A flyback converter solution where two outputs are better than one

Flyback converters are widely used in applications requiring isolation between the primary and secondary. The flyback converter's single primary switch and output rectifier provide a cost-effective solution for a single output. Often, more than one output voltage is required. Typically, the flyback converter generates an isolated output, such as 5V, and a non-isolated point-of-load (POL) converter generates a second output, such as 3.3V, from the 5V output (Figure 1). If the POL IC has internal FETs, at least an input capacitor and an output inductor are required. A more cost-effective alternative is to add a second winding to the flyback transformer and a second rectifier to generate the second output (Figure 2). Compared to the POL IC, input capacitor, and inductor, the second winding plus the rectifier only adds a few cents to the design cost.

Figure 1. Single isolated output with a POL converter for the second output.

Figure 2. Dual isolated outputs with additional transformer windings/rectifiers.

One output is typically regulated by a feedback control loop, while the other output tracks the regulated output via the transformer turns ratio. Synchronous rectification is often implemented at the output, minimizing voltage drop variations across the rectifiers when loaded and providing good cross-regulation. Minimizing transformer leakage inductance is also critical for good cross-regulation.

There are two methods for driving synchronous rectifiers. The first and simplest method is to add a gate drive winding to the flyback power transformer (Figure 3). This is called a "self-driven" technique and only adds a few cents to the transformer cost. A disadvantage is that there is little control over the timing between the switching of the primary FET and the synchronous FET. Both the main FET and the synchronous FET are on for a short period of time, creating a short circuit that leads to shoot-through current, increased power dissipation, and reduced efficiency.

Figure 3. "Self-driven" method of controlling synchronous FETs.

A second method for driving synchronous rectifiers is to add a separate gate drive transformer (Figure 4). This costs more than adding a winding to the flyback power transformer, but still less than adding a POL converter. The advantage is that the synchronous FETs can now be directly controlled on and off, which reduces shoot-through current and improves efficiency. Using a pulse-width modulation (PWM) controller IC with two gate drivers and adjustable delay, such as TI's UCC2897A or TPS23754, further reduces or eliminates shoot-through current, achieving even higher efficiency.

    The TPS23754 and TPS23756 devices feature a combined Power over Ethernet (PoE) powered device (PD) interface and a current-mode DC/DC controller optimized for isolated converters. This PoE interface supports the IEEE 802.3at standard.

    The TPS23754 and TPS23756 support multiple input voltage ORing options, including maximum voltage, external adapter preference, and PoE preference. These features allow designers to determine which power source carries the load in any situation.

    The PoE interface features new extended hardware classification required for compatibility with high-power midspan power sourcing equipment (PSE) in accordance with the IEEE 802.3at standard. A signature detect pin can also be used to force the PoE supply to shut down. A single resistor can be used to set the classification to any of the defined classes.

This DC/DC controller features two complementary gate drivers with programmable dead time. This simplifies the design of active-clamp forward converters or optimized gate drivers for efficient flyback topologies. The second gate driver can be disabled if a single-MOSFET topology is required. The controller also features internal soft-start, a bootstrap startup supply, current-mode compensation, and a 78% maximum duty cycle. A programmable and synchronizable oscillator optimizes designs for efficiency and simplifies the controller's use as an upgrade path for existing power supply designs. Precisely programmable blanking with a default period simplifies common current sense filter design trade-offs.

Figure 4. Separate gate drive transformer used to control synchronous FETs.

The following links provide examples of dual-output flyback converters with synchronous rectifiers. The first two examples use a self-driven approach to control the synchronous FETs. The third example, while having only one output, demonstrates a gate drive transformer and a PWM controller IC with dual gate drivers and adjustable delay for controlling the synchronous FETs. Figure 4 shows how this can be expanded to dual outputs.

    The TI Designs Class 3 Dual-Output Isolated Flyback Converter for PoE Applications Reference Design is an isolated Class 1 Power over Ethernet (PoE) flyback converter with dual 5V/3.3V outputs. It uses the TPS23753A powered device (PD)/PWM controller with a single gate driver. Self-driven synchronous rectification provides very good efficiency and cross-regulation between the outputs.

    The High-Efficiency 36-60V Input Isolated Synchronous Flyback Reference Design with Dual Outputs is an isolated flyback converter for telecom applications with 22W dual 5V/3.3V outputs. It uses the LM5020 PWM controller with a single gate driver. Self-driven synchronous rectification provides very good efficiency and cross-regulation between the outputs.

    The Class 4 Efficiency-Optimized Flyback Converter for PoE Applications is an isolated Class 4 PoE flyback converter. It has a single 5V output but uses a separate gate drive transformer and the TPS23754 PD/PWM controller with dual gate drive. The converter alone has an efficiency of 92%.