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HIPCHECK 13th November 2012 04:00 PM

Need help on "pass" transistor math
I would like the math involved to calculate the power consumption/heatsink for a "pass" transistor used in cc-ccs-x-bosoz psu. I am not able to buy a new transformer now, and I am trying to use something on hand.

Papa Pass article for BZLS uses 2x30V transformer

60 * 1.4 = 84V then drop 4V on mosfet (pass transistor) and regulate 80V down to 60V (dropping 20V) and it is ok to use IRF610.

I have the twisted x bosoz power supply unit and 36-0-36V tranformer

72 * 1.4 = 100.8 then drop 4V on mosfet (pass transistor) and regulate 96.8V down to 74V (dropping 22V) it should still be ok.

Does this equal around 3W-4W power consumption?

I also have a pile of new batteries for RC cars (1600mah 9.6V) I could series several of them together and use them for supply I guess, any input on doing this? something with a virtual ground?

Thank you for looking,


sakellogg 13th November 2012 05:22 PM

looks like it for both chan.
80ma per chan. 22v....

going to have to change the zeeners to get to 60v
but, should be doable.

unless im looking at wrong zen...

flg 13th November 2012 05:41 PM

Maybe I need to read it again but, 4V on the MOSFET and 22V regulated/dropped/what ever = 26V on the MOSFET?
However, since you have the voltage, using the correct load R's or modifying the Iq, might work better at the higher voltage. I had a BOSOZ modified for +80 -20 with CCS at the bottom and resisitors at the Load, 50mA per side. Sounded great. I think your talking CCS load also but it's worth a try IMO!

HIPCHECK 13th November 2012 06:16 PM

Thank you for both replies, Any confusion is my fault for not posting circuit info.

I finally found some time to search the entire thread found a reference to this same question. Starting @ post 559

The math I used was:

36-0-36 = 72V * 1.4= 99.8V before CRC, dropping 4V across IRF610 as effect of "pass" transistor (maybe 1V in zener string) leaving me with 96V and needing to drop 22V to the 74V Rail I need.

I used 74V rail because the current source and cascode will use a few volts also.

22V drop on pass transistor x 8ma biasx2ch
22V*0.16A=3.5W round up for using 1 psu for 2 channels so I figure 5W or so MAX.

I just wanted to get some input..I think I have it.


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