document pyro current limits in an appendix

[fw/altos] / doc / altusmetrum.xsl

diff --git a/doc/altusmetrum.xsl b/doc/altusmetrum.xsl
index 0785f65a9a8219ebaae8d4e3ffe553a7234cbd3c..cd78d5f2b9cafa8f5934f519b17fce57bedb3b84 100644 (file)
--- a/doc/altusmetrum.xsl
+++ b/doc/altusmetrum.xsl
@@ -5830,6 +5830,58 @@ NAR #88757, TRA #12200
         </para>
       </section>
   </appendix>
+  <appendix>
+    <title>Igniter Current</title>
+      <para>
+       The question "how much igniter current can Altus Metrum products 
+       handle?" comes up fairly frequently.  The short answer is "more than
+       you're likely to need", the remainder of this appendix provides a
+       longer answer.
+      </para>
+      <section>
+        <title>Current Products</title>
+        <para>
+         The FET switches we're using on all of our current products that 
+         have pyro channels are the Vishay Siliconix Si7232DN.  These parts 
+         have exceptionally low Rds(on) values, better than 0.02 ohms!  That 
+         means they aren't making a lot of heat... and the limit on current 
+         is "package limited", meaning it's all about how much you can heat 
+         the die before something breaks.
+       </para>
+       <para>
+         Cutting to the chase, the Si7232DN specs are 25 amps *continuous* at
+         20V at a temperature of 25C.  In pulsed mode, they're rated for 40A.
+         However, those specs are a little mis-leading because it really is 
+         all about the heat generated... you can get something like 85A 
+         through one briefly.  Note that a typical commercial e-match only 
+         needed about 13 microseconds to fire in tests on my bench a couple 
+         years ago!
+       </para>
+       <para>
+         So a great plan is to use something like an e-match as the initiator 
+         and build up pyrogen(s) as required to actually light what you're 
+         trying to light...  But if you want to use a high-current igniter, 
+         we can probably handle it!
+       </para>
+      </section>
+      <section>
+        <title>Version 1 Products</title>
+        <para>
+         The FET switches used on TeleMetrum v1 and TeleMini v1 products
+         were Fairchild FDS9926A.  The Rds(on) values under our operating
+         conditions are on the order of 0.04 ohms.  These parts were rated
+         for a continuous current-carrying capacity of 6.5A, and a pulsed 
+         current capacity of 20A.
+       </para>
+       <para>
+         As with the more modern parts, the real limit is based on the heat
+         generated in the part during the firing interval.  So, while the 
+         specs on these parts aren't as good as the ones we use on current
+         products, they were still great, and we never had a complaint about
+         current carrying capacity with any of our v1 boards.
+       </para>
+      </section>
+  </appendix>
   <appendix>
     <title>Release Notes</title>
     <simplesect>