Unlike the conventional pulse-width modulation (PWM) technique in which power is processed by controlling the duty cycle and interrupting the power flow in the converter circuit, resonant switching processes power in a sinusoidal form with the switching devices being softly commutated. The result is that resonant switching dramatically reduces the switching losses and noise generated compared to PWM. Resonant switching also allows passive components to be reduced in size by increasing the switching frequency. For this reason, resonant converters are attractive for various applications where size is a concern. Among the many resonant converters available, the LLC resonant converter is the most popular topology thanks to its many advantages, including its ability to regulate the output over entire load variations with a relatively small variation of switching frequency. LLC resonant converters can also achieve zero voltage switching (ZVS) for the primary side switches and zero current switching (ZCS) for the secondary side rectifiers over the entire operating range and the magnetic components can be integrated into a transformer.
Figure 1. Voltage mode controlled LLC resonant converter
Most LLC resonant converters use voltage mode control in which the error amplifier output voltage directly controls the switching frequency. This has drawbacks, however, as the compensation network design of the LLC resonant converter is relatively challenging since the frequency response has very complicated characteristics with four poles that change location with input voltage and load condition. Fig. 1 shows the circuit diagram of an LLC resonant converter employing conventional voltage mode control. The control signal is generated by comparing the output voltage with the reference voltage, which controls the switching frequency of two switches in the half-bridge circuit. The limitation of this control method is that the LLC resonant converter responds to the change of the control very sensitively as shown in Fig 2 since it is a multiple pole system, which necessitates a slow feedback control loop design.
Figure 2. Response of voltage mode controlled LLC resonant converter to the change of control signal
The peak current mode control, commonly used in PWM converters to simplify the multiple-pole system into a one-pole system, cannot be used for an LLC resonant converter because the switch current waveform does not increase monotonically. Fortunately, multi-resonant converters can use the charge control technique to improve their dynamic performance. Charge control compares the total charge of the switch current to the control voltage to modulate the switching frequency as shown in Fig 3. In Fig. 4, the LLC resonant converter responds to the change of the control very stably. Since the charge of the switch current is proportional to the average input current over one switching cycle, charge control provides a fast inner loop and offers excellent transient response including inherent line feed-forward.
Figure 3. Charge control circuit for LLC resonant converter
Recognizing the value of current mode control, we developed our new FAN7688 to fully exploit its advantages. The FAN7688 is an advanced pulse frequency modulated (PFM) controller for LLC resonant converters that features current mode control based on a charge control in which the triangular waveform from the oscillator is combined with the integrated switch current information to determine the switching frequency. This provides a better control-to-output transfer function of the power stage simplifying the feedback loop design while allowing true input power limit capability. Fig. 5 shows a typical application circuit using the FAN7688.
Figure 4. Response of charge current mode controlled LLC resonant converter to the change of control signal
To learn more about how Fairchild is enabling manufacturers to improve reliability and efficiency for power supply applications, watch our FAN7688 video and visit the FAN7688 page for more information.
Figure 5. Typical application circuit
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