around the world about loweringElectronicVarious initiatives for system energy consumption are prompting single-phase AC inputpower supplyDesigners use more advanced power technology. To achieve higher power levels, these initiatives require efficiencies of 87% and above.due to standardflybacktype (flyback) and dual switchPositiveThese high efficiency stages are not supported by traditional power topologies such assoft switchreplaced by resonant and quasi-resonant topologies.

working principle

Figure 1 shows the use of three different topologies (quasi-resonant flyback, LLC resonant, and asymmetric using soft switchinghalf bridgetopology) of the switchVoltageandcurrentwaveform.

Figure 1: Comparison of Quasi-Resonant, LLC and Asymmetric Half-Bridge Topologies

outputdiodecurrent drops to zero

Ramp change when primary side is coupled back to secondary side

body diode conducts untilMOSFETturn on

The three topologies employ different techniques to reduceMOSFETThe turn-on loss of , the calculation formula of turn-on loss is as follows:

In this formula, ID is the drain immediately after turn-oncurrentVDS is on the switchVoltageCOSSeff is the equivalent outputcapacitancevalue (including stray capacitance effects), tON is the on-time, and fSW is the switching frequency. .

As shown in Figure 1, a MOSFET in a quasi-resonant topology has zero drain current when it is just turned on. Because the converter operates in discontinuous conduction mode, the switching losses are determined by the voltage at turn-on and the switching frequency. Quasi-resonant converters turn on when the leakage voltage is minimal, thereby reducing switching losses. This means that the switching frequency is not constant: at light loads, the first minimum leakage voltage comes earlier. Previous designs always turned on at the first minimum value, and the efficiency at light loads decreased as the switching frequency increased, negating the benefit of the lower turn-on voltage.at Fairchildsemiconductore-Series™ Quasi-Resonantpower supplyIn switching, the controller simply waits the minimum time (thus setting the upper frequency limit), and then turns on the MOSFET at the next minimum value.

Other topologies use zero-voltage switching techniques. In this case, the voltage VDS in the above formula will drop from the bus voltage of typically around 400V to around 1V, which effectively eliminates the turn-on switching losses.By reversing the current through the bodydiodeFlow through the MOSFET, and then turn on the MOSFET to achieve zero-voltage switching. The voltage drop across the diode is typically about 1V.

A resonant converter achieves zero-voltage switching by generating a sinusoidal current waveform that lags the phase of the voltage waveform, which requires a square-wave voltage to be applied to the resonant network, the fundamental frequency component of which causes the sinusoidal current to flow (higher-order components are generally negligible). ). Through resonance, the current lags the voltage, enabling zero-voltage switching. The output of the resonant network is rectified to provide a DC output voltage. The most common resonant network consists of a transformer with a special magnetizing inductance, an additional Inductor and a capacitor, hence the name LLC.

asymmetricalhalf bridgeThe converter is throughsoft switchtechnology to achieve zero voltage switching. Here, the voltage generated by the bridge is a square wave with a duty cycle well below 50%. Before applying this voltage to the transformer, a coupling capacitor is required to remove the DC component, and this capacitor also acts as an additional energy storage unit. When both MOSFETs are turned off, the energy in the leakage inductance of the transformer causes the voltage polarity of the half-bridge to reverse. This voltage swing is eventually clamped by the associated MOSFET body diode with the sudden primary current.

selection criteria

These energy optimisation results lead to outstanding efficiency. For a 75W/24V supply, the quasi-resonant converter design can achieve efficiencies in excess of 88%. With synchronous rectification (plus an additional analog controller and a PFC front end) it is more likely to increase the efficiency to over 90% on a 90W/19V supply. At this power stage, although LLC resonant and asymmetric half-bridge converters can achieve higher efficiencies, quasi-resonant converters are commonly used in this power range due to the higher implementation costs of these two solutions. For applications ranging from 1W auxiliary power supply to 30W set-top box power supply and even 50W industrial power supply, the e-Series integrated power switch series is very effective. Above this power stage, the FAN6300 quasi-resonant controller with external MOSFETs is recommended, which provides additional flexibility to handle very high system input voltages, and in addition helps optimize price/performance due to the wide selection of external MOSFETs.

