Switching with 3V3

Hi

I would like to switch a 3V3 current. Currently I used this mosfet: DMN26D0UT N-Mosfet

My Problem is, the voltage drop of approx. 1V , which leaves me with 2.3V on the power circuit...

Any hints how I can switch the current without this huge loss?
Should I go for a circuit with 2 P-Mosfets?

Please point me to the right direction, I'm absolutely stuck on this.

Thanks

From the data sheet https://www.mouser.ch/datasheet/2/115/ds31854-88548.pdf:

Low On-Resistance:
• 3.0 Ω @ 4.5V
• 4.0 Ω @ 2.5V
• 6.0 Ω @1.8V
• 10 Ω @1.5V

If you need to draw the same current (which I guess from what you said is about 250mA), you have to choose a mosfet with a lower RDSon (Ohms) at 3.3volts or less.

Show your circuit diagram.

As 6V6gt says, you need to post your schematic. But generally, an N-channel mosfet is used to switch the low side of the load (the connection to ground). But it seems you are switching the high side, for which you would normally use a single P-channel mosfet. Something like a DMP1045U would work well, and has very low ON resistance.

MOSFETs have something called On-Resistance or Rds(on). Basically they act like a series resistor in your circuit. So they will drop a voltage of V = IRds(on). Your MOSFET has a huge Rds(on). At 4.5v it has a resistance of 3 Ohms. So if 300mA of current goes through it it will drop 30.3 ~ 1 volts.

You need to get a MOSFET with lower on-resistance. It’s a very important factor in selecting MOSFETs and is usually stated at the top of the datasheet.

Thanks alot for the hints!

I will post my schematic tomorrow to depict my problem. I already understood the problem is the Rds resistance - I will try to look at this.

My problem is the module I'm using is switching high side, which in turn need to switch on the current flow to the load - and currently I'm getting poor 2.2V with a required minimum of 3 V...

It took me quiet some time to figure out where my problem lies...

When you put a load on an N channel MOSFET, you are creating the FET equivalent of the BJT "emitter follower". In both cases, the voltage drop between the gate and source (or base and emitter) is subtracted from the load circuit. This is why you need a common source configuration, which you get from a P channel device on the high side, or an N channel device on the low side. What you have, is a common drain configuration. It is no good for power switching. You would have the same problem with any voltage, not just 3.3V.

Have a look at this device:

Fig 5 shows that with a Vgs of 3V and a Id around 250mA Vds is less than 0.1V.

You can get very very low on resistance with low gate voltage. There are hundreds of MOSFETs to choose from. But you pay with large Gate capacitance for this. It is not a problem for on/off switching but it is important for PWM to have the capacitance as low as possible.

That is not an appropriate MOSFET for this application. The only time I would use that FET is if I desperately needed fast switching time (input capacitance is tens of pf!). It is absolutely not for power - it's like, what you'd use in a level shifter or something, where you're driving the gate with a data line.

For switching power to something with low Vgs, try an AO3416 or PMV20XNE - both of those are absolute beasts, with like 30-some mOhm Rds(on) at 1.8V on the gate...

Hi

to give you more details a short explanation:
I'm trying to build a PCB with an active GPS antenna, which needs to be powered.
The first version had a always-on power supply to the antenna, which is somewhat inefficient...
The second version had a power supply via the mosfet mentioned in first post, but this one was not supplying enough power...

Thanks to your ver helpfull hints and explanations I switched to the DMG2302UK MOSFET :slight_smile:

This one has only 90mOhms resistance, so it should be fine I hope..

The Nets are:

GPS_LNA GPIO for switching current to antenna
3V3_MAIN 3V3 Power supply
ANT_PWR Power supply to antenna
ANT_GPS Power injection to antenna trace

I know I can buy this from some store, but I wanted to build a GPS module myself :slight_smile:

Okay, the DMG2302 is worse than the ones I suggested, while the chinese clone SI2302's are cheaper (I think they are made by MicroCommercialCorp, which has cloned a bunch of Vishay's parts using the same part numbers. I think they may actually have some sort of deal with Vishay - either that, or they're just doing what China Inc. has a reputation for. Expensive from western vendors, ie, same price as Vishay's, but mysteriously they're cheap as dirt from chinese vendors on aliexpress/ebay. They seem to meet spec though... I wonder if there are royalties that aren't being paid on the ones sourced from china?)

