Help with "Logic Level" MOSFET Switch

Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
I need to switch one of those cheapy eBay floodlights. The lights are 12V and the lamps at 10W. I would like to have the option of expanding to 20W. The lights have their own current limiting circuitry. No more specs other than that sorry.

The light will be switched with a PIC18FK22. Zero volts on the binary out to turn the light off, 5V to turn the light on. The time period to switch the light on or off is not important.

Since it is a battery operated light, I would like the circuit to be as efficient as possible.

I did some searching and came up with the following. The LED in the schematic actually represents the floodlight unit. I choose the IRF540 simply because it is a "logic level" MOSFET I stumbled across in my searching and it seems to more than able to handle the current I need to switch. Here is the datashheet.

Will this work? Is there a better choice than the IRF540 for my needs?


upload_2015-12-14_21-51-8.png
 

Bernard

Joined Aug 7, 2008
5,335
With a gate- source of +-20 V, does not look like a logic level FET.
My only LL N ch. FET, FDD6530A SMT is only good to 21A.
 
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GopherT

Joined Nov 23, 2012
8,012
I think you may have an IRF 9540 (P-Channel equiv) in your pile of parts. If not, let me know. That and any common NPN will let you do this with 12 volt gate current and logic level switching...

(Where the 1kHz is actually your Microcontroller).

image.jpg
 
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Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
I think you may have an IRF 9540 (P-Channel equiv) in your pile of parts. If not, let me know. That and any common NPN will let you do this with 12 volt gate current and logic level switching...

(Where the 1kHz is actually your Microcontroller).

View attachment 96531
I have something pchannel at home. Need to check. Is there any losses by going this way?
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
IRF540 is not a logic level, and IRL540 does, but you still can try the IRF540.
I knew there was something wrong when I entered that! I need to check my notes at home to see if it is really the irl I found. Thanks. Could I get away with just the MOSFET with the irl540?
 

GopherT

Joined Nov 23, 2012
8,012
I knew there was something wrong when I entered that! I need to check my notes at home to see if it is really the irl I found. Thanks. Could I get away with just the MOSFET with the irl540?
Yes, the IRL can work by itself. As you drew in earlier post.

You should add a 100k resistor between gate and source to insure the gate isn't floating or susceptible to static charge damage when the device is starting up or unpowered.
 

GopherT

Joined Nov 23, 2012
8,012
I have something pchannel at home. Need to check. Is there any losses by going this way?
Just the 0.5 mA from the Microcontroller keeping the NPN base high. P-channel MOSFETS generally have slightly higher on-state resistance, so a bit more heat. You'll have to compare the exact devices head-to-head.
 

ScottWang

Joined Aug 23, 2012
6,843
I knew there was something wrong when I entered that! I need to check my notes at home to see if it is really the irl I found. Thanks. Could I get away with just the MOSFET with the irl540?
If you want to using IRL540 then it should be fine.
I tried a 5V level 350 Khz from ne555 to the IRF540, the output for a light current load, it's works fine, so you can try it for your current I=20W/12V=1.667A.
 

ian field

Joined Oct 27, 2012
6,539
I need to switch one of those cheapy eBay floodlights. The lights are 12V and the lamps at 10W. I would like to have the option of expanding to 20W. The lights have their own current limiting circuitry. No more specs other than that sorry.

The light will be switched with a PIC18FK22. Zero volts on the binary out to turn the light off, 5V to turn the light on. The time period to switch the light on or off is not important.

Since it is a battery operated light, I would like the circuit to be as efficient as possible.

I did some searching and came up with the following. The LED in the schematic actually represents the floodlight unit. I choose the IRF540 simply because it is a "logic level" MOSFET I stumbled across in my searching and it seems to more than able to handle the current I need to switch. Here is the datashheet.

Will this work? Is there a better choice than the IRF540 for my needs?


