This invention relates generally to LLC converters and, more particularly, to driving LLC converters at their resonant frequency.
LLC converters are most efficient when operating at their resonant frequency. Small deviations of the switching frequency of an LLC converter can have significant impacts on its efficiency. Consequently, maintaining operation of an LLC converter at, or close to, its resonant frequency is important in maintaining an efficient system. Current approaches to driving an LLC converter at its resonant frequency focus on monitoring the secondary side of the LLC converter to determine its resonant frequency. The information gathered at the secondary side is then provided to a driving mechanism for the LLC converter. The driving mechanism is on the primary side of the LLC converter. Consequently, monitoring an LLC converter from the secondary side introduces additional complexities (e.g., an increased number of components) and costs.
Embodiments of the invention are illustrated in the figures of the accompanying drawings in which:
As previously discussed, maintaining operation of an LLC converter at its resonant frequency helps maximize the efficiency of the LLC converter. Current approaches rely on measurements taken on the secondary side of the LLC converter. Typically, these systems sense zero crossings of the current flowing through the secondary side of the rectifier. These approaches measure current on the secondary side because the current on the secondary side is isolated from the magnetizing current from the transformer. That is, the current can be monitored without having to account for the magnetizing current. While this allows for a direct measurement of current (i.e., the current is isolated from the magnetizing current), it introduces additional costs and complexities. One reason that measuring current on the secondary side of the LLC converter introduces additional costs and complexities is due to the fact that information must be transmitted back to the primary side of the LLC converter through an isolation barrier. The additional componentry necessary to facilitate this transmission increases the complexity of the circuit, and thus, the cost of the circuit.
Embodiments of the inventive subject matter allow an LLC converter to be driven at its resonant frequency without measuring current at the secondary side of the LLC converter. Put simply, embodiments of the inventive subject matter obviate the need for the additional componentry, and cost, associated with monitoring information on the secondary side of the LLC converter and transmitting that information to the primary side of the LLC converter. Instead of monitoring the secondary side of the LLC converter, embodiments of the inventive subject matter monitor current on the primary side of the LLC converter. However, as discussed above, the current on the primary side includes both a sinusoidal portion and a magnetizing current produced by the transformer. Because the current on the primary side of the LLC converter includes both a sinusoidal portion and a magnetizing current, the sinusoidal portion of the current is isolated from the magnetizing current. In some embodiments, the sinusoidal portion of the current is isolated from the magnetizing current by performing mathematical calculations (e.g., calculating the second order derivative) on the current. Once the sinusoidal portion of the current is isolated, zero crossings of the sinusoidal portion of the current can be detected and the half bridge switches can be driven based on the zero crossings. In some embodiments, a resonant frequency determination unit is utilized to monitor current on the primary side of an LLC converter and determine a resonant frequency for the LLC converter.
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The resonant frequency determination unit monitors the primary side current via a sensor 308. The current on the primary side 302 (i.e., the resonant current) includes a sinusoidal portion and a magnetizing portion (i.e., the magnetizing current) caused by the transformer. To drive the LLC converter at its resonant frequency, the controller 306 should switch the half bridge switches 316 when the primary side current crosses the magnetizing current. To find this crossing point, the resonant frequency determination unit 314 first isolates the sinusoidal portion from the magnetizing portion. In the example LLC converter, the resonant frequency determination unit 314 utilizes the magnetizing current suppressor 312 to isolate the sinusoidal portion from the magnetizing portion. In one embodiment, the current suppressor 312 isolates the sinusoidal portion from the magnetizing portion by taking the second order derivative of the primary side current.
After isolating the sinusoidal portion of the primary side current, the resonant frequency determination unit 314 determines the intersections of the primary side current and the magnetizing portion. In the example LLC converter, the resonant frequency determination unit 314 utilizes a detector 310 to determine the intersections. As previously discussed, the zero-crossings of the sinusoidal portion correspond to the intersections of the primary side current and the magnetizing current. Consequently, in one embodiment, the detector 310 can be a comparator that detects the zero-crossings of the sinusoidal portion to determine the intersections of the primary side current and the magnetizing current. Based on the zero-crossings of the sinusoidal portion, the resonant frequency determination unit 314 determines the resonant frequency of the LLC converter.
In some embodiments, the resonant frequency determination unit 314 determines the resonant frequency of the LLC converter on a cycle-by-cycle basis. In such embodiments, the resonant frequency determination unit 314 can adapt to changes in the LLC converter to ensure that the controller 306 continues driving the half bridge switches 316 at the resonant frequency.
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At block 502, current is monitored. For example, a resonant frequency determination unit can monitor the current on the primary side of an LLC converter. The resonant frequency determination unit can monitor the current via a sensor. The sensor can be any type suitable for measuring or monitoring current, such as an ammeter. The flow continues at block 504.
At block 504, a portion of the current is isolated. For example, the resonant frequency determination unit can isolate a portion of the current. In an LLC converter, the monitored current (i.e., the primary side current or resonant current) is the combination of a sinusoidal portion associated with the half bridge switches and a magnetizing portion associated with the transformer. In some embodiments, the resonant frequency determination unit isolates the sinusoidal portion of the current. As one example, the resonant frequency determination unit can isolate the sinusoidal portion of the current by determining the second order derivative of the monitored current. In some embodiments, the resonant frequency determination unit includes a current suppressor that isolates the sinusoidal portion of the current. The flow continues at block 506.
At block 506, zero-crossings are determined. For example, the resonant frequency determination unit determines zero-crossings of the sinusoidal portion. As previously discussed, the zero-crossings of the sinusoidal portion correspond to the intersections of the primary side current and the magnetizing current. In an LLC converter, the zero-crossings are indicative of the resonant frequency of the LLC converter. In some embodiments, the resonant frequency determination unit includes a detector that determines the zero-crossings. The flow continues at block 508.
At block 508, a resonant frequency is determined. For example, the resonant frequency determination unit can determine the resonant frequency. In an LLC converter, the resonant frequency determination unit determines the resonant frequency based on the zero-crossings of the sinusoidal portion. To drive the LLC converter at its resonant frequency, the half bridge switches should alternate at the zero-crossings.