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# TPS2413 - Ability to detect open circuit from power supply

Other Parts Discussed in Thread: TPS2413

We are OR'ing 4 power supplies together using the TPS2413. I had originally designed the circuit using a 47.5K resistor for the Rset value due to that we wanted our power supplies to be fully on when they were at a very light load. I have been playing around with resistor values and I am currently at 220K to be able to detect when a power supply is off. Right now when we are at a total overall load of 710W at 5V = 142 Amps = 36 Amps per channel = 17 Amps per mosfet I am seeing that when I disconnect a power supply simulating an open circuit that my gate voltage is still on (15VDC) and my Vin is still 5V for that channel. I am wondering at what value do I need to set the resistor value to to be able to detect when a power supply is off.

It seems that the tradeoff is that the power supplies are no longer fully on when there is a light load situation. Using the 220K resistor when the load is at 50W (our minimum field load) = 2.5A per channel = 1.25A per mosfet I am only seeing 2 or 3 of the 4 channels are on. I will also see that the gate voltage is chopping as I described in this thread when the power demand is increased. At around 200W (10A per channel) all 4 power supplies are fully on. Is this chopping of the gate of concern at low currents? At what threshold should I be concerned about the gate voltage chopping? What are the negative side effects of a chopping gate? Will I have premature failure of my mosfet transistors?

The mosfets that I am using that are in parallel are PSMNOR9-25YLC which have an Rdson of 1mohm. So my Rdson is 0.5mohm per channel.

We have the option of bringing the gate to ground which then sets the gate voltage to 0 and Vin of that power supply to 0 if it is off, but we would rather not turn off our mosfet gates just to check if the power supplies are still on.

Thank you for your help in selecting a Rset value. Please let me know if there are any negative consequences of using an Rset higher than 220K.

• Scott,

Received you E2E request. We are shorthanded today. Will review on Monday.

Brian
• Scott,

This is in review.

Brian

• Scott,

Couple of things: Rset can be open circuit, which would default the reverse current detect (fast turn off) to +3mv. With 220k, you are set at 1mv. There is ~ +/2 2mv tolerance on the fast turn off so with 1mv, you could really be at -1mv to turn off the gate.

The rest of the discussion can be pretty involved.

I would like to get your work email and we can more efficiently communicate that way and post the final here when done. Can you provide?

Brian
• Thank you for your response. The +/- 2mv tolerance could very likely be what I am seeing. Is this tolerance from part to part (TPS2413), is it based on temperature, or can it be change within the same part based on other transients?

Should I be concerned when/if I see a gate that is chopping and is not fully on? Should I be concerned about MOSFET life etc.?

My email is sphillips@nevco.com my direct phone number is 618-659-seven five two one

Thank you again