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Implement digital power control using LLC resonant converters

Posted: 07 Sep 2012 ?? ?Print Version ?Bookmark and Share

Keywords:microcontrollers? analog systems? communications?

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 multichannel applications, and programmable burst-mode capabilities for stand-by and light-loads.

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

Figure 1b: Digital control system.

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 C including proportional integral derivative (PID) and two-pole two-zero (2P2Z) C 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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