Until recently, power supply efficiency standards were better than 80%. New initiatives are pushing for efficiency of 87% and above. Furthermore, traditional full-load efficiency measurements are no longer acceptable. The current standard measures efficiency at 25, 50, 75, and 100 times the rated load and determines the average. Similarly, maximum permissible standby power levels are being tightened. The EU recommends standby power levels below 500 mW for all devices and below 200 mW for televisions.
Outside of specialized high-efficiency power supply designs, typical AC input power supplies for 1 to 500 watt applications use "hard-switching" flyback and dual-switching forward topologies. However, these are being superseded by quasi-resonant flybacks, LLC resonant converters, and asymmetric half-bridge topologies. In this article, we will discuss the differences between quasi-resonant and resonant operation, and their optimal applications.
Basic principles
Both quasi-resonant and resonant topologies function by reducing turn-on switching losses in the circuit. Figure 1 shows the differences in turn-on switching waveforms for flyback, quasi-resonant flyback, and LLC resonant converters operating in continuous conduction mode (CCM).
The CCM flyback converter has the highest switching losses. For a wide input voltage range design, VDS is approximately 500 to 600 volts, which is the sum of the input voltage VDC and the reflected output voltage VRO. When the converter operates in discontinuous conduction mode (DCM), the first term of the switching losses drops to zero because the drain current drops to zero. We can further reduce the losses of the quasi-resonant converter by turning on the first (or subsequent) minimum in the voltage waveform. The dashed line in the figure represents the drain waveform of the quasi-resonant converter when it turns on at the first valley.
If the turns ratio of a quasi-resonant flyback converter is 20 and the output voltage is 5 volts, then VRO will be 100 volts. Therefore, for a bus voltage of 375 volts, the switch will open at 275 volts. If the effective output capacitance COSSeff is 73 pF and the switching frequency fSW is 66 kHz, the power loss is 0.18 watts. For a standard CCM flyback converter, the switches are out of sync, and the drain voltage rings. In the worst case, the drain voltage is higher than VDC. The power loss is...
The resulting loss is 0.54 watts. Therefore, for a discontinuous-mode flyback converter, power loss fluctuates between 0.18 and 0.54 watts, depending on timing. Factors affecting timing are input voltage and output current; favorable factors result in higher efficiency. This is often seen as an anomaly in the full-load efficiency curve of a discontinuous-mode flyback converter. Here, the input voltage varies with a constant output current (and voltage). As we move along the switching point, the efficiency curve will show fluctuations. Variations in the primary inductance between different batches will also exhibit variations, thus affecting efficiency.
Resonance conversion
On the other hand, resonant converters use different techniques to reduce switching losses. Returning to the turn-on loss equation (Equation 1), if VDS is set to zero, there are no losses at all. This principle is called zero-voltage switching (ZVS). It is used in resonant converters, especially LLC resonant converters, as shown in Figure 1.
Zero-voltage switching is achieved by forcing the current flowing through the switch in the opposite direction. When the switching current reverses, the body (or external anti-parallel) diode clamps the voltage to a low value (e.g., 1 volt). This is far below the 400 volts of a typical flyback converter mentioned earlier.
Achieving this requires a resonant circuit. Two MOSFETs generate square waves and apply them to the resonant circuit. If we choose an operating point above the resonance point, the current flowing into the resonant circuit will approximate a sine wave because higher-order components are typically well attenuated. The sinusoidal current waveform lags behind the voltage waveform. Therefore, when the voltage waveform reaches its zero-crossing point, the current is still negative, allowing for zero-voltage switching.
Basic Topology
The circuit diagrams and block diagrams of quasi-resonant and LLC resonant converters are described below. The circuit diagram of a quasi-resonant converter looks very similar to that of a flyback converter, except that there is a detection circuit to help determine the timing of the minimum voltage.
The LLC resonant converter (named for the three components in the resonant circuit: the transformer's magnetizing inductance L<sub>m</sub>; the transformer's leakage inductance L<sub>lk</sub>; and the resonant capacitor Cr) differs significantly from a two-switch forward converter. The required large leakage inductance means the transformer is wound in a manner that increases its normal leakage inductance, or the designer adds an inductor. The LLC has a half-bridge structure on the primary side, but unlike a two-switch forward converter, no diodes are needed there. And no resonant capacitor is used in a two-switch forward converter. Two output diodes are connected to the output of the center-tapped transformer. These rectify the AC output of the resonant circuit to a DC voltage. A large output inductor, which is necessary for two-switch forward applications, is not required.
For a given output power, the quasi-resonant flyback transformer has the largest size because the converter stores all energy on the primary side before transferring it to the secondary side. This is not the case with the two-switch forward converter, which transfers energy from the primary side to the secondary side when the switch is open. Like the flyback converter, the two-switch forward converter uses only one pole. The LLC converter uses both poles, so it is generally smaller for a given power level all else being equal.
Frequency and gain
The benefits of quasi-resonant and LLC resonant switches include reduced conduction losses. The disadvantage is that the frequency increases as the load decreases. The turn-off losses of both converters worsen with increasing frequency, where t<sub>OFF</sub> is the turn-off time. This reduces efficiency at lighter loads. For example, Fairchild's FSQ0165RN quasi-resonant FPS power switch uses a special frequency clamping circuit to counteract this inherent drawback. The controller waits for the shortest time corresponding to the maximum frequency before turning on the next available minimum.
Another limitation of LLC resonant converters is their very limited dynamic range of gain. At higher resonant frequencies (100 kHz in this case), the frequency does not change with load variations. However, the dynamic range of gain is low, ranging between 1.0 and 1.4. If 1.2 represents a system gain with a 220 VAC input to achieve the desired output voltage, the dynamic range would allow for an input voltage range of 189 to 264 VAC. Therefore, universal input operation is not easily achieved with this topology. However, typical European power supplies are possible through careful design to allow for hold-up time conditions.
The dynamic range of the gain can be improved by increasing the leakage inductance relative to the magnetizing inductance. The trade-off is reduced efficiency under light loads due to the higher magnetizing current. In practice, we use a second inductor to increase the leakage inductance; however, if the leakage inductance is too large, there are practical limitations to achieving a repeatable leakage magneto-inductance ratio.
application
Quasi-resonant flyback and LLC resonant converters are increasingly being used in embedded AC input power supplies. Practical operating ranges from a few watts to approximately 100 watts. Full-load efficiency ranges from around 81% for a 7-watt, 12-volt integrated solution to over 88% for a 70-watt, 22-volt supply using a quasi-resonant controller with external MOSFETs. Low-power examples have standby power well below 150 mW; higher-power examples have standby power less than 350 mW. Using a lower output voltage quickly reduces efficiency below this level. A 5-watt, 5-volt supply will waste at least 10% of the rated output power in the output diodes.
Another advantage of the quasi-resonant topology is that EMI is significantly lower than in hard-switching applications. The frequency naturally varies with the ripple on the 400-volt input capacitor, resulting in spectral spread. Furthermore, common-mode EMI noise is reduced due to switching at lower voltages, thereby lowering switching noise.
The practical operating range of LLC resonant converters is approximately 70 to 500 watts. The FSFR2100 with a PFC front end has been used to achieve power supplies from 200 to 420 watts. For applications up to 200 watts, a heatsink is typically not required (on the FSFR2100). It is generally recommended to use Schottky diodes at the output, which typically require a heatsink.
Synchronous rectification methods can be used to eliminate the need for heat sinks. However, the control signals for MOSFETs are not easily generated. Typical peak efficiency for applications using Schottky diodes was in the mid-1990s, depending on the input voltage, output voltage, and output power.
