1、EIA TEB22 79 W 3234b00 000714b 8 W - NOVEMBER 1979 TEPAC ENGINEERING BULLETIN NO42 2 O ELECTRONIC INDUSTRIES ASSOCIATION Enprikeering hpartment 2001 Eye Street, N.W., Waehington, D.C. 20006 L EIA TEE22 79 = 3234600 0007347 T PubW by ELECTRONIC INDUSTRIES ASSOCIATION En#neering Deputment 2001 Eye Str
2、eet, NOW.# WaAam, Do C M e EIA TEB22 79 m 3234600 0007L48 I m TEPAC ENGINEERING BULLETIN No. 22 MAGNETIC DEFLECTION YOKES WILLIAM O. ADAMS Syntronic Instruments, Inc, Member, TEPAC JT-20 Committee 11-1-78 MAGNETIC DEFLECTION YOKES To many CRT display system designers a deflection yoke represents a v
3、ital but troublesome component, The yoke is vital because it is an in-line component directly related to key system performance parameters. It is troublesome because there seems to be an air of mystery surrounding the yoke selection process. The fact that a yoke performs a vital role cannot be chang
4、ed, However, the air of mystery surrounding the yoke selection process can be removed, and in its place a logical, orderly selection process may be substituted. The result being a much more effective means of optimizing the overall performance of the CRT display system. The following O paragraphs, p
5、rovide the information and guidelines necessary for a user to predict the basic performance characteristics of a deflection yoke during the planning or design phase of a CRT display system. -1- I EIA TEB22 79 m 3234600 0007150 T m V TWO BASIC TYPES OF DEFLECTION YOKES The majority of all deflection
6、coils fall into one of two basic categories: (1) parallel (toroidal) coils where each winding contributes an equal chare of the total magnetic flux, (2) series coils where each winding shares the same magnetic flux. Parallel coils have no “ears“ giving the advantage of lower resistance for equal cur
7、rent sensitivity, However, the external field equals the useful internal field doubling the inductance, This external field can be troublesome and usually results in poor settling for graphic displays. The flux conPiguration requires tighter tolerances for equal geometry. Of the two basic types, the
8、 series coil provides greater flexi- bility and is more adaptable to present day applications. A typical series coil.having a distributed winding is illustrated in figure A, and is commonly referred to as a saddle coil. Figure B illustrates a series coil having a semi-distributed winding, and is com
9、monly referred to as a stator coil since it is most often used with slotted or stator cores. As can be seen in figures A e( B, both the saddle and the stator coils have bent-up portions, normally referred to as “ears“ in both the front and the rear, -2- ,- y-= I I EIA TEB22 79 m 3234b00 0007151 1 m
10、. FIGURE A FIGURE B D I S TR I B UT ED II SAD DL E I CO I L SEM I - D I S T R I B UT ED CO I L - 2-A - - EIA TEB22 79 3234600 0007152 3 M The advantages of saddle yokes YS. stator yokes are indicated as follows: SADDLE YOKE ADVANTAGES (A) higher inductance-to-recistance ratio (B) lower crosstalk fro
11、m horizontal to vertical winding STATOR YOKE ADVANTAGES (A) more efficient (lower energy constant) (B) better spot growth performance repeatabi ity (C) better geometry performance repeatabi 1 i ty (D) more flexability in impedance choice for custom design In general, the stator yoke is more often us
12、ed for current high performance display systems because of the advantages indicated. -3- EIA TEB22 79 = 323VbOO 0007353 5 YOKE RELATED PARAMETERS First of all a few comments are in orderwith regard to why the overall role of the deflection yoke is vital in , a CRT display system. The following key s
13、ystem parameters are briefly discussed with respect to their relationship to the def1ectio.n yoke. RESOLUTION Resolution is of course, an all important parameter for any CRT display system. It is dependent upon two primary factors: (1) the spot size at the center of the CRT, (2) the center-to-edge s
14、pot growth characteristics of the system. The first is independent of the yoke. The second is determined almost entirely by the center-to-edge spot growth performance of the yoke. BRIGHTNESS Brightness and resolution are closely interrelated in that the greater the brightness required the greater th
15、e burden on both the yoke and the CRT to produce the speci fi ed resol uti on. I EIA TEB22 79 = 3234600 0007354 7 m -. DISPLAY SIZE The display size on the face of theJRT determines the I maximum deflection angle which in turn is one of the . primary factors in determining the basic yoke form. The d
16、isplay size and aspect ratio is significant in determining both the resolution and geometric perf,ormance of the deflection yoke. GEOMETRY The specified geometric performance requirement of a given system can be limited by the performance of the deflection yoke. In turn the geometric specification m
17、ay-also significantly affect the spot growth performance of the yoke. WRITING SPEED The specified writing speed and/or retrace time are primary factors in determining the maximum inductance that can be used in the deflection yoke, The in.ductance, in turn, along with the accelerating voltage, determ
18、ines the deflection current required to deflect the CRT beam through a given angle. EIA TEB22 79 323L)bOO 0007155 9 W l. DEFLECTION YOKE THEORY The above comments indicate that there is a .significant interrelationship between the requirements which the key system parameters place upon the deflectio
19、n yoke. This suggests that there are several trade-offs to be considered in order to achieve optimization, and in fact, this is qui te true. The following paragraphs are, therefore, intended to provide a logical approach to yoke selection by relating general yoke theory to display system parameters.
