Quasi-Resonant, Zero-Current Switching DC/DC Converters

Today’s switching converters are commonly packaged as high-density modular ‘bricks.’ In selecting a DC/DC converter component, a power system designer routinely considers factors such as size, efficiency, and price, but, rarely, topology. In view of the large number of competing topologies – basic power conversion designs – in use today, some understanding of converter topologies can provide valuable insights to enhance the selection process.

A simplified diagram of the quasi-resonant, ZCS DC/DC converter topology is shown in Figure 1. It is called a single-ended forward converter because the energy is transferred from source to load during the ‘on’ time of a single solid-state switch. It is quasi-resonant because it virtually eliminates switching losses by switching at zero current, as a resonant converter does, but, unlike a resonant converter, the energy stored in capacitor Cr is not returned to the inductor, Lr.

The converter consists of the following primary elements:

Main Switch, Q1: When the switch is turned on, an approximately half-sinusoidal waveform of current flows through the switch, transferring a quantized energy packet from the input source to the LC circuit.

Transformer: The transformer provides voltage scaling, electrical isolation from primary to secondary, and magnetic storage in the leakage inductance, an essential function in this topology.

LC Circuit: The leakage inductance in the transformer, Lr, as the inductor in series with the secondary, and the capacitor, Cr, create an LC circuit. The inductor stores energy as current, (LrI2), and the capacitor stores energy as voltage, (CrV2).

Dual Diode Output Rectifier: Both sections of the dual diode, D1 and D2, work together to force energy transfer from the input to the output. The diode D1 conducts when the main switch conducts and allows energy to transfer from the leakage inductance to capacitor Cr. Its rectifier action also prevents the reverse transfer of energy. Diode D2 provides a path for the output inductor, Lo, current as needed. It also prevents voltage reversal on Cr after the energy delivered from the leakage inductance into Cr has been delivered into Lo.

Core Reset Circuit: It is desirable to ‘reset the core’ of a single-ended forward converter because, most importantly, it enables more complete use of the available dynamic flux swing of the magnetic material making up the transformer core. This results in more available power processing from a core of a given size.

Low-Pass Filter: The output LC filter is present primarily to ensure that the output ripple voltage across the load is small. Because of its relatively large inductance, Lo at full-load current will store more energy than any other storage element in the circuit. With steady-state operation, the energy delivered to the load must be matched by the energy pulses delivered from Cr .

The quasi-resonant, zero-current switching DC/DC converter processes power by delivering a sequence of energy transfer cycles. For a given input voltage, each resonant sequence transfers an identical quantum of energy, and these quanta can be delivered at different repetition rates, thus varying the total output power (or voltage) delivered to the output. This energy is, in turn, averaged or smoothed by the Lo-Co output filter. When more output power is needed, the repetition rate is increased.

The emergence of DC/DC converters packaged as modular components has subordinated consideration of topologies in spite of the large number available. While no one topology is superior to every other in every respect, understanding some of the inherent attributes – such as increasing losses with increasing frequency associated with other topologies – can provide helpful design insight.

Quasi-resonant converters, for example, in contrast to resonant converters, deliver unidirectional energy flow from the source to the load. Efficiency can be expected to be high, and they are inherently stable.

Zero-current switching is enabled by the leakage inductance from the main transformer coupled with the secondary side resonant capacitor. The main switch turns on and off at the zero current crossings, which eliminates the high dI/dt along with much of the switching noise associated with many other topologies. The ‘soft’ switching of the sine wave also minimizes the parasitic noise of components that is associated with ‘hard’ switching square waves.

Figure 2. A reset circuit using a magnetizing current mirror fully utilizes the core area, operating in the first and third quadrants. This permits a higher duty cycle and results in a smaller reset switch and lower losses.

One version of the core reset circuit provides a magnetizing current mirror that, in effect, enables the converter to operate in the first and third quadrants of the familiar B-H curve (Figure 2). Such a circuit provides operating advantages such as maximum available flux from the transformer core, minimal voltage stress on the main switch, and consequently, a smaller, less expensive main switch. These advantages contribute to high efficiency and power density and low cost of the converter. Other core reset circuit designs used with single-ended forward converters operate only in the first quadrant, resulting in limited available duty cycle.

Converter features associated with a recent version of the topology include a wide trimming range and a fault tolerant architecture. These converters can be adjusted or programmed from 10% to 110% of the nominal output voltage using fixed resistors, potentiometers, or voltage DACs. A 12 VDC output, for example, can be trimmed over the range of 1.2 VDC to 13.2 VDC. They also feature a patented N+M fault tolerant architecture.

One module in a parallel array will automatically assume control at start-up, and the other modules will synchronize to it. Modules communicate with high-speed pulses over a primary-side bus. In the unlikely event that the controlling module fails, another module will automatically be elected master and the system will continue operating without interruption.

Robert Marchetti is Senior Product Manager, DC/DC Converters, with Vicor Corp.