9/13/2005 Peak Inerse Voltage.doc 1/6 Peak Inerse Voltage Q: I m so confused! The brdge rectfer and the fullwae rectfer both prode full-wae rectfcaton. Yet, the brdge rectfer use 4 juncton dodes, whereas the full-wae rectfer only uses 2. Why would we eer want to use the brdge rectfer? A: Frst, a slght confesson the results we dered for the brdge and full-wae rectfers are not precsely correct! Recall that we used the juncton dode CV model to determne the transfer functon of each rectfer crcut. The problem s that the CV model does not predct juncton dode breakdown! If the source oltage becomes too large, the juncton dodes can n fact breakdown but the transfer functons we dered do not reflect ths fact! Q: You mean that we must rework our analyss and fnd new transfer functons!? Jm tles The Un. of Kansas ept. of EEC
9/13/2005 Peak Inerse Voltage.doc 2/6 A: Fortunately no. Breakdown s an undesrable mode for crcut rectfcaton. Our job as engneers s to desgn a rectfer that aods t that why the brdge rectfer s helpful! To see why, consder the oltage across a reersed based juncton dode n each of our rectfer crcut desgns. Recall that the oltage across a reerse based deal dode n the full-wae rectfer desgn was: 2 = 2 so that the oltage across the juncton dode s approxmately: = + 0.7 = 2 + 0.7 Now, assumng that the source oltage s a sne wae = Asnωt, we fnd that dode oltage s at t most negate (.e., breakdown danger!) when the source oltage s at ts maxmum alue A. I.E.,: = 2A + 07. mn Of course, the largest juncton dode oltage occurs when n forward bas: = 07V. max Jm tles The Un. of Kansas ept. of EEC
9/13/2005 Peak Inerse Voltage.doc 3/6 A (t) 0.7 t 2A + 07. (t) Note that ths mnmum dode oltage s ery negate, wth an absolute alue ( mn = 2A 07. ) nearly twce as large as the source magntude A. We call the absolute alue of the mnmum dode oltage the Peak Inerse Voltage (PIV): mn PIV = Note that ths alue s dependent on both the rectfer desgn and the magntude of the source oltage. Q: o, why do we need to determne PIV? I m not sure I see what dfference ths alue makes. Jm tles The Un. of Kansas ept. of EEC
9/13/2005 Peak Inerse Voltage.doc 4/6 A: The Peak Inerse Voltage answers one mportant queston wll the juncton dodes n our rectfer breakdown? If the PIV s less than the Zener breakdown oltage of our rectfer dodes (.e., f PIV < VZK ), then we know that our juncton dodes wll reman n ether forward or reerse bas for all tme t. The rectfer wll operate properly! Howeer, f the PIV s greater than the Zener breakdown oltage of our rectfer dodes (.e., f PIV > VZK ), then we know that our juncton dodes wll breakdown for at least some small amount of tme t. The rectfer wll NOT operate properly! Q: o what do we do f PIV s greater than V ZK? How do we fx ths problem? A: We hae two possble solutons: 1. Use juncton dodes wth larger alues of V ZK (f they exst!). 2. Use the brdge rectfer desgn. Q: The brdge rectfer! How would that sole our breakdown problem? Jm tles The Un. of Kansas ept. of EEC
9/13/2005 Peak Inerse Voltage.doc 5/6 A: To see how a brdge rectfer can be useful, let s determne ts Peak Inerse Voltage PIV. Frst, we recall that the oltage across the reerse based deal dodes was: = so that the oltage across the juncton dode s approxmately: = + 0.7 = + 0.7 Now, assumng that the source oltage s a sne wae = Asnωt, we fnd that dode oltage s at t most negate (.e., breakdown danger!) when the source oltage s at ts maxmum alue A. I.E.,: = A + 07. mn Of course, the largest juncton dode oltage occurs when n forward bas: = 07V. max Jm tles The Un. of Kansas ept. of EEC
9/13/2005 Peak Inerse Voltage.doc 6/6 A (t) 0.7 t A + 07. (t) Note that ths mnmum dode oltage s ery negate, wth an absolute alue ( mn = A 07. ), approxmately equal to the alue of the source magntude A. Thus, the PIV for a brdge rectfer wth a snusodal source oltage s: PIV = A 07. Note that ths brdge rectfer alue s approxmately half the PIV we determned for the full-wae rectfer desgn! Thus, the source oltage (and the output C component) of a brdge rectfer can be twce that of the full-wae rectfer desgn ths s why the brdge rectfer s a ery useful rectfer desgn! Jm tles The Un. of Kansas ept. of EEC