Pulse Transformer – Construction, Working Principle & Applications

Introduction

In today’s modern electronic and power-electronic systems, signals are often in the form of pulses. The special transformer that is used to transfer such pulse signals safely, provide isolation and forward without distorting the shape is called pulse transformer.

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This transformer is different from a normal power transformer because its job is not to handle continuous AC power, but to handle short-duration voltage or current pulses.

What is a Pulse Transformer?

A pulse transformer is a transformer that is specially designed to transfer short-duration voltage or current pulses instead of continuous AC. It transmits the pulse signal from the input side to the secondary side through magnetic coupling, as well as provides electrical isolation (DC isolation).

A pulse transformer is a transformer that handles voltage or current as a pulse and ensures clean pulse transfer with isolation.

In simple terms, the job of a pulse transformer is to ensure that the shape, timing, and amplitude of the input pulse are on the secondary side without disturbing much.

Key Hallmarks of Pulse Transformer

The input voltage given in a pulse transformer is not in a continuous but discontinuous pulse form. This is the reason why its construction, core material and winding style are different from normal power transformers.

Pulse Transformer usually has:

• Ferrite core is used (for high-frequency pulses)
• Tight coupling winding is done
• Special attention is paid to rise time, pulse width, and distortion

Construction of Pulse Transformer

The construction of a pulse transformer is different from a normal power transformer because instead of continuous AC, high-frequency, fast-rising voltage pulses have to be faithfully transferred. For this reason, special attention is paid to core selection, winding style and insulation.

Core Material (Why Ferrite is Used)

Ferrite cores are almost always used in Pulse transformers. The main reason for this is that ferrite material gives much better performance on high frequency pulses.

The permeability of the Ferrite core is quite high, which establishes the required magnetic flux in fewer turns. Also, the electrical resistivity of the ferrite is very high, so eddy current losses become negligible. In high-frequency pulse operation, the silicon steel core generates heat and can distort the waveform, while in the ferrite core:

Fast rise-time pulses transfer easily

• The risk of core saturation remains low.
• Losses are very low
• Pulse shape reproduces more accurately

For this reason, ferrite-based pulse transformers are preferred in SMPS, gate-drive circuits, digital electronics, and radar systems.

Primary / Secondary Winding Styles

The winding design of a pulse transformer is the most critical part of its performance. The objective here is to keep leakage inductance to a minimum and inter-winding coupling to a maximum.

In bifilar winding, the primary and secondary wires are wrapped side by side. This makes the magnetic coupling between the two windings very strong.

The advantage of this technique is that:

• Leakage inductance is greatly reduced
• Pulse rise-time remains sharp
• High-frequency distortion is low

Bifilar winding is mostly used in gate-drive pulse transformers, where precise pulse shape is essential.

In layered winding, the windings are wrapped in different layers, such as the primary layer first, then the insulation, then the secondary layer.

This method is used where:

• High voltage isolation is required.
• Turns ratio is complex
• Multiple secondary outputs required

Although the leakage inductance in layered winding is slightly higher than the bifiler, the insulation strength is better.

Multi-Winding Examples

Pulse transformers are often built in multi-winding configurations. This means that on the same ferrite core, A primary winding and two or more secondary windings. Maybe.

Such a design is used when multiple isolated outputs are required from a single pulse source. For example:

• In Thyristor or IGBT gate-drive circuits
• Multiple control signals to give with isolation
• To distribute clock or trigger pulses in digital systems

Every secondary winding is electrically isolated, but the magnetic coupling is done through the common core. This increases safety, noise immunity and system reliability.

Working Principle of Pulse Transformer

The working principle of pulse transformer is based on basic transformer law, but here the operation takes place with short-duration voltage pulses instead of continuous AC. The entire process is explained step-by-step below with a reference to the waveform.

Step 1: Apply to the Input Pulse Primary

When a rectangular or square voltage pulse is applied to the primary winding, the current does not increase instantaneously due to the winding being inductive.

As soon as the rising edge of the pulse arrives, the current in the primary starts building up slowly. In the ideal case, the input pulse is square, but in practical circuits, its rise time and fall time are finite.

