which mosfet package, soic-8 vs TO-252-3?

Thread Starter

charosenz

Joined Sep 12, 2017
2
I am looking to power an N Channel Mosfet from pin 3 of a 555 timer to light 4 LED arrays . Each array has 12 LEDs made of of 4 strings of 3 LED. They will be flashed with an "on" time of about .2 seconds with about a 1 hz rate. VCC will be 12-13v.

I am debating on whether to use the larger, TO-252-3 package Mosfets, but the multi-channel SOIC-8 package mosfets are tempting, because they are smaller, and half the price....

One consideration is not only will the mosfet be able to handl the current, but more importantly how much performance (brightness) of the arrays will be lost to consumption by the Mosfets.

Suggestions on which package is welcome, and of course which Mosfet would be great too, either on the T0-252 or SOIC-8....

I am not an EE, or even close. This is my first post. I have built several PCB for fun using EasyEDA and have a basic understanding of electronics....

One SOIC-8 that I am looking at is the SI9148BDY. Can someone clarify when it states the continuous current rating is - say - 7amps, does that mean for all channels combined, or that each of the 3 channels can take this amount of current?

Thank you!

Charlie
 

Jack J. He

Joined Apr 24, 2017
9
When looking for a MOSFET you need to take into account what is the maximum voltage they can withstand, what is the maximum current that they can withstand, what gate voltage you need to switch it and how much heat it will produce. When it states that the max continuous current is 7 amps it means the maximum current from drain to source is 7A. It doesn't mean you should pass that much current through it though because it might become really hot if you don't have any means to dissipate it. I don't think you need to worry too much about the brightness of the LEDs as long as you choose a mosfet with a gate threshold voltage (Vgth) well below you timers output voltage.

For example this one:
http://www.mouser.com/ds/2/389/sts8c5h30l-956712.pdf
This would work for your voltage level. To switch it properly your 555 timer pin would need to output at least 4.5V. It has a maximum N-channel current of 8A meaning that you can pass a maximum of 8A from drain to source. Be careful though because if you do that, the mosfet would heat up by about 80C above ambient temperature. It is still below its maximum operating temperature but it really is not ideal.

Also, I can't find anything when googling the SOIC-8 you posted.
 
Last edited:

crutschow

Joined Mar 14, 2008
23,824
Generally you want to limit the MOSFET power to about 1 watt for a surface-mount package.
Thus the ON-resistance of the MOSFET should be such that I²R x duty cycle ≤ 1W where I is the MOSFET current and R is its ON-resistance.
 

Thread Starter

charosenz

Joined Sep 12, 2017
2
When looking for a MOSFET you need to take into account what is the maximum voltage they can withstand, what is the maximum current that they can withstand, what gate voltage you need to switch it and how much heat it will produce. When it states that the max continuous current is 7 amps it means the maximum current from drain to source is 7A. It doesn't mean you should pass that much current through it though because it might become really hot if you don't have any means to dissipate it. I don't think you need to worry too much about the brightness of the LEDs as long as you choose a mosfet with a gate threshold voltage (Vgth) well below you timers output voltage.

For example this one:
http://www.mouser.com/ds/2/389/sts8c5h30l-956712.pdf
This would work for your voltage level. To switch it properly your 555 timer pin would need to output at least 4.5V. It has a maximum N-channel current of 8A meaning that you can pass a maximum of 8A from drain to source. Be careful though because if you do that, the mosfet would heat up by about 80C above ambient temperature. It is still below its maximum operating temperature but it really is not ideal.

Also, I can't find anything when googling the SOIC-8 you posted.
Thank you. In a case like the SOP you listed above, are the ratings for the whole chip, or for each channel? I.E if it states it can take 7A, does that mean it can take 7A on each channel, with a total of 14A for the two channels combined (as long as you have proper heat sinks in place?
 

crutschow

Joined Mar 14, 2008
23,824
In a case like the SOP you listed above, are the ratings for the whole chip, or for each channel? I.E if it states it can take 7A, does that mean it can take 7A on each channel
The data sheet shows the following current maximums.
That is for each operating at the same time.
But note that the total dissipation for both operating at their maximum currents is 2.8W.
upload_2017-9-14_10-57-54.png

I can find no info on the SI9148BDY.
 
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