“Peak current-mode control” in an LLC converter

Other Parts Discussed in Post: UCC256301

In 1978, when Cecil Deisch worked on a push-pull converter, he faced a problem of how to balance the flux in the transformer and keep the core from walking away into saturation caused by slightly asymmetrical pulse-width modulation (PWM) waveforms. He came up with a solution of adding an inner-current loop to the voltage loop and let the switch turn off when the switching current reached an adjustable threshold. This is the origin of peak current-mode control.

Since then, peak current-mode control technique is widely employed in PWM converters. Compared to conventional voltage-mode control, peak current-mode control brings many advantages. For example, it changes the system from second order to first order, simplifying compensation design and achieving high loop bandwidth with much better load transient response. Other advantages include inherent input-voltage feed forward with excellent line transient response, inherent cycle-by-cycle current protection, easy and accurate current sharing in high-current multiphase designs.

However, when it comes to inductor-inductor-capacitor (LLC) converters, peak current-mode control becomes infeasible. The reason is obvious: because the resonant current in LLC is sinusoidal, the current is not at its peak when the switch turns off. Turning off the switch at the peak-current instant will cause the duty cycle to be far away from the required 50% for LLC.

Because of this, while peak current-mode control has already been widely used in other topologies, voltage-mode control is still dominant in LLC applications. When power engineers enjoy the high efficiency of LLC, they also experience the poor transient performance caused by conventional voltage-loop control.  Because LLC is a high nonlinear system, its characteristics vary with operational conditions. Therefore, it is very difficult to design an optimized compensation, the loop bandwidth is usually limited and the load/line transient response may not be able to meet a strict specification.

Is there any way to employ peak current-mode control in LLC? Let’s take a close look at how peak current-mode control works in PWM converters. In a PWM converter, the switching current is sensed, typically through a current transformer (CT), which is then compared to a threshold to determine the PWM turn-off instant. The CT output is a sawtooth waveform, and the input electric quantity is proportional to the magnitude of this sawtooth wave. This means you are actually controlling the electric quantity going into the power stage. Since the input electric quantity represents the input power, and input power equals output power (assuming 100% efficiency), the peak current mode controls the output power by controlling how much electric quantity goes into the power stage in each switching cycle.

So can you use the same concept in LLC? The answer is yes. One intuitive way is to integrate the input current in each half switching cycle, this can be done by connecting the CT output to a capacitor, where the capacitor voltage represents the integration of the input current. Luckily, there’s already an integration circuit in a LLC circuit. In LLC, when the top switch turns on, the input current charges the resonant capacitor, causing the resonant capacitor voltage to increase. The voltage variation over this half period represents the net input current charged to the resonant capacitor. By controlling the voltage variation on the resonant capacitor, you can control how much input power goes into the resonant tank and thus control the output power.

The UCC256301 adopts this charge-control concept through a novel control scheme called hybrid hysteretic control (HHC), which combines charge control and traditional frequency control – It is charge control with an added frequency compensation ramp, just like conventional peak current-mode control with slope compensation.

Figure 1 shows the details of HHC. There is still a voltage loop; however, instead of setting the switching frequency, its output sets the comparator thresholds VTH and VTL. A capacitor divider (C1 and C2 in Figure 1) senses the resonant capacitor voltage, and an internal current source (ICOMP) charges (when the high-side gate is on) or discharges (when the low-side gate is on) the capacitor divider. Comparing the sensed voltage signal (VCR) with VTH and VTL determines the gate-drive waveform.


Figure 1: HHC in the UCC256301

Figure 2 shows how to generate the gate waveform. When VCR drops below VTL, turn off the low-side gate; after some dead time, turn on the high-side gate. When VCR reaches VTH, turn off the high-side gate; and after dead time, turn on the low-side gate.

Figure 2: Gate waveform in the UCC256301

Just like peak current-mode control in PWM converters, HHC in the UCC256301 offers excellent transient performance by changing the LLC power stage into a single-pole system that simplifies compensation design and achieves higher bandwidth.

Figures 3 and 4 compare the load transient response with HHC and conventional voltage-mode control, respectively. With the same load transient, the voltage deviation is much smaller than conventional voltage-mode control.  

Figure 3: Load transient with HHC control

With such superior transient performance, you can reduce the output capacitance while still meeting a given voltage regulation requirement, allowing for a reduced bill of materials count and a smaller solution size.

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