Summary: The challenge of low power LED design is to achieve a balance between power regulation set by regulatory standards, energy conversion guided by regulatory standards, and payload control, which includes dimming fidelity, often set by market acceptance . The FL7730 and FL7732 strike this balance better, performing all three functions with a single circuit.
Key words: MOSFET; LED driver; low power; energy conversion; dimming
Low-power LED solutions typically consist of a single string of LEDs or an LED driver with at least a single input and output control point. These drivers must perform the basic functions of LED power supplies, such as power regulation, energy conversion, and load control. High-power solutions typically employ specialized circuitry for these functions. For cost and space reasons, low-power solutions below 35 W, such as light bulbs, must perform all three functions with as little circuitry as possible.
The challenge of low power LED design is to achieve power regulation set by regulatory standards, energy conversion guided by regulatory standards, and priority load control set by market acceptance (which includes constant current regulation and dimming fidelity). The FL7730 and FL7732 strike this balance better, performing all three functions with a single circuit. With these solutions, there is a significant interaction between all three functions. To understand these solutions, focus on power conditioning and power conversion starting at the highest level. The interaction of load control does participate in power regulation, but in order to meet power regulation standards and market requirements, it is important to understand how power regulation is implemented (eg EMI requirements and power factor). Next is to understand load control such as fault protection, constant current control and dimming.
Flyback is used in most low power isolated LED power supplies, however not all flybacks are alike. Operating and regulating the flyback affects system performance and cost. Typical flyback circuits for low power supplies have no power factor correction and usually have high voltage electrolytic capacitors after a bridge rectifier. These supplies are typically secondary-side regulated (SSR), meaning they have optocouplers, a reference voltage, and a fast loop bandwidth of 1 kHz to react to load charge. This type of flyback is not suitable for LED lighting, because the flyback circuit has no power factor correction and is usually set up with a constant voltage power supply, while LEDs are more suitable for constant current driving.
This method typically utilizes an SSR galvanostatic control scheme, which directly measures load current and voltage. Good constant current control of a few percent is achieved despite the power dissipation when measuring the load current. In addition, optocouplers are also required. These supplies operate with slow feedback loops around 20 Hz, and since they are not dynamic, they can work well for LED loads. Whereas the energy storage performed by the high voltage electrolytic capacitor in the classic flyback is accomplished by the low voltage (LED string voltage) capacitor.
To address the cost constraints of a single-stage flyback PFC, some solutions attempt to use a primary-side regulation (PSR) flyback with a passive PFC. This approach reduces power dissipation on the SSR and voltage stress on the MOSFET, however, the process uses high-voltage capacitors and other components on the primary side, limiting power factor, lifetime, and size.
It can be known from formula (2) that the output current is determined by the peak current of the diode and the discharge time of the stored energy of the transformer. The output current (Iout), the average steady-state diode current, is estimated using the peak Inductor current and the inductor current discharge time (Tdis), where the current is measured by the current sense resistor at the MOSFET source, and the inductor current discharge time is given by VS pin measured. Because the output current (Iout) is the average value of the steady-state diode current, proper selection of the current sense resistor (RCS) can measure the peak drain current value of the peak detection circuit. Iout can be calculated from the inductor discharge time and can be measured in the VS pin. As the diode current tends to zero, the voltage on the VS pin quickly begins to drop, these measurements and the known transition period (TS) are the main factors in the TRUECURRENT control module.
This output information is compared with an accurate internal reference to generate an error voltage (VCOMI) that determines the duty cycle of the MOSFET (Q1) in constant current mode operation. It is through this innovative technology that the FL7732 and FL7730 precisely control the constant current output. Figure 2 and Table 1 show the measurement results from the evaluation board, which show that the constant current deviation over a wide output voltage range (11 V to 28 V) is less than 2.1% at each line input voltage.
The first key addition to the lighting is the line compensator, which receives line voltage information from the VS pin and uses it to modify the peak current circuit. This innovative solution enables extremely tight tolerance and constant current regulation over the entire input voltage range. Figure 2 and Table 2 show measurements from the evaluation board, which show that the constant current deviation in wide line regulation (90 V to 265 V) is less than 2.1% at rated output voltage (24 V).
The second key function of lighting is dimming control. As shown in Figure 3, the duty cycle of the AC line voltage is converted to a DC voltage using a simple resistor divider network and RC filter placed on the dimming pin of the FL7730. A dual angle control block is used for bias current sensing measurements and as input to the TRUECURRENT calculation block. It will be equal to a low RMS input voltage compared to high dimming control with a specific dimming angle. Controlling LED intensity with this method is simple and effective, and is useful for nearly all forms of dimming control, even the most difficult TRIAC-based dimming. Alternatively, use a simpler DC input solution or a PWM input solution, both of which can be filtered to produce a DC voltage. The relationship between the voltages on the dimming pin (pin 5) is shown in Figure 4.
More details on load control and dimming will be explained in future articles. More details on TRIAC based dimming can be found in the application note http://www.fairchildsemi.com/an/AN/AN-9745.pdf.
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