Optimizing a Synchronous Rectification Scheme in CLLLC
Abstract
This paper analyzes the problem with a synchronous rectification scheme, that is, when the secondary rectifier switch and the primary active switch are turned on at the same time, the problem of turning on rectifier switch in advance occurs when the switching frequency is greater than the resonant frequency.
This paper proposes that the formula for time domain analysis can be used to calculate the delay of the turn-on time of the rectifier switch relative to the turn-on time of the switch in primary side, so as to confirm the normal opening of the rectifier switch.
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1 Background
In energy storage systems (ESS), bidirectional isolated DC/DC is usually used to charge and discharge batteries.
Among the different isolated bidirectional DC/DC topologies, LLC resonant converters can easily implement soft switching over the full-load range, offering significant advantages in high-efficiency applications. The CLLLC topology is the basis of the LLC topology, which adds a resonant tank on the secondary side to enable symmetric gain in both forward and reverse mode.
In traditional bidirectional CLLLC resonant converters, as shown in Figure 1-1, the body diodes of the MOSFET form an uncontrolled rectification network. Compared to fast recovery diodes, body diodes can seriously reduce efficiency due to the larger reverse recovery current, longer reverse recovery time, higher voltage drop, and lower rated on-current.
The most effective way to solve this problem is to use synchronous rectification (SR) technology. By replacing the body diode with a MOSFET in a rectified network, the current now flows through the MOSFET channel. The MOSFET has a small on-resistance which effectively reduces rectified conduction losses.
Figure 1-1 CLLLC Topology in ESSTraditional synchronous rectification strategies can be divided into the following categories:
- Current type. Current sensors use current type to obtain a voltage in phase with the rectifier current as the synchronous rectification driving signal. The current-type synchronous rectification strategy can be used in most topologies and is simple to implement, but requires one or more current sensors, increasing the cost and volume.
- Voltage type. By detecting the drain-source voltage of the rectifier switch tube as a reference signal for synchronous rectification drive switches, this strategy has been applied to many commercial synchronous rectification chips, such as UCC24624. The voltage-based synchronous rectification strategy does not require an additional current sensor, but the detection of the drain-source voltage signal is affected by the parasitic inductance of the switch package and the inductance on the sensing path.