LLC SWITCHING CONVERTERS FROM PRIMARY SIDE

[0001] This relates generally to LLC converters, and more particularly to driving LLC converters at their resonant frequency.

BACKGROUND

[0002] 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.

SUMMARY

[0003] Described examples include systems, methods and apparatus for determining a resonant frequency of an LLC converter via a primary side of the LLC converter. In one example, a circuit includes the LLC converter and a resonant frequency determination unit. The resonant frequency determination unit is configured to: monitor electrical current on the primary side; isolate a portion of the electrical current; based on the isolated portion of the electrical current, determine a crossing point; and based on the crossing point, determine a resonant frequency of the LLC converter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Embodiments of the invention are illustrated in the figures of the accompanying drawings in which:

[0005] FIG. 1 depicts an example LLC converter 100 including a resonant frequency determination unit 1 10, according to some embodiments; [0006] FIGS. 2 A and 2B are waveforms associated with an LLC converter, according to some embodiments;

[0007] FIG. 3 depicts the primary side of an example LLC converter including a current suppressor 312 and a detector 310, according to some embodiments;

[0008] FIG. 4 is a flow chart depicting example operations for determining a resonant frequency for an LLC converter by monitoring current on the primary side of the LLC converter, according to some embodiments.

DETAILED DESCRIPTION OF EXAMPLE EMB ODEVIENT S

[0009] As described hereinabove, 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.

[0010] 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 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. FIG. 1 provides an overview of an example LLC converter including a resonant frequency determination unit, while FIG. 3 provides additional information regarding an example resonant frequency determination unit.

[0011] FIG. 1 depicts an example LLC converter 100 including a resonant frequency determination unit 110, according to some embodiments. The LLC converter 100 has a primary side 102 and a secondary side 104. The resonant frequency determination unit 110 is located on the primary side 102. The resonant frequency determination unit 100 is able to determine the resonant frequency of the LLC converter 100 based on measurements taken at the primary side 102 of the LLC converter 100. More specifically, the resonant frequency determination unit 110 monitors the primary side current of the LLC converter 110 via a sensor 108. As previously discussed, the primary side current includes both a sinusoidal portion (i.e., the resonant current) and a ramp current (i.e., a magnetizing current caused by the transformer). The resonant frequency determination unit isolates the sinusoidal portion of the primary side current so that the resonant frequency can be detected. In one embodiment, the resonant frequency determination unit 110 isolates the sinusoidal portion of the primary side current by taking the second order derivative of the primary side current. The resonant frequency determination unit 110 analyzes the sinusoidal portion of the primary side current to determine the resonant frequency of the LLC converter 100. In some embodiments, the resonant frequency determination unit 110 determines the resonant frequency by detecting the point at which the sinusoidal portion crosses the magnetizing portion (i.e., the zero-crossing). The controller 106 receives resonant frequency information from the resonant frequency determination unit 110 and drives the LLC converter 100 at the resonant frequency.

[0012] While the discussion of FIG. 1 provides an overview of a circuit capable of determining a resonant frequency for an LLC converter via the primary side of the LLC converter, the discussion of FIGS. 2A - 2B provides additional information regarding the primary side current of an LLC converter.

[0013] FIG. 2A depicts a first waveform 202 and a second waveform 204. The first waveform 202 indicates the voltage across a first switch of an LLC converter, and the second waveform 204 indicates the voltage across a second switch of the LLC converter. As can be seen from FIG. 2A, the first switch is on during a first period 206 and the second switch is on during a second period 208. When an LLC converter is driven at its resonant frequency, switching from first period 202 to the second period 208 occurs at the point in time in which the sinusoidal portion of the primary side current crosses the magnetizing portion of the primary side current, as depicted in FIG. 2B.

[0014] FIG. 2B depicts a resonant current waveform 222 (i.e., the sinusoidal portion of the primary side current) and a magnetizing current wave form 224. The primary side current of an LLC converter is a composite of both the resonant current waveform 222 and the magnetizing current waveform 224. When operating at its resonant frequency, the LLC converter switches between the high side and low side switches on the primary side when the resonant current reaches the magnetizing current. This occurs when the resonant current waveform 222 and the magnetizing current waveform 224 intersect, as indicated by an intersection marking 226.

[0015] While the discussion of FIGS. 2A - 2B describes waveforms associated with an LLC converter, the discussion of FIG. 3 provides additional information regarding an example LLC converter including a resonant frequency determination unit.

[0016] FIG. 3 depicts the primary side 302 of an example LLC converter including a current suppressor 312 and a detector 310, according to some embodiments. The primary side 302 also includes half bridge switches 316 and a controller 306. The resonant frequency determination unit 314 monitors current on the primary side 302 (i.e., the primary side current) of the LLC converter and, based on the primary side current, determines the resonant frequency of the LLC converter.

[0017] The resonant frequency determination unit monitors the primary side current via a sensor 308. The current on the primary side 302 includes a sinusoidal portion (i.e., the resonant current) and a magnetizing portion 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 sinusoidal portion crosses the magnetizing portion of the primary side 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 a 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.

[0018] After isolating the sinusoidal portion of the primary side current, the resonant frequency determination unit 314 determines the crossing points of the sinusoidal portion and the magnetizing portion. In the example LLC converter, the resonant frequency determination unit 314 utilizes a detector 310 to determine the crossing points. In one embodiment, the detector 310 can be a comparator that compares the sinusoidal portion and the magnetizing portion to determine the crossing points. Based on the crossing points, the resonant frequency determination unit 314 determines the resonant frequency of the LLC converter.

[0019] 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 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.

[0020] While the discussion of FIG. 3 describes a specific example of a resonant frequency detection unit, the discussion of FIG. 4 provides example operations for determining the resonant frequency of an LLC converter via the primary side of the LLC converter.

[0021] FIG. 4 is a flow chart depicting example operations for determining a resonant frequency for an LLC converter by monitoring current on the primary side of the LLC converter, according to some embodiments. The flow begins at block 402.

[0022] At block 402, 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 resonant frequency determination unit monitors the current at a resonant capacitor. The flow continues at block 404.

[0023] At block 404, 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 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 406.

[0024] At block 406, a crossing point is determined. For example, the resonant frequency determination unit determines a crossing point between the sinusoidal portion of the current and the magnetizing portion of the current. In an LLC converter, the crossing point is indicative of the resonant frequency of the LLC converter. In some embodiments, the resonant frequency determination unit includes a detector that determines the crossing point. The flow continues at block 408.

[0025] At block 408, 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 crossing point. To drive the LLC converter at its resonate frequency, the half bridge switches should alternate at the point at which the sinusoidal portion is equal to the magnetizing current.