In Power-over-Ethernet (PoE) applications, high efficiency is a primary concern because input power is limited. Low-power systems are limited to 13W, while high-power systems are limited to 25.5 W. Telecom supplies which typically require higher power levels, up to 200W, also need high efficiency for thermal reasons. Employing an active reset in a forward converter allows for very high efficiencies, while still using a single-ended topology.
First of all, the active clamp recaptures the leakage energy of the transformer, preventing that energy from being dissipated. More importantly, the active reset conditions the transformer’s winding voltage waveforms in a way that suits them for employing self-driven synchronous rectifiers.
Proper design of an active clamp forward converter requires careful selection of the transformer turns ratio and inductance, as well as setting the switching delays appropriately. Setting the switching delays too short will result in cross-conduction of the synchronous rectifiers, increasing losses and reducing the efficiency. Delays that are too long will result in excessive conduction of the body diodes on the synchronous rectifiers. Losses associated with the reverse recovery of the body diodes can be just as severe as those due to cross-conduction.
Output voltages over 5V typically require some extra conditioning circuitry to protect the gates of the synchronous rectifiers. An example of this type of conditioning circuit is shown in Figure 1. The zener D8 limits the gate voltage. Q6 allows higher current to charge the gate. D7 provides a low impedance path to quickly turn the synchronous rectifier off.
Figure 1. Output voltages over 5V require gate drive circuitry to protect the gates.
Of course the design of an active clamp forward converter can’t be completely described in a 400-word blog post. If you are interested in diving deeper into this topic, please download the following paper that I had the privilege of co-authoring with my friend and colleague David Strasser from the 2010-2011 Power Supply Design Seminars (SEM1900). This topic and many others can be found here: www.ti.com/psds
Also, we have many different active clamp forward reference designs in the TI Designs PowerLab reference design library. Here are a few examples:
- PMP6584 – Class 4- High power PoE, active clamp forward converter 8V@3A:
- PMP7452 – Class 4- 3.3V/7.6A High Efficiency Active Clamp Forward Converter for PoE Applications:
- PMP9656 – 48V-60VDC Input, 12V/250W Active Clamp Forward Reference Design:
- PMP10395 – 18-60VDCin, 6V/31A Active Clamp Forward Reference Design: