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VSSA Lateral MosFet Amplifier

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R

Renioo

Member
Joined 2012
I got everything ! VSSA assembled and all the stuff, but there is a problem. VSSA is not running. I use 2x26V trafo (as above) connected to PSU (each 26V section to the seperate bridge and then to seperate caps and to VSSA). There should be around 35V on each VSSA, but there is 37V and 34V on each, setting the trimmer does change the offset (and voltage too) but there is nothing on input bias. Like it does not consumpt the power. PSU are grounded together in the star ground (with GND from VSSA too). Any ideas ?
 
C

cvtaudio

Member
Joined 2006
Lazy Cat said:
As miksi and me correctly found out R19, R20 installed are standard Yageo SMD2010 resistors and not Vishay as I specified to the producer of PCB. Vishay resistors are current pulse proved type and Yageo are obviously not.

R19, R20 faces current pulse at the power on sequence when C14, C15 are empty of charge. This happens only when SMPS400A180 is used since this power supply starts with full +/-45 V voltage at the moment when TL494 starts to oscillate mosfets on primary. DC voltage on secondary goes from zero to max in a matter of micro seconds so the current pulse to charge C14, C15 is higher than R19, R20 resistors can withstand.

In classic linear power supply rising of the secondary smoothed DC voltage is slow, also the charge current for C14, C15, so there should be no problem with R19, R20.
Click to expand...

LC,
what do you suggest if a SMPS is being used ? Should we replace these resistors and could you please provide a part number ?

Thanks !
 
Lazy Cat

Lazy Cat

Member
Joined 2007
cvtaudio said:
LC,
what do you suggest if a SMPS is being used ? Should we replace these resistors and could you please provide a part number ?

Thanks !
Click to expand...

Yes change R19, R20 and use VISHAY ones, since these are specified as 10 Ap pulse current peak to withstand. Installed ones are YAGEO 2 Ap max. Guys from production company didn't quite follow my BOM where it was specified VISHAY 2010 10 Ohm/0,75 W type. Sorry for that inconvenience.
 
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