A recent McKinsey mobility survey shows that automobile owners are prioritizing battery range, charging speeds, and reliability when considering an electric vehicle (EV). Automakers are responding to these consumer preferences by developing more resilient power systems with higher power density and advanced battery management systems (BMS) that maximize space while improving performance.

Regardless of manufacturer or vehicle type, EV architecture development prompts digital technology innovations. Yet tried-and-true analog technologies such as integrated magnetics offer measurable benefits, with inductor-inductor-capacitors (LLCs) providing stable voltage regulation and a consistent response to the load changes needed for EV charging.

LLC resonant circuits operating within switched-mode DC/DC power converters deliver wide output voltage control, soft switching in the primary, low voltage in the secondary, and slight changes in switching frequency—all requirements for EVs.

Because resonant converters have soft switching capabilities and can handle high voltages with nearly 98% efficiency, these devices can rapidly charge EVs while minimizing energy losses. Compact LLC resonant converter modules enable easy scalability and adaptability for different voltage requirements.

LLC resonant circuit

As Figure 1 shows, LLC resonant converters include MOSFET power switches (S₁ and S₂), a resonant tank circuit, a high-frequency transformer, and a rectifier. S₁ and S₂ convert an input DC voltage into a high-frequency square wave.

Figure 1 An LLC resonant half-bridge converter with power switches S₁ and S₂, a resonant tank circuit, a high-frequency transformer, and a rectifier. Source: Texas Instruments

The resonant tank circuit consists of a resonant capacitor (C_r), a resonant inductor (L_r) in series with the capacitor and transformer (T₁), and a magnetizing inductor (L_M) in parallel with the capacitor and transformer. Using two inductors allows the tank circuit to respond to a broad range of loads and to establish stable control over the entire load range.

Oscillating at the resonant frequency (f_R), the resonant tank circuit eliminates square-wave harmonics and outputs a sine wave of the fundamental switching frequency to the input of T₁. Operating the circuit at a switching frequency at or near f_R causes the resonant current to discharge or charge the capacitance just before the power switch changes state.

By shaping the current waveform, the resonant tank circuit causes S₁ and S₂ to turn on at 0 V (zero voltage switching) and turn off at 0 A (zero current switching). The resultant soft switching increases efficiency, decreases energy losses, reduces stress on power systems, and eliminates voltage and current spikes that cause electromagnetic interference (EMI). Soft switching also enables LLC resonant converters to handle a wide range of input and output voltages.

T₁ provides input/output isolation. Electrically isolating the input and output circuits prevents ground loops and minimizes interference. Isolation also prevents voltage fluctuations or transients from propagating and allowing voltage variations. After T1 scales the voltage up or down, the rectifier (D1, D2, and CO) converts the sine wave into a stable DC output.

How LLC solutions support high power density

LLC resonant converters support the growing demand for higher-power-density solutions. Since these converters operate at high switching frequencies while maintaining high efficiency, designers can integrate smaller and lighter transformers and inductors into the LLC package.

Integrating L_r and T₁ into a single magnetic unit increases the converter’s power density and circuit efficiency. For EV designers, the size, weight, and cost savings gained make it possible to incorporate more functionality into limited spaces. Optimizing the T₁ winding and core structure allows the converter to operate within thermal limits.

Strategically and selectively integrating protection features and intelligent control capabilities into analog controllers reduces system complexity while maintaining performance. Using LLC converters allows manufacturers to move beyond the basics toward adaptive power systems and advanced control methods.

Input power proportional control (IPPC) represents a growing focus on the intelligent power management available through LLC resonant circuits.

As shown in Figure 2, IPPC widens the control range of an LLC converter by modulating the switching frequency and comparing the input power to a control signal. By regulating the output voltage and current, the feedback loop directly controls the converter’s input power.

Figure 2 Simplified application schematic for the Texas Instruments UCC25661-Q1 LLC controller implementing the IPPC scheme. Source: Texas Instruments

With the control signal proportional to the input power, the signal becomes limited in range and limits the converter’s power output. As a result, the control signal works as a load monitor regardless of any variations in the resonant converter’s output voltage, preventing unwanted system shutdowns while also protecting valuable system components.

