“If the DIM senses a PWM signal from the MCU to control dimming, the IC enters CC mode. The feedback voltage is modulated by reading the DIM and the LED current is modified according to the DIM duty cycle while keeping the CV output constant. As in CC mode, the MCU Vo pin must also be high enough, so the IC’s minimum LED dimming depth is limited to 5% to provide enough voltage to drive the MCU. The Vo power loop is decoupled from the main LED current loop by setting the ratio of the current sense resistors (Rs1 and Rs2).
This article will focus on power solutions for smart lighting, especially smart light bulbs.
Government initiatives to install smart lighting systems, popularity of wireless control and growing affordability, and growing awareness of the benefits of smart lighting systems are also key factors driving the growing demand for smart lighting solutions globally. According to a 2017 study by Research and Markets, the total global smart lighting market size was USD 6.8 billion in 2017 and is expected to grow at a CAGR of 25.44% to USD 21.2 billion by 2022 (2).
Referring to the dismantling reports of some commercial smart bulbs (such as Cree Connected Bulb (3) or Philips Hue (4)), the main structure of smart bulbs is basically the same (see Figure 1). A constant current (CC) LED driver (such as the MPS MP4027) drives a constant voltage (CV) step-down driver (such as the MPS MP15x and MP17x) and the main LED; provides 5V or 3.3V simultaneously for Zigbee/Wifi/Bluetooth/other transceivers The controller IC and main microprocessor provide power. Microprocessors and transceiver ICs that allow end users to remotely control lighting using mobile devices. The control module sends pulse width modulation (PWM) or other digital signals (such as I2C) to control the dimming of the LED driver.
Figure 1: The current mainstream smart lighting power supply structure
Although this topology looks ideal, its power stage faces two challenges: further reducing the solution size and further reducing standby power consumption.
A19 and PAR bulb sizes are industry standard and will not grow. Adding smart lighting to a regular light bulb means designers have to squeeze more electronics into the same space, a big challenge, and space constraints may force designers to use fewer LEDs or fewer LED power supplies to simplify thermal management. Sometimes, designers must customize the shape of the PCB to accommodate all the required electronics, resulting in increased assembly costs.
In addition, customers and legislators are increasingly aware of the importance of standby power consumption. Designers are expected to design smart lighting devices with ultra-low standby power consumption, making products attractive or marketable under stringent energy consumption standards. For example, European CoC Tier 2 requires that all external power supplies rated below 49W have standby losses of no more than 75mW. It is expected that in the near future, smart lighting products will also be expected to meet certain energy codes (5). Currently, many smart bulbs have high standby losses, which are unavoidable due to the use of wireless circuit power, so standby losses can only be saved from the power stage.
To further reduce the size of smart lighting circuits and reduce standby losses, Figure 2 presents a better integrated total power solution. In this scheme, the CV and CC circuits can be combined into a single-chip solution, thereby reducing the circuit BOM and standby losses.
Figure 2: Smart light bulbs transition from existing power levels to new power solutions
By adopting the solution shown in Figure 2, circuit designers can save a control IC, an external Inductor, and other resistors and capacitors, thereby reducing the PCB space and BOM cost of the smart light bulb power section by at least 20 to 30 percent (See Figure 3).
Figure 3: Existing power stage and new power solution BOM comparison
In addition, this solution can achieve system standby power below 20mW because its power supply section is highly integrated, and at no-load or light-load conditions, more IC modules can be enabled when the power is turned off with an optimized minimum operating frequency is dormant.
Figure 4 summarizes the operating modes of the MP4057A. During startup, VDD is powered by the N_Forward winding. The IC then determines whether it should run in CV mode (powering the MCU only when the LED is off) or CC mode (powering both the MCU and the LED) by monitoring the DIM pin (controlled by the MCU itself).
In CV mode (DIM pin is low), VCC powers the IC. The ratio of N_flyback and Ns windings can be adjusted so that the LED does not light up in CV mode. In addition, an intelligent frequency modulation algorithm ensures low standby power consumption and fast transient response to LED load changes, and ensures no voltage drop across the MCU power supply and no LED flickering during CV/CC transitions.
Figure 4: Control Method Reference
If the DIM senses a PWM signal from the MCU to control dimming, the IC enters CC mode. The feedback voltage is modulated by reading the DIM and the LED current is modified according to the DIM duty cycle while keeping the CV output constant. As in CC mode, the MCU Vo pin must also be high enough, so the IC’s minimum LED dimming depth is limited to 5% to provide enough voltage to drive the MCU. The Vo power loop is decoupled from the main LED current loop by setting the ratio of the current sense resistors (Rs1 and Rs2).
As for LED current control, the solution implements a constant-on-time (COT) control mode, which guarantees a high power factor. The on/off transition occurs during valley switching (detected via the Zero Current Detect (ZCD) pin) to eliminate conduction losses and diode reverse recovery losses, also known as transition mode control. The IC also includes other features such as minimum off time (to improve EMI), leading edge blanking, over voltage protection (OVP), over current protection (OCP), over temperature protection (OTP) and more.
Figure 5 shows the LED load efficiency and power factor performance, and Figure 6 shows the PWM dimming performance with excellent linearity. Figure 7 shows the 8W reference design board for all PAR lamps. In addition, MP4057A provides 3.3V power supply for MCU, while MP4057B provides 5V power supply.
Figure 5: LED Load Efficiency and Typical Power Factor
Figure 6: PWM Dimming Rate
Figure 7: EV4057A-K-00A Evaluation Board:
230Vac/50Hz Input, Isolated Flyback Converter, VLED=21V, ILED=0.37A, Vo=3.3V, Io=50mA
To sum up, IP-protected solutions have higher system density and lower cost than existing solutions.