Powering a more efficient supply

Improving the efficiency of power supplies, during normal operation and on standby, is critical if products are to meet the continued trend for reducing energy consumption.

Demands for energy efficient power supplies, however, can conflict with the need to reduce the mechanical form factor of such products. This is because reducing the size of the power supply typically requires an increase in the switching frequency, to allow for smaller passive components. An increased switching frequency in conventional square-wave hard-switched converters generally results in increased switching losses. This can make it very challenging to design power supplies that meet both efficiency and footprint requirements.

 

 

The resonant techniques used by softer switching converters can be a solution to this problem. A resonant converter differentiates from a hard-switched, fixed-frequency converter with the addition of a strategically placed L-C resonant network somewhere in the powertrain. In an LLC resonant converter, the resonant network is comprised of the transformer leakage inductor (L_lk), the resonant capacitor (C_r), and the magnetizing inductor (L_m). The low-pass filtering action of the resonant network results in the processing of the power through the powertrain in a sinusoidal manner. This typically results in a lower EMI signature for the converter. Furthermore, the main switches can be softly commutated in a resonant converter, resulting in dramatically lower switching losses during higher-frequency operation. Characteristic waveforms of an LLC converter powertrain are shown in Figure 2, illustrating the soft switching of the FETs and the sinusoidal current in the primary.

 

 

As can be seen in Figure 2, the body diode of the FET (Q2) conducts briefly before the FET is activated. This allows for the parasitic capacity of the FET to discharge its off-state voltage before the FET is enabled, allowing zerovoltage switching to take place. This dramatically reduces the switching losses in the FETs. It should also be noted that the secondary diodes switch in a zero-current fashion, further reducing switching losses to allow higher-frequency operation.

The LLC-type resonant converter also offers other advantages: First, the resonant inductance of the resonant network is comprised of the transformer leakage inductance. This means that the required leakage/resonant inductance can be built into the transformer, thereby eliminating a component.

Since LLC resonant converters are variable-frequency systems, the feedback circuit will vary the switching frequency of the converter in response to line and load changes to keep the output voltage constant. In other types of resonant converters this might cause a problem, since large frequency changes would be required in order to maintain regulation. The LLC converter, however, is designed to operate around the higher resonant frequency (fo). Therefore, as the load changes (shown as changes in Q in Figure 3) the frequency will remain very close to fo. Minimal frequency deviation is required to maintain regulation in an LLC converter, even at a no-load condition where the LLC converter maintains regulation and operates with zero-voltage switching.

 

 

In Figure 3, Q is defined as characteristic impedance of the resonant network divided by the reflected load impedance, which can be expressed as:

Fairchild Semiconductor’s new line of integrated controllers enable soft switching with the implementation of LLC-type resonant half-bridge converters. Fairchild’s FSFR2100 incorporates high-performance SuperFET™s with fast recovery body diodes which feature a 120ns reverse recovery time. This allows the design of LLC converters up to 300kHz. The SuperFET™s used in the FSFR2100 have on-resistance ratings as low as 0.38Ω, and offer power supply efficiencies as high as 95%. This means that LLC converters using the FSFR2100 can be used to deliver up to 200W without a heatsink. The FSFR2100 also incorporates Over-Current Protection (OCP), Over-Voltage Protection (OVP), Abnormal Over-Current Protection (AOCP) and Thermal Shutdown (TSD), offering additional protection features to the key combination of low footprint and high efficiency.

 

Fairchild Power Supply Design Note