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Energy Suggestions #117: Measure your LLC resonant tank earlier than testing at full working circumstances

Half-bridge sequence resonant converters obtain excessive efficiencies and excessive energy densities for converters over 100 W. The commonest resonant topology (Determine 1) is a resonant tank comprised of a sequence magnetizing inductor; resonant inductor; and a capacitor (abbreviated as LLC). The collection of parameter values determines the form of the resonant tank’s acquire curve, which impacts how the resonant converter performs in a system.

Determine 1 A half-bridge LLC power-stage with cut up resonant capacitors, the collection of parameter values determines the form of the resonant tank’s acquire curve which must be verified earlier than making use of power to the circuit. Supply: Texas Devices

After getting settled on a set of parameters and chosen the parts, it is rather necessary to confirm the acquire curve earlier than making use of power to the circuit. On this energy tip I’ll describe a method for measuring a resonant tank’s acquire curve and find out how to interpret the outcomes, together with examples that present each the strengths and limitations of this system.

A frequency response analyzer injects a small AC sign to any circuit, then measures the voltages at two factors within the system to find out the sign acquire and part delay throughout a decided frequency vary. Whereas this tools is mostly used for testing a management loop, you too can use a frequency response analyzer to measure the acquire of the facility stage of an LLC converter. Determine 2 reveals the wiring diagram for such a measurement.

 Determine 2 Wiring diagram for connecting a resonant tank to a frequency response analyzer to recreate acquire curve plots. The acquire plot of the facility stage might be obtained by displaying the magnitude of channel 2’s voltage divided by channel 1’s voltage. Supply: Texas Devices

The half-bridge LLC has a pair of resonant capacitors, with one tied to the enter voltage and the opposite tied to major floor. To run the check on this circuit, the resonant capacitors have to be in parallel with one another and in sequence with the first winding. The injection sign and channel 1 measurement of the analyzer join throughout the primary-side parts from the switching node of the half bridge to the opposite finish of the resonant capacitors. The secondary channel of the analyzer, channel 2, connects throughout the secondary winding, with a resistor added to approximate the loading circumstances. After sweeping the frequency of the injected AC sign, you may plot the acquire of the facility stage by displaying the magnitude of channel 2’s voltage divided by channel 1’s voltage. Determine 3 reveals an instance check outcome.

Determine 3 A pattern LLC resonant tank acquire curve measurement that may be noticed from the check setup proven in Determine 2. Supply: Texas Devices

You possibly can convert the power-stage acquire to a voltage acquire relying on the transformer turns ratio and the configuration of switches and windings on the first and secondary sides of the facility stage. A half-bridge LLC energy stage is usually proven with a center-tapped secondary winding and two output rectifiers. On this instance, the output voltage is roughly the product of the enter voltage, turns ratio, and acquire of the resonant tank on the working frequency. Different choices for secondary configurations drawn in Determine 4 allow the resonant tank to transform to larger output voltages. Word that if the first facet is configured with a full bridge, you will want to multiply these ratios by an element of two.

Determine 4 Configuring the secondary-side with a full-wave rectifier doubles the transferred power (a); a double-ended secondary configuration achieves a fourfold voltage acquire (b). Supply: Texas Devices

A profit to this system is which you can take the measurement immediately on the PCB and account for power-stage parasitic parts within the check outcomes. The TI E2E™ design help boards article, “Why is Your LLC Resonant Converter Frequency Method, Method Off,” makes use of an alternate mannequin to elucidate how the development of the transformer introduces extra inductances into the circuit (Determine 5). You possibly can design round these inherent parasitic parts or combine them into your design. For instance, you should use the leakage inductance because the resonant inductor, which saves price and improves effectivity by eradicating a bodily element from the design. It’s doable to simplify the optimization of a resonant tank design with this technique through the use of this fast check.

Determine 5 Transformer mannequin utilizing leakage inductance as a resonant component for an LLC converter, permitting designers to both design round inherent parasitic parts or combine them into the design. Supply: Texas Devices

Utilizing synchronous rectifiers on the secondary will additional enhance the LLC converter effectivity. Doing so will decrease conduction losses, which are likely to dominate the entire loss attribute for that element; nonetheless, the selection of MOSFETs can change the form of the acquire curve. Decrease-resistance MOSFETs could have a bigger output capacitance. The turns ratio of the transformer can amplify this capacitance, which might then turn into a difficulty in some circumstances. As I discussed, testing the acquire curve in circuit helps account for extra parasitic parts all through the facility stage. Determine 6 highlights the consequences of MOSFET output capacitance that will have gone unnoticed within the preliminary resonant tank design.

Determine 6 On this design, parasitic capacitance added a resonance at round 300 kHz that was not supposed to be there within the design part. Supply: Texas Devices

Nonetheless, utilizing a frequency response analyzer can not account for all design parasitic parts. For instance, the measurement is not going to present the consequences of secondary windings that aren’t well-coupled to one another on a center-tap construction. Free couplings between the first and secondary windings will kind leakage inductance, and that is helpful to an extent in LLC designs. Secondary windings that aren’t well-coupled to one another will lower the efficiency of the facility stage, nonetheless. It isn’t doable to look at the consequences of this in AC evaluation, however will probably be obvious when monitoring the secondary winding voltages.

For instance, the design in Determine 7 had an accurate acquire curve. However when trying on the voltage throughout the secondary windings, the extent began out larger and drooped to a stage decrease than the output voltage. Ideally, these voltage waveforms ought to look extra like a sq. wave. Free coupling additionally creates a big leakage spike on the secondary rectifier turnoff edge. Because the load will increase, the distorted results of the secondaries loosely coupled to one another turn into extra noticeable and restrict the doable output. energy.

Determine 7 The unfastened coupling in a transformer design is noticeable in switching waveforms however was not obvious in its acquire curve. Supply: Texas Devices

Even after reconfiguring this transformer design to have the secondary windings higher coupled to one another, the ensuing resonant inductance and magnetizing inductance remained the identical. There was no visible distinction within the acquire curve measurements, as anticipated. However the switching waveforms in Determine 8 illustrated a noticeable enchancment with the brand new design.

Determine 8 An improved transformer design with higher coupling mitigated voltage droop whereas sustaining the acquire curve form. Supply: Texas Devices

With the secondary windings reconfigured, the switching waveforms look nearer to expectations; the waveform is extra square-shaped, with the blocking voltage equal to the output voltage. The leakage spike by the turnoff edge was additionally eradicated.

The 2 transformer designs have been successfully the identical, requiring no extra parts. But the adjustments had a big influence on total effectivity.

When designing a resonant converter, begin your analysis by validating the acquire curve of the resonant tank. Whereas it can not detect all faults, you’ll get some perception into the achievable acquire, in addition to the anticipated working frequency vary.

John Dorosa is a Methods Engineer in Texas Instrument’s Energy Design Providers workforce centered on industrial functions and has developed over 100 distinctive SMPS reference design boards to fulfill customized energy necessities.





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