Mosfet with Low Idss ? - Page 1
Forum section: electronics / projects

author: namster
author_rank: Regular Contributor
timestamp: August 25, 2019, 10:18:02 pm
content: i need to reverse a precision current with Symetrical value , to cancel thermoelectric EMF , so I tried to use a H bridge but this did not give the expected result a leakage current has alteared the Reference Current ! , i think that was caused by Idss Current of Mosfet the IRFz44 has a Idss of 40µA wich mosfet can i use ? or there is another solution ?note : i have simulate the Idss with a 1k Resistor !

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2019, 10:50:28 pm
content: Why 1k resistors in parallel? Tried without?Why IRFZ46? That's a fuckoff huge transistor for switching a 1mA current. Why not 2N7002, say?Tim

author: namster
author_rank: Regular Contributor
timestamp: August 25, 2019, 11:06:28 pm
content: i add this 1k resistor because the simulator don't take Idss in consideration , there is another technique to simulate it ?i used a IRFZ46 because That's what I had available , 2n7002 is a good choise but i have 4 Range of Current ( 1 mA 10mA 100mA and 1A) the absolute Maximum current of 2n7002 is 200 mA .

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2019, 11:43:30 pm
content: Oh, then just go with like Si2304BDS. Something smaller. I suppose even IRF510 would be something.You aren't likely to find a part that guarantees low leakage, especially not at high currents, but typical leakage is much better than the worst case figure. The reason is it takes time to measure low currents, and time is money.Tim

author: exmadscientist
author_rank: Frequent Contributor
timestamp: August 26, 2019, 05:14:06 am
content: Quote from: T3sl4co1l on August 25, 2019, 11:43:30 pmFor what it's worth,PhilipsNXPNexperia has a series of "low-leakage" FETs. The specs aren't particularly impressive (25 nA guaranteed) but, hey, that's something. I seem to recall another manufacturer also has such a line but at the moment I can't remember which one.Nexperia's catalog lists these parts as PMZ600UNEL / PMZB600UNEL (N-Ch), PMZ950UPEL / PMZB950UPEL (P-Ch), PMDXB600UNEL (Dual N-Ch), PMDXB950UPEL (Dual P-Ch), and PMCXB900UEL (dual complementary).Oh, and they're in one of those satanic tiny little 1mm by 0.6mm DFN packages. Just in case you need an excuse to buy a JBC Nano station....

author: namster
author_rank: Regular Contributor
timestamp: August 26, 2019, 10:56:27 am
content: thank you for this information ,another solution is to reduce the current of Test like the Keithley 580 wich use a 100mA Current for a range of 200mOhm , the parts cited by exmadscientist still missing in my country that's why i will try tu use 2n7002 .

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2019, 04:04:15 pm
content: The simplest solution is to not use power MOSFETs that are larger than necessary.A higher performance alternative is to use a much smaller MOSFET or JFET to switch a bipolar transistor in a pseudo-Darlington configuration. This can get the gate leakage down to 10s of picoamps or less so collector leakage becomes the limit to performance.Things get more complex after that to remove or compensate for gate and collector/drain leakage. Typically residual leakage can be reduced by another 1 or 2 orders of magnitude if required.Quote from: exmadscientist on August 26, 2019, 05:14:06 am25 nanoamps is often the most sensitive fast test on commonly available ATE (automated test equpiment) so it shows up in a lot of datasheets. Few customers will pay for a more sensitive but slower test.

author: IDEngineer
author_rank: Super Contributor
timestamp: August 26, 2019, 07:13:00 pm
content: You can also drop leakage current by driving the gate beyond the source. For an N-channel enhancement mode FET, driving the gate more negative than the source will forcefully pinch off the channel and can drop the leakage current by a couple more orders of magnitude. Of course this means you need a negative supply rail that you may not otherwise have, but the required current is negligible so there are a lot of options for developing what is effectively a negative voltage potential (rather than an actual current source).Check the Vgs stats on the FET's you're considering. They generally have a +/- range around the source. Most people only think about and use the "turn on" direction, but the other direction is safe and gives you a whole extra dimension of channel control.

author: namster
author_rank: Regular Contributor
timestamp: August 28, 2019, 12:21:11 pm
content: I took a while to answer , i have tried to simulate a H bridge with take into consideration the leakage current but the result aren't credible ,@David Hessi simulate the circuit in atachment and the lekage current was about 1.2nA compared to the Mosfet that is 20μA the result was realy impressive .@IDEngineerAlso Leakage will drop significantly with a lower Vds/Vds(max) ratio and a Tj/Tj(max) ratio , you gived a very effective apporch but i think the principal probleme is the high side Mosfet of h bridge that is controlled with a bootstrap capacitor .