Electronics > Beginners

Mosfet application doubt

**Microdoser**:

--- Quote from: fourfathom on September 25, 2023, 08:15:07 pm ---

--- Quote from: Veketti on September 25, 2023, 07:38:45 pm ---Excellent, makes sense. So the conclusion is that Vds = 12V and Vgs = 5V is perfectly fine as long as the Id is within limits (<4A for continuous current)?

--- End quote ---

No. If you have a Vds of 12V and and an Ids of 4A that means the transistor is dissipating 48 Watts. This is not going to end well for that tiny SOT-23 package with its specified maximum power dissipation of about 1W.

How do you plan to use this FET? Typical applications are low-power linear or higher-current switching (where the Vds is near zero when current is flowing).

--- End quote ---

Well if you use something like a GSFD8003, they have a rated power dissipation of 80W, of course at what voltage and current they can do that, and what cooling setup they would need is a different matter, and they are only rated at 3A, but they are a tiny TO-252 (DPAK) package. My point being that you can get far more than 1W rated tiny Mosfets.

**TimFox**:

Also note that different maximum ratings, such as VDS, ID, and power dissipation are quoted by the manufacturer, but that does not mean that you can run the device at its maximum voltage and maximum current, simultaneously, if the resulting power dissipation exceeds the temperature-derated power rating with your heat sink (or none).

For switching MOSFETs, one typically runs near maximum current at very low voltage, and maximum voltage at very low current, and then looks at the SOA to see what happens during the switching transition from ON to OFF, respectively.

**thm_w**:

--- Quote from: Microdoser on September 25, 2023, 09:40:47 pm ---Well if you use something like a GSFD8003, they have a rated power dissipation of 80W, of course at what voltage and current they can do that, and what cooling setup they would need is a different matter, and they are only rated at 3A, but they are a tiny TO-252 (DPAK) package. My point being that you can get far more than 1W rated tiny Mosfets.

--- End quote ---

Sure, but OP linked a SOT23 package.

**Veketti**:

Ah, yes what a brainfart. I'm considering load switching and sure the Vds wouldn't be 12V. I made some calculations with tweaked values for better understanding. I checked the datasheet and according to that Rdson is roughly 22mohm for Vgs=5V. So in my calculations this circuit draining 4A Vds would be 0.088V and power dissipated by the FET 0.352W which is well between the limits. Is my calculations and understanding correct that this is ok?

**Microdoser**:

--- Quote from: thm_w on September 26, 2023, 01:00:38 am ---

--- Quote from: Microdoser on September 25, 2023, 09:40:47 pm ---Well if you use something like a GSFD8003, they have a rated power dissipation of 80W, of course at what voltage and current they can do that, and what cooling setup they would need is a different matter, and they are only rated at 3A, but they are a tiny TO-252 (DPAK) package. My point being that you can get far more than 1W rated tiny Mosfets.

--- End quote ---

Sure, but OP linked a SOT23 package.

--- End quote ---

SOT23 packages do go up to 5W that I can find, and a small design change, not being fixed on a particular package, gives much more freedom of choice. Their link was only an example, not a fixed choice.

--- Quote from: Veketti on September 26, 2023, 05:35:45 am ---Ah, yes what a brainfart. I'm considering load switching and sure the Vds wouldn't be 12V. I made some calculations with tweaked values for better understanding. I checked the datasheet and according to that Rdson is roughly 22mohm for Vgs=5V. So in my calculations this circuit draining 4A Vds would be 0.088V and power dissipated by the FET 0.352W which is well between the limits. Is my calculations and understanding correct that this is ok?

--- End quote ---

Ohms law is your friend when looking at things like dissipated power, yes. The dissipated power isn't the voltage x amps running through it (total power), but the voltage drop x current (power dissipated). A FET controlling 12V@4A isn't dissipating 48W, or there would be no power left over for components in the rest of the system. All the power would be turned into heat. If it has a resistance of 22mOhm when active, then you have calculated correctly and the dissipated power is closer to 1/3W.

As well as using Ohms law that way, you can also use P=I2×RDS(on)

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