Unitek UNIBOND 2, User Manual

Page: 30 / 49

R88 i s t h e i n p u t r e s i s t o r t o t h e a m p l i f i e r a n d R132 and 8131 - R133 a r e r e s p e c t i v e l y
the v o l t a g e and c u r r e n t feedback r e s i s t o r s . R 1 3 2 i s t w i c e t h e v a l u e o f R88 s o t h a t
there i s a v o l t a g e g a i n o f two. T h e remaining components form a power a m p l i f i e r.
R112. R115. R118, R121 a n d R124 h e l p t h e t r a n s i s t o r s t o share c u r r e n t . R e s i s t o r s
R108, R111. R114. R117. R120. R123 and R126 a l s o h e l p t h e t r a n s i s t o r s t o share c u r -
rent and a r e simultaneously used t o monitor t h e transformer primary c u r r e n t b y means
U9 and Q15 p r o v i d e t h e v o l t a g e g a i n and v o l t a g e l e v e l s necessary t o d r i v e t h e output
stage, Q16 t h r u Q22. R 9 8 determines t h e l o o p g a i n and R97. C18 and C19 p r o v i d e f o r
feedback loop s t a b i l i t y . R 1 0 0 , R104 and R105 l i m i t t h e c u r r e n t and power d i s s i p a -
tion i n Q15. R 1 0 1 a n d R102 p r o v i d e t h e proper i n i t i a l b i a s f o r Q15. R 1 0 3 a l l o w s a
path f o r c o l l e c t o r t o base leakage c u r r e n t s f o r Q16 - Q22. R e s i s t o r s R106, R109,
of averaging r e s i s t o r s R107, R 11 0 , R 11 3 , R116, R 11 9 . R122 and R125. T h i s c u r r e n t i s
d i r e c t l y r e l a t e d t o t h e secondary c u r r e n t b y t h e transformer r a t i o and must b e s c a l -
ed by t h a t f a c t o r . S i n c e t h e primary windings a r e switched from s e r i e s t o p a r a l l e l
connection a t Pulse Amplitude s e l e c t i o n s over 1 . 9 9 v o l t s . t h e c u r r e n t feedback must
also b e changed by a f a c t o r o f two. T h i s i s accomplished b y means o f Q14 and R93 i n
conjunction w i t h
feedback r e s i s t o r s R131 and R133. P o t e n t i o m e t e r R4 and r e s i s t o r R5
attenuate t h e c u r r e n t feedback s i g n a l . r e q u i r i n g a lower v a l u e o f RL. t h e Weld Load
Resistance, t o p r o v i d e s u f f i c i e n t c u r r e n t feedback t o balance t h e l o o p . T h i s
e f f e c t i v e l y reduces t h e output impedance. R e s i s t o r R2 l i m i t s t h e d i s s i p a t i o n i n t h e
core b i a s c i r c u i t .
R4.
Figure 27 - Pulse A m p l i f i e r w i t h Vo l t a g e + Current Feedback.
7 . 0 3 . H O W THE FEEDBACK LOOP DETERMINES OUTPUT IMPEDANCE
Probably t h e most common method o f measuring output impedance i s t o connect a r e s i s -
tance across t h e o u t p u t t h a t causes t h e unloaded output v o l t a g e t o drop t o o n e - h a l f o f
i t s o r i g i n a l v a l u e . T h i s i s because i t forms a v o l t a g e d i v i d e r o f equal r e s i s t a n c e s
as i l l u s t r a t e d below:
- 2 5 -