LLC applications for LEVs

Light electric vehicles (LEVs) include mopeds, scooters, bikes, and golf carts. Adopting the LLC topology for onboard and external DC/DC converters in an LEV improves the charger efficiency within the battery power and voltage ranges, regardless of the charging architecture. Using an LLC resonant converter also supports the high-power density and efficiency requirements of an LEV while reducing EMI and noise.

When compared to traditional flyback converters and parallel-resonant converters, LLC converters offer specific advantages for LEVs.

One advantage exists through the operation of LLC converters at the wide input and output voltages that match LEV charging requirements. Wide-output LLC converters with IPPC work well for LEVs by supporting constant current and constant voltage charging.

Instead of going into burst mode with a low battery voltage, the converter maintains the operating mode and minimizes ripple into the battery. The stable operating mode shortens the time needed to charge the battery and extends battery life.

LLC applications for PHEVs and EVs

Plug-in hybrid electric vehicle (PHEV) and EV architectures can use LLC resonant circuits for the DC/DC converter, BMS, onboard charger (OBC), and traction inverter subsystems. Along with the high efficiency established through zero-voltage switching, resonant converters provide high power density and decreased switching losses.

Figure 3 is a block diagram of a DC/DC converter combined with an active power factor correction (PFC) circuit. The PFC brings the input current and voltage waveforms in phase and increases the system efficiency.

After applying AC power to the input of the PFC stage, the boost voltage from the PFC combines with the filtered voltage at the DC-link capacitor and becomes the input for the DC/DC converter.

Figure 3 DC/DC converter block diagram. Source: Texas Instruments

EV battery management systems monitor and control state of charge, state of health, and residual capacity to maintain the safe operating range of the battery cells.

Within these broad functions, the BMS monitors the voltage, current, and temperature of the batteries and protects against deep discharge, overcharging, overheating, and overcurrent conditions. The cell balancing function of a BMS ensures that each cell in a battery pack has a uniform charging and discharging rate.

Resonant converters provide precise energy management, scalability, and the isolated power needed for a BMS as represented in Figure 4. As EVs incorporate more loads, the power requirements for high- to low-voltage conversion increase and require a higher power density.

Figure 4 An isolated DC/DC converter isolates the high-voltage battery from the low-voltage battery. Source: Texas Instruments

The LLC topology is a good fit for OBC applications because it addresses the need to adapt the output voltage according to the battery’s charging voltage range. LLC resonant converters simply adjust the voltage with the switching frequency.

While traction inverters convert energy stored in the battery into instantaneous multiphase AC power to drive traction motors, LLC resonant circuits operate within the subsystems that support inverters. These subsystems provide input power protection, signal isolation, isolated and non-isolated DC/DC power supplies, current and voltage sensing, and signal isolation.

LLC innovation for LEVs, PHEVs, and EVs

The high efficiency and compact size of LLC resonant circuit modules maximize vehicle range while cutting costs. LLC converters do have limitations, however, when meeting the output capacity requirements of evolving technologies. Limited power capacity and performance degradation under dynamic conditions require a different approach.

As next-generation zone EV architectures become standard, newer PHEVs and EVs will rely on multiple LLC converters distributed throughout the vehicle within the zone control module to optimize distribution, preserve output power stability, and deliver higher power capacity.

New EVs will also use bidirectional DC/DC LLC converters to connect the high-voltage battery with a low-voltage supply, improve charger efficiency, facilitate charging from and discharging to the grid, and reduce space and costs.

Other improvements include producing LLC resonant converters with two transformers and integrating lightweight planar transformers into the tank circuits.

Dual transformer converters may provide wider-range output voltages while maintaining high efficiency while charging. Using planar transformers in converters reduces the weight and size of converter modules.

EV consumer acceptance

Each innovation represents another step toward widespread consumer acceptance of EVs. In turn, EV adoption reduces greenhouse gas emissions, improves local air quality, and reduces the impact on human health.

Andrew Plummer is a product marketing engineer in Texas Instruments’ high-voltage power business. He focuses on growing the automotive, energy infrastructure and aerospace and defense sectors. He graduated with a bachelor’s degree in electrical engineering from the University of Florida.

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How LLCs unlock innovation in automotive electronics