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18th August 2015, 07:28 PM  #1 
diyAudio Member
Join Date: Aug 2004
Location: sulmona

half bridge secondary voltage
Hi to all
I have in mind to build a SMPS for a tube tester. I considered half bridge topology and calculated the power transformer turns with the formula finded in Marty Brown book (you can see in the picture). I have 66 turns primary and 270 turns secondary for 650V, 5 turns for 12,6V, 41 turns for 100V and 8 turns for 20V, repectively used for anodic voltage, filament, negative bias and aux voltage for SG3524. Now if I understand well, the secondary of the anodic section has a turn ratio Ns/Np=4,09 so the voltage on this will be (230*1.41)*4=1297V!! The filter inductor must drop 647V!!! I think this can be a great problem for inductor dissipation and even for the insulation of the secondary... What if I use a lower turn ratio? say only 2? And what does happen if I use the 12,6V output to feedback the SG3524? the 650V secondary will have voltage variation as the load on the filament vary? thanks in advance! Luigi 
18th August 2015, 07:51 PM  #2 
diyAudio Member
Join Date: Dec 2008

No problem with turn ratio of 2.
You will get 440480Vac, so 620680Vdc when rectified full wave. 
18th August 2015, 08:19 PM  #3 
diyAudio Member
Join Date: Sep 2006

Halfbridge = voltage/2, which means you should be fine, taking into account inductive and other losses (won't be regulated though, and without any minimal bleeding, could rise to impressive voltage levels due to ringing)
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18th August 2015, 09:39 PM  #4 
diyAudio Member
Join Date: Aug 2004
Location: sulmona

mmmm I'm a bit confused...
In an example from Brown book I have a half bridge SMPS with 38 turns primary at 230Vac (324Vdc), a secondary of 5 turns for 28V (Ns/Np=0,131) and an aux secondary of 2 turns for 12V. As I can see in the formula for the filter inductance, in the numerator appear (VinmaxVout), Vinmax is the raw output voltage given by the turn ratio and is 47V (in the picture), (230V*1.41)*0,131 = 42V ... where is the error?!? 
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