pulse transformer and its applications

HỒ CHÍ MINH UNIVERSITY OF TECHNOLOGY

PULSE TRANSFORMER AND ITS APPLICATIONS Instructor: Assoc Prof. Dr. NGUYỄN Hoàng Việt Student: HUỲNH Lê Duy – 40900382 PFIEV-09Télécom

Huỳnh Lê Duy 12/23/2012

HUYNH Le Duy [PULSE TRANSFORMER AND ITS APPLICATIONS]

I

ABSTRACT

Pulse transformer is the designed for operating at high frequency, therefore the transformer can

physically more compact, as well as transfer more power than same-size normal transformer.

The pulse transformer core has better quality due to the Foucault current at high frequency. This

essay also studies the response of the pulse transformer with an ideal pulse and some of its

application.


II

Table of Contents

Table of Contents ............................................................................................................................ II

Table of Figure .............................................................................................................................. III

1. INTRODUCTION .................................................................................................................. 1

2. EFFECT OF FREQUENCY ................................................................................................... 2

2.1. Foucault current................................................................................................................ 2

2.1.1. Power dissipation ...................................................................................................... 2

2.1.2. Skin effect ................................................................................................................. 3

2.2. Transformer size ............................................................................................................... 4

3. CONSTRUCTION .................................................................................................................. 5

3.1. Core .................................................................................................................................. 5

3.1.1. Ferrite cores .............................................................................................................. 5

3.1.2. Permalloy cores ......................................................................................................... 6

3.2. Windings .......................................................................................................................... 6

4. CHARACTERISTICS ............................................................................................................ 8

4.1. Pulse response .................................................................................................................. 8

4.1.1. Equivalent circuit ...................................................................................................... 8

4.1.2. Front-Edge Response .............................................................................................. 10

4.1.3. Response at the Top of the Pulse ............................................................................ 11

4.1.4. Trailing-Edge Response .......................................................................................... 12

4.2. Efficiency ....................................................................................................................... 13

5. APPLICATION .................................................................................................................... 14

5.1. Pulse Transformer in Triggering Circuits ...................................................................... 14

5.2. Some real life pulse transformer .................................................................................... 16

5.2.1. Signal Pulse transformer ......................................................................................... 16


III

5.2.2. Power pulse transformer ......................................................................................... 16

6. REFERENCES ..................................................................................................................... 18

6.1. Bibliography ................................................................................................................... 18

6.2. Notes............................................................................................................................... 18

Table of Figure

Figure 1 : Symbol of the pulse transformer .................................................................................... 1

Figure 2: Lamination of conductors parallel to the field lines reduce eddy currents ..................... 3

Figure 3: Distribution of current flow in a cylindrical conductor, shown in cross section. For

alternating current, 63% of the electrical current flows between the surface and the skin depth δ 3

Figure 4: Some kind of ferrites cores .............................................................................................. 5

Figure 5: Permalloy cores ............................................................................................................... 6

Figure 6: Litz wire ......................................................................................................................... 7

Figure 7: Hysteresis loop ................................................................................................................ 8

Figure 8: Flat-topped pulse ............................................................................................................. 9

Figure 9: Transformer coupling ...................................................................................................... 9

Figure 10: Equivalent circuit. ....................................................................................................... 10

Figure 11: Circuit for step-up transformer .................................................................................... 10

Figure 12: Influence of transformer constants on front edge of pulse R1=R2 ............................ 11

Figure 13: Circuit for top of pulse ................................................................................................ 12

Figure 14: Droop at top of pulse transformer output voltage ....................................................... 12

Figure 15: Circuit for Trailing-Edge ............................................................................................. 12

Figure 16: Trailing-Edge response................................................................................................ 13

Figure 17 Complete output circuit ............................................................................................... 14

Figure 18 Pulse transformer connection of 2 SCR ....................................................................... 15

Figure 19: Pulse Transformers (1000 Series) | Murata-PS ........................................................... 16

Figure 20: Corona free pulse transformer ..................................................................................... 16

Figure 21: Pulse charging transformer ......................................................................................... 17


1

1. INTRODUCTION

A pulse transformer is a transformer that is optimized for transmitting rectangular electrical

pulses (that is, pulses with fast rise and fall times and relatively constant amplitude) to the load

with its shape and other properties unchanged. It is designed for operating at high frequency

from many kHz to hundreds kHz. It can transfer more power compare to the same-size normal

transformer. Pulse transformers find its applications in communication, power electronics, digital

electronics, and fast pulse generation and so on.

In power electronics, Pulse transformer is useful for triggering thyristor, triacs and so on. Pulse

transformer covers a wide range of pulse and power levels.

