The best performing technology for medium-power applications using PD 3.1
Conventional flyback topology has inherent limitations when applied to offline applications and high-voltage outputs such as PD 3.1 EPR at 48V. The primary challenge is the excessive voltage stress on the main switch, caused by reflected voltage from the secondary. The voltage across the main switch can be expressed as Vin + n·Vo, where Vin is the input voltage, n is the turns ratio between the primary and secondary windings, and Vo is the output voltage. For a 48V application, the combined reflected and input voltages can easily exceed 600V.
Figure 1 (Prior Art)
Figure 2 (Prior Art)
The two-transistor flyback topology alleviates this stress on the primary switches. Figure 1 presents this topology, while Figure 2 illustrates the key waveforms. These include the control signals for M1 and M2, current through the devices, the control signal for the synchronous rectifier (SR), and voltage across M2. At t0, when M1 and M2 turn off, the leakage inductance energy flows back to Vin through Dc1 and Dc2. Once this energy is transferred to Vin (at t1 in Figure 2), parasitic oscillations emerge across the switches, resulting in voltage ringing. This interference can disrupt both the primary controller and the synchronous rectifier’s IC. At t2, the SR turns off once all magnetizing current energy from the transformer is delivered to the secondary. This issue becomes more pronounced as the difference between Vin and n·Vo increases. Another drawback of the conventional two-transistor flyback is its operation in hard-switching mode.
Rompower’s ZVS two-transistor flyback topology overcomes these limitations. In this design, both switches achieve zero-voltage switching, and the leakage inductance energy is directed to the secondary, with a small portion enabling ZVS. This eliminates voltage spikes and ringing across the devices. Furthermore, the constant switching frequency is highly advantageous for EMI optimization. Figure 3 shows the waveforms of this topology, while Figure 4 depicts the voltage across the primary switches and the SR current in a 240W AC-DC adapter (Vo = 48V @ 5A).
Figure 3 (Novel Technology)
Figure 4
This novel topology combines simplicity, low cost, and outstanding performance. For 240W AC-DC adapters, it achieves 94.3% efficiency at 90Vac— the highest recorded efficiency for this class—along with a power density exceeding 27 W/in³, currently the best in the market. Unlike hybrid flyback solutions, which suffer from optimization constraints, Rompower’s ZVS two-transistor flyback topology is simple to implement, easy to control, and achieves ZVS under all operating conditions, including burst mode.
