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pulse_transformer

Pulse transformer

Stan Zurek, Pulse transformer, Encyclopedia-Magnetica.com, {accessed on 2019-10-20}
reviewed by Jeff Jones, 2014-10-02

Pulse transformer (for some applications also gate transformer1), gate drive transformer2), trigger transformer3), wideband transformer4)5) or signal transformer6)) - is a type of transformer optimised and designed for transmission of voltage pulses between its windings and into the load. Pulse transformers can be used for signal transmission (galvanic isolation), low-power control circuits, as well as the main components in high-power switched-mode power supplies.

Typical pulse transformers 7)8) used in low-power electronic circuits

by S. Zurek, E. Magnetica, CC-BY-3.0

 

Low-power pulse transformers are used for controlling switching elements like power semiconductors (transistors, thyristors, triacs, etc.) which are connected to a different voltage level and direct driving is not possible due to unfavourable potential difference or safety implications. In such applications the name gate drive transformer is also used.9) The distinction is based mostly on the actual purpose of the transformer, where the transformer is used to directly drive a transistor gate it is referred to as a “gate drive transformer”, if used only as a means of transmitting rectangular voltage signals then as a “pulse transformer”.10)

However, in a general sense a “pulse transformer” is any transformer capable of transmitting voltage pulses (often rectangular) with adequate signal fidelity.11) Requirements like high-permeability core, low leakage inductance, low inter-winding capacitance etc. are common also to the power transformers in several switched-mode power supplies. Therefore, extremely high power transformers (rating MW or even TW) can be referred to as “pulse transformers”. 12) 13)

As with most transformers, pulse transformers can utilise several functionalities simultaneously: pulse fidelity, voltage level transformation, impedance matching, galvanic isolation, DC isolation, etc.14) 15)

Technical requirements

Technical requirements are always specific to a given application hence it is not possible to have a universal configuration. However, there are features which are favourable for most implementations, and some of them are given as examples below.

Galvanic isolation

A pulse transformer usually has galvanic isolation between its windings. This allows for the primary driving circuit to operate at a different electric potential from the secondary driven circuit. The isolation can be very high, e.g. 4 kV for small electronic transformers.16) This is especially true for very high-power applications in which the output voltage can reach 200 kV.17)

The galvanic isolation also allows meeting safety requirements if one part of the circuit is unsafe to touch, due to the danger of higher voltage, even if for a brief period of time (e.g. if current path is broken in series with inductance).

Pulse transformation

For a gate driving applications usually a rectangular voltage pulse with fast rising and falling edges is required. The frequency bandwidth must be high enough for a given application, so that the delay in signal transmission is acceptably small and there are no severe distortions of the signal.

The frequency bandwidth and signal fidelity are dictated mostly by the non-ideal and parasitic parameters of the transformer: inter-winding capacitance, self-capacitance of each winding, equivalent resistance, etc.

Combination of these parameters can cause a number of effects on the transformed pulse: overshoot, droop, back swing, rise time and fall time, which appear as unwanted signal distortions.18)

A good quality pulse transformer should have low leakage inductance and distributed capacitance as well as high open-circuit inductance.

The transformed pulse will be only a poorer copy of the input pulse. So if the driving circuit produces a non-ideal pulse then the pulse shape will suffer from additional distortions.19)

Typical pulse transformer waveforms suffering from overshoot, droop and some ringing

by S. Zurek, E. Magnetica, CC-BY-3.0

 
Typical good pulse transformer waveforms, critically damped and without overshoot or droop

by S. Zurek, E. Magnetica, CC-BY-3.0

 
Typical pulse transformer waveforms suffering from overdamping

by S. Zurek, E. Magnetica, CC-BY-3.0

 

Windings and turns ratio

In most low-power or applications the turns ratio is around unity 1:1 (or similar like 1:2). Only when the level of signal must be changed to a different voltage then a significantly different turns ratio will be used, as it is the case for most transformers in forward converters (low or high power).

Pulse transformer can have more than two windings, which can be used for instance to drive several transistors simultaneously, so that any phase shifts or delays between signals are minimised.

Typical configurations of low-power pulse transformers pulse_transformers_diagrams_magnetica.jpg

by S. Zurek, E. Magnetica, CC-BY-3.0

 
Simplified example with a two-winding 1:1 pulse transformer driving a gate of a MOSFET (T)

by S. Zurek, E. Magnetica, CC-BY-3.0

 
Simplified example with a three-winding 1:1:1 pulse transformer driving gates of thyristors (T1 and T2)

by S. Zurek, E. Magnetica, CC-BY-3.0

 
Multi-winding pulse transformer wound on a toroidal core, for a specialised application ASCII���User comments

by S. Zurek, E. Magnetica, CC-BY-3.0

 
Pulse transformer wound on a toroidal core, ready for potting

by TCT, Copyrights ©

Saturation

A pulse transformer can operate correctly only when the magnetic saturation is avoided. For this reason most electronic pulse transformers have a rating (volt-second product) given in their data sheets.20)21). This value indicates what amount of energy can be safely applied to the primary winding before the saturation occurs.

