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Using a P FET for polarity protection like this means that in case you flip + and -, the gate will be inactive, meaning that GND which is now connected to drain will get disconnected - no current w...
Answer
#3: Post edited
by
Lundin
·
2023-05-11T08:18:03Z (over 1 year ago)
Using a P FET for polarity protection like this means that in case you flip + and -, the gate will be active meaning that GND which is now connected to drain will get disconnected - no current will flow.- If you put a TVS before the MOSFET, you cancel this out. Instead the TVS will short + and - and take the blow instead, so the polarity protection no longer works as intended.
- No matter the TVS picked, something will get fried in case the TVS is placed before the MOSFET. In fact, in case you have some gigantic 6.6kW peak pulse TVS, it's so damn rugged that it will probably survive that short. Meaning something else will give instead, like a wire or a trace - and there you have a big problem.
- Therefore the TVS should be placed _after_ the MOSFET and zener. The MOSFET is close to an ideal diode in this circuit so it will happily forward a spike and then the TVS will take the spike before it does any damage.
- You could however place a non-polarized cap before drain to even out the supply, without affecting anything else.
- There is no need for a fuse unless you have EX/intrinsic safety requirements, in which case you'll also need multiple TVS.
- Using a P FET for polarity protection like this means that in case you flip + and -, the gate will be inactive, meaning that GND which is now connected to drain will get disconnected - no current will flow.
- If you put a TVS before the MOSFET, you cancel this out. Instead the TVS will short + and - and take the blow instead, so the polarity protection no longer works as intended.
- No matter the TVS picked, something will get fried in case the TVS is placed before the MOSFET. In fact, in case you have some gigantic 6.6kW peak pulse TVS, it's so damn rugged that it will probably survive that short. Meaning something else will give instead, like a wire or a trace - and there you have a big problem.
- Therefore the TVS should be placed _after_ the MOSFET and zener. The MOSFET is close to an ideal diode in this circuit so it will happily forward a spike and then the TVS will take the spike before it does any damage.
- You could however place a non-polarized cap before drain to even out the supply, without affecting anything else.
- There is no need for a fuse unless you have EX/intrinsic safety requirements, in which case you'll also need multiple TVS.
#2: Post edited
by
Lundin
·
2023-05-11T08:11:18Z (over 1 year ago)
- Using a P FET for polarity protection like this means that in case you flip + and -, the gate will be active meaning that GND which is now connected to drain will get disconnected - no current will flow.
- If you put a TVS before the MOSFET, you cancel this out. Instead the TVS will short + and - and take the blow instead, so the polarity protection no longer works as intended.
No matter the TVS picked, something will get fried. In fact, in case you have some gigantic 6.6kW peak pulse TVS, it's so damn rugged that it will probably survive that short. Meaning something else will give instead, like a wire or a trace - and there you have a big problem.- Therefore the TVS should be placed _after_ the MOSFET and zener. The MOSFET is close to an ideal diode in this circuit so it will happily forward a spike and then the TVS will take the spike before it does any damage.
- You could however place a non-polarized cap before drain to even out the supply, without affecting anything else.
- There is no need for a fuse unless you have EX/intrinsic safety requirements, in which case you'll also need multiple TVS.
- Using a P FET for polarity protection like this means that in case you flip + and -, the gate will be active meaning that GND which is now connected to drain will get disconnected - no current will flow.
- If you put a TVS before the MOSFET, you cancel this out. Instead the TVS will short + and - and take the blow instead, so the polarity protection no longer works as intended.
- No matter the TVS picked, something will get fried in case the TVS is placed before the MOSFET. In fact, in case you have some gigantic 6.6kW peak pulse TVS, it's so damn rugged that it will probably survive that short. Meaning something else will give instead, like a wire or a trace - and there you have a big problem.
- Therefore the TVS should be placed _after_ the MOSFET and zener. The MOSFET is close to an ideal diode in this circuit so it will happily forward a spike and then the TVS will take the spike before it does any damage.
- You could however place a non-polarized cap before drain to even out the supply, without affecting anything else.
- There is no need for a fuse unless you have EX/intrinsic safety requirements, in which case you'll also need multiple TVS.
#1: Initial revision
by
Lundin
·
2023-05-11T08:10:47Z (over 1 year ago)
Using a P FET for polarity protection like this means that in case you flip + and -, the gate will be active meaning that GND which is now connected to drain will get disconnected - no current will flow. If you put a TVS before the MOSFET, you cancel this out. Instead the TVS will short + and - and take the blow instead, so the polarity protection no longer works as intended. No matter the TVS picked, something will get fried. In fact, in case you have some gigantic 6.6kW peak pulse TVS, it's so damn rugged that it will probably survive that short. Meaning something else will give instead, like a wire or a trace - and there you have a big problem. Therefore the TVS should be placed _after_ the MOSFET and zener. The MOSFET is close to an ideal diode in this circuit so it will happily forward a spike and then the TVS will take the spike before it does any damage. You could however place a non-polarized cap before drain to even out the supply, without affecting anything else. There is no need for a fuse unless you have EX/intrinsic safety requirements, in which case you'll also need multiple TVS.