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TO-251 or SOT23 for mosfet low current?

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earckens

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For use in a low-power H-bridge (less than 12V 100mA) connected to outdoor liquid sensing level probe I hesitate between TO-251 and SOT23 package.
I see following pro's and con's for TO-251:
Pro:
- good power dissipation
- high surge voltage resistance (600V vs 30V for SOT23)
(hence suitable for outdoor environment use, better voltage surge protection)
- higher surge current (see line above)
- more rugged
Con:
- bulky, takes >4x more space on pcb
- hard to replace in double sided pcb
- unit cost

What would be the preferred choice for this application?
 

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kubeek

Well-Known Member
Most Helpful Member
That is a rather large difference, why not something in the middle like TO223? As for surge resistance, I would prefer to use a properly rated TVS instead of relying on the transistors.
Or are you limited to these two specific transistors?
 

earckens

Member
That is a rather large difference, why not something in the middle like TO223? As for surge resistance, I would prefer to use a properly rated TVS instead of relying on the transistors.
Or are you limited to these two specific transistors?
TVS: I have included 18V varistors, would that do?
TO223: valid proposal! But I have hundreds in storage of the others, the TO223 I would have to order (had some bad stock keeping practice) (and delay in project :nailbiting:).
 

dknguyen

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There's no reason to pick the TO-251 device unless you just already have them:

-600V source-drain voltage is way more than you need (not so bad on its own except that it means the part is more expensive for something you don't need while sacrificing performance in other areas that are more important to you such as gate charge, gate voltage, thermal resistance, or on-resistance).
-through-hole device (could be good or bad)
-large device that still has similar junction to ambient thermal resistance to the smaller SMD device
-has hundreds of times higher on-resistance
-higher on-resistance with same thermal resistance and same gate charge (i.e. same switching losses) means that it handles less current than the smaller SMD device
-not a logic level gate (dealbreaker if it's required, but not bad to have if you don't need it)
-similar total gate charge as the smaller device (similar switching losses)

The TO-251 device is going to heat up about 10C above ambient passing that 100mA. That's not very hot, but c'mon, we're talking 100mA running through something that is meant to be a switch, not a resistor so a 10C rise for 100mA is way hotter than it needs to be.
 
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earckens

Member
There's no reason to pick the TO-251 device unless you just already have them:

-600V source-drain voltage is way more than you need (not so bad on its own except that it means the part is more expensive for something you don't need while sacrificing performance in other areas that are more important to you such as gate charge, gate voltage, thermal resistance, or on-resistance).
-through-hole device (could be good or bad)
-large device that still has similar junction to ambient resistance to the smaller SMD device
-has hundreds of times higher on-resistance
-higher on-resistance with same thermal resistance and same gate charge (i.e. same switching losses) means that it handles less current than the smaller SMD device
-not a logic level gate (dealbreaker if it's required, but not bad to have if you don't need it)
-similar total gate charge as the smaller device (similar switching losses)

The TO-251 device is going to heat up about 10C above ambient passing that 100mA. That's not very hot, but c'mon, we're talking 100mA running through something that is meant to be a switch, not a resistor so a 10C rise for 100mA is way hotter than it needs to be.
Thank you!
 

dknguyen

Well-Known Member
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Do note that the SMD device has a maximum gate voltage of 12V. The through-hole device is 30V. Not suprising since the through hole device expects at least 10V of gate voltage.
 

earckens

Member
Do note that the SMD device has a maximum gate voltage of 12V. The through-hole device is 30V. Not suprising since the through hole device expects at least 10V of gate voltage.
A switching power supply with Vout of 12V is used; it is probably safer to use something like 2N7002 (Vgs=20V), it's source current is limited to 200mA but in my case the maximum current in any mosfet smd would be about 30mA.
Thank you for reminding me of that point.
 

dknguyen

Well-Known Member
Most Helpful Member
A switching power supply with Vout of 12V is used; it is probably safer to use something like 2N7002 (Vgs=20V), it's source current is limited to 200mA but in my case the maximum current in any mosfet smd would be about 30mA.
Thank you for reminding me of that point.
Yeah, I wouldn't drive a FET gate near it's maximum.
 
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