ST’s Single-chip Digital-power Controller Zeros in on LED Lighting
Using its patented time-shift control (TSC) method in its LLC resonant converter, the new digital power controller also packs a combination of a power-factor correction controller and a start-up generator.
LLC resonant converters (where the LCC is two inductances (L) and a capacitor (C)) are used in a wide range of industrial applications. These converters offer benefits such as zero voltage switching (ZVS), zero current switching (ZCS), to name a few.
With those benefits in mind, manufacturers tap into them to fabricate digital power controllers for energy-saving solutions in high-power LED lighting and LCD TVs.
In a bid to achieve stability margin in LLC converters, a TSC methodology in LLC converters design has been put forward by STMicroelectronics (ST). This methodology is featured in ST’s latest digital power controller to achieve soft switching.
Close-up view of ST's latest digital power controller, the STNRG012. Image used courtesy of STMicroelectronics
Before diving into ST’s new product, let’s discuss some control methods used in LLC converters and how they compare to one another.
TSC vs DFC vs HHC
The first thing to consider is direct frequency control (DFC), the traditional control method used in LLC converters. In this control method, a feedback signal from a compensation circuit is used to measure the frequency of the gate drive signals.
The feedback signal also directly controls a local oscillator which the switching frequency of a bridge circuit is locked into. While a DFC is simple to design, it is characterized by low bandwidth. It can also be difficult to achieve an acceptable load transient response and stability in its compensation circuit.
Schematic of an LLC resonant half-bridge converter. Image used courtesy of Claudio Adragna
On the other hand, the TSC method enables an easy-to-design compensator. This method also makes the power stage in LLC converters turn to a low-Q second-order system or close to a first-order system.
Additionally, the input ripple rejection is improved in LLC converters using the time-shift control method. Overall, the converter reliability is improved.
Finally, another control method adopted in the industry is the hybrid hysteretic control (HHC). This method utilizes both frequency control and charge control.
Like in TSC, hybrid hysteretic control simplifies the design of a compensator as the power stage is turned into a first-order system. HHC is also characterized by high bandwidth and boasts inherent input feedforward, which could lead to an outstanding input line transient response compared to the DFC method.
With a bit of light shown on those methods for creating LLC converters, let's take a look at what ST's device is bringing to the table.
Introducing ST’s STNRG012
Featuring the TSC method, STMicroelectronics has announced its latest digital power controller, STNRG012, which packs a ton of features into one chip.
Some of those features include:
- A multi-mode power factor correction (PFC) controller
- An LLC resonant half-bridge converter
- An 800 V rated start-up generator circuit
Each of these features is managed by a digital engine that executes optimized control algorithms for efficiency and reliability. In the half-bridge converter, the TSC methodology controls the amount of time elapsing from a zero crossing of the tank current to the switch-off of the MOSFET.
Block diagram of the STNRG012. Image used courtesy of STMicroelectronics
The STNRG012 features a robust architecture and has four high-speed state machine event-driven (SMEDs) peripherals that control the PFC and LLC resonant half-bridge circuits. The SMEDs are programmable components in the device, and it is used to control events such as power manager generated events for protection, input and output events, timer events, etc.
Additionally, the SMEDs allow users to configure the PFC controller to alternate between different working modes: transition mode, valley skipping, and discontinuous mode.
Schematic of the STNRG012. Image used courtesy of STMicroelectronics
In addition, the STNRG012 has a non-volatile memory (NVM) that houses the digital algorithm and the essential operating parameters, which allow users to customize settings and program the device during production.
The device is packaged in a 20-pin SO package, and, with its advanced distortion mitigation, it claims to be an advanced solution for line-powered applications up to 300 W and LED lighting applications ranging from home lighting to street lighting.
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