td = 400 ms`. Could I use some TVS diodes effectively for protecting against this ?
  • My targeted power switches `Rds(on) = 30 - 40 mohm` and for a led light of 200 mA normal draw @ 12V I would throw in `60 ohm` of dynamic reistance even for voltage spikes (?).
  • My targeted TVS dynamic resistance `Rd = 1/Pdc +/- 50% ~= 1/8 = 125 mohm` (formula pricked up from [here](https://electronics.stackexchange.com/questions/299373/determining-dynamic-resistance-of-tvs-diode-from-datasheet)). Multiply that by 3 for a stack of 3 TVS diodes -> `375 mohm`.
  • It appears at a first glance that the TVS stack is sufficient to help out a power switch make it through load dump events unharmed. Of course, the logic part must also tolerate the clamping voltage of these 3 series TVS diodes. Does anything escape my thinking ?
  • **edit** Diagram:
  • ![Image_alt_text](https://electrical.codidact.com/uploads/ckab52uldk0qr13eh8r69lioi9ar)
  • Power switches used in automotive are usually rated to withstand clamped load dump events, of the 35 - 40 V peak variety. AFAICT there are two main clamping mechanisms - first one for the logic part and second one for the MOSFET DS channel within. These power switches will generally (re)open the DS channel once the second clamping mechanism has engaged.
  • Since manufacturers do not boast about their power switches being able to withstand full (unclamped) load dumps it must be the case that the FETs within won't take that kind of energy / peak power.
  • Worst case as per ISO 16750-2: `Ri = 0.5 ohm, Vs = 79 V, tr = 5 ms, td = 400 ms`. Could I use some TVS diodes effectively for protecting against this ?
  • My targeted power switches `Rds(on) = 30 - 40 mohm` and for a led light of 200 mA normal draw @ 12V I would throw in `60 ohm` of dynamic reistance even for voltage spikes (?).
  • My targeted TVS dynamic resistance `Rd = 1/Pdc +/- 50% ~= 1/8 = 125 mohm` (formula pricked up from [here](https://electronics.stackexchange.com/questions/299373/determining-dynamic-resistance-of-tvs-diode-from-datasheet)). Multiply that by 3 for a stack of 3 TVS diodes -> `375 mohm`.
  • It appears at a first glance that the TVS stack is sufficient to help out a power switch make it through load dump events unharmed. Of course, the logic part must also tolerate the clamping voltage of these 3 series TVS diodes. Does anything escape my thinking ?
  • **edit** Diagram:
  • ![Image_alt_text](https://electrical.codidact.com/uploads/igcdobagisowbwy04xvhhzjc41x0)
#2: Post edited by user avatar kellogs‭ · 2023-08-24T15:48:37Z (over 1 year ago)
  • Power switches used in automotive are usually rated to withstand clamped load dump events, of the 35 - 40 V peak variety. AFAICT there are two main clamping mechanisms - first one for the logic part and second one for the MOSFET DS channel within. These power switches will generally (re)open the DS channel once the second clamping mechanism has engaged.
  • Since manufacturers do not boast about their power switches being able to withstand full (unclamped) load dumps it must be the case that the FETs within won't take that kind of energy / peak power.
  • Worst case as per ISO 16750-2: `Ri = 0.5 ohm, Vs = 79 V, tr = 5 ms, td = 400 ms`. Could I use some TVS diodes effectively for protecting against this ?
  • My targeted power switches `Rds(on) = 30 - 40 mohm` and for a led light of 200 mA normal draw @ 12V I would throw in `60 ohm` of dynamic reistance even for voltage spikes (?).
  • My targeted TVS dynamic resistance `Rd = 1/Pdc +/- 50% ~= 1/8 = 125 mohm` (formula pricked up from [here](https://electronics.stackexchange.com/questions/299373/determining-dynamic-resistance-of-tvs-diode-from-datasheet)). Multiply that by 3 for a stack of 3 TVS diodes -> `375 mohm`.
  • It appears at a first glance that the TVS stack is sufficient to help out a power switch make it through load dump events unharmed. Of course, the logic part must also tolerate the clamping voltage of these 3 series TVS diodes. Does anything escape my thinking ?
  • Power switches used in automotive are usually rated to withstand clamped load dump events, of the 35 - 40 V peak variety. AFAICT there are two main clamping mechanisms - first one for the logic part and second one for the MOSFET DS channel within. These power switches will generally (re)open the DS channel once the second clamping mechanism has engaged.
  • Since manufacturers do not boast about their power switches being able to withstand full (unclamped) load dumps it must be the case that the FETs within won't take that kind of energy / peak power.
  • Worst case as per ISO 16750-2: `Ri = 0.5 ohm, Vs = 79 V, tr = 5 ms, td = 400 ms`. Could I use some TVS diodes effectively for protecting against this ?
  • My targeted power switches `Rds(on) = 30 - 40 mohm` and for a led light of 200 mA normal draw @ 12V I would throw in `60 ohm` of dynamic reistance even for voltage spikes (?).
  • My targeted TVS dynamic resistance `Rd = 1/Pdc +/- 50% ~= 1/8 = 125 mohm` (formula pricked up from [here](https://electronics.stackexchange.com/questions/299373/determining-dynamic-resistance-of-tvs-diode-from-datasheet)). Multiply that by 3 for a stack of 3 TVS diodes -> `375 mohm`.
  • It appears at a first glance that the TVS stack is sufficient to help out a power switch make it through load dump events unharmed. Of course, the logic part must also tolerate the clamping voltage of these 3 series TVS diodes. Does anything escape my thinking ?
  • **edit** Diagram:
  • ![Image_alt_text](https://electrical.codidact.com/uploads/ckab52uldk0qr13eh8r69lioi9ar)
#1: Initial revision by user avatar kellogs‭ · 2023-08-24T11:47:32Z (over 1 year ago) 
Power switches and automotive load dump, unclamped
Power switches used in automotive are usually rated to withstand clamped load dump events, of the 35 - 40 V peak variety. AFAICT there are two main clamping mechanisms - first one for the logic part and second one for the MOSFET DS channel within. These power switches will generally (re)open the DS channel once the second clamping mechanism has engaged. 

Since manufacturers do not boast about their power switches being able to withstand full (unclamped) load dumps it must be the case that the FETs within won't take that kind of energy / peak power.

Worst case as per ISO 16750-2: `Ri = 0.5 ohm, Vs = 79 V, tr = 5 ms, td = 400 ms`. Could I use some TVS diodes effectively for protecting against this ?

My targeted power switches `Rds(on) = 30 - 40 mohm` and for a led light of 200 mA normal draw @ 12V I would throw in `60 ohm` of dynamic reistance even for voltage spikes (?).

My targeted TVS dynamic resistance `Rd = 1/Pdc +/- 50% ~= 1/8 = 125 mohm` (formula pricked up from [here](https://electronics.stackexchange.com/questions/299373/determining-dynamic-resistance-of-tvs-diode-from-datasheet)). Multiply that by 3 for a stack of 3 TVS diodes -> `375 mohm`.

It appears at a first glance that the TVS stack is sufficient to help out a power switch make it through load dump events unharmed. Of course, the logic part must also tolerate the clamping voltage of these 3 series TVS diodes. Does anything escape my thinking ?
transistor-switch load overvoltage power TVS