Implementing resonant mode using digital control

The LLC resonant power stage has become an extremely popular power topology in the industry for high power applications that require an efficient power conversion up to and around a kilowatt. Resonant mode is a best fit choice as the second stage of the AC-DC converter as it is typically preceded by the boost Power Factor Correction (PFC) stage that provides a stable high voltage input.

In traditional Pulse Width Modulation (PWM) duty cycle controlled power converters the switching frequency is fixed while the duty cycle varies to maintain a regulated output voltage. In resonant power conversion the opposite occurs—duty cycle is fixed to 50% (minus deadtime) the frequency is allowed to vary.

Some of the advantages of the series resonant converter are:
 • High efficiency
 • ZVS/ZCS (Zero Voltage/current switching) of primary/secondary switches (depending upon operating mode)
 • Increased power density
 • Low noise and EMI due to ZCS
 • No transformer saturation due to DC current block by resonant capacitor
 • Good cross regulation between multiple outputs
 • Flatter efficiency curve across the load range

However, there are some drawbacks that complicate the operation of the converter are:
 • Soft start and light load operation
 • Synchronous rectification drive
 • Short circuit behaviour
 • Difficult to design under a wide input operating range.

This article shows how the digital implementation of the resonant topology using an advanced digital controller (figure 1) makes the control, design, and optimisation an easy task and solves the common problems described above. The topology under consideration is the half bridge LLC resonant converter with MOSFETs as switches. However, the same concepts apply to the full bridge SRC (Series Resonant Converter) or even the LCC resonant topology.

Resonant mode basics
Resonance can be defined as the excitation (or stimulus) that provides a maximum transfer of energy from one part (or element) of the system to another. In an ideal (lossless) system, this transfer of energy can continue indefinitely. Resonance occurs in several places in nature and such as electrical, mechanical, acoustical etc.

Figure 2 to figure 5 show an LCR circuit operating above and below resonance. It can be seen that only when the operating frequency is at resonance we can get maximum current flowing in the circuit. Another observation is when the excitation frequency (f_S) is greater than the resonant frequency (f_RES) the input current lags the input square voltage waveform which can be approximated by its first harmonic or fundamental by a sine. The reverse happens when f_S < f_RES , and the voltages and currents are in phase when f_S= f_RES. Due to the nature of the plots, we can conclude that when f_S > f_RES the voltage drops to zero before the current waveform then we can achieve ZVS turn-on of a switch (see figure 6). The opposite is true for f_S < f_RES.