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I'm using a MOSFET for a sample and hold circuit controlled by a microcontroller. I want to control the charging and discharging of a capacitor by a solar cell. For this I'd be using two MOSFETs. The solar cell can have a large area and can source upto 3.2 A of current. This flows through the drain of the MOSFET. I want this flow to be switched ON and OFF by applying a gate voltage from an MCU. I can boost the MCU output to required gate voltage levels, but what would be a suitable MOSFEET for my application? I'd need approximately 250 ms ON time and 750 ms off time during switching. The IC (or transistor) package may be any one of the packages that may be connected to a breadboard easily that is NOT surface mount.

EDIT: Strictly speaking, the purpose isn't sample and hold but its very similar. I'd like to characterize the solar cell IV by sampling the voltage across the cap and the current flowing to it while charging.

Schematic here.

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    \$\begingroup\$ Let me try and understand this... You want to (every 1 second) connect a solar cell to a capacitor and when you do you want to measure the charged voltage of the cap (after 250ms) and the current flowing into the cap to charge it up. Then presumably, you'll discharge the cap (over the 750ms period) to start again with all initial conditions. Yes? \$\endgroup\$ – Andy aka Dec 26 '13 at 20:03
  • \$\begingroup\$ I've added the schematic. That's what I was working with until I realized IRF7201 came only in SMD. I want to take about 3000 samples of voltage and current during the charging phase. I'll monitor the voltage across the 0.03 ohm resistance to get the 'current' samples. So I'm not just trying to measure the charged voltage of the cap, but the voltage levels during the time its charges. \$\endgroup\$ – Analon Dec 27 '13 at 1:31
  • \$\begingroup\$ If the cap is ground connected you can more easily measure it during charging and much more so when charger is turned off. With a low side FET the cap + side goes to full Vcharge when FET is off. When FET is on the V reading may be affected by Icharge x Rdson. But, if a high side FET is used Vcap is always able to be read wrt ground. \$\endgroup\$ – Russell McMahon Dec 27 '13 at 5:35
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While a schematic of the application, and the solar panel voltage, would make it simpler to provide a definitive answer, here is how one could select a suitable MOSFET:

  • MOSFET should (ideally) switch on hard at the MCU's GPIO output voltage. Let's say 3.3 Volts, so we are looking for a logic level MOSFET, one designed for 3.3 Volt operation. The Gate-Source threshold voltage VGS(th) for such MOSFETs would typically be below 1 Volt.
  • MOSFET Drain-Source voltage rating should be significantly higher than the expected voltages in the application. If the solar panel in question operates at say 10 Volts maximum, I would look for a MOSFET rated for VDS of 20 Volts, fairly common.
  • MOSFET drain current rating should be significantly higher than the expected drain current. For 3.2 Amperes, I would limit my search to MOSFETs rated to 5 Amperes or higher
  • The power dissipation at the MOSFET during full conduction needs to be dealt with, either via heat sinks, other forms of cooling, or simply by keeping the power dissipation well below the rating for the MOSFET package without heat-sink. Since through-hole was specified, the common MOSFET package of TO-220 would typically be quite safe at 0.5 Watt dissipation, and would run hot but probably not too badly at 1 Watt, without a heat sink. To achieve this, one would look for a MOSFET with on resistance RDS(on) of under 49 milliOhms (501 mW) or at worst, 100 milliOhms. A heat sink allows far greater latitude, of course.
  • Given the rather relaxed timing requirements, switching losses are not much of a concern, nor is switching speed or gate capacitance, really.

So, given the above parameters, a search on Digikey yields at least 79 results as I just checked. On sorting by lowest price for single unit purchases, a couple of options are:

Of course, if SMD were an option, many, and less expensive, options would open up, including some excellent devices by Alpha Omega and International Rectifier.


As pointed out by Russell, if providing a gate drive voltage higher than the MCU power rail is not a problem, and a P-channel MOSFET is preferred, then a similar Digikey MOSFET search yields several P-channel MOSFETs that meet the specifications above, setting aside the VGS(th) point. For instance:

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  • \$\begingroup\$ He says that he can provide requisite gate drive voltages so limitation to MCU supply voltage is not necessary. That being so, use of a high side P Channel FET makes measurement task much easier as cap is ground referenced. \$\endgroup\$ – Russell McMahon Dec 26 '13 at 22:44
  • \$\begingroup\$ @RussellMcMahon Could you elaborate on the advantage of choosing a 'high side P channel FET'? \$\endgroup\$ – Analon Dec 27 '13 at 1:34

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