Cost-effective sync rectification in ac/dc supplies

BY HELEN DING

International Rectifier

www.irf.com

Synchronous rectification is being used to improve performance in an increasing number of ac/dc power supply applications, as tougher ecodesign targets drive designers to achieve further gains in energy efficiency. For optimum efficiency, cost, and reliability, an economical solution is needed to control the synchronous rectifier MOSFETs without allowing reverse current to flow during turn-off.

As power supply designers seek further improvement in efficiency, more and more are choosing to use a synchronous rectifier in place of the traditional high-speed or Schottky diode in flyback or forward converters, as shown in Fig. 1 . This requires control of the MOSFETs (synchronous rectifier MOSFET for both Forward and Freewheeling), to turn the device on and off at the correct times to maintain the flow of current through the load.

 Fig. 1: Schematic of forward converter with synchronous rectifier. Diodes shown are the MOSFET body diodes.

When the converter is operating in discontinuous-conduction mode (DCM), the current in the output inductor is allowed to decay to zero while the synchronous rectifier MOSFET is turned on. The secondary-side gate-control signals may be coordinated by sensing the MOSFET drain-to-source voltage (V_DS ). Controllers designed to operate this way are available from several manufacturers. The IR1169 SmartRectifier controller uses IR’s proprietary High-Voltage IC (HVIC) technology to connect directly to the MOSFET drain and source. This approach minimizes demand for additional components, thereby reducing bill-of-materials (BOM) costs.

The IR1169 has intelligent controls built in that sense and control the secondary synchronous rectifier MOSFET from initial startup and during the operation properly control its signals to prevent unwanted reverse current flows as well as improving overall efficiency and prevent stress on converter components.

At higher output-power levels, operating the converter in continuous-current mode (CCM) significantly improves energy efficiency by preventing the inductor current from decaying to zero. In CCM, the dI/dt slope at turn-off is much steeper than is the case in DCM. The IR1169 internal circuitry ensures proper operation in such a condition in order to avoid possible reverse-current flow.

For proper control, a timely monitoring of the gate-control signal generated by the primary-side MOSFET controller is needed. However, since the isolated topology prevents making a direct connection, most controllers need to sense this signal via a pulse transformer. This approach has a number of drawbacks. The use of an additional magnetic component adds cost to the design while also reducing system reliability and occupying a large PCB area. Some secondary-side controllers require a special signal, and therefore can only be used with a specific primary-side controller.

The IR1169 secondary-side controller has integrated circuitry that allows an ordinary primary-side gate-drive signal to be connected using a simple and low-cost capacitor circuit. This approach eliminates the need for a pulse transformer, and so allows designers to build smaller, more economical and more reliable power supplies.

A forward converter using two IR1169s to control the forward and synchronous rectifier MOSFETs is shown in Fig. 2 . A high-voltage safety-qualified Y capacitor is connected to the IR1169 and the gate-drive output of the primary-side controller. A standard Y capacitor is also placed between the primary ground and secondary ground to help close the current-flowing loop. When the primary controller generates a gate-drive pulse, this signal is received by the IR1169 and it properly controls and synchronizes the MOSFET, preventing any shoot-through current condition.

 Fig. 2: Synchronous rectifier capable of CCM operation without pulse transformer.

The synchronous rectification improves system efficiency and reduces power losses, compare with Schottky diodes solution. It enables the use of a smaller heatsink, whereby reducing the size and weight of power supply.

Internal circuitry of the IR1169 allows VDS sensing in DCM or CCM mode. Hence this controller is suitable for high-efficiency synchronous rectification in power supplies for cost-sensitive equipment such as small PCs and home electronics in the range of 150-300 W, as well as high-power applications such as enterprise computing and telecom or networking infrastructure equipment up to 600 W and higher.

Maximizing energy efficiency has become an important goal for power supply designers, and is encouraging adoption of more sophisticated operating principles such as synchronous rectification. Cost and reliability continue to be key concerns, however, and both are served by minimizing the number of components required. High-voltage IC technology allowing direct sensing of MOSFET voltages, and synchronization circuitry capable of eliminating the pulse transformer in isolated synchronous rectifiers, each provide valuable help enabling designers to achieve this goal.