OEM&Lieferant Ausgabe 2/2021

94 New Products and Markets Why Push-Pull Transformers are an Optimal Choice for High Energy Storage Applications By Cathal Sheehan, Sr. Technical Market Manager Magnetics Division, Bourns Electronics, Ireland Providing isolated low voltage bias power to ICs such as microcontrollers, analog-to-digital converters, isolated gate drivers or voltage monitoring ICs in high voltage systems is usually accomplished with an isolated DC-DC converter. If the high voltage system is spread out over several modules, the architecture may call for a parallel DC bus on the low voltage side with multiple isolated low power DC-DC converters for each module. Because it is used multiple times, an efficient and cost-effective topology is the best approach. This article highlights the design benefits of using push-pull transformers and uses Bourns® Model HCTSM8 series transform- er as an example. This series is AEC-Q200 compliant and available with a wide range of turns ratios as standard. Multiple turns ratios are an important feature enabling the same basic circuit topology to be replicated across a system with the same components and PCB layout. With the Model HCTSM8, designers are able to select the right rein- forced transformer part number based on the specified output voltage for powering a microcontroller or an isolated IGBT gate driver. Electrical Advantages Push-pull transformers are known to oper- ate well with low voltages and low variations in input and output. This characteristic is ideal for a microcontroller bias or gate driver IC that has constant power levels and input voltages. Unlike typical flyback and forward topologies, the push-pull topology offers high efficiency at a stable input and output current. In addition, flyback transformers can cause EMI problems and often require closed loop control for stable operation even though they can efficiently handle wide input rang- es. Conversely, a push-pull transformer can operate very simply in open loop. Compared to the components required for closed loop control, open loop control only requires 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. Space-Saving Push-pull transformers are typically of- fered in a smaller footprint than flyback transformers. And, push-pull transformers usually have physically smaller ferrite cores Images/Graphics: © Bourns Figure 1 compares the dimensions of the Bourns® Model HCTSM8 transformer to a transformer with a split core such as an E13. compared to flyback transformers. Plus, there is no gap required in the ferrite core of a push-pull transformer, and, therefore, the effective permeability remains high and the magnetizing inductance can be quite high for a low number of turns. Given a suffi- ciently high switching frequency and low DC voltages, the flux generated (Volt Seconds per Turn) remains well below the satura- tion point. Contrast this result with the split ferrite core in a flyback transformer where more turns are needed to ensure the current does not saturate the transformer. If there are tight space considerations and restrictions, the DC resistance will inevita- bly increase with the higher number of turns, resulting in reduced efficiency. It is advised to look for a push-pull transformer with a to- roidal core as there is no need for a gap, and it is well known for providing good coupling between windings. Figure 2 shows a typical circuit generating diagram showing plus and minus voltages when using the Model HCTSM8 transformer.