Quasi-resonantflybackThe two topologies use one low-side MOSFET; the other two topologies require two MOSFETs in a half-bridge configuration. Therefore, at lower power levels, the quasi-resonant flyback is the most cost-effective topology. At higher power levels, the size of the transformer increases and the efficiency and power density decrease, and two zero-voltage switching topologies are often considered.

System design is affected by four factors: input voltage range, output voltage, ease of synchronous rectification, and implementation of leakage inductance.

Figure 2 compares the gain curves of the two topologies.For illustration purposes, we assume that supported inputs are requiredVoltagefor 110V and 220V.for asymmetrichalf bridgetopology, this is not a problem. Under the operating conditions we set, the gains are 0.2 and 0.4 at 220V and 110V, respectively. At 220V, the efficiency is lower because the magnetizing DCcurrentincreases as the duty cycle decreases. For the LLC resonant converter, the maximum gain is 1.2, note that the full load curve is very close to resonance. A gain of 0.6 will result in extremely high frequencies and poor system performance. In conclusion, LLC converters are not suitable for wide operating ranges. With external adjustment of the leakage inductance, the LLC converter can be used in the European input range at the expense of higher magnetizing currents; it works best with a PFC front end.The asymmetric half-bridge structure has a PFC stage at the input, soCircuitCan work over a wide input voltage range.

Figure 2: Gain Curves for Asymmetric Half-Bridge and LLC Converters

For output voltages above 24V, we recommend an LLC resonant converter.high outputdiodeThe voltage causes asymmetric half-bridge converters to be less efficient because diodes with higher voltage ratings have higher forward voltage drops. Below 24V, an asymmetric half-bridge converter is a good choice.Because at this time the output of the LLC convertercapacitanceThe ripple current is much larger, which increases as the output voltage decreases, increasing the cost and size of the solution.

Both of the above topologies can use synchronous rectification.For asymmetric half-bridge topologies, this is very simple to implement (see FairchildsemiconductorApplication Note AN-4153).For LLC controllers, a special analog circuit is required to detect incomingMOSFETthe technique is simpler if the switching frequency is limited to the second resonant frequency (100kHz in Figure 2).

Finally, both designs rely on the leakage inductance of the transformer: in LLC converters to control the gain curve (Figure 2); and in asymmetric half-bridge converters to ensure low loadsoft switch. For most applications, we recommend using two separate inductors for this purpose. Leakage inductance is a parameter that is not easy to control in transformers. Furthermore, to achieve an unusual leakage inductance, a non-standard coil bobbin is required, which increases the cost. For an asymmetric half-bridge configuration, if a standard transformer is used, the resonant switching speed is at least 10 times the switching frequency, resulting in higher losses. In conclusion, for LLC converters, it is recommended to use another common ferrite inductor; for asymmetric half-bridge converters, it is recommended to use only one high-frequency ferrite inductor.

Figure 3 shows the circuit schematic of the asymmetric half-bridge converter. The diagram is very similar to the LLC resonant converter with one difference: the LLC resonant converter does not require an output inductor, and the asymmetric half-bridge controller requires frequency setting rather than PWM control.

Figure 3: Asymmetric Half-Bridge Converter Based on FSFA2100

The efficiency of the 192W/24V asymmetric half-bridge converter is around 93%. The AN-4153 360W/12V current doubler version also has a full load efficiency of over 93% at a rated load of 20%-100%.

200W/48V with PFC front end power supplyUnder these conditions, the efficiency of the LLC resonant converter is around 93%. Through synchronous rectification, the efficiency can be increased to 95%-96% at this power stage.