View attachment 96526
AFAICR: There is an IRFL540 which is logic level, the IRF540 is just a bog standard MOSFET.
 

ian field

Joined Oct 27, 2012
6,539
With a gate- source of +-20 V, does not look like a logic level FET.
My only LL N ch. FET, FDD6530A SMT is only good to 21A.
The 20V is the absolute max, I've seen a report that claims anything over 15V starts shortening the life expectancy by a small amount.

Around 8 - 10V is usually sufficient with most average power MOSFETs to achieve the headline RDSon figure.

A few of the LL MOSFETs in my collection will *WITHSTAND* 20V on the gate, but a few of them will turn fully on with as little as 1.6V on the gate.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
Just the 0.5 mA from the Microcontroller keeping the NPN base high. P-channel MOSFETS generally have slightly higher on-state resistance, so a bit more heat. You'll have to compare the exact devices head-to-head.
What is considered a low on state resistance? Mouser has a IRL540 with an RDSON of 44 mohms.

I found this on another forum.

The advantage of logic level MOSFETs is that their source-drain saturates with at a low gate voltage. The disadvantages are that they tend to have higher gate capacitance/gate charge (take longer to turn on for a given amount of drive current), have higher on-resistance, have lower maximum tolerable gate voltages, and cannot be made to have source-drain breakdown voltages as high as standard MOSFETs. You are basically trading the advantage of lower gate drive voltage for performance hits in every other area to varying degrees. Sometimes though the ability to use a logic-level gate voltage simplifies things enough to justify this.

Basically, if you can easily provide the gate voltage necessary for a standard level MOSFET choose a standard MOSFET over a logic level MOSFET (all things being equal). If you cannot provide such a voltage easily, then you have to start thinking about the complexity and performance tradeoffs of supplying that gate drive voltage or using a logic-level FET.

Am I better to go another route? Maybe an opto-isolater ?
 

GopherT

Joined Nov 23, 2012
8,012
What is considered a low on state resistance? Mouser has a IRL540 with an RDSON of 44 mohms.

I found this on another forum.

The advantage of logic level MOSFETs is that their source-drain saturates with at a low gate voltage. The disadvantages are that they tend to have higher gate capacitance/gate charge (take longer to turn on for a given amount of drive current), have higher on-resistance, have lower maximum tolerable gate voltages, and cannot be made to have source-drain breakdown voltages as high as standard MOSFETs. You are basically trading the advantage of lower gate drive voltage for performance hits in every other area to varying degrees. Sometimes though the ability to use a logic-level gate voltage simplifies things enough to justify this.

Basically, if you can easily provide the gate voltage necessary for a standard level MOSFET choose a standard MOSFET over a logic level MOSFET (all things being equal). If you cannot provide such a voltage easily, then you have to start thinking about the complexity and performance tradeoffs of supplying that gate drive voltage or using a logic-level FET.

Am I better to go another route? Maybe an opto-isolater ?
If you are ordering something, I recommend the smd part, SI4666DY from vishay. Lower on resistance, and logic or standard gate voltages. To 25V DS voltage and cheap. Handles up to 16 amps constant. Even at 3.3V gate voltage, they work great! I've used them in high amperage motors and no problem at 5 amps, 12 volts. Not even warm. I left about 1 sq inch of copper as heat sink on the drain pins.

So, the comment from the other forum was true about 20 years ago but modern MOSFETS are great. Even at logic level.


image.jpg
 
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Thread Starter

spinnaker

Joined Oct 29, 2009
7,837
Unfortunately it looks like it only comes in SMD. I would rather not deal with it unless I have to.

Digikey has a really cool metric for special feature FETs. One is the "logic level" feature. I found the 2SK4066. 4.7 mOhm @ 50A, 10V. Good candidate?
 

GopherT

Joined Nov 23, 2012
8,012
Ah looks like they are non stock!
If you really want TO220, use the info from the 3D printer guys. They have been controlling 100 to 200 watt 12 V loads to heat the build platform with logic level MOSFETS. They have proven the following three to work well.

FDB8880
IRL3803 and
STP55NF06L <-- we used this one. It is fine.

All seem to do well without a heat sink up to 10 amps. The one you posted looks good too.
 
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