Voltage level shifting

Transcription

1 rek Applicaion Noe Number 1 r. Maciej A. Noras Absrac A brief descripion of volage shifing circuis. 1 Inroducion In applicaions requiring a unipolar A volage signal, he signal may be delivered from a bi-polar volage /generaor and reposiioned relaive o a zero vol reference (an earh ground) using a volage level shifing circui. In addiion, he volage level shifing echnique is frequenly used o generae high volage oupus from lower volage A sources. he maximum volage (A or ) produced a he oupu of a single sage of he level shifing circui is nominally equal o he peak-o-peak value of he A signal delivered by he /generaor. 2 A volage shifing An example of an A volage shifing circui diagram (Villard circui) is shown in Figure 1. Figure 2 shows he inpu and he oupu volage of he circui. In order o move he volage signal oward negaive values, i is necessary o reverse he diode shown on he schemaic in Figure 1. Addiionally, if he capaciors 1 and 2 are polar, hey also need o be reversed. (A source) 1 volage shifing secion load 2 Figure 1: Villard volage level shifing circui. - 2 Figure 2: Shifed signal. he seady sae curren drawn from he A source (, for example) follows he dependency: 1 2 I = d (1) d For a sinusoidal A volage inpu = sin (2πf) (2) he curren is: 1 2 I = 2πf cos (2πf) (3) he oupu volage ou across he capacior 2 is: 1 ou = () Example onsider he following example: =1 sin (2πf) V; f=1 Hz; 1=2=3 nf. he maximum curren is 2 ma (eq. 3) and he volage across he 2 is 1 V (eq. ). hese resuls are shown in Figure 3. When he capacior 1 becomes smaller (1=31.5 nf), he oupu volage value drops (Figure ) according o equaion. urren I also decreases, as he oal capaciance of he circui decreased. REK, IN. 19 Walnu Sree Lockpor, NY 19 el: (71) all: 1 8 FOR REK FAX: (71) sales@rekinc.com Web: opyrigh 213 REK, IN. 8/MAN Rev. 2 page 1 of 5

2 rek Applicaion Noe Number inpu volage volage across 1 volage across 2 2 curren hrough he load =3 nf, 2=3 nf, f=1 Hz ime, s Figure 3: Simulaion resuls for he circui shown in Figure 1, where f=1 Hz, 1=2=3 nf. When considering a choice of capaciors for he volage shifing circui, wo facors have o be aken ino accoun: oal capaciance of 1 and 2 conneced in series( ), and he raio of a capaciive divider formed by boh capaciors ). I is imporan o pay aenion on he volage raing of he capaciors. he frequency of he inpu volage signal also influences choice of capaciors. For example, if he frequency is changed from 1 Hz o 1 khz, he curren drawn from he source muliplies by he facor of 1 (Figure 5). apacior values have o be lowered by he facor of 1 o ge back o he 2 ma curren level. 1 ( 1+2 volage generaion By combining several sages of he basic circui from Figure 1, a high volage signal can be obained. Figure 7 presens a wo sage volage muliplier generaing posiive oupu volage. A similar, wo sages negaive volage circui is shown in Figure (A source) he oupu volage ou from he capaciive divider is maximized for 1>>2. For example, le 1=12, nf and 2=1 nf. Resuls of a simulaion wih hese 1 and 2 values are shown in Figure. Figure 7: Posiive volage muliplier. REK, IN. 19 Walnu Sree Lockpor, NY 19 el: (71) all: 1 8 FOR REK FAX: (71) sales@rekinc.com Web: opyrigh 213 REK, IN. 8/MAN Rev. 2 page 2 of 5

3 rek Applicaion Noe Number inpu volage volage across 1 volage across 2 2 curren hrough he load =31.5 nf, 2=3 nf, f=1 Hz ime, s Figure : Simulaion resuls for he circui shown in Figure 1, where f=1 Hz, 1=31.5 nf, 2=3 nf. -.. n * 2 (A source) Figure 8: Negaive volage muliplier. 2 - Figure 9: muliplier signal. he number of sages ha can be used in his kind of design is limied by curren capabiliies of REK, IN. 19 Walnu Sree Lockpor, NY 19 el: (71) all: 1 8 FOR REK FAX:(71) sales@rekinc.com Web: opyrigh 213 REK, IN. 8/MAN Rev. 2 page 3 of 5

4 rek Applicaion Noe Number inpu volage volage across 1 volage across 2 2 curren hrough he load =3 nf, 2=3 nf, f=1 Hz ime, s Figure 5: Simulaion resuls for he circui shown in Figure 1, where f=1 Hz, 1=2=3 nf. he circui. he oupu volage has an A volage ripple δ (Figure 1) given by equaion [1]: n * 2 no load wih load δ = I f n (n + 1), (5) =1/f δ where I is he load curren and n is he number of sages. A volage drop (Figure 1) due o he load can be calculaed using formula [1]: Figure 1: Volage ripple and volage drop. References = I ( 2n 3 f 3 - n ) () [1] E. Kuffel, W. S. Zaengl, and J. Kuffel. High Volage Engineering: Fundamenals. Newnes, 2 ediion, 2. REK, IN. 19 Walnu Sree Lockpor, NY 19 el: (71) all: 1 8 FOR REK FAX:(71) sales@rekinc.com Web: opyrigh 213 REK, IN. 8/MAN Rev. 2 page of 5

5 rek Applicaion Noe Number 1 2 inpu volage 2 volage across volage across curren hrough he load 2 curren hrough ime, s 1=12 nf, 2=1 nf, f=1 Hz Figure : Simulaion resuls for f=1 Hz, 1=12 nf, 2=1 nf. REK, IN. 19 Walnu Sree Lockpor, NY 19 el: (71) all: 1 8 FOR REK FAX:(71) sales@rekinc.com Web: opyrigh 213 REK, IN. 8/MAN Rev. 2 page 5 of 5