Hello Everyone,

I have been working on a small circuit which is more or less a PWM with a delay. In the circuit I am using a mosfet " KIA50N03BD "which is 30V drain to source and can handle up to 50 AMP (my application only pulls 2 ~2.5 AMP and MAX 12V). When I am testing the circuit the circuit does not get into the PWM mode and the mosfet heats very fast. When I swapped the mosfet to "IRF3205" which is 55V and 110AMP type. The circuit works normal.

My question is, what could be the difference between both mosfets that is casing the problem. Aren't both mosfets supposed to work given that both are over my requirements of 12V and maximum 2.5 AMPs?

I would appreciate an help to explain what could make the 2 mosfets behave differently.

Thank you.
Fady

 

You should check the data sheets for the devices. The first one you mention probably needs a higher voltage than you are providing on the gate to turn it fully on.

 

You need to post a circuit and it would be considerate to post links to the two data sheets.

 

To answer the question posed in the thread title; no, they are not all created equal. They have large differences in the threshold voltage, Vgs(th), the point where they begin to turn on and large differences in the channel resistance, rds(on) when they are fully turned on. All of this information is manifest in the datasheets, but you have to look for it an know what to pay attention to.

 

Thank you guys for the prompt reply.

Here are the links for the datasheets
https://alltransistors.com/adv/pdfview.php?doc=irf3205pbf.pdf&dire=_international_rectifier
https://alltransistors.com/adv/pdfview.php?doc=kia50n03bd.pdf&dire=_kia

Also I am attaching the circuit design.

Appreciate all your inputs.

Thank you.
Fady

 

You should check the data sheets for the devices. The first one you mention probably needs a higher voltage than you are providing on the gate to turn it fully on.

Thank you for your reply.

Actually the mosfet which works has a higher Vgs(th) (4 V) and the one that doesnt work has Vgs(th) of 3V. So I dont think that the gate voltage is the issue.

 

To answer the question posed in the thread title; no, they are not all created equal. They have large differences in the threshold voltage, Vgs(th), the point where they begin to turn on and large differences in the channel resistance, rds(on) when they are fully turned on. All of this information is manifest in the datasheets, but you have to look for it an know what to pay attention to.

Hello,

Thank you for your reply.
I actually checked both the Vgs(th) and the rds(on) for both mosfet and they are very close. Vgs(th)= 4V (for the mosfet that works) VS Vgs(th) = 3V (for the mosfet that does not work). Also the rds(on)= 8mOhm (the mosfet that works) VS rds(on)= 6.5mOhm.
Are there any other aspects that I should look into?
Thank you

 

You need to post a circuit and it would be considerate to post links to the two data sheets.

Thank you for the prompt reply.

Here are the links for the datasheets
https://alltransistors.com/adv/pdfview.php?doc=irf3205pbf.pdf&dire=_international_rectifier
https://alltransistors.com/adv/pdfview.php?doc=kia50n03bd.pdf&dire=_kia

 

Are you operating the circuit from 5 volts?
CD40xx series gates only provide an output source current of 1 mA at Vdd =5 V.
Although the circuit parallels two outputs for increased current, it may not be sufficient. People almost always grossly underestimate a Mosfet’s gate drive requirements, while operating in PWM mode.

If indeed you are operating at 5 volts, could you increase the voltage? Let’s say, at least 9 to 12 volts? If you don’t have access to a variable supply, use batteries instead.

 

An 18-wheeler and a step van both have the ability to carry a 1000-pound load.

(But one will hardly notice the load)

 

Thank you for the prompt reply.

Here are the links for the datasheets
https://alltransistors.com/adv/pdfview.php?doc=irf3205pbf.pdf&dire=_international_rectifier
https://alltransistors.com/adv/pdfview.php?doc=kia50n03bd.pdf&dire=_kia

That is not a link to the datasheets. That is a link to site where one may search for the datasheets.

Please provide two links. Each of which pulls up one the actual datasheets directly when clicked.

 

Hello,

Thank you for your reply.
I actually checked both the Vgs(th) and the rds(on) for both mosfet and they are very close. Vgs(th)= 4V (for the mosfet that works) VS Vgs(th) = 3V (for the mosfet that does not work). Also the rds(on)= 8mOhm (the mosfet that works) VS rds(on)= 6.5mOhm.
Are there any other aspects that I should look into?
Thank you

Yes, you should look at the total gate charge in coulombs. This is the amount of charge you have to add to the gate to turn it on, and the amount of charge you need to remove from the gate to turn if off. The thing that helps you add and remove charge from the gate of a MOSFET is a current source/sink. A CD4000 series CMOS part in not exactly a standout choice in that department. When switching a gate there is a thing called the "Miller plateau" where, as Vgs begins to rise, the voltage remains constant for a time before rising to its final value. During this time the MOSFET is operating in the linear region, basically shorting the power supply to ground and dissipating a bit of heat. The same plateau exists when turning the device off. Using a push-pull driver for the gate usually solves this problem.

 

The datasheet links worked for me.
There is huge difference in gate charge, but it should favour of the device which is giving the problem, not the device from the well-known and well-respected manufacturer!
Needless to say, for any type of rapid switching circuit, the driver circuit is totally inadequate. It needs to be able to deliver Amps not milliAmps. Use a proper low-side driver such as an MCP1402.

Would you really have expected all N-channel MOSFETs to have been created equal?

 

Actually the mosfet which works has a higher Vgs(th) (4 V) and the one that doesnt work has Vgs(th) of 3V. So I dont think that the gate voltage is the issue.

Why are you talking about Vgs(th)? Unless you're using the mosfets as a variable resistor. The Vgs(th) is not a turn on voltage for those mosfets, but the turn off voltage. Using the Vgs(th) you have to expect the mosfet to get hot, it's operating in the resistive state not as a switch.

 

The circuit schematic was posted without a Very important power supply voltage.
Does the power supply voltage drop when there is a load?