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In Reply to: RE: Low DCR versus higher inductance chokes posted by xaudiomanx@aol.com on October 05, 2008 at 17:18:44
Proper power supply design requires some DCR and enough inductance to keep the first cap's charging current constant.
Other power supply designs will work but not as well.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Tre,
Define the terms "some DCR" and "enough inductance." 20 ohms? 100 ohms? 200 ohms? 10H? 5H? 15H? A friend showed me the formula to find out how much inductance to use on a circuit in a particular part of the crcuit or the whole circuit.
Thanks!
Paul
The formula for critical inductance is for the input (first part after the diodes) choke. Lc = ((voltage / current) + DCR of choke))/1000
So if your B+ will be 400 and the current draw is 80ma. with a choke that has 60 ohms DCR you need a 5.06Hy choke.
boris' math works out to be almost the same. He's right about using a choke larger than the formula just to make sure.
The DCR of the choke has to do with the value of the caps and the DCR of the power transformer. I don't have a formula for that but the Duncan PSUDII is very helpful.
Too little DCR and the supply will ring (the voltage will not only drop when more current is drawn, it will also "bounce").
Tre'
PS Low DCR is a good thing as long as the ringing can be controlled and other, more important things have not been compromised.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
For a 60hz input, critical inductance is Lc > RL/1130 (From RDH4), where RL is the load resistance. You can usually approximate it by RL = VL/IL where VL = 0.9*(VAC-Vdrop), IL is load current, where VAC is the AC input voltage to the filter (per leg) and Vdrop is the diode drop voltage (you can approximate here). You can do all the math here, or a few button clicks in PSUDII :).
Whatever value you find, it's best to use 25%+ minimum inductance to ensure it won't drop into DCM and deregulate (B+ will increase). Just make sure your inductor resistance isn't too high that winding dissipation will risk an open-coil. Usually < 5 watts on an average sized choke is the maximum you'd want (hard to say what 'average' is, but I'm looking at a 2.5" x 3" x 1" lamination stack choke here that has 75 ohms and is rated for 300mA -> 6.75 watts @ max DC current). Having some resistance helps with damping and ringing with a changing load. Hope this helps!
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