Step 2: Developing Magnetic Flux in the Core

As soon as the voltage is applied to the primary, magnetic flux (Φ) is generated inside the ferrite core. The important thing is that flux is not proportional to the voltage, but proportional to the time-integral of the voltage.

That is, the longer the pulse is applied, the longer the flux will increase. If the pulse width is too high or the core/turns are not selected correctly, then a situation of core saturation may occur.

Step 3: EMF Induce in Secondary due to Flux Change

As the core flux changes with time, according to Faraday’s Law, EMF induces in secondary winding. The rising edge of the input pulse generates a sharp voltage pulse in → secondary.

In the flat-top region of the pulse, the output voltage remains almost constant, and at the falling edge, the pulse of opposite polarity is visible in the secondary.

According to the Turns ratio, the amplitude of the output pulse is determined.

Step 4: Transfer to Output Pulse Load

On the secondary side, the connected load gets an isolated replica of the input pulse.

This isolation is the greatest strength of the pulse transformer—the signal transfer, but the DC component does not pass on the secondary side.

For this reason, pulse transformers are widely used in gate-drive, digital triggering and control circuits.

Practical Non-Ideal Effects

The volt-second limit is the most critical parameter in pulse transformer design.
If the V × t (volt-second product) of the applied pulse exceeds the core capacity, the ferrite core goes into saturation. In this condition, the output pulse becomes distorted, the rise time slows, and gate triggering may become unreliable.

Key Characteristics of Pulse Transformer

The performance of a pulse transformer depends on certain electrical parameters. These parameters directly affect pulse size, speed, and reliability. Below each parameter is explained in the heading + simple paragraph.

Rise Time

Rise time is the time in which the output pulse reaches its maximum value (usually 10% to 90%) from its minimum value. The shorter the rise time, the sharper the pulse edge. Fast rise time is very important in high-speed digital circuits and gate-drive applications, because slow rise time causes switching delays and extra power loss.

Fall Time

Fall time is the time in which the pulse drops from its maximum value to the minimum value (90% to 10%). If the fall time is too slow, the turn-off process is delayed and switching losses can increase. In practical designs, it is tried that the rise time and fall time are almost equal, so that the pulse remains symmetric.

Pulse Width

Pulse width is the effective ON time of pulse, i.e. how long the pulse remains at a high level. The higher the pulse width, the more volt-second stress will be on the core. Long pulse width increases the risk of core saturation, so pulse width is always selected according to the core and turn design.

Pulse Droop

Pulse droop means the gradual downward bending of the flat-top of the pulse. This effect comes due to the limiting of magnetizing inductance. If the droop is too high, there may be an under-voltage problem in the gate-drive or logic circuits. In Good Pulse transformer design, the droop is usually kept below 5–10%.

Leakage Inductance

Leakage inductance is caused by imperfect magnetic coupling between the primary and secondary windings. The rise time of the pulse slows down as the leakage inductance increases and overshoot or ringing may appear. For this reason, techniques such as bifilar winding are used in pulse transformers so that leakage inductance is minimal.

Magnetizing Inductance

Magnetizing inductance is the inductance that is required to magnetize the core. The higher the magnetizing inductance, the lower the pulse droop and the longer the pulse can be transferred safely. In the event of low magnetizing inductance, the core quickly approaches saturation.

Turns Ratio

Turns ratio is the ratio of primary and secondary turns. This ratio determines the amplitude of the output pulse. If the turn ratio is 1:1, then the voltage of the output pulse will be equal to the input pulse. Gate-drive pulse transformers often use a 1:1 ratio to keep waveform distortion low.

Volt-Second Product

The volt-second product is the most critical parameter of the pulse transformer, defined by the product of voltage and pulse width. Core saturation is not decided by voltage, but by volt-second product. If the applied volt-second value exceeds the capacity of the core, the transformer goes into saturation and the pulse is severely distorted.

Excessive leakage inductance in the pulse transformer increases the rise time of the output pulse. This means that fast switching devices such as SCR, IGBT or MOSFET may not receive the required sharp gate pulse, which can increase switching delay and power loss.

