The efficiency of an LLC converter power stage drops rapidly with falling output power. To maintain reasonable light load efficiency it is necessary to operate the LLC converter in burst mode. In this mode the LLC converter operates at relatively high power for a short burst period and then switching is stopped for a time period. During the burst period excess charge is transferred to and stored in the output capacitor. During the burst off period this stored charge is used to supply the load current. The burst mode operation is critical to meet strict standby power consumption requirements. But a challenge with burst mode operation is the audible noise performance. Due to the need to stop switching for certain time, there will be a pattern of switching pulses with frequencies that fall within the audible frequency range from 20 Hz to 20 kHz. Some device variants of UCC25640x includes a burst mode with soft on and soft off periods at the first few and last few switching pulses, minimizing the audible noise during standby operation.
Figure 7-17 describes the timing diagram of the burst mode operation. Two burst mode thresholds are used (BMTH for burst mode exit and BMTL for burst mode entry). The details of how to program these two thresholds are described in Section 188.8.131.52 section. The FBreplica from the FB chain is compared with the two thresholds, and determines the burst mode operation.
|Burst Soft On and Soft Off Step||Fraction used for Reduced Vcomp During Soft On|
The HHC control makes the implementation of the burst soft on and soft off very straight forward. Due to the frequency control portion in HHC, changing the Vcomp can directly impact the LLC switching frequency to control the resonant current magnitude within switching cycles. Also the last pulse of each burst on period is turned off when the VCR node voltage equals the common mode voltage VCM. In HHC control, this is approximately equivalent to resonant capacitor voltage equal to VIN/2. This operation keeps the resonant capacitor voltage to approximate VIN/2 for each burst off period, thus enabling the burst pattern to settle as soon as possible during the burst on period.
As described in Section 184.108.40.206, in order to avoid nuisance ZCS detection at light load condition, ZCS is disabled at light load condition. This is achieved by using the BMTH threshold. When FBreplica is below BMTH, indicating a light load operating condition, ZCS detection is disabled. When FBreplica is above BMTH, ZCS detection will resume.
Some device variants disables the burst soft on and soft off feature, for applications that have less concern about audible noise, but want smaller output ripple during burst mode. A smaller burst packet size is also used. The timing diagram of the burst mode operation for when burst soft on and soft off is disabled is described in Figure 7-18.
UCC25640x can also disable the burst mode by changing the BW pin resistance, as described in Section 220.127.116.11. When burst mode is disabled, the switching does not stop even when FBreplica becomes lower than BMTL. The pick higher block still works by picking the higher value of BMTL and FBreplica for the control effort Vcomp. In this case, BMTL will limit the maximum switching frequency of LLC.
When burst mode is disabled, FBreplica needs to be higher than BMTL at steady state. Otherwise, LLC output voltage will lose regulation as it continues deliver more energy than what is demanded from the feedback. For a transient period, it is ok for FBreplica to be lower than BMTL.