In any event, MCC has some solid technology for low Vgs, low Rds(on), high current FETs. They've released a MCU90N02 that is just a BEAST - not ROHS though - and in DPAK package (ie, easy to hand solder - sure, the western manufacturers can beat those specs, but they do it only in packages like "POWER-TDFN8" that are hard to hand solder, and straight up cannot be reworked without mad hot air (and have fun with that if you put a properly thermal pad under it)), I'm going to be releasing boards with 4 of those on it in november... I love their systematic part naming too (on the parts that aren't clones, where they just copy the part number of the original manufacturer)

Edit: WOAH WOAH WOAH!!!!
Those are N-channel FETs. You are using them like they were P-channel FETs! No wonder you see the results you do! It's not turned on at all, because when you drive the gate low to turn it on, that turns it off, and it's conducting through the body diode. If you try to turn it off, you'll find that it doesn't turn it off!

You want a SSM3J358 or SSM3J332 or something like that... (If you don't want to solder them, I sell breakout boards, $~6 for a 6-pack with the chips mounted, and a 10k gate-source resistor)... https://www.tindie.com/products/drazzy/simple-sot-23-mosfet-six-pack-logic-level/

Some other suggestions:

If you don't need to turn it on and off very frequently you could use a relay. It's a simple switch with no voltage drop. Or you could use a solid state relay which is smaller and faster but more expensive.

If you are running off a higher voltage with a regulator you could try a buck converter with enable pin like this: Pololu Step-Down Voltage Regulator D15V70F5S3

pourduino:
Some other suggestions:

If you don't need to turn it on and off very frequently you could use a relay. It's a simple switch with no voltage drop. Or you could use a solid state relay which is smaller and faster but more expensive.

If you are running off a higher voltage with a regulator you could try a buck converter with enable pin like this: Pololu Step-Down Voltage Regulator D15V70F5S3

As I would Like to keep it small, it is no suitable solution.
But it is a very interesting approach...

And I want to try my new Chinese SMD oven and hot air gun :slight_smile:

DrAzzy:
Edit: WOAH WOAH WOAH!!!!
Those are N-channel FETs. You are using them like they were P-channel FETs! No wonder you see the results you do! It's not turned on at all, because when you drive the gate low to turn it on, that turns it off, and it's conducting through the body diode. If you try to turn it off, you'll find that it doesn't turn it off!

You want a SSM3J358 or SSM3J332 or something like that... (If you don't want to solder them, I sell breakout boards, $~6 for a 6-pack with the chips mounted, and a 10k gate-source resistor)... https://www.tindie.com/products/drazzy/simple-sot-23-mosfet-six-pack-logic-level/

I'm still having a hard time following this... after your post I changed to the SI2342DS-T1-GE3 by Vishay

Am I now on the right path or is there a basic error in my schematic?

mparx:
Am I now on the right path or is there a basic error in my schematic?

Read this:

For an N-Channel MOSFET, the source connects to ground, and the drain connects to the negative side of the load.

For a P-Channel MOSFET, the source connects to the voltage source, and the drain connects to the load’s positive side.

mparx:
I'm still having a hard time following this... after your post I changed to the SI2342DS-T1-GE3 by Vishay
https://www.vishay.com/docs/63302/si2342ds.pdf

Am I now on the right path or is there a basic error in my schematic?

The one I suggested is a P-channel model. It should suit your needs.

windoze_killa:
The one I suggested is a P-channel model. It should suit your needs.

Yes, but then I would need to cascade the MOSFET ?!?
Doesn’t P-Channel conduct on low?

Could you not just change your code to be active low instead of active high?

Also can you post a schematic, would help heaps.

mparx:
The Nets are:

GPS_LNA GPIO for switching current to antenna
3V3_MAIN 3V3 Power supply
ANT_PWR Power supply to antenna
ANT_GPS Power injection to antenna trace

I know I can buy this from some store, but I wanted to build a GPS module myself :slight_smile:

Hi

the schematic is in Post #10 ;-).
Nothing much changed from there.

I can't change the logic to low, because the GPS_LNA signal is coming from the GPS module itself.
So from your post I assume, cascading is the only way out...

Or an inverter.

Or read the signal for the GPS and output the opposite from the micro.