TX1 Series Push-Pull Isolation Transformers

Introduction

Push-pull isolated switching power supplies are able to utilize the smallest possible transformers. Using push-pull switching drives the transformer primary in alternating directions and therefore utilizes the core very efficiently and reduces peak magnetizing current with less inductance than unipolar excitation. Low peak current means small transformer size, low conduction loss and inherently low EMI.

With push-pull supplies being forward-mode converters, V_OUT in an output-regulated supply depends on the transformer turns ratio and the regulation scheme. In a non-regulated push-pull supply, V_OUT depends only on Vin and the transformer turn ratio. Therefore, even without output regulation the output voltage is inherently well regulated if Vin is from a tight-tolerance voltage source. In any case, push-pull switching power supplies offer simplicity of design, small transformer size, and excellent power density.

Selecting a Transformer

While push-pull architecture has been in use for a long time and these benefits are well documented, Coilcraft TX1 Series transformers represent an advancement in transformer optimization for high frequency 1 MHz switching. Combined with SN6507 transformer drivers from Texas Instruments, TX1 transformers offer a low EMI solution, with capability to operate over a much wider voltage range and offer higher power density.

Coilcraft’s current lineup of TX1 Series Transformers for Texas Instruments SN6507 is show below.

Typical Push-Pull Schematics for Texas Instruments SN6507

Courtesy of Texas Instruments Company

Unregulated Output

Regulated Output

There are three main considerations for selecting a transformer.

1. Turns ratio must be selected to achieve desired output voltage.
2. Volt-time product rating to be greater than the expected volt-seconds applied to transformer primary.
3. Transformer power rating is greater than the desired supply output.