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Over current protection devices question

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Chris Wilson:
With devices like this, limited to a max of 36V how do you apply the technology to say an RF amp running on 60v or even rectified 240V mains? And secondly, with the amp powered, when a signal is applied, let's say the digital amateur mode, WSPR commences, there is a high current for a short time. how do you allow for this yet provide effective over current protection for rapidly failing devices like FET's? Thanks

http://www.ti.com/product/INA300

PartialDischarge:

--- Quote from: Chris Wilson on September 18, 2018, 10:31:18 am ---With devices like this, limited to a max of 36V how do you apply the technology to say an RF amp running on 60v or even rectified 240V mains?

--- End quote ---

That device would be useful only in a low side application if your Vcc goes beyond 36V. You can easily build your own high side with the appropriate components using for example the circuit on page 11 from http://www.ti.com/lit/an/snoa620/snoa620.pdf , there are other circuits out there



--- Quote --- And secondly, with the amp powered, when a signal is applied, let's say the digital amateur mode, WSPR commences, there is a high current for a short time. how do you allow for this yet provide effective over current protection for rapidly failing devices like FET's? Thanks

--- End quote ---

You can't have it both ways. Best you can do is filter the sensed current and allow for a peak in current during a certain amount of time. Anyway semiconductors are much faster in failing that you'd be in protecting them so a small delay would be ok anyway.

dmills:
You use a high side current sensing arrangement, either a resistor and INA-Something or something from the likes of LEM, then a comparator, D flipflop and a small transistor to yank the RF sands bias supply or trip the power supply offline. 


I would note that there is a nasty trap if you yank the drain voltage on an amp built with significant amounts of RF feedback, the loss of feedback makes the gate voltage swing increase and risks puncturing the gates.

Personally I like a PIN diode switch in the drive circuitry in combination with killing the bias voltage, gets it shut down in a few microseconds and transient protection diodes can take the 10W or so of drive for that long.

Regards, Dan.

Chris Wilson:

--- Quote from: dmills on September 18, 2018, 12:57:38 pm ---You use a high side current sensing arrangement, either a resistor and INA-Something or something from the likes of LEM, then a comparator, D flipflop and a small transistor to yank the RF sands bias supply or trip the power supply offline. 


I would note that there is a nasty trap if you yank the drain voltage on an amp built with significant amounts of RF feedback, the loss of feedback makes the gate voltage swing increase and risks puncturing the gates.

Personally I like a PIN diode switch in the drive circuitry in combination with killing the bias voltage, gets it shut down in a few microseconds and transient protection diodes can take the 10W or so of drive for that long.

Regards, Dan.

--- End quote ---

Thanks Dan, problems are it's Class D so no bias supply, and the huge caps in the supply mean I would have to trip after them, not a big problem save aesthetics modding the PCB. I imagine a relay is too slow, what device will switch off 60V or more at 40 Amps plus, very fast?

dmills:
So just yank the gate drive (An and gate between the gate pulse generator and the mosfet driver for each fet)?

A mosfet makes a decent switch if you really wanted to, but you usually get into needing a little boost supply charge pump to keep the gate where it needs to be, or use a P Channel part.

Relays are actually usually fine providing your power supply has a current limited output, less so if you have a honking great cap waiting to vaporise the bond wires, they are of course way too slow to catch drain or gate over voltage.

Regards, Dan.

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