LLC Resonant Converters Increase Efficiency in DC-to-DC Applications

Many power supplies spend much of the time at loads far below their maximum and/or where they have the greatest efficiency, so improved efficiency is frequently required in normal and lower-power modes. For example, the 80 PLUS® Initiative requires 115V power supplies to have a minimum of 80% efficiency at 20%, 50% and 100% of rated load. Higher efficiencies at these operating points get a Bronze, Silver, Gold or Platinum rating. For 230V supplies, the minimum Bronze classification requires efficiencies of 81% at 20% load, 85% at 50% load and 81% at 100% load.

The U.S. Department of Energy has extended its thrust for higher-efficiency products to the data center, with the ENERGY STAR Data Center Energy Efficiency Initiative. This initiative addresses all of the high energy consuming aspects of a facility, including the information technology (IT) equipment and supporting infrastructure items of uninterruptible power supplies (UPS) and more.

Purchasing specifications requiring certification to these or other accepted energy-conscious standards dictate that suppliers meet these levels or lose business. This creates a strong mandate for higher efficiency. Reduced operating costs alone are a sufficiently strong incentive. Applications in the medium to high power range (200 to 1000W), such as telecom, are increasingly implementing power supplies with lower losses to control the operating cost for powering and cooling equipment.

To achieve the highest efficiencies, many designers are turning to digital control, which also provides design flexibility, high performance and high reliability. With low pin count digital signal controllers (DSCs), such as the dsPIC® DSCs available from Microchip Technology, sophisticated control is possible using the digital signal processing (DSP) capabilities and intelligent power peripherals of these devices. Before adding digital control, the basics of resonant converters need to be understood.

Benefits of Resonant Converters

Operating a converter in resonant mode, at the point where the impedance between the input and output of the circuit is at its minimum, provides improved efficiency. With a resonant converter, by supplying the MOSFETs with either a sinusoidal voltage or a sinusoidal current and switching in close proximity to the zero crossing of the sinusoidal voltage or current, the power dissipated in the MOSFETs can be significantly reduced.

Switching the MOSFET when the drain to source voltage is near zero, Zero Voltage Switching (ZVS), and transitioning from one MOSFET state to another while the current through the switch is zero, Zero Current Switching (ZCS), minimizes MOSFET switching losses. This soft-switching approach also reduces noise in the system and provides improved electromagnetic interference (EMI) performance. ZVS is preferred in high-voltage, high-power systems.

In a resonant-switch converter, reactive elements (capacitors and inductors) are added around the switch to generate the sinusoidal voltage or current. The three main classes of resonant converters are series resonant converter (SRC), parallel resonant converter (PRC) and a combination of the two, the series-parallel resonant converter (SPRC). Fig. 1 shows the high-level resonant converter block diagram and the three types of resonant-tank circuits.

As its name implies, in the series resonant converter, the load is connected in series with the tank’s inductor and capacitor. The gain from the resonant tank is ≤ 1. While the SRC can operate at no load, its output voltage cannot be regulated. For ZVS, the circuit needs to operate above resonance in the inductive region. At low line voltage, the SRC operates closer to resonant frequency.

In the PRC, the load is connected in parallel with the resonant capacitor. The PRC can operate at no load output and, unlike the SRC, its output voltage can be regulated at no load. For ZVS, the PRC also needs to operate above resonance in the inductive region. Similar to the SRC, at low line, the PRC operates closer to resonant frequency, however, the PRC differs by having high circulating currents. The series inductor and parallel capacitor provide inherent short-circuit protection.

In an SPRC, the tank circuit is a combination of the series and parallel converters and can be either a LCC or LLC configuration. Similar to the SRC and PRC, a SPRC LCC design cannot be optimized at high input voltage. As a result, the preferred alternative for many applications is an LLC. The LLC resonant tank is shown in Fig. 1.

The LLC converter can operate at resonance, at nominal input voltage, and is able to operate at no load. In addition, it can be designed to operate over a wide input voltage. Both zero voltage and zero current switching are achievable over the entire operating range.

The performance of a resonant converter is measured by several parameters. The quality factor (Q), of a resonant circuit is a dimensionless parameter that describes the amount of dampening in the circuit. It is defined as the ratio between the power stored and the power dissipated in the circuit. A higher Q indicates a narrower bandwidth for the resonant tank.

Quality is a key parameter in the tank circuitís gain, also called the voltage conversion ratio or M. By considering the families of M curves that are generated when varying either l, the normalized frequency, or Q. it is possible to obtain an indication of a resonant converter’s performance before all the parameters have been computed. M is defined as:

M(fsw ) = f ( fn, l, Q)

Where:

fn = normalized frequency, f/fr

l = the inductance ratio, Lr/Lm

Q = quality, a function of the output impedance

As shown in Fig. 2, there are two resonant frequencies, one due to the presence of Lr and Cr, the series inductor and capacitor at fn=1, and the second one due to the parallel inductor (Lm), Lr and CR at fn~ 0.5.