For small transformers these values can be specified as e.g. “200 µVs”22), so a 10 V pulse could be applied for 20 µs. After the pulse is applied the circuit must provide the transformer with a means of resetting the flux, otherwise saturation might occur after repeated pulsing due to DC offset or “flux walking”. This can be achieved for example by AC coupling through a series capacitor23) or limiting the duty cycle to less than 50%.

In some applications presence of strong magnetic field might cause problems for operation of pulse transformers with high-permeability cores. The core could saturate which might lead to partial or full loss of signal fidelity. In order to avoid such problems some magnetic shielding might be required.24)

Leakage inductance and parasitic capacitance

High-power pulse transformer with conically shaped windings for minimising parasitic capacitance 25) pulse_transformer_cone_20mw.jpg by Dominik Bortis, Copyrights

Leakage inductance is an unwanted parameter and its value should be as low as possible. This is achieved firstly by using high-permeability magnetic core. As it is for other transformers, the type of material depends on the required frequency bandwidth and other operating parametres. Some applications use soft ferrite, mumetal, 26), nanocrystalline, amorphous or thin gauge electrical steel.27)

Reduction of leakage inductance is also achieved by tightly wound coils, e.g. as in bifilar winding.28)29). Leakage inductance of typical electronic pulse transformers is less than 1% of the total primary inductance.30)31)32)

The leakage inductance stores energy which must be removed after which driving pulse. This can worsen the energy efficiency but also increase time delay in the gate driving applications.33)

As in any transformer, minimising leakage inductance by winding primary and secondary winding close to each other usually leads to increased inter-winding capacitance, which is also an unwanted parameter.

On the other hand, higher insulation between the windings might require larger distance between them and this lowers the parasitic capacitance, but increases the leakage inductance.

Although not always specifically referred to as “pulse transformers” there are devices used for the purpose of pulse transmission. Such devices might not have a magnetic core at all, so that the leakage inductance will be proportionally much greater. However, their capability is sufficient for achieving the pulse transformation as required in a given application.34) Therefore, because of the very function they are used for they can also be referred to as “pulse transformers”.

Power rating

Pulse transformers are build in a wide range of power ratings, from “on-chip transformers” built into integrated circuits35) to megawatt devices, and greater.36)

As with other transformers, the cost of pulse transformers is related to their power rating.

The lowest power devices are designed to just transfer information about the signal, not power as such, so the resulting rating is for instance 3 V and 50 µA of average rating.37)

Larger devices are used for transmission of usable power through the transformer, for instance in energy-harvesting applications or small isolated power supplies. The power can be from μW to several watts.38)39)

Transformers in forward or full-bridge switch-mode converters are used is to transform rectangular voltage pulses, with ratings up to hundreds of kW of continuous power.40)41)

For such applications like magnetrons or klystrons the pulse transformers must be capable of delivering pulses with MW power.42)43)44)

Pulse transformer with 20 MW rated power45) pulse_transformer_20mw.jpg by Dominik Bortis, Copyrights
Pulse modulator with 20 MW rated power to drive a klystron through a pulse transformer46) pulse_generator_20mw.jpg by Dominik Bortis, Copyrights

Equivalent circuit

Quality of transmission of fast transients in pulse or wideband transformers depends on such parasitic parameters like: resistive losses, leakage inductance, inter-winding capacitance and winding self-capacitance.47) These parameters can be included in the equivalent circuit as shown in the figure:

Typical equivalent circuit of a wideband or pulse transformer48)

by S. Zurek, E. Magnetica, CC-BY-3.0

 

The equivalent circuit can be designed to reflect different performance parameters important for a given application. For instance in balun type transformers the leakage inductance is the limiting factor for high-frequency operation and this can be modelled with the help of appropriately designed equivalent circuit, which can take several forms, depending on the target application.49).

Examples of practical applications

There are many applications where pulse transformers are used. Certain features or capabilities can be utilised:50)51)

  • as components for AC coupling various parts of circuit
  • to change amplitude of a pulse
  • to change impedance level (e.g. for impedance matching)
  • for fast pulse transmission
  • to invert polarity of the pulse
  • to provide positive and negative pulses simultaneously (using multiple or centre-tapped windings)
  • to provide DC separation between parts of a circuit
  • to differentiate a pulse
  • for coupling of pulse amplifiers
  • for transmitting digital pulses
  • to provide galvanic separation between circuits

Some examples of specific applications:

  • simultaneously firing gates of multiple thyristors53)
  • galvanic isolation of gate drivers and gates of power transistors54)
  • low-power galvanically isolated power supplies55)
  • transmission of wideband signals56)
  • very high-power pulse transmission and voltage transformation from pulse modulator (through pulse transformer) to klystron57)58)

See also

References


pulse_transformer.txt · Last modified: 2019/06/03 18:30 (external edit)

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