Energy Ideas # 141: Ideas and tips for reaching extensive working ranges with LLC resonant converters

May 27, 2025

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Energy Ideas # 141: Ideas and tips for reaching extensive working ranges with LLC resonant converters

Inductor-inductor-capacitor (LLC) resonant converters have a few interesting traits for purposes requiring an remoted DC/DC converter akin to minimal switching losses, no reverse restoration when working under the resonant frequency and the power to tolerate giant leakage inductance throughout the transformer.

The problem

A major problem when designing an LLC converter with a large working vary is the conduct of the achieve curve with respect to equal load resistance. It is because as the standard issue (Qe), will increase, the utmost attainable achieve decreases Conversely, the minimal attainable achieve will increase as Qe decreases. That is proven in Determine 1 under.

Determine 1 LLC achieve curves exhibiting that, as Qe will increase, the utmost attainable achieve decreases. Supply: Texas Devices

This conduct makes it tough to keep up cheap root-mean-square (RMS) currents within the energy stage and an affordable switching frequency vary. The inductance ratio (Ln) must be decreased to scale back the required frequency vary; nonetheless, a decrease inductance ratio will increase the magnetizing present within the energy stage. This text will talk about 5 suggestions for designing an LLC converter with a large working vary.

Utilizing a reconfigurable rectifier

One potential technique to prolong an LLC converter’s working vary is to implement a reconfigurable rectifier, as proven in Determine 2.

Determine 2 reveals an LLC converter with a reconfigurable rectifier, which may be reconfigured as both a full-bridge or a voltage-doubler. Supply: Texas Devices

On this construction, you’ll be able to configure the rectifier as a full-bridge or a voltage-doubler rectifier by utilizing a comparator to have a look at the output voltage to determine the mode of operation. When working as a full-bridge rectifier, Equation 3 calculates the input-to-output switch perform.

When working as a voltage doubler rectifier, the input-to-output switch perform is:

Determine 3 reveals the switching frequency versus output voltage for an LLC utilizing the above strategy to attain a 140-V to 420-V output voltage vary from a set 450-V enter. This information is collected with an 800-mA load on the output. Discover the soar at 200 V the place the comparator switches from full-bridge to voltage-doubler mode.

Determine 3 Switching frequency versus output voltage in LED driver reference design. Supply: Texas Devices

Minimizing winding and rectifier capacitance

If the working level drops under the minimal achieve curve, the LLC controller is compelled to function in burst mode to maintain the output voltage in regulation. Burst mode ends in larger low-frequency output ripple voltage. This can be a concern for purposes requiring very low output ripple at gentle load and on the minimal output voltage.

In such circumstances, the winding capacitance throughout the transformer and the output capacitance (Coss) or junction capacitance (Cj) of the rectifiers have to be minimized. These parasitic capacitances will trigger the achieve curve to invert when working above the resonant frequency. Figure 4 reveals the standard first harmonic approximation (FHA) calculation of an LLC achieve curve at gentle load and the identical LLC achieve curve when accounting for winding the capacitance and Coss of the rectifiers used within the energy stage.

Determine 4 The impression of parasitic capacitance on the LLC achieve curve at gentle load. Supply: Texas Devices

Cautious consideration to the winding stackup throughout the transformer and number of the rectifier elements minimizes this achieve curve inversion impact. Utilizing extensive bandgap units akin to SIC diodes or GaN excessive electron mobility transistors (HEMTs) because the rectifier can lead to significantly decrease Coss in comparison with Si MOSFETs or diodes.

Utilizing LLC controllers with a high-frequency skip mode

A high-frequency skip mode can obtain a decrease achieve in contrast to what’s achievable with regular switching. Under is an instance from a 100-W half-bridge LLC converter with an enter vary of 70 V to 450 V. In Determine 5, the resonant present is proven in inexperienced, and the first facet swap node is proven in blue.

On the appropriate facet, the LLC converter is working in a high-frequency skip mode, omitting each fourth switching cycle. The switching frequency is 260 kHz, however it’s sub-modulated at a 77 kHz burst frequency.

Determine 5 The 100-W LLC converter switching conduct at 70V and 450V inputs with resonant present in inexperienced and the first facet swap node in blue. Supply: Texas Devices

Managing auxiliary bias voltages

Producing the mandatory bias voltages for the first and secondary sides of the ability provide may be completed by together with auxiliary windings on the LLC transformer. For LLC converters with a variable output voltage, the auxiliary winding voltages will change because the output voltage adjustments. That is very true for LLC transformers utilizing sectioned bobbins the place the auxiliary windings have poor coupling to the secondary windings. When utilizing a easy low-dropout regulator (LDO) construction to control the bias voltage, the effectivity will drop because the output voltage will increase. It could require a bigger bodily bundle to deal with the ability dissipation.

In Determine 6, N_aux1 and N_aux2 are sized in order that on the lowest output voltage, or the VCC bias voltage, is supplied by D1, Q1, and D4. Because the output voltage will increase, the voltage on C2 is proscribed to the breakdown voltage of Zener D3 minus the gate-source threshold voltage of Q1. Because the output voltage is elevated additional, the voltage generated by N_aux2 turns into excessive sufficient to provide VCC, and Q1 is compelled off because the gate-source voltage decreases under the turn-off threshold.

Determine 6 Utilizing auxiliary windings together with an LDO construction to generate the mandatory bias voltages for the first and secondary facet of the ability provide. Supply: Texas Devices

This strategy is extra environment friendly than a single winding + LDO however requires two aux windings. An alternate strategy that requires just one aux winding is to make use of a buck converter or increase converter as a substitute of an LDO.

Managing trickle-charging for deeply discharged batteries

LLC converters used as battery chargers should safely recuperate deeply discharged batteries by making use of a small charging present till the battery pack voltage is excessive sufficient to soundly take the complete charging present. LLCs can’t regulate right down to a 0-V output with a small output present and subsequently battle to fulfill this requirement.

This may be managed by together with a small fixed present circuit with a bypass FET in parallel, as proven in Determine 7. When in trickle-charge mode, the bypass FET turns off, and the output present is equipped by LM317 configured to control the output present. This enables the minimal output voltage of the LLC converter to be higher than 0 V, even with an output voltage of 0 V. This strategy permits the LLC transformer to generate the mandatory bias voltages on the first and secondary facet and keep away from needing a separate bias provide when the output voltage is 0 V. As soon as the battery pack voltage has risen to a high-enough degree, a FET with a discrete charge-pump circuit bypasses the constant-current circuit.

Determine 7 LLC with a trickle charging circuit that may safely recuperate deeply discharged batteries. Supply: Texas Devices

Broad LLC operation

Whereas reaching a large working vary with an LLC converter might look tough due the character of the LLC topology, a number of methods exist for acquiring a large working vary simpler to attain. The 5 easy suggestions and tips listed below are analog-control pleasant and don’t require extra complicated, digital-control implementations.

Ben Lough is a Techniques Engineer in Texas Instrument’s Energy Design Companies workforce targeted on energy issue correction and remoted DC/DC conversion. He acquired his MS in Electrical and Laptop Engineering from Ohio State College in 2016 and BS in ECE from Ohio State in 2015.