Windings and Insulation

Pulse transformers generally have single-layer concentric windings with solid insulation between sections. For high load impedance, a single section each for primary and secondary as in Fig. 166 is favorable, as the effective capacitance is lowest. For low load impedance, more interleaving is used to reduce leakage inductance. To reduce capacitance to a minimum, pie-section coaxial windings may be used. In these, coil capacitance is kept low by the use of universal windings, and intersection capacitance between windings is low because the dielectric is air. Such coils are more difficult to wind, require more space, and therefore are used only when necessary.

Coil sections can be wound with the same polarity as in Fig. 166 or with one winding reversed. Effective capacitance between P and S is given below for three turns ratios. Capacitance is. based on 100 μμf measurable capacitance.

Turns Ratio	Effective Capacitance	Referred to Primary
N1/N2	Same Polarity	Reversed Polarity
1:5	533	1200
1:1	0	133
5:1	21	48

From this it can be seen that the polarity exemplified in Fig. 166 is preferable for reducing effective capacitance, but that the percentage difference is greatest for turns ratios near unity and less as the ratio increases.

Attention to the insulation so far has centered around capacitance. The insulation also must withstand the voltage stress to which it is subjected. It can be graded to reduce the space required. Low-frequency practice is adequate for both insulation thickness and end-turn clearances.

Small size is achieved by the use of solventless varnish. Small size with consequent low capacitance and low loss results in higher practicable impedance values and shorter pulses.

In order to utilize space as much as possible, or to reduce space for a given rating, core-type construction is often used. Low capacitance between high-voltage coils is possible in such designs. It is advantageous in reducing space to split the secondary winding into non-symmetrical sections. Although the leakage inductance is higher with non-symmetrical windings, there is less distributed capacitance when the high-voltage winding has the smaller length. Lower capacitance obtains with two coils than with a shell-type transformer of the same interleaving. In core-type transformers high-voltage windings are the outer sections. It is preferable to locate terminals or leads in the coil directly over the windings in order to maintain margins. Insulating barriers may be located at the ends of the windings to increase creep-age paths.

Autotransformers, when they can be used, afford opportunity for space saving, because there are fewer total turns and less winding space is needed. Less leakage inductance results, but not necessarily less capacitance; this always depends on the voltage gradients.

Initial distribution of voltage at the front edge of a pulse is not uniform because of turn-to-turn and winding-to-winding capacitance. In a single-layer coil the total turn-to-turn capacitance is small compared to the winding-to-ground capacitance, because the turn capacitances add in series but the ground or core capacitances add in shunt. Therefore a steep wave of voltage impressed across the winding sends current to ground from the first few turns, leaving less voltage and less current for the remaining turns. Initially, most of the pulse voltage appears across the first few turns.

After a short interval of time, some of the current flows into the remaining turns inductively. Before long the capacitive voltage distribution disappears, all the current flows through all the turns, and the voltage per turn becomes uniform. This condition applies to most of the top of a pulse. Between initial and final current distribution, oscillations due to leakage inductance and winding capacitance may appear which extend the initially high voltage per turn from the first few turns into some of the remaining turns.

Winding capacitance to ground is evenly distributed along the winding of a single coil, and so is the turn-to-turn capacitance. If a rectangular pulse E is applied to one end of such a winding, and the other end is grounded, the maximum initial voltage gradient is⁽²⁾

where

N = number of turns in winding
α = √(c_g/c_w)
C_g = capacitance of winding to ground
C_w = capacitance across winding = turn-to-turn capacitance/N.

Maximum gradient ~ αE / N

[136]

or the maximum initial voltage per turn is approximately α times the final or average voltage per turn.

If the other end of the winding is open instead of grounded, equation 136 still governs. This means that maximum gradient is independent of load. If there is a winding N₁ between the pulsed winding N₂ and ground, α depends on C₁_₂ and C₁ in series. The initial voltage in winding N₁ is⁽¹⁾

[137]

where

E₁ = initial voltage in winding N₁
E = pulse voltage applied across N₂
C_1-2 = capacitance between N₁ and N₂
C₁ = capacitance between N₁ and core.

Thus the initial voltage in winding N₁ is independent of the transformer turns ratio. It is higher than the voltage which would appear in N₂ if N₁ were pulsed, because then current would flow from N₁ to ground without any intervening winding. If winding N₁ is the low-voltage winding (usually true), applying pulses to it stresses turn insulation less than if N₂ is pulsed.

Reinforcing the end turns of a pulsed winding to withstand better the pulse voltages is of doubtful value, because the additional insulation increases α and the initial gradient in the end turns. Increasing insulation throughout the winding is more beneficial, for although α is increased the remaining turns can withstand the oscillations better as inductance becomes effective. Decreasing winding-to-core capacitance is better yet, for then α decreases and initial voltage gradient is more uniform.

(1)	See "Surge Phenomena," pp. 227-281.
(2)	For the development of this expression see "Surge Phenomena," British Electrical and Allied Industries Research Association, 1941, pp. 223-226.