1. Higher Ripple at Very Light Load
At very light load to no load, an LLC resonant converter will operate in very high frequency to maintain regulation. This is due to the flatness of the gain that requires a bigger frequency before it can create a change to the gain. In most controllers, this is attained by a technique called burst mode. Burst mode means switching on and off. The side effect of burst mode is the occurrence of low-frequency output ripple voltage.
2. High Current Stress in the Tank Components at Startup
Once the actual switching frequency hits the resonance frequency of series L and C, an LLC converter has a very smooth response and low current flowing to the tank circuit. However, at start-up the series capacitor is not charged yet and it will behave not a capacitance but just a wire. The series inductor is also described the same. Moreover, the transformer will not act as a transformer right away upon the application of power. These actions will result in a higher inrush current. The inrush current will be in short duration, but the components must be selected carefully to withstand it.
3. Higher Voltage Stress on the Primary FET at Start-up
At start-up, the series capacitor will charge and act as a wire. The transformer will be acting as an inductor for a short period of time. Thus, the circuit will look purely an inductive one that being driven by Q1. When Q1 is off, the inductive kickback will be seen by it. This inductive kickback is as high as:
Vkickback = VIN + L x di/dt
Where L is the sum of Lres and the inductive value of the transformer primary
4. EMI Performance Suffers When the Converter Operates Outside Resonance
The main advantage of a resonant converter is the so-called soft switching. With this, EMI is improved too. However, when the operation goes outside the resonance, the EMI advantage is gone and challenges take place. This is true for LLC converter too.
When LLC converter operates below the resonance (actual switching frequency is less than the resonant frequency, gain is more than 1), the primary current (the current in the tank circuit) is distorted, there is a discontinuity. In the discontinuity section, the converter primary current is no longer sinusoidal. This will result to some harmonics and may trigger higher EMI.
At operation below resonance also, the primary FET currents are not a perfect sinusoid since the primary current is distorted. This results to harmonics also. EMI could result to a higher EMI noise. On the output side, the secondary current is also showing discontinuity rather than a perfect full wave. This also can result to an elevated EMI.
No doubt that there is a corresponding EMI increase at operation below the resonance due to the current imperfections. However, the effect of operating above resonance is worse. Operating above resonance means the actual switching frequency is higher than the resonant frequency (gain is less than 1). At operations above resonance, the primary current is distorted too but this is different from the operations below resonance. The current is at its peak (or near peak) and instantly pull down to zero. There is obviously hard switching.
In this operation, harmonics are more pronounced than operating below resonance. Moreover, when an LLC operated above resonance, the actual switching frequency is normally higher than the resonant frequency. At worst scenario, the actual switching frequency is already overlapping the first band of the conducted EMI. With the harmonics present, it may also impact radiated EMI spectrum. This is a real problem!
5. The Need to Put Minimum and Maximum Clamp Frequency
LLC resonant converter is a frequency-controlled converter. The frequency is going to adjust to control the converter. The frequency cannot go too low to ensure the converter will not operate in the capacitive region. On the other hand, the frequency cannot go too high to avoid EMI issues. Therefore, there is a need to put a minimum and maximum clamp frequency. This seems simple but it’s actually tricky. The minimum clamp frequency must not too low in order to still pass hold-up requirement when operating below the resonance. In the same manner, the maximum clamp frequency must not be too low in order to ensure regulation at very light load to no load.
6. Hold-up is Very Difficult to Achieve at Operation Below Resonance
LLC converter has only one single point to operate at resonance. This is set by selecting the series inductor and capacitor and the transformer turns ratio. These components are related to a gain equation. Below is the mathematical equation of a half bridge LLC gain.
Gain = 2 x n x Vout / Vin
Where;
N – transformer turns ratio
Vout – secondary voltage
Vin – voltage applied to the primary MOSFET
For more detailed discussion on gain, read Simplified LLC Resonant Converter Operation for Beginners
For a resonance to happen, the gain must be unity. In a variable output power supply, the component selection and the unity gain is set at the nominal output most of the time. Therefore, at the maximum output voltage trim, the gain will be more than unity based on the gain equation. This results to a decrease in the switching frequency.
During the hold-up period, the output voltage is held steady while the input voltage (VIN) decreases. As a result, the gain will further increase above unity. The switching frequency will further decrease until it hits the minimum frequency limit of the LLC and can no longer sustain the regulation. Thus, not meeting the hold-up requirement.
LLC converter has a minimum frequency limit to ensure that its operation will not go to capacitive region. This limit is often set near the peak gain. When the converter enters capacitive region, the ZVS is gone. This time the current leads voltage. When viewed at the MOSFET, at ZVS, the FET voltage is already at a diode drop level when the current starts to flow. Thus, there is only very small power dissipation. However, when at capacitive region, the current and voltage overlap, and it will result to a very huge power dissipation that can destroy the MOSFET right away.
7. Current Stress on the Output Capacitors are High at Operations Below the Resonance
When the operation goes below resonance, the shape of the secondary current is no longer a perfect full wave. Instead, the current stays at zero for a significant time before rising again. This behavior results to higher peak current. A higher peak current will result to a higher RMS current in the output capacitors. This is critical to be addressed. Otherwise, the capacitor’s life will be shorter if it may not fail immediately.
8. Thermal Issue when Operating Below Resonance
The peak level of the devices current at operations below resonance is high due to the discontinuity of the primary current. The magnetizing current will have higher rms value resulting to higher transformer loss. The primary FET currents are higher resulting a higher power dissipation. All of these will add up and result to a real thermal issue if not addressed.
9. Efficiency Suffers when Operating Below Resonance
At operations below the resonance, the power loss of the primary FETs, transformer and secondary diodes are higher compared to operating at resonance. Therefore, the overall efficiency will decrease.
10. MOSFET Voltage Stress at Operation Above Resonance is High
At above resonance, the current in the primary FETs is forcefully pulled zero when it is near the top. This results to ringing in the drain to source voltage of the FET.