20、 The path of an electron thr0ugh.a magnetic deflecting field is illustrated insfigure A. For purposes of the illustration, the magnetic field is considered to be uniform between the boundaries a 81 b and zero elsewhere, The field is also considered to be normal to the plane of the paper and in a dir
21、ection which would cause the deflection direction shown (flux exiting the paper). -6- EIA TEB22 79 m. - 3234b00 0007156 O m FIGURE A The expression for the force (F) on a body with a charge (e) moving with a velocity (v) in a uniform magneti,c field having a density (H) is-given by equation (1). The
22、 centripetal force (F) on an electron of mass (m) traveling at velocity (v) over a curved path of radius (r) is given by equation (2). Y EIA TEB22 79 m 3234b00 0007357 2 m The force indicated in equation (1) must equal the force indicated in equation (2). The radius traveled by the electron is there
23、fore described in equation (3). _. .mv r- He From figure A it can be shown that the angle fl is equal to the deflection angle 8. If R is the length of the uniform magnetic field the angle 8 can be. expressed as in equation (4). A SIN e = (4) Substituting for the radius from equation (3) the deflecti
24、on angle can be expressed as in equation (5). . .RHe SIN e = - mv (5) The velocity of an electron can be approximated in terms of the accelerating anode potential (E ) as indicated in equation (6). b EIA TEB22 79 m 3234600 0007158 m I Substituting for the velocity in equation (5) the deflection angl
25、e can be expressed as indcated in equation (7). Assuming, as in a deflection yoker that H is proportional to a deflection current I, then equation (8) would apply for a yoke having a given form factor when used with a given accelerating vol tage. I = K SIN e (8) Actual deflection yokes do not provid
26、e a perfectly uniform magnetic field nor do they provide a field having discrete boundaries. most yokes does closely approximate that of equation (8). However, the performance of From equation (7) it can be seen that the sine of the deflection angle O is inversely proportional to the square root of
27、the accelerating voltage Eb. ance in a deflection yoke is directly proportional to the number of turns squared. ampere turns, the deflection current for a given deflection angle is inversely proportional to the square root of the The induct- Since H is proportional to the inductance. I* EIA TEB22 79
28、 3234600 0007359 b O These proportionalities are illustrated in equation (8A). ESTIMATING YOKE INDUCTANCE The maximum required deflection angle and the outside diameter of the CRT neck determine the maximum length and minimum inside diameter of the yoke. Once these parameters have been established a
29、nd the operational accelerating voltage of the CRT has been determined there is a relation- ship between yoke inductance and current that is extremely useful in estimating the maximum inductance for a given application. This relationship giving the maximum stored energy in the yoke, is illustrated i
30、n equation (9). e Where: L = Yoke inductance in microhenries I = Yoke current in amperes required to deflect a beam through a given - 10 - EIA TEB22 79 3234600 0007LbO 2 * half axis angle (e) with a given. accelerating voltage (Eb) K = Yoke energy constant in m for the same angle 0 and acceleratihg
31、voltage (Eb) crojoules The yoke energy constant along with the following information will enable one to estimate the maxlmum ugeable iriduc.tance, 1, The type of driving circuitry (1 inear or resonant). 2. The minimum time required to deflect the beam through the angle 8 (see EQ 9). 3. The maximum a
32、llowable induced voltage across the yoke. Knowing the above, the maximum useable inductance for a linear drive circuit can be computed through the following process. The maximum induced voltage developed by the yoke can be expressed as in equation (10). - 11 - f- , EIA TEB22 79 m 3234600 0007LbL 4 m
33、 I Where: e = Induced voltage in volts t = Yoke inductance in henries e -di -. = Rate of change of current dt in amperes per second If in equation (lo), the maximum applied voltage, E, is substituted for e and divided by the minimum time, T, required to deflect the beam through the angle 8 is substi
34、tuted for di/dt, equation fil) fpllows. . E 2r 2 LMAX = . , 2K . Where: E = Maximum allowable induced voltage in volts T = Time in microseconds to position the beam through one half axis angle K = Yoke energy constant in microjoules for one half axis deflection at a specified operational acceleratin
