Principios formativos que orientan la interacción en el aula

Power System

The VAX 8800 power system consists of modu­ lar u n i ts of switching power regulators operat­ i ng at 5 0 kilohertz (KHz) . The total three-phase ac power required for a typical appl ication con­ figuration is about 5 k ilowatts ( KW) . The hard­ ware implementation uses u n i ts from a fam i ly of

products called the Modu lar Power System , or MPS, designed by Digita l . These u n i ts yield low a n d t i gh t l y c o n t ro l l e d d i ffe r e n t i a l ( n o r m a l mode) noise l eve ls for the d e power that sup­ pl ies voltages to run logic.

Through t h e i r h ig h e l ec t r i c a l effi c i e n cy of power conversion , such swi tc h i ng power sys­ tems have made possible the sma l l sizes and l ow weights of present computers . This power c i r­ cuitry, however, has current spikes (dijdt) as high as 1 000 amperes per m icrosecond (!is) and voltage slew rates (dVjdt) as high as 2 0 0 0 volts ( V ) p e r 11s . These h i g h s l e w ra t e s , a c o n s e ­ quence o f t h e purs u i t of h i g h efficiencies , can produce sign i ficant noise problems . The rest of this section discusses five of the most i mportan t noise sources that w e identified a n d resolved i n the power system .

Noise Currents

When high-voltage s lew rates are present across parasitic capacitances ( i . e . , u n intentional capac­ i ta nce t h a t is presen t as a conseq u e n c e of a physical metallic structure) , a noise curre n t Ill wi l l be generated :

Ill = C" dVjdt

in which Cp is the parasitic capacitance .

O n e s i g n i f i c a n t sou rce of c o m m on - m od e n o i s e i n t h e M P S reg u l a to rs i s t h e p a ras i t i c capa c i tance between the p r i m a ry w i n d i ngs i n the high-frequency power transformer and the solid-foil safety shield between the primary and secondary windings. The use of t h is shield , con­ nected to a sheet-metal "safety groun d , " is one way of complying with the i nternational safety regulations. 1

Digital Technical journal

No. 4 Februtn:y I 'J8 7

During normal switch i ng-converter operation , voltage pulses with rise times of approx i mately 1 000 V per 11s are applied to the pri mary. These pu lses cause capa c i t i v e l y cou p l e d n o ise cur­ rents with peak a m p l i tu des of approxi mately 200 m i l l ia m peres to be s e n t i n to the syst e m chassis, or safety ground . Figure 1 shows a sche­ matic representation of this process . The para­ sitic l eakage inductance associated with the pri­ m a ry w i n d i n g c o m p r i s e s a s e r i e s - reso n a n t circu i t with the shield capacitance. This noise cur ren t has a decay i n g exponential waveform with a frequency in t he range of 5 to 1 0 mega­ hertz (MHz) and a repetition rate of twice the swi tch i ng frequency. Since many power con­ verters are u sed in t h e VAX 8800 syst e m a n d they are a l l synchro ni zed to a common clock , t he noise curren ts tend to add . Current a m p l i ­ tudes a s h i gh as 2 amperes were observed .

The most pract i ca l way to reduce this noise sou rce was to i nsert a damping resistance, Rd , that would redu ce the Q of this resonant circui t a t the specific frequency range . Q is tradi tion­ a l ly defi ned as the ratio of reactive i mpedance to resistance, and represents a measure of reso­ nant efficiency. The i n ternational safety regula­ tions, however, strictly l i m i t the fau l t-current i m pedance in this pat h . To meet both req ui re­ ments, we i nserted a ferri te bead on the shield ground lead . This bead is made of ceramic ferro­ magnetic m aterial that is e l ectrica l ly l ossy . I t acts a s a s m a l l i n d u ct a n ce a t low frequencies and as a nearly pure resistance a t h igh frequen­ cies. The bead does not block the fau l t currents from a short circui t but does redu ce the noise current to the des i red leve l . The noise a m p l i ­ tude i s reduced by two t o fou r t i mes a n d t h e r i n g frequency reduced t o about 1 MHz. Thus a pote n t i a l ly s e r i o u s c a u s e of com m o n - m o d e noise current i n the system is redu ced a t t h e source t o acceptable l evels.

I n n ew d e s i g n s , m o r e e ffe c t i ve s c h e m e s i nvol v i n g d iffer e n t s h i e l d confi gu ra t i o n s a n d interconnections could b e employed .

Power L ine Filter

One of the more subtle (and i ronic) sources of com mon- mode noise curren t origina tes i n t he power fi I ter designed to reduce the e lectrical noise emanating from the power l i ne . Figure 2 d e p i cts a sc h e m a t i c of a typ i c a l l i n e fi l te r ,

89

A Logical Grounding Scheme for the VAX 8800 Processor

PRIMARY

I I I

L_

T1

(lp)

PRIMARY

VOLTAGE

(Vp)

NOISE

CURRENT

(In)

PRIMARY SECONDARY

01

Ls

L1p � 1 .2 x

1 06H

primary leakage inductance

I I I

l

_ _

l E--J

I --1.- "'T' _I L - - - -

l_ __ i E--__j

c1

Cp

�

C,

�

200

x

10-12

picofarads primary and secondary parasitic capacitance to shield

Ro

is the damping resistance provided by a lossy ferrite bead

Resonant frequency of In is Fo � _[2,.-

(L1p

x

C0) 11\1

=

10.3 MHz

Resonant impedance

Ro

�

(L10/C0) 112

= 775 ohms

With

Ro

= 0,

In

(peak) =

V0

(peak)/Ro � 200 milliamps

Wilh

Ro

=

500

ohms @ 10

MHz, In

(peak) = 1 1 8 milliamps

Figure 1 Parasitic Capacitance of the Power Transformer

i ncluding the parasitic, or l eakage, i nductance of the common-mode choke, L 1 • The "Y" capac­ itors, C1, are connected from e i ther side of the power l i n e tO the chassis, forming a high-Q res­ onant circuit with this leakage inductance. The load current for this power fi l ter is domi nated by t h e d i sco n t i n u o u s c u r re n t p u lses of t h e swi t c hi ng power converte rs , w h i c h provid e

90

excitation for this resonant circui t . The result is a resonanr current pulse i n to the chassis with each ha lf-cyle of curren t in the power l ine.

O t h e r c o n s i d e ra t i o n s of s i g n a l i n t e g r i t y de mand that an i n d u c tOr be p l aced i n series with the power ground wire in the fi l ter before that wire is connected to the chassis. The resulting ground i mpedance forces the resonant common-

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