Volt-Second Limit and Core Saturation

What is a Volt-Second Product?

The volt-second product is understood in very simple terms as the product of voltage and pulse width. This means that the core of the transformer does not see what the voltage is, but rather how long that voltage was applied. If the voltage is low but the pulse is very long, there will still be as much stress on the core as a high voltage short pulse can do. For this reason, pulse width cannot be ignored in pulse transformer design.

Why does core saturation happen?

When pulse is applied to the primary winding, the magnetic flux inside the core increases with time. If the volt-second product of the applied pulse exceeds the magnetic capacity of the core, the core reaches its maximum flux density. This condition is called core saturation. After saturation, the flat-top of the output pulse begins to deteriorate, the waveform is distorted, and the primary current suddenly increases too much, creating a risk of heating and failure.

How to Check Volt-Second Limit?

A simple but very powerful relation is used to check saturation in a pulse transformer:

V × t ≤ N × A × ΔB

This relationship suggests that the volt-second demand of the applied pulse should be less than the capacity of the core. Here V is the applied pulse voltage, t is the pulse width, N is the primary turns, A is the cross-sectional area of the core and ΔB is the allowable flux swing. If the value of the left side is larger than the right side, then core saturation cannot be avoided.

Simple Numerical Example

Suppose the pulse voltage is 40 V and the pulse width is 5 μs. The primary turns are 20, the core area is 1 cm² (i.e. 1×10⁻⁴ m²) and the allowable flux swing is 0.2 Tesla.

In this case, the volt-second demand of the pulse would be 40 × 5×10⁻⁶ = 200×10⁻⁶ V·s, while the core capacity would be 20 × 1×10⁻⁴ × 0.2 = 400×10⁻⁶ V·s.

Because the demand is less than the capacity, the transformer will work in the safe zone. If the pulse width is increased to 10 μs, the demand will reach the limit of the core and the risk of saturation will arise.

Practical Thumb Rule

In field practice, always remember that core saturation is not from voltage, but from volt-second product. So keep a margin of 20–30% when designing, increase the turns for the long pulse or choose the core size is larger, and keep the pulse width under control with the high voltage pulse.

Design Checklist — Practical Pulse Transformer Design

Pulse transformers are not just a textbook topic, but an important part of real-world electronics and power-electronics systems. Pulse transformers can be practically used in SCR firing circuits, SMPS gate drives, and isolation-based control systems, where it is important to safely separate high-voltage and low-voltage circuits.

The design checklist below is written to practically design the pulse transformer. This content is directly usable for students, technicians, and field engineers—and applies equally to gate-drive, trigger, and digital pulse applications.

Core Selection (First Step to Choosing the Right Core)

The ferrite core is always preferred for pulse transformers, as the ferrite gives low loss over high-frequency pulses. The MnZn ferrite is most common for normal pulse and gate-drive applications, while the NiZn ferrite is better in very high-frequency pulses. The cross-section area of the core is selected according to the pulse width and voltage—the longer the pulse or the higher the voltage, the larger the core is safe to take. Limiting flux density to around 0.2–0.25 Tesla in design is considered good practice to avoid core saturation.

Primary Turns (NP) Selection — Saturation Avoidance

The selection of primary turns is always done keeping in mind the volt-second limit. Minimum turns are calculated according to pulse voltage and pulse width, then safety margin is added to it. When the turns are too low, the core quickly goes into saturation and the waveform distorts. In practical designs, it is always recommended to select primary turns with a 20–30% margin.

Secondary Turns (NS) and Turns Ratio

Secondary turns directly decide the amplitude of the output pulse. The higher the turn ratio, the more proportional the output pulse will be the step-up or step-down. Gate-drive pulse transformers often have a 1:1 turn ratio to keep the pulse shape clean and symmetrical. In multi-secondary designs, it is necessary that the coupling of all secondary windings is balanced, so that pulses of the same quality are obtained on all outputs.

Practical Example — SCR / Thyristor Gate Triggering Circuit

This example shows that the practical use of pulse transformers is SCR/SCR. How is Thyristor Gate Triggering In. This application is very critical, because here the pulse transformer not only transfers the signal, but also ensures reliable firing, isolation and device safety.

