Optimal digital power control using LLC resonant converters

The authors describe a digital power control implementation using line level control (LLC) resonant converters based on a flexible, 32-bit, low-cost, high-performance microcontroller. Key elements of digital power control are explored; including duty cycle control, dead-band adjustment in real time, frequency control, and adaptive thresholds for maintaining different safe operation regions.With the availability of new low-cost, high-performance microcontrollers (MCUs), the benefits of digital power control can be introduced to a wide range of embedded, industrial and control applications. Traditional analog systems are susceptible to factors such as drift, aging of components, variations caused by temperature and component tolerance degrading. Developers are also limited to classical control implementations. In addition, analog-based systems offer little flexibility to accommodate different environmental operating conditions or even simple changes in system requirements.

When designed using a digital approach, portions of the power system can be implemented in software, resulting in a level of flexibility that enables a single architecture to provide optimal performance across a range of applications and operating conditions. With software-based control algorithms, developers can:

  • Ensure precise and predictable system behavior through configuration – both in the factory and at power up – to adjust for component tolerance issues
  • Improve efficiency through the use of advanced algorithms (i.e., non-linear, multi-variable, etc.), which are not feasible to implement in analog-based systems
  • Maintain performance over an extended system lifetime through dynamic recalibratio
  • Support multiple systems with a single controller
  • Increase system reliability through self-diagnostics
  • Enable intelligent management through a communications link
  • Simplify system design by allowing developers to work with model tools and C rather than having to rework analog designs with every requirement change
  • Reduce system cost by supporting other system functions on the same MCU

This article describes a digital power control implementation using LLC (line level control) resonant converters based on a flexible, 32-bit, low-cost, high-performance microcontroller. Key elements of digital power control will be explored; including duty cycle control, dead-band adjustment in real time, frequency control, and adaptive thresholds for maintain different safe operation regions.

Tuning of the voltage compensator using coefficients during an active load will show the flexibility of the implementation, and the use of programmable soft start/stop capabilities and slew rate control will demonstrate how to avoid inrush current and reduce audible noise. Finally, developers will learn how hybrid burst mode control dramatically increases light-load and stand-by efficiency.

Digital control with microcontrollers

Consider the right MCU to provide all of the necessary performance and peripherals needed to control a system with a single stand-alone controller. MCUs with ample headroom and specialized peripherals will enable developers to implement more advanced control algorithms to further improve performance while lowering system cost.

Few microcontrollers have an architecture optimized for digital control applications with advanced architectural features to enhance high-speed signal processing. The main CPU core needs built-in DSP capabilities such as a single cycle 32 x 32-bit multiply and accumulate (MAC) unit to greatly speed processing of computations. Integrated control peripherals, such as the analog-to-digital converter (ADC) and PWMs, are designed to be very flexible and easily adapt to almost any use with very little software overhead. For example, the ADC has a programmable auto-sequencer that cycles through samples in a specific order so that values are ready when the application needs them. With more intelligent control peripherals and a powerful CPU core, control loops run tighter, both improving the dynamic nature of control algorithms and resulting in better disturbance behavior.

Microcontrollers need to provide the significant PWM features needed for real-time digital control including:

  • Duty cycle control for soft start-up avoids inrush currents and enables various burst mode configurations to enhance light-load efficiency
  • Real-time dead band adjustability guarantees ZVS at all operating points and optimizes efficiency
  • Trip-zone and internal comparator options enable instantaneous PWM disabling to ensure system reliability and safety
  • High-resolution frequency adjustment capabilities down to 150 ps for precise output voltage regulation

Unlike analog controllers, systems using microcontrollers can be easily customized to achieve optimal performance through the use of programmable voltage/current regulators like PID and 2P2Z. Developers can prevent catastrophic faults by setting certain thresholds for safe operating region boundaries, which are tied to programmable soft-start/stop capabilities. Other capabilities enabled through digital control include avoiding inrush current, reducing audible noise, limiting the slew rate using a programmable soft transient option, sequencing and programmable delay time for multi-channel applications, and programmable burst-mode capabilities for stand-by and light-loads.

LLC resonant converters
One of the well-known digital power topologies is the resonant convertor. While offering high efficiency and low noise, the most common resonant topologies have several significant limitations. For example, the converter is theoretically incapable of regulating under no- or light-load conditions and wide frequency variation is necessary to regulate the output over full load range. Under light-load conditions, small resonant currents cause a loss of zero voltage switching (ZVS). In addition, re-circulating energy will degrade high line or light-load efficiency.

