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Im trying to switch this common converter:http://www.ebay.com.au/itm/281807832840?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT enter image description here

With: https://www.fairchildsemi.com/datasheets/2N/2N7000.pdf

Connected direct to battery it works fine and does not pull a great deal of current.

But when switched with a mosfet, loads, such as an LED, cause the voltage coming out and into the converter to drop and the current consumed by the converter to increase to compensate. (I figure with these things the larger the voltage gap to get up to 5v the greater the current it pulls)

The reason I need to switch (eventually with a micro) is to get around its idle current because running off batteries.

Direct to batteries, measurement at Step-Up:

  • No load: 3.7mA, inV: 2.48v outV: 4.98v
  • With LED: 7.4mA, inV: 2.48v outV: 4.98v

Via Mosfet:

  • No Load: inV: 24mA, inV: 0.38v outV: 4.34v
  • With an LED: 24.4mA, inV: 0.36v outV: 3.44v

As far as i can tell this mosfet should be suitable for this sort of voltage and current range. But I am new to mosfet, so not sure how to work out what the problem is.

outputoutput voltage of mosfet at turn on

Eventually I want to drive a 5V sensor that pulls more current than an led ~ 10mA idle and a 3v3 voltage regulator that drives a rf module (~20mA). But the more load i add the worse this situation gets very quickly to the point that the step up is pulling like 80mA and only putting out about 3V.

schematic

simulate this circuit – Schematic created using CircuitLab

**Update" From the data sheet would this be the graph of interest to me, showing that with the voltage range i have i cant pull much current ?

enter image description here

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There are two issues with your design. First and most important, replace that 100 Ohm resistor which is way too small with 10K. Second, the Vgs On of the mosfet could be bigger than the battery voltage. You may want to use another battery or another FET

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  • \$\begingroup\$ Yes, those 24mA input current is solely from those 100 Ohm (2.4/100). \$\endgroup\$ – Roker Pivic Apr 6 '16 at 8:59
  • \$\begingroup\$ Im pretty sure you are right about the VGS, i just ran it at 3v3 and it works ok. In regards to the 24mA reading that is made going into the step-up so the 100 Ohm pulldown has no effect on that. \$\endgroup\$ – Hayden Thring Apr 6 '16 at 9:09
  • \$\begingroup\$ I have added the graph that i believe is most relevant would you agree ? \$\endgroup\$ – Hayden Thring Apr 6 '16 at 9:15
  • \$\begingroup\$ Looking at other mosfets and posts it looks like i might be better with a transistor than a mosfet, so im going to look at some of them. \$\endgroup\$ – Hayden Thring Apr 6 '16 at 9:40
  • \$\begingroup\$ Hayden even if it is the step up, the pull down will consume another 24mA and that is uncalled for. I think it is a good idea to use an NPN, and also use a much bigger pull down transistor. \$\endgroup\$ – Claudio Avi Chami Apr 6 '16 at 13:44
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The trick about selecting power MOSFETs for switching regulators is to drive them appropriately. You need to select a MOSFET with a low RDSon, a low gate charge, and a low Ton/Toff, and you need to deliver enough current into the gate to switch it fast - not in terms of switching frequency, but in terms of the edge itself. It might seem counter-intuitive, but the key is that most losses in a switching converter comes from the switching itself. When a MOSFET is going from off to on, the RDSon becomes gradually lower until it is fully on. The faster you get it from off to fully on, the least losses your system will suffer.

Designing efficient SMPS is an artform in itself!

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