Circuit Concept (Pulse Transformer-Based Gate Drive)

The SCR gate provides isolation between the pulse transformer control circuit (low-voltage side) and the power SCR (high-voltage side) in the triggering circuit. A control pulse is applied on the primary side, and the same pulse is electrically isolated from the secondary side to reach the Gate–Cathode terminals.

Due to this isolation, the control electronics are protected from high-voltage transients and noise.

Typical practical values:

• Control pulse voltage: 10–12 V
• Pulse width: 8–15 µs
• Required SCR gate current: 100–200 mA (short pulse)

Turns Ratio Selection (as per Gate Requirement)

It is usually 2–3 V sufficient to trigger the SCR gate. For this reason, most of the 1:1 turn ratio is chosen in pulse transformers.

The advantage of the 1:1 turns ratio is that:

• Pulse shape distortion remains minimal
• Timing accuracy is good
• The design is simple and predictable

If the control voltage is too low, the slight step-up ratio can be taken, but 1:1 is the most common and safe choice in standard industrial designs.

Selection of Magnetizing Inductance (Lm) — According to Pulse Width

Suppose the gate pulse width is 10 μs. Throughout this duration, the gate current should remain reasonably constant, so that the SCR can turn on reliably.

This is possible only if the magnetizing inductance of the pulse transformer is high enough.

Having Low Lm:

• Pulse droop increases
• Gate current starts to fall
• SCR can misfire or delayed firing

Therefore the magnetizing inductance in SCR gate pulse transformers is usually placed in the milli-henry range, keeping the droop less than 5–10%.

Volt-Second Limit Check (Most Ignored Step)

Now let’s do a core saturation check, which competitors often skip.

Let’s say:

Primary pulse voltage = 12 V

Pulse width = 10 µs

Volt-second demand will be:

V × t = 12 × 10 × 10⁻⁶ = 120 × 10⁻⁶ V·s

The design ensures that the capability of the transformer core is:

N × A × ΔB ≥ 120 × 10⁻⁶ V·s

And there is always a 20–30% safety margin in it.

If this check is ignored, the SCR firing angle becomes unstable and the transformer heating problem arises.

Gate Circuit Components (Typical Practical Values)

On the secondary side, some basic components are installed to protect and control the SCR gate. The gate resistor is usually kept at 10–47 Ω, keeping the gate in the current range. The reverse diode is installed across the gate–cathode to prevent gate junction damage from negative pulses.

These small components play a big role in the long-term reliability of the SCR.

RC Snubber Recommendation (Overshoot Control)

Leakage inductance in the pulse transformer can cause overshoot and ringing in the gate pulse.

To suppress this, the RC snubber network is installed on the primary or secondary side.

Typical starting values:

• Resistor: ≈100 Ω
• Capacitor: 0.01–0.1 µF (polypropylene)

The RC snubber gate reduces overshoot, prevents false triggering, and also reduces EMIs.

Industry Reference Note (Why This Matters)

This approach is widely followed in industrial SCR drive designs.

For detailed gate-drive recommendations and waveform limits, engineers usually refer to the application notes of STMicroelectronics and Dynex Semiconductor, where pulse transformer based triggering is explained with practical numbers.

Final Practical Insight

The pulse transformer in SCR gate triggering is not just an isolation device, but a core element of reliable firing systems.

The right turn ratio, adequate magnetizing inductance, volt-second limit check, and RC snubber—all four of these together give the SCR a stable, repeatable, and safe operation. This is the reason why pulse transformer design is never taken lightly in SCR applications.

It is not practical to set the pulse transformer’s turns ratio too high in SCR gate triggering circuits.
A high turns ratio increases leakage inductance and stray capacitance, resulting in a weak and slow gate pulse. Therefore, a compact ferrite core and tight magnetic coupling are preferred in real-world designs.

