LLC Resonant Converter Topologies for the DC-DC Stage of OBC
This article presents popular LLC and LLC derived bidirectional converter topologies described in the literature.
Selection of dc-dc converter scheme for On-Board Charger (OBC) is based on efficiency, performance and power density targets, for which resonant converters are preferred choice. This article presents popular LLC and LLC derived bidirectional converter topologies described in the literature.
A typical OBC architecture, as in Figure 1.1, has a bidirectional front-end ac-dc stage followed by an isolated bidirectional dc-dc converter charging the high voltage battery. The designers must meet the performance, efficiency, and power density targets for the entire range of grid and battery voltages. For the ac-dc stage, Totem-pole PFC is the preferred solution. The charging algorithms are implemented in the dc-dc stage. The dc-dc is switched at high frequency and requires a topology with soft switching in both directions, even with the use of wide bandgap devices.
Figure 1.1: Typical OBC power train
The Phase Shifted Full Bridge  is a suitable topology but suffers with issues like limited Zero Voltage Switching (ZVS) range, loss of duty to get ZVS, snubbers for secondary devices etc. The Dual Active Bridge also operates with ZVS but has best performance for fixed output. For high power, resonant converters are preferred as they offer soft switching in all devices even at high frequency with low EMI.
Low component count, utilization of transformer leakage inductance for resonance and absence of snubber/clamp circuitries are other added advantages. FETs based rectifier makes converter bidirectional. This article describes LLC and LLC derived topologies for dc-dc and describes OBC design challenges with these converters.
Resonant DC-DC Converter for Bi-OBC
The specifications for dc-dc stage of a typical 6.6kW OBC is shown in Table 2.1. The design is for the highest power and the current and thermal stress are determined for charging mode. Note that the efficiency requirement is stringent in both modes.
|Battery Charging Module|
|Input Voltage||400V||With 40Vpk-pk ripple at double
|Output voltage nominal||330V|
|Output voltage range||200V - 450V|
|Output current max||20A||For 330V and below|
|Input voltage nominal||330V|
|Input voltage range||200V – 450V|
|Output voltage||400V||Input to grid tie inverter stage|
|Output power max||3.3kW|
|Efficiency target||>98%||For overall efficiency of 96%|
Table 2.1: Specifications of dc-dc stage of OBC
LLC Resonant Converter
An LLC power stage is sown in Figure 2.1. The circuit has two full bridge circuits separated by an isolation transformer. The transformer ratio is set for nominal operating voltage. The resonant tank gain is function of resonant elements (Lm, Lr and Cr), load and switching frequency.
Figure 2.1: LLC converter power stage
LLC converter design procedure is not direct and finalizing the optimum resonant tank component values will need some iterations. The design steps are summarized below
1. Set transformer turns ratio (N) based on nominal operating input and output voltages (400V input and 330V output)
2. Determine the maximum and minimum gain requirements from converter parameters in Table 2.1. The maximum gain is evaluated with maximum output voltage and minimum input (which is the minimum voltage considering the line frequency ripple content in PFC output). Similarly, peak of input voltage is to be used for minimum gain calculation
3. Calculate the switching frequency ranges. This will be an iterative process and will need tuning of tank parameters Q (quality factor) and M (ratio of Lm to Lr)
Set a resonance frequency value. High frequency is preferred to reduce the size of transformer. Also, output filter capacitance and resonant capacitance value reduce with frequency. However, the transformer and FET turn OFF losses must be monitored while deciding the frequency.
Determine the maximum Lm value at resonance which is required to discharge Coss of FETs and aid in ZVS turn ON of primary devices
Set a value for M to start with. A high value of M indicates high magnetizing inductance and low circulating energy but the gain achievable is limited. For low value of M, high gains can be achieved at narrow frequency ranges. The resultant magnetizing inductance is less and associated circulating current and losses are high. A value between 6 to 10 is good enough to start with .
Select Q based on the maximum gain requirement at full load. If the gain is not enough, then the M value must be reduced. The gain range should be achieved for entire load range or Q range.