Different operating modes of the LLC include at resonance, below resonance and above resonance. At resonance, the MOSFETs are switched at the resonant frequency within a very narrow timing window, as determined by the selected components. This produces very low losses.

Below resonance, the circuit behavior is similar to that at resonance, but the tank current is limited by the magnetizing current for a portion of the cycle. If MOSFETs are used for synchronous rectification in the secondary instead of diodes, the gates must be turned off at the right time. This usually requires a current-sensing technique, such as measuring the voltage drop across the MOSFETs.

Above resonance, instead of being limited by the magnetizing current, the tank current is higher than the magnetizing current. In this region, the synchronous switches can be turned on and off at the same time as the primary switches simplifying their control.

Since zero-voltage switching is used, an inherent benefit of the LLC resonant supply is low electromagnetic and radio interference.

Highly Efficient Digital Control Topology

With today’s digital signal controllers, full digital control of the power-conversion and system-management functions of the LLC resonant converter can easily be implemented.

A more complete description of the LLC circuit components and sections than shown in Fig. 1 includes the DC input, switch network, LLC resonant tank, transformer, rectifier, filter and load. The addition of digital control for an LLC resonant converter is shown in Fig. 3. This design represents one that could be specified for telecom circuits. In these applications, the LLC converter is widely used as the DC/DC converter following a power factor correction (PFC) circuit in an AC-DC system. The typical PFC output voltage is approximately 400V, and can be directly fed into the LLC converter. The wide input range allows the use of smaller bulk caps. The design specifications are summarized in the Table titled “The Reference Design Specification”.

A dsPIC33FJ GS provides the digital computing power in the resonant converter. Its 40 MIPS performance and Intelligent Power Peripherals make it ideal for this application. The peripherals include high-speed PWM (16-bit with 1 ns period resolution, phase-shiftable outputs and more.

The switching circuit in the reference design uses a half-bridge topology, so the half-bridge voltage swings between 0V and Vd = 400 Vdc nominal. The resonant tank circuit is made up of a capacitor, an inductor, and the magnetizing inductance of the isolating transformer. Since the design uses the magnetizing inductance of the transformer as the parallel inductor and the leakage inductance of the transformer as the series inductor, overall systems costs are reduced by eliminating the external inductors.

If correctly tuned to the switching frequency, the resonant tank presents finite impedance to the fundamental frequency and very high impedance to all other harmonics. The impedance of the tank causes a phase shift between the voltage and current, which allows ZVS to occur. The ZVS of the primary MOSFETs is shown in Fig. 4.

The secondary side has been designed using a synchronous rectifier, instead of diodes, to reduce conduction losses at the secondary. This reduces both the forward resistance (Rf) and the losses due to the diode forward voltage. Fig. 5 shows the switching waveform for the synchronous rectifier.

For synchronous rectification, the digital control initiates the switching of the FETs without requiring current-sensing circuitry on the secondary. This results in improved efficiency and reduced cost over a full-wave rectifier design. Fig. 6 shows the efficiency over the load current range. Additional benefits include flexibility of compensator design, since the DSC also implements soft-start for duty-cycle control.

Digital Solution Flexible

Since the power-conversion control is implemented with easily reprogrammable software, the digital solution provides designers the freedom to innovate and easily modify or adapt their design. In addition to the ability to add new, cost-effective and value-creating features, the precise digital control also improves the power supply’s reliability. The use of a reference design can reduce development time, improve time-to-market, and ease manufacturing issues that can arise from a design that is started from scratch.

The performance advantages of an LLC resonant converter make this design approach an excellent choice for increased energy efficiency in medium to high-power telecom applications. At the same time, the addition of digital control provides the design flexibility, high performance and high reliability that designers have come to expect in electronic systems. To easily implement both, a reference design provides the easiest way to evaluate the system and reduce time-to-market or, perhaps more appropriately stated, the time-to-higher efficiency.

Engineers seeking greater details of the LLC circuit can refer to Microchip Technology’s “AN1336, DC/DC LLC Reference Design Using the dsPIC DSC”. The design provides an in-depth explanation of the individual circuit components and functions. Details are available on Microchip’s website at http://www.microchip.com.

Note: The Microchip name and logo, and dsPIC, are registered trademarks of Microchip Technology Inc. in the U.S.A., and other countries. All other trademarks mentioned herein are property of their respective companies.

References:

1. DC/DC LLC RESONANT CONVERTER Reference Design

2. AN1336, “DC/DC LLC Reference Design Using the dsPIC® DSC”

3. Masters class 2010 presentation: “Digital Power Conversion Using dsPIC® DSCs: LLC Series Resonant Converter”