Advantages of Pulse Transformer

• Excellent electrical isolation between control circuit and power circuit, which increases safety and noise immunity.
• Supports fast rise and fall time, so suitable for high-speed switching and digital pulses.
• Low loss in high-frequency operation, because the ferrite core is used.
• The pulse shape remains reasonably preserved (with proper design), which is essential for gate-drive and triggering circuits.
• Compact size and lightweight, compared to power transformer.
• Multiple isolated outputs are possible (multi-secondary windings).
• No DC transfer, which avoids ground loop and DC offset problems.

Disadvantages of Pulse Transformer

• Not suitable for continuous AC or power transfer.
• There is a risk of core saturation if the volt-second limit is not followed.
• Design is sensitive—a balance of leakage inductance, magnetizing inductance, and insulation is essential.
• Droop can occur at long pulse width, if the magnetizing inductance is low.
• There is a possibility of overshoot and ringing, especially on fast edges (snubber may be needed).
• Wideband/high-speed designs can be costlier than simple power transformers.

Applications of Pulse Transformer

• Gate Drives / SCR Triggering
  To give isolated, fast and reliable gate pulses to power devices (SCR, TRIAC, IGBT, MOSFET).
• SMPS (Switched Mode Power Supplies)
  To transfer high-frequency control and drive pulses with noise-free isolation.
• Radar Systems
  To generate and isolate high-voltage, fast-rise pulses with accurate timing.
• Digital Signal Isolation
  Logic, clock and trigger pulses to isolate while avoiding ground-loop and EMI.
• Test & Measurement Equipment
  Oscilloscope triggers, pulse generators, and timing circuits for clean reference pulses.
• Laser Drivers
  Laser diodes/flash lamps to deliver precise, short-duration high-current pulses.
• Medical Pulse Equipment
  To deliver controlled, isolated electrical pulses in medical imaging and therapy systems.

Conclusion

Pulse transformers are designed to accurately and safely isolate fast, short-duration pulses, where core selection, winding layout, and volt-second limit are the most crucial factors.
Without proper design, practical problems such as pulse distortion, mis-triggering, and core saturation become unavoidable.

FAQ

Reference List — Pulse Transformer

• Pulse Transformers – Operating Principles & Uses
  » Application note from a transformer manufacturer giving design insights and classification of pulse transformer types. Pulse Transformers Operating Principles (Gowanda Electronics)
• Gate Drive Transformers & Pulse Transformer Use
  » Pulse transformer use in gate drive circuits and switching applications (Talema example). Gate Drive Transformers & Pulse Use (Talema)

• Design of High Voltage Pulse Transformer (IITK)
  » Technical design ideas for high voltage pulse transformer (research paper/PDF). Pulse Transformer Design (IIT Kanpur PDF)
• Pulse Transformer Definition (Wikipedia)
• 
  » Pulse transformer definition & how it fits into transformer categories. Pulse Transformer (Wikipedia – Types of Transformers)
• Pulse Transformer Applications Overview (PDF)
• 
  » A document covering high frequency operation, pulse shape, and various real applications (download available). Pulse Transformer Applications (PDF)
• What is a Pulse Transformer? (Function, Design & Uses)
  » Explains waveform preservation, isolation, core material, and winding details. Pulse Transformer Design & Applications (Velatron)
• Pulse Transformers: Types, Advantages & Applications
  » Pulse transformer types (signal/power), pros/cons, and performance parameters. Pulse Transformers Types & Applications (CustomCoils)

Reference Sources & Content

The information explained in this article is based on standard Electrical & Electronics Engineering textbooks, manufacturer application notes, and academic research papers, such as:
– P.S. Bimbhra
– B.L. Theraja
– V.K. Mehta & Rohit Mehta
– Technical notes from STMicroelectronics, Talema, and Dynex Semiconductor

About the Author

Pranjul Yadav is an Electrical Engineer and Electrical Field Professional with real-world experience in academic teaching and on-site electrical work.
He has taught electrical subjects to ITI, Diploma, and B.Tech level students and has been working on practical electrical systems for several years.

The content published on this website is based on standard electrical engineering principles, trusted textbooks, and real-world experience.

Disclaimer:

Pulse transformer design values ​​(turns ratio, core selection, insulation level) may vary depending on the application.
When implementing them in practical circuits, it is important to follow manufacturer datasheets and safety standards.