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| Inrush current burns switches |
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| ogden:
--- Quote from: rachdatu on June 14, 2019, 09:56:50 am ---I built an inrush current limiter for my ebike and it works. I then tried to use the same circuit for these BPS units and it did not work. It just blows the FETs and I don't know why. --- End quote --- It's thermal runaway. VGS for paralleled FETs most likely will differ slightly, they do not start conduction all at the same VGS voltage. When you parallel FET's, especially w/o current-balancing resistors, one will open first, alone dissipating all power and explode, then next FET starts conduction and explode and so on. Some FETs are better for "linear applications", other are notoriously bad. Basically you shall check which FETs are used in high power programmable DC loads and use those, preferably with some current balancing resistors in form of prolonged PCB traces or just wire. There are few discussions about FETs in linear mode here. --- Quote from: rachdatu on June 14, 2019, 09:56:50 am ---For your info, I built the "Inrush Limiter 3" from Richard Fechter with 2 irfb4110. --- End quote --- What are values of components, what is resulting dV/dt? Pay close attention to "Fig 8. Maximum Safe Operating Area" of the irfb4110 datasheet, make sure you operate FET *deep* into "safe operation area". |
| TheWelly888:
That reminds me of my old Amstrad PC (1991 vintage) which had such a high inrush current on powering up that it welded the contacts of the mains socket switch! |
| wraper:
--- Quote from: ogden on June 14, 2019, 12:45:54 pm ---It's thermal runaway. VGS for paralleled FETs most likely will differ slightly, they do not start conduction all at the same VGS voltage. When you parallel FET's, especially w/o current-balancing resistors, one will open first, alone dissipating all power and explode, then next FET starts conduction and explode and so on. Some FETs are better for "linear applications", other are notoriously bad. Basically you shall check which FETs are used in high power programmable DC loads and use those, preferably with some current balancing resistors in form of prolonged PCB traces or just wire. There are few discussions about FETs in linear mode here. --- End quote --- With MOSFETS in linear operation simply using current balancing resistors is not nearly enough. Each MOSFET should have individual gate drive with current sense. In this regard they are worse than BJT. On the other hand when using them as switches, you can simply connect them in parallel. |
| ogden:
--- Quote from: wraper on June 14, 2019, 01:00:13 pm ---With MOSFETS in linear operation simply using current balancing resistors is not nearly enough. --- End quote --- For DC load indeed you want to balance dissipated heat as evenly as possible. Yet this is not programmable DC load. Refer to chapter "5. Partially enhanced (linear mode) power sharing": https://assets.nexperia.cn/documents/application-note/AN11599.pdf --- Quote ---MOSFETs developed specifically for linear mode operation are available from NXP. They can simplify the design of paralleled groups of MOSFETs intended for linear mode operation. --- End quote --- |
| wraper:
--- Quote from: ogden on June 14, 2019, 01:36:34 pm --- --- Quote from: wraper on June 14, 2019, 01:00:13 pm ---With MOSFETS in linear operation simply using current balancing resistors is not nearly enough. --- End quote --- For DC load indeed you want to balance dissipated heat as evenly as possible. Yet this is not programmable DC load. Refer to chapter "5. Partially enhanced (linear mode) power sharing": https://assets.nexperia.cn/documents/application-note/AN11599.pdf --- Quote ---MOSFETs developed specifically for linear mode operation are available from NXP. They can simplify the design of paralleled groups of MOSFETs intended for linear mode operation. --- End quote --- --- End quote --- You either need closely matched MOSFETS for that to work reasonably well or prohibitively high resistance source resistors for low voltage applications if you just use random MOSFETS. |
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