May 2020

30 POWER CONVERTER DESIGN www.ti.com Issue 2 2020 Power Electronics Europe www.power-mag.com double-ended variant of the forward topology with both MOSFETs ground- referenced, eliminating the need for external bootstrap circuitry. Similar to the single-ended forward converter, the voltage stress at the FETs is two times the input voltage. The MOSFETs switch at a 50 % duty cycle during alternate half cycles, driving the center-tapped winding of the transformer. The push-pull transformer driver is a prevalent isolated bias power-supply solution for many reasons. It offers flexibility and the ability to support multiple outputs. The open-loop configuration provides design simplicity by eliminating the feedback loop. The push-pull transformer offers lower primary-secondary capacitance, which enables a reduction in common-mode noise compared to flyback and Fly-Buck converters. Additionally, the push-pull topology more efficiently uses the transformer core magnetizing current, resulting in a smaller magnetic solution compared to flyback and Fly-Buck converters. Although the transformer driver carries a number of advantages, it is also important to take into account the trade-offs. Unlike the flyback and Fly- Buck converters, the transformer driver cannot support a wide input voltage range, and instead requires a tightly regulated input voltage. Meeting the output-voltage regulation requirements for feedback can be challenging due to the absence of a closed loop and may require a low- dropout post-regulator (LDO). Power modules Power modules have existed for decades. These solutions are widely available and offer significant integration compared to discrete implementations. Power modules exist in many varieties, with input voltage, output voltage, output power, number of outputs, isolation rating and regulation options. Figure 5 shows the block diagram of the inner workings of one power module. The topology includes a transformer driver similar to the discrete version. Some devices may integrate an output LDO for regulation. With many options available, a power module can be used in most isolated bias converter applications. They greatly simplify the design process because there is no need to specify, design or choose a transformer; to include an input and output decoupling capacitor is sufficient to start the design. Other options like synchronization, output voltage selection, enable and error signaling are available as well. Some flexibility with modules will be lost, specifically to configure the number of outputs and transformer turns ratios. The selection of modules rated for a 125°C ambient temperature range is less than for the 55°C and 85°C options. Similarly, the number of modules available with fully reinforced insulation ratings is less than those modules available with functional or basic isolation. A next-generation bias solution Innovations in transformer design and higher frequency topologies have enabled IC designers to integrate a transformer and Silicon into one IC. For the end user a small, lightweight isolated DC/DC bias power supply is available without having to design a transformer or compromise on system performance. Figure 6 shows the block diagram of the UCC12050. Though it looks similar to a power module with integrated power stage and rectifier, a closer look at the operation shows that the switching frequency is much higher compared to power modules. This allows significant height and weight reduction versus lower switching frequency alternatives. The internal topology control scheme runs closed- loop without an LDO or external feedback components. The UCC12050 brings many benefits to the wide variety of isolated DC/DC bias supply applications. It is designed with an EMI-optimized transformer with only 3.5 pF of primary-to-secondary capacitance and a quiet control scheme. On its own the solution can pass CISPR32 Class B on a two-layer PCB without ferrite beads or LDOs. The device is rated for reinforced isolation of 5 kVrms and 1.2 kVrms working voltage and will operate at 125°C ambient temperature. The family of devices also includes UCC12040, which is rated for basic isolation of 3 kVrms and 800 Vrms working voltage. UCC12050 is targeted for 5-V input, 3.3-V to 5.4-V output applications requiring 500 mW. Applications requiring higher input or output voltages will need to provide pre-or-post conversion. Also, for designs requiring power above the UCC12050’s derating curve, alternative topologies should be explored. Literature Power Through the Isolation Barrier: The Landscape of Isolated DC/DC Bias Power Supplies; Texas Instruments, APEC 2020 Presentation. Figure 6 : UCC12050 isolated DC/DC bias power supply Figure 5: Block diagram of the inner workings of a power module