35、g vol tage o EIA TEB22 79 m 3234600 0007362 6 m c t Maximum alwmbleyoke .I inductance in microhenries Following the same logic, the maximum yoke inductance for use with a resonant circuit can be computed. AssumSng that the deflection current follows a sine wave when in the resonant mode, equation (1
36、2) can be used. 2E2T2 IT K LMAX = (i23 Whe.ike: ; All units are the same as in equation (il) Once the inductance has been determined by using equation (11) or (12), the corresponding current required can be computed using equation (13). I 1. -2K I = - Where: 1 = Current in amperes to deflect the bea
37、m through one half axis angle at the specified accelerating vol tage K = Same as in equation (il) L = Same as in equation (li) -I - EIA TEB22 79 = 3234600 0007Lb3 B INDUCTANCE SELECTION SUMMARY In summary, the inductance sel ecti on proces s i s repeated in the following step-by-step procedure. STEP
38、 1 Select a CRT based upon the system per: formance requirements requirement based upon the CRT neck size. Obtain a yoke ehergy constant from a deflection yoke manufacturer based upon the I.D. & maximum deflection angle. A more accurate energy constant can be obtajned if the yoke manufacturer is inf
39、ormed of the overali application and any significant performance requirements _. that are related to the yoke. STEP 4 Establish the operational anode voltage to be used. . STEP 5. Determine the maximum half-axis deflection angle associated with the horizontal & vertical axes. EIA TEBZ 79 m 3234b00 0
40、007164 T m STEP 6 Compute the energy constants applicable for the operatio anode voltage and the half axis angles determined in steps 4 and 5. I STEP 7 Determine the minimum time required to deflect the beam through the halfbaxis angle defined in step 5. STEP a Establish the maximum allowable induce
41、d voltage across the yoke. STEP 9 Compute the maximum allowable yoke inductance by using equation (il) or (12) PERFORMANCE CHARACTERISTICS Primary yoke characteristics are discussed in the fb$%owing paragraphs. INDUCTANCE - The factors related to yoke inductance are as follows. (i) For equal horizon
42、tal and vertical deflection factors there will be a natural unbalance of inductance. This is due to the fact that the - 15 - EIA TEB22 79 = 3234600 O007365 3 = b windings for one axis must be either nearer to the core or deeper in the core slot than the windings for the other axis. In addition the l
43、ength of nesting coils is unequal. To achieve both a balanced inductance and a balanced deflection factor, the coils must be inserted in the yoke core non-symmetrical ly, and consequently the spot growth performance of the yoke will be-degraded. . (2) The upper limit of inductance for a given yoke t
44、ype is determined by the smallest wire size that is practical for production of the yoke. 0 (3) The lower limit of inductance for a given yoke is determined by the smallest number of turns that will allow for a satisfactory turns distribution. RESISTANCE - Por a yoke having a given form factor resis
45、tance Js related as follows. (1) The coil resistance for a given inductance is dependant upon the volume available for copper. - 16 - EIA TEB22 79 W 3234600 O007366 3 W (2) The inductance-to-resi stance ratio is essentially constant. (3) Increasing the inductance-to-resistance ratio tends to decreas
46、e the resonant frequency. DEFLECTION FACTOR - The deflection factor for a yoke must be given with respect to a specified deflection angle and accelerating voltage. The relationship between deflection factor,inductance, deflection angle and accelerating voltage .are shown in equation (8A). RESONANT F
47、REQUENCY - The resonant frequency of a deflection yoke is dependant upon the following factors. (1) For a given type yoke, the coil distributed capacitance is relatively constant. Therefore, the resonant frequency of a yoke coil is inversely proportional to the square root of the coil inductance. (2
48、) Increasing the inductance-to-resistance ratio will tend to increase the distributed capacity and reduce the resonant frequency. , i - EIA TEB22 79 E 3234600 0007167 5 E (3) Encapsulation significantly increases. the distributed capaclty. RESIDUAL MAGNETISM - The residual magnetism characteristic o
49、f a yoke is primarily dependant upon the core material. RECOVERY (SETTLING TIME)- If we define recovery or settling time as the amount of time required for the flux field of the yoke to reach a specified percentage of final value after the current in the coil has reached the final value, then recovery or settling time is primarilydependant upon the properties of the core material. The use of ferrite cores has decreased settling times by more than an arder a of magnitude when compared to iron cores. DIELECTRIC BREAKDOWN - The specified minimum diele
