Issue link: https://resources.mouser.com/i/1442856
| 5 | Why Push-Pull Transformers are an Optimal Choice Push-pull transformers are known to operate 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. Any variations in input and output current tend to waste energy as the power dissipated in the switches remains constant. 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 ranges. Conversely, a push-pull transformer can operate very simply in open loop. 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. In fact, the driver can be a microcontroller that may already be in use. If a microcontroller is used as the driver, then additional NPN transistors and resistors are necessary to provide the gate drive of the push-pull MOSFETs. There are other reasons that justify the choice of a push- pull transformer. The shape of the output current is regular and not pulsating, which tends to stress the diodes and capacitors. For a relatively low-current solution, diodes offer a cost-effective addition that can help ensure compensation and balancing of the transformer. If the magnetizing current is imbalanced, the additional current in the winding will cause the drain-to-source resistance of the driver MOSFET to increase. This increases the voltage drop across the MOSFET and reduces the voltage across the winding, thereby equalizing the imbalance. Space-Saving Specifying a push-pull transformer for low-voltage applications offers several space-saving benefits. They typically are offered in a smaller footprint than flyback transformers. And because push-pull transformers are designed as "pure" transformers, they usually have physically smaller ferrite cores 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 sufficiently high switching frequency and low DC voltages, the flux generated (Volt Seconds per Turn) remains well below the saturation point. Contrast this Figure 1 Illustration of the operation of a push-pull converter with the red and green lines indicating output current