Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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 Posted: Tue Dec 24, 2013 12:15 pm Post subject: Soild State Relays |
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1) International Rectifier sells "Intelligent Solid State Switches" These take most of the pain out of building simple and reliable SSRs. Google them. These are available in both "high side" and "low side" types. See attached.
2) Don't forget the heat sink. Assuming the mosfet is either fully on or fully off, the heat sink is merely I X I X Rds(on). Example: FDD8445S http://www.farnell.com/datasheets/49165.pdf, is a 25A, 40V Fet in a remarkably tiny D-pak. It's Rds(on) is 6.7 mΩ (Typ), VGS = 10V (the gate drive voltage), Id=25A (50A at 50% duty cycle). So to keep the thing under 100 degC you need a heat sink that will dissipate 25 X 25 X 0.0067 = 4.2 W minimum. Really a simple thing to do. And it costs $1.00.
3) The discussion about "What is the most common failure mode...?" for SSRs neglected to ask "What is the most common method of abuse...?", but there is a general rule for these devices:
Large over-currents (slowly applied) tend to cause "short" failures.
Large over-voltages (suddenly applied) tend to cause "open" failures.
But YMMV...
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| Description: |
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Download |
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High-Side Low-Side Fets.pdf |
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84.74 KB |
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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