The frequency range for associated gains should be small and the minimum frequency should have low impact on the magnetics size and losses. Reiterate design of Q and M to meet the gain and frequency range criteria
4. With value of M and Q, the values of Lr, Cr and Lm are finalized.
The LLC converter has bidirectional power flow capability. But in discharging mode, the magnetizing inductance directly appears across the battery followed by the Lr and Cr which yields an series resonant converter type configuration. The gain curves for a LLC in charging and discharging curve from  is shown in Figure 2.2. The discharging curves show no voltage gain from converter and will result in unregulated output. In , LLC is switched at resonant frequency in discharging mode and additional boost converter stage after LLC regulates input to PFC stage. The boost stage is bypassed with a relay in battery charging mode. This method however adds to component cost and system size.
Figure 2.2: LLC charging and discharging mode gain curves
CLLLC Resonant Converter
A bidirectional CLLLC resonant converter with 5 resonant elements is shown in Figure 2.3. The resonant tank is symmetrical, and the converter has approximately similar gain curves for charging and discharging mode.
Figure 2.3: CLLLC converter power stage
The design method for CLLLC power stage is similar to the LLC converter. The resonant elements in secondary are all referred to primary and the equivalent circuit yields the transfer function. To simplify design step, reflected Lrs is assumed same as Lrp and a ratio of reflected Crs to Crp is set. The equivalent M and Q values are tuned to meet the gain and frequency range criteria in both modes. When deciding the M value, it is ensured that the gain curves are monotonically decreasing without multiple peaks to enable linear control over the entire operating frequency range.
The CLLC resonant converter is derived from CLLLC wherein the secondary side resonant inductance is eliminated. The Crs is however required to tune the gain curves for discharging mode. If the transformer leakage inductance is also to be made use of, then equivalent configuration becomes CLLLC type. Design example and experimental results with CLLLC for 3.5kW OBC is presented in .
CLLLC with Variable DC Link Voltage
The frequency variation for output regulation deviates the converter from resonance, the point for which the converter is optimized. To keep the frequency swing to minimum, the dc bus voltage is varied based on output voltage required. The transformer ratio is adjusted such that at minimum output voltage corresponds to 400V dc bus and then dc link is varied linearly as per set output reference. The design in  presents the gain curves as in Figure 2.4 shows significant reduction in frequency range.
Figure 2.4: Gain curves for fixed dc bus and variable dc bus CLLLC
Resonant converter is undoubtedly the preferred choice for dc-dc conversion for OBC. With modern day wide bandgap devices, designers can easily target the high efficiency at high frequencies. Popular resonant converter configurations based on LLC converter are described in the article. The design methods in literature to suit bidirectional OBC specifications are presented.
About the Authors
Milind Dighrasker holds a Master's Degree in Power & Control at IIT Kanpur and a Bachelor's Degree in Electrical Engineering at Government Engineering College Bilaspur. He co-founded Enstin lab Pvt Ltd, a Design services company. He currently works there as the CTO responsible for leading and enabling a highly customer-focused technology team to develop innovative and energy-efficient products.
Raj (Thiagarajan) Venkatachalam holds a Master's Degree in Power and Applied Electronics at the Indian Institute of Science and a Bachelor's Degree in Industrial Electronics at PSG College of Technology. He specializes in leading technically qualified teams through all phases of development from technical requirements elicitation to product roll-out, including cost analysis and planning. He is currently the CEO and Co-Founder of Enstin Labs Pvt. Ltd. since July 2017.
Vishwas Kedlaya holds a Master's Degree in Industrial Electronics at Sri Jayachamarajendra College of Engineering and a BTech in Electrical and Electronics Engineering at NMAM Institute of Technology. He currently works as the Technical Lead at Enstin Labs Pvt. Ltd. since January 2019.
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- Li, Hongbin et al. “A 300-kHz 6.6-kW SiC Bidirectional LLC Onboard Charger.” IEEE Transactions on Industrial Electronics 67 (2020): 1435-1445.
- Application note AN2012-09, “Resonant LLC Converter: Operation and Design 250W 33Vin 400Vout Design Example”, Infineon Technologies
This article originally appeared in the Bodo’s PowerSystems magazine.