Small pulse transformers, is used in pulse generators, can deliver only a few Volts at pulse

widths of a few microseconds. Large pulse transformers used in linear accelerator, radar and so

on can deliver wider range of power from 50 to 100MW at 200 to 300kV with pulse duration of

microseconds.

Figure 1 show the symbol of the pulse transformer. But it is able to use the same symbol with

normal transformer with notification of the frequency the transformer is used.

Figure 1 : Symbol of the pulse transformer


2

2. EFFECT OF FREQUENCY

2.1. Foucault current

2.1.1. Power dissipation

The biggest problem of operating at high frequency is the Foucault current. Foucault currents

(also called Eddy currents) are electric currents induced within conductors by a changing

magnetic field in the conductor. It will overheat the core and waste power. Look at the Power

dissipation of Foucault currentsi:

2 2 2 2

6

pB d fP

k D

Where

P is the power lost per unit mass (W/kg),

Bp is the peak magnetic field (T),

d is the thickness of the sheet or diameter of the wire (m),

f is the frequency (Hz),

k is a constant equal to 1 for a thin sheet and 2 for a thin wire,

ρ is the resistivity of the material (Ω m), and

D is the density of the material (kg/m3).

From the equation, we found that a higher frequency, power lost will increase. To minimize the

power losses, stacking layers of thin steel laminations is used to construct the core and high

resistivity material is used.

Transformers operate at normal frequency (50-60Hz) is constructed by silicon steel lamination.

But cores made by this material cannot operate at high frequency because Silicon steel has low

resistivity. Thinner laminations reduce losses but are more laborious and expensive to construct.

Higher Foucault current will overheat the transformer and waste power. Therefore, higher

resistivity material is used.


3

Figure 2: Lamination of conductors parallel to the field lines reduce eddy currents

2.1.2. Skin effect

In very fast-changing fields, the magnetic field does not penetrate completely into the interior of

the material. The skin effect causes the effective resistance of the conductor to increase at higher

frequencies where the skin depth is smaller and therefore decrease the Power dissipation of the

Foucault current

Figure 3: Distribution of current flow in a cylindrical conductor, shown in cross section. For

alternating current, 63% of the electrical current flows between the surface and the skin depth δ


4

The penetration depth can be calculated from the following equation:

1

f

Where:

δ is the penetration depth (m)

f is the frequency (Hz)

μ is the magnetic permeability of the material (H/m)

σ is the electrical conductivity of the material (S/m).

Skin effect is useful in decreasing the losses at high frequency so high permeability material is

used in pulse transformer core

2.2. Transformer size

Transformer universal EMF equation if the flux in the core is purely sinusoidal:

4.44E f N B A

Where:

E is the sinusoidal root mean square voltage of the winding;

is the frequency

N is the number of turns of wire;

B is the peak magnetic flux density;

A is the cross-sectional area of the core.

For the same voltage E, increasing the frequency allows decreasing the number of turn N and by

this decreases the size of the core too. By operating at higher frequencies, transformers can be

physically more compact because a given core is able to transfer more power without reaching

saturation and fewer turns are needed to achieve the same impedance.


5

3. CONSTRUCTION

3.1. Core

Pulse Transformer is similar to normal transformer: it has primary winding(s) and secondary

winding(s) which is wound around a core. But due to its operating point a high frequency, the

cores are made of ferrites or of special high permeability silicon steel such as Hipersil or

Permalloy ii

3.1.1. Ferrite cores

Figure 4: Some kind of ferrites cores

Ferrite is used for its properties of high magnetic permeability coupled with low electrical

conductivity (which helps prevent Foucault currents). Ferrites are ceramic compounds of the

transition metals with oxygen, which are ferromagnetic but nonconductive. The most common

soft ferrites are:

Manganese-zinc ferrite MnZn, with the formula MnaZn(1-a)Fe2O4. MnZn have higher

permeability and saturation induction than NiZn.

Nickel-zinc ferrite (NiZn), with the formula NiaZn(1-a)Fe2O4. NiZn ferrites has higher

resistivity than MnZn, and are therefore more suitable for frequencies above 1 MHz

The notable advantage of ferrite makes them the most widely used is the manufacturing method

is simple and cheap


6

3.1.2. Permalloy cores

Figure 5: Permalloy coresiii

Permalloy is a nickel-iron magnetic alloy, with about 20% iron and 80% nickel. The alloy is

called by the proportion of Nikel, for example Permalloy75 is permalloy with 75% atom is niken

(or Ni75Fe25).

