bourns hct transformer app note

What is the Right Transformer for High Voltage Storage Designs?

Leveraging their extensive power conversion experience, application experts at Bourns believe the answer is a reinforced push-pull transformer.  We outlined the design benefits of using push-pull transformers for high voltage energy storage applications such as in battery management systems (BMS) for automotive and industrial designs in a useful new application note.

What makes a push-pull transformer an ideal choice for these applications?  This type of transformer is known to operate well with low voltages and low variations in input and output. These characteristics are well-suited for a microcontroller bias or gate driver IC that has constant power levels and input voltages. Unlike flyback and forward topologies, the push-pull topology offers high efficiency at a stable input and output current. Any variations in input and output current tend to waste energy as the power dissipated in the switches remains constant.

Other drawbacks of the use of typical flyback transformers in these applications is that they can cause EMI problems and often require closed loop control for stable operation even though they can efficiently handle wide input ranges. A push-pull transformer, on the other hand, is able to operate very simply in open loop control. Compared to the number of components required for closed loop control, open loop control only requires a combination of a driver with a fixed duty cycle along with two MOSFETs, a transformer whose turns ratio is selected to suit the desired output, two Schottky diodes and two ceramic capacitors. With the design more streamlined, the driver can be a microcontroller that is probably already in use. 

There are additional reasons that justify the choice of a push-pull transformer in these applications, including the shape of the output current and a smaller footprint that offers space-saving advantages.  And significant for high voltage applications, the relatively high inductance factor of a push-pull transformer with a toroidal core means it is possible to achieve high magnetizing inductances without a high number of turns.

An example using the Bourns® Model HCTSM8 series transformer is shown in the application note. It demonstrates how an elevated degree of isolation from high voltage hazards can be achieved with the Model HCTSM8 series. The series’ ferrite toroid core construction that supports a high coupling factor and efficiency is highlighted along with creepage/clearance distance and withstanding voltage features.

The note also covers the benefits of multiple turns ratios in push-pull transformers to enable the same basic circuit topology to be replicated across a system with the same components and PCB layout. With a transformer series like the Model HCTSM8, designers are able to select the right reinforced transformer part number based on the specified output voltage for powering a microcontroller or an isolated IGBT gate driver.

See the application note for more benefits of push-pull transformers in terms of electrical and mechanical advantages, maximum creepage/clearance, enhanced insulation, safety testing and more.

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