(Click for enlarged view)

Vishay released a 500-kHz half-bridge dc-to-dc controller and synchronous rectifier driver that work together as a chipset to enable 25-W to 300-W telecom and computer power supplies. The Si9122A half-bridge controller and SiP11203/11204 secondary synchronous rectifier driver offer greater than 90% dctodc conversion efficiency even for output voltages below 3.3 V in power supply modules and network cards for fixed telecom systems, central office switching equipment, distributed power systems, intermediate bus converters, and brick converters.

Si9122A Half-Bridge Controller

Designed to operate within the fixed telecom voltage range of 36 V to 75 V and withstand 100-V, 100-ms transients, the Si9122A is capable of controlling and driving both the low and high-side switching devices of a half bridge circuit and also controlling the synchronous rectifier MOSFETs on the secondary side of the bridge. The Si9122A uses a pulse transformer to drive these low-on-resistance secondary MOSFETs, enabling much better conversion efficiency compared to implementations using Schottky diode rectifiers. On-chip control of the dead-time delays between the primary and secondary synchronous signals keeps efficiencies high and prevents shorting of the power transformer. An external resistor sets the switching frequency from 200 kHz to 500 kHz. Monitoring and control circuitry in the Si9122A enables users to set the maximum current in the primary circuit and allows the system to operate a constant current into a short circuit. This same circuitry also provides constant current into large capacitive loads during start-up or when paralleling multiple power supplies. The Si9122A is available in TSSOP-20 or MLP65-20 packages.

SiP11203 and SiP11204 Secondary Synchronous Rectifier Drivers

The SiP11203/11204 integrate seven functions typically implemented with discretes in isolated converters: the synchronous rectifier drivers, the error amplifier, the precision voltage reference, the opto-isolator driver, control of the start-up, control of the shutdown, and overvoltage protection. As such, they enable a simpler compact power supply module when used in conjunction with the Si9122A and other isolated dc-to-dc controllers. The SiP11203/11204 and Si9122A interface via a pulse transformer to provide isolated control of the synchronous MOSFETs. The error signal, supplied by the SiP11203/11204, is fed to the Si9122A via an opto-isolator. Together the transformers and the opto-isolator provide isolation between the input and the output. Both ICs are capable of being powered via conventional bias supplies (output inductor winding or power transformer winding), or from the same pulse transformer supplying the gate timing signals, and both parts generate a regulated supply for powering the error amplifier and control circuitry. The two product versions provide a choice of synchronous rectifier turn-off (SiP11203) or turn-on (SiP11204) in the event of an overvoltage condition.

During power-up, the SiP11203/11204 ensure that the synchronous rectifiers are held off until the supply voltages are sufficient to guarantee effective operation of the driver circuits. During the soft-start interval, a gradual ramp-up of the synchronous rectifier conduction time is assured. The ICs also allow control of the discharge rate of the synchronous rectifier driver outputs during power-down. Both devices are rated for a 5.5V to 13V operating range, feature a regulated internal logic voltage of 5 V, and provide a 2.2A source and 4A sink drive capability. They are designed for high-current applications in any topology requiring secondary-side drivers. Each driver is available in a compact MLP4416 package.