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.
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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 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.
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).
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.
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)
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.