It is notable for its very high magnetic permeability, which makes it useful as a magnetic core

material in electrical and electronic equipment. The name permalloy is made with “perm” in

permeability and “alloy”. In addition to high permeability, its other magnetic properties are low

coercivity, near zero magnetostriction, and significant anisotropic magnetoresistance. The low

magnetostriction is critical for industrial applications, allowing it to be used in thin films where

variable stresses would otherwise cause a ruinously large variation in magnetic properties

3.2. Windings

By operating at high frequency, fewer turns are needed to achieve the needed impedance. For

small power and signal transformers, in which currents are low and the potential difference

between adjacent turns is small, the coils are often wound from enamelled magnet wire, such as

Formvar wire. Larger power transformers operating at high voltages may be wound with copper

rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard. High-


7

frequency transformers operating in the tens to hundreds of kilohertz often have windings made

of braided Litz wire to minimize the skin-effect and proximity effect losses

Figure 6: Litz wire iv


8

4. CHARACTERISTICS

Pulse Transformers are different from ordinary transformers in that they have a square hysteresis

loop. This indicates that the magnetization can be only in two statesv. Than change from on state

to another takes place almost instantaneously in pulse transformer, whereas the change is only

gradual in ordinary transformers.

Figure 7: Hysteresis loop

Pulse transformer work with square waveforms and pulse waveforms. When a pulse is applied to

its primary, a pulse transformer produces a sharp pulse across its secondary, instantaneously. It

also provides isolation between 2 primary and secondary, therefore can be used in applications

which require isolation between the main circuit and that which supply the pulses.

Pulse transformer needs to have low values of leakage inductance and distributed capacitance,

and a high open-circuit inductance to minimize distortion of the pulse shape. In power-type pulse

transformers, a low coupling capacitance (between the primary and secondary) is important to

protect the circuitry on the primary side from high-powered transients created by the load

4.1. Pulse responsevi

4.1.1. Equivalent circuit

We analyse the response for a square- or flat-topped pulse impressed upon the transformer:


9

Figure 8: Flat-topped pulse

With the input is a pulse shown in Fig. 6, and a generalized circuit for the amplifier is shown in

Fig. 7. The equivalent circuit for this amplifier is given in Fig. 8. At least this is applied to the

front edge of the pulse.

Figure 9: Transformer coupling

Front edge of the pulse is rising suddenly from zero to an steady value E. This change is sudden,

so the transformer Open Circuit Inductance (OCL) is considered as infinite impedance and is

omitted in Fig. 10


10

Figure 10: Equivalent circuit.

Where R1,R2 represents the source and the load resistance, Winding capacitances are shown as

C1 and C2 for the primary and secondary windings, respectively, LS is leakage inductance of the

transformer, the pulse source is made by an ideal switch

4.1.2. Front-Edge Response

The step-up transformer is illustrated in figure 11, C1 may be neglected due to small number of

primary winding.

Figure 11: Circuit for step-up transformer

When the front of the wave is applied on the transformer, it is simulated by the closing of switch

S. We got the equation, which expresses the rise of voltage e from zero to its final steady value

Ea = ER2/(R1 + R2):

2 2 1 22

1 2 1 2 1 2

exp( )1 1 exp( )

ER m m t me m t

R R m m m m


11

Which 2

1,2 11 1/m m k k1= 2Sm L C and 1

2 2

1

2 2s

Rm

L R C

Figure 12 shows the rate of rise of the transformed pulse for R1=R2

Figure 12: Influence of transformer constants on front edge of pulse R1=R2

4.1.3. Response at the Top of the Pulse

When the pulse top is reached, Ea require the transformer OCL for its maintenance at this value.

If the pulse stayed on indefinitely at the value Ea, an infinite inductance would be required to

maintain it, this is not practical. There is always a droop at the top of a pulse. The equivalent

circuit during this time is shown in figure 13, in which the inductance L is the OCL of the

transformer, and R1 and R2 remain the same as before. Since the rate of voltage change is

relatively small during this period, capacitances C1 and C2 disappear. Also, since leakage


12

inductance usually is small compared with the OCL, it is neglected. The responded is provided in

figure 14. Several curves are given for several types of pulse depended on R1 and R2

Figure 13: Circuit for top of pulse

Figure 14: Droop at top of pulse transformer output voltage

4.1.4. Trailing-Edge Response

At instant b in figure 8, it is simulated that the switch S in figure 10 is opened suddenly. The

equivalent circuit now reverts to that shown in figure 15

Figure 15: Circuit for Trailing-Edge


13

which Le is the OCL, RI,CD total resistance and capacitance referred to the primary. Figure 15

show the trailing edge response. The equation for which is:

1 1 2 2

1 2

2 exp 2 expaEe m m m t m m m t

m m

Which

2

1,2 3 3

/1 Magnetizing current1 1 1/ , , , 2 ,

2 2 Load current

e D

I D e D

I

L Cm m k m R C k T L C

R

Figure 16: Trailing-Edge response

4.2. Efficiency

Pulse transformers have high efficiency. By using high resistivity and permeability it reduce the

power losses by Foucault current. Copper losses are also reducing because less winding is

require than those normal transformer.