The LLC resonant topology’s simple structure overcomes the drawbacks of conventional resonant topologies. Advantages of the LLC resonant topology include:

  • Full ZVS operation for primary side switches is possible because the magnetizing inductance (Lm) of the transformer is relatively small when compared to an ideal transformer
  • High-efficiency and high-power density from no-load to full-load ZVS due to reduced switching losses without degrading output voltage regulation
  • Low electromagnetic interference (EMI) and reduced filtering requirements due to ZVS, and switching takes place under conditions of zero drain voltage
  • No need for external parallel series inductors because of an integrated transformer. Magnetizing and leakage inductors also serve as a part of the topology
  • Reduced turn-off losses since switches are turned off under low-current conditions
  • Low-voltage stresses (limited to two times output voltage) and zero current switched (ZCS) operation on the secondary rectifier due to the absence of a secondary filter inductor. In addition, ZCS of secondary diodes removes its reverse recovery problem

Resonant converter drivers are designed to adjust the switching frequency of the half-bridge to regulate the output. However, one can achieve better operating efficiency of the overall system by using a low-cost microcontroller to adjust the frequency, duty cycle and dead-band. Figure 1 shows a variable input, variable output LLC converter system. Digital control methods support the use of any regulator – including proportional integral derivative (PID) and two-pole two-zero (2P2Z) – thereby simplifying customization of the system.

Embedded comparators and trip zones within the microcontroller need to provide programmable protection in case of a short circuit, overload, overvoltage, brown-out, etc. In the control software, soft-start/stop capabilities avoid inrush current and reduce audible noise. A programmable soft transient option limits the slew rate while the system follows a given reference voltage level. A smoother startup profile without causing overshoots or high inrush current is achieved through gain adjustment by means of hybrid duty cycle and frequency control. Light-load efficiency is increased by running the system in burst mode, which involves on/off control of the half-bridge pulse width modulators (PWMs). Finally, the additional peripherals on the microcontroller should allow the user to control the synchronous rectifiers.

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Fig. 1a System-level block diagram of an LLC resonant converter

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Fig. 1b digital control system

On the secondary side, various combinations of diode circuits or synchronous rectification methods improve the overall efficiency as shown. The microcontroller can be located at the primary or secondary side, depending on application requirements.

The transformer leakage and magnetizing inductances serve as a part of LLC topology to minimize the cost and size. Alternatively, leakage inductance can be implemented externally during prototyping to simplify design and troubleshooting. In addition, the ability to use an external inductor provides flexibility to optimize resonant tank design to address specific manufacturing difficulties and design trade-offs. Some common resonant tank design trade-offs are system efficiency, operating frequency, output accuracy, conversion ratio, conduction vs. switching losses, system frequency resolution, maximum/minimum achievable frequency and variable input-output requirements.

Software flow
Figure 2 shows single-stage LLC converter control software flow, which is partitioned into two sections: high-speed, high-priority code used for control related algorithms and low-speed, low-priority code used for initialization and background tasks.

The high-speed code is typically written for maximum efficiency in order to enable greater-bandwidth control loops. This code is called using interrupt service routines (ISRs) which are able to interrupt the background tasks when called. For an LLC converter, which operates with variable switching frequency, two ISRs running asynchronously might be used. One ISR would be used to handle the control loop algorithm and called at a fixed frequency to avoid violating sampling and control theory. A second ISR would be used to handle the PWM modules updates and called at the PWM switching frequency (variable) in order to allow simultaneous updates and minimize delay between control loop calculation and update.

The slower background loop is executed in the remaining time interval when no ISRs are active. This is where system tasks such as instrumentation, soft-start, on/off delays, protection mechanisms, active load control and communications are executed. A task state machine has been implemented as part of the background code. Tasks are arranged in groups (A1, A2, A3…, B1, B2, B3…, C1, C2, C3…) and executed according to three CPU timers that are configured with user-defined periods of 1 ms, 5 ms, and 7.5 ms respectively. Tasks are executed in a “round-robin” manner within each group. For example, if group B executes every 5 ms and has 3 tasks, each “B task” will execute once every 15 ms. “Slow” tasks can be written in C, whereas the more time critical resonant converter control algorithm is written in assembly code.

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Figure 2: LLC resonant converter control software flowchart