Which MOSFET for DCSSR

P

ptyzzer

New member
Looking at the DCSSR on the WIKI it suggests using either a RFP14N05L or STF20NF06 MOSFET. Looking in the UK I can only find the RFP14N05L MOSFET on http://uk.rs-online.com
When I look for similar devices there are are whole lot of parameters or characteristics to compare. Obviously working voltage and current carrying capability are obvious but what other characteristics matter when looking for an alternative MOSFET for the DCSSR. I will be controlling 12V dc at no more than 6A using some dumb RGB strips like these http://www.lightingever.co.uk/12v-led-rgb-strip-150-units-5050-leds-waterproof-5m.html ( 2 units per channel ).
Suggestions / guidance will be very much appreciated, thanks.
 
I used the STP16NF06L. I see that the website you quoted carries has a listing for. Since it is rated for 16 amps and I only load it to less than 1 amp, I don't have to worry about adding heat sinks.
 
Well, ... since you can find the RFP14N05L in the UK why look for additional substitutes that requires further research? It's a 14 amp device @ 50V so it is well within your requirements.
 
You should use a heat-sink with the RFP14N05L, as it will end up dissipating somewhere around 3W if left on continuously with a 6A load (according to the graphs in the datasheet, it will typically saturate at 6A with about a 0.5V RDS).
 
[Kev] RS components does carry a listing for the MOSFET but I'd rather see if I can find a cheaper alternative, but I don't know which of the characteristics I need to be concerned about other than Voltage & current.

[Lightup] Thanks for the reply, I would look for alternatives but I don't know which of the characteristics I need to be concerned about.

[P.Short] I would like to be able to do this without a heat-sink if I can, mainly for space, happy to use a higher current rated device. Due to lack of knowledge, what does RDS relate to?

Thanks for your replies guy's.
 
Hi Barnabybear,

Thanks for that. What characteristics of the device make it a suitable alternative?
I don't quite understand MOSFET's other than they appear to be power transistors of a sort.
 
ptyzzer:
The basic characteristics are voltage, current, power, gate drive and thermal design.
It was a course in my schooling so a blurb here wouldn't do full justice, as you are getting into design details. I haven't done this for some decades. :)
You need to be able to decipher data sheets of the device you are going to use. You can start the process and have reviews done here.
A solution can take time (and money in buying various parts) as some potential solution may use several very cheap devices and wire them in parallel. But you have to evaluate all the angles to determine if it really is cheaper, or worth the bother.

I don't have the cheapest solution for you waiting to be built.

Generally you pick a working design already tested by members here and go with it.

Edit:
There is also a software way of designing by using, say LTSpiceIV.
http://www.linear.com/designtools/software/#LTspice
How close it comes to the real world would need to be evaluated by building a prototype and testing it.
 
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ptyzzer said:
[Kev] ....... but I'd rather see if I can find a cheaper alternative.......
Click to expand...

Yeah. Not sure what your cost is, but I paid $0.25 each two years ago for 100 pieces. They are now $0.33 each for 100 pieces from Avnet, more if you buy less quantity.
My 16 channel DC SSR boards cost less than $1.00 per channel (actual cost of board and components at the time).
 
Okay, Thanks for your help guys, I think I'll go with Lightup's suggestion of try a tested design and see how I get on. I've got 11 months to do it.

Happy Christmas everybody.
 
This comment will be somewhat conservative and over-simplified. The conservative element here is that I think that the circuit should be designed using the worst-case characteristics of the part (not the typical characteristics), and should be designed for the worst case situations. In this case, I'm assuming that the controller firmware locks up with a 100% duty-cycle and maximum load at a temperature of 40° C (104°F).

First, for that part in a TO-220 package the maximum allowable power dissipation should be (175° - 40°)/(80°C/W) = 1.6875W in order to keep the junction temperature within the 175° maximum temperature. This is not conservative at 40°C, but is quite conservative at 0°C. At 0°C the allowable power dissipation is 2.1875W.

Second, looking at Figure 7 in the datasheet (from mouser), the voltage drop from drain to source is about 0.4V (just an eyeball estimate) when the current is 6A (at almost any gate voltage above 4V). This equates to a power dissipation of about 2.4W (a little different from what I wrote above, as my eyeball estimate is different this second time around).

If you want to live dangerously, go without a heatsink, and hope that the software doesn't lock up sometime at night or when you are away for several hours. Otherwise, either use a heatsink or limit the current to a much smaller value than 6A, or move to the North Slope of Alaska or the South Pole. Reducing the current to about 4A should decrease the maximum power dissipation to about 1.2W, which would be fairly safe (for the part, but not for your skin) to operate without a heat sink,
 
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