Efficiencies of over 90 per cent are common in pulse transformers, and with high-permeability

materials over 95 per cent may be obtained. vii


14

5. APPLICATION

Some applicationviii

:

Power transformer supply 50Hz AC power

Converter: Fly-back, Push-Pull

AF transformer are used in audio stage of amply, radio, TV

Supply pulse to trigger SCRs, magnetism, etc.

Change the amplitude of a pulse

Invert the polarity of a pulse

Differentiate a pulse.

5.1. Pulse Transformer in Triggering Circuits

Pulse transformers are often used to couple a trigger pulse generator to a thyristor for obtaining

electrical isolation between the two circuits. The transformer used in thyristor control is either

1:1 two-winding or 1:1:1 three-winding types. Figure 17 show a complete output circuit to fire a

thyristor correctly

Figure 17 Complete output circuit ix

Figure 18 shows different ways of using a transformer to drive an inverse parallel pair of SCRs.

Full isolation is provided by the three winding transformer in figure 17(a). In application where

isolation is not required, a two winding transformer may be used in a series mode, Figure 17(b)

or a parallel mod figure 17(c). The pulse generator must supply sufficient energy to trigger both


15

SCRs, and the pulse transformer (plus any additional resistors) must supply sufficient gate

current to both SCRs under worst condition of unbalanced gate impedances

Figure 18 Pulse transformer connection of 2 SCRx

If the SCRs is trigger at 50Hz, The signal is go through an And gate with a high frequency pulse

to prevent the magnetic saturation of the core.

A trigger transformer is primarily use to enhance the efficiency.xi


16

5.2. Some real life pulse transformer

5.2.1. Signal Pulse transformer

Figure 19: Pulse Transformers (1000 Series) | Murata-PS xii

The 1000 series are intended for wideband and pulse operations. They are also suitable for signal

isolation and use in small isolated power supplies. The compact footprint makes them ideal for

applications where space is at a premium.

5.2.2. Power pulse transformer

Figure 20: Corona free pulse transformer

Application

Ground Based Radar

Corona free pulse transformer

Input Voltage

11,700 V Pulse

Output Voltage

117,000 V Pulse

Frequency


17

250 pps

Duty Cycle

0.192%

Figure 21: Pulse charging transformer xiii

This transformer is used in the doubling project (sled) at Stanford linear accelerator center (slac).

The unit is rated at 270 kilovolts, 75 megawatts, for a 7 microsecond pulse. By using precision

winding and construction techniques it was possible to limit the rise time to 0. 5 microseconds

(10-90), while maintaining less than 0. 15% p-p flat top ripple. The tapered (constant gradient)

secondary windings limit the leakage inductance to almost half of what it would be with the

conventional construction.


18

6. REFERENCES

6.1. Bibliography

[1] M D Singh, K B Khanchandani, Power Electronics 2e, Tata McGraw-Hill Education.

[2] Reuben Lee, Electronic Transformers and Circuits second edition, john willey & sons, inc,

1955.

[3] B. Somanathan Nair, Electronic Devices And Applications, PHI Learning Pvt. Ltd..

[4] Prasad Rajendra, Fundamentals Of Electrical Engineering 2Nd Ed., PHI.

6.2. Notes

i http://en.wikipedia.org/wiki/Eddy_current

ii Fundamentals Of Electrical Engineering 2Nd Ed. [4]

iii http://www.alibaba.com/product-gs/241500451/permalloy_core_Magnetic_core_Tape_wound.html

iv http://en.wikipedia.org/wiki/Litz_wire

v Electronic Devices And Applications p27 [3]

vi Electronic Transformers and Circuits second edition p297 [2]

vii Electronic Transformers and Circuits second edition p314 [2]

viii Electronic Devices And Applications p27 [3]

ix Power Electronics 2e p76 [1]

x Power Electronics 2e p77-78 [1]

xi Power Electronics 2e p78 [1]

xii http://www.ttiinc.com/object/Murata-Power-Pulse-Transformer-1000-Series.html

xiii http://www.stangenes.com/pulse-transformers.html

pulse transformer and its applications

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