As personal computers (PCs) and televisions adopt a slimmer and elegant form factor, the power supply must be reduced in thickness, and adapters for ultra-portable computers must also become compact and lightweight travel companions.
To help meet these goals, quasi-square wave resonant (QR) power supplies provide high switching energy efficiency, and also help reduce electromagnetic interference (EMI), which simplifies electromagnetic shielding or suppression. The flyback and power factor correction (PFC) combination controller manages the power supply, allowing designers to reduce the number of components. In addition, the controller can also improve standby energy efficiency by turning off the PFC under low load/no load conditions. The roadmap for such devices is moving toward further enhancements in functional integration and enhanced switching performance to reduce audible noise.
Combined QR/PFC controller
There are a variety of combined devices in the market that include PFC and quasi-resonant flyback flyback control in a single package. High voltage integrated circuit (IC) technology allows direct startup using a rectified AC line circuit. The logic circuit controls the flyback and PFC switch waveforms, as well as other functions such as soft start, overcurrent, overvoltage and overtemperature protection, safe restart, and demagnetization detection.
During normal operation, the PFC minimizes reactive power (reacTIve power) and prevents line voltage distortion to improve energy efficiency. The PFC is mandatory for appliances with power above 70 W; however, PFC is not required for power classes below 70 W, during which the energy consumption of the PFC circuit may reduce energy efficiency. In order to save these energy consumption, some flyback/PFC combination controllers on the market can turn off the PFC circuit in light load and off mode, and thus improve energy efficiency.
ON Semiconductor's NCP1937 combined PFC and quasi-resonant flyback controllers offer a novel feature that allows the user to set the PFC shutdown threshold based on the percentage of output power to full load power. The internal circuit produces a current proportional to the output power that is adjusted and averaged using external resistors and capacitors to produce a voltage proportional to the output power. This IC combines this voltage with an integrated PFC off timer and a reference voltage that varies with line voltage to coordinate PFC shutdown and reactivation. This allows the PFC section to be turned off during 25% to 50% load at low line voltages and 50% to 75% load at high line voltages. The PFC section is also turned off during power-up until the flyback soft start timing expires. In addition, the circuitry required to turn off the PFC by disconnecting the feedback connection is also integrated into the device. Traditional combined devices require an external feedback connection that includes a MOSFET circuit to open or close the loop as required. Figure 1 shows the PFC on/off control circuit.
Figure 1. PFC turn-on/off control IC.
This feature provides an example of how advances in high-voltage IC technology enable manufacturers to offer higher levels of integration, allowing power supply designers to eliminate additional external circuitry and further increase energy efficiency over a full load range. The digital functions provided by ICs with high voltage capability are usually limited. Today, higher-end high-voltage processes with smaller digital feature sizes allow for additional on-chip functionality, enabling designers to build smarter and higher-accuracy power solutions with the smallest form factor possible.
Key details of improvement
ON Semiconductor's 700 V high-voltage process also enables the controller to integrate most of the circuitry used to discharge the X2 input filter capacitor (a function required by a safety agency standard) when AC line voltage is removed. This saves PCB space and the energy consumption of the external resistor network for X2 capacitor discharge. The NCP1937 controller contains two high-voltage startup circuits and uses a novel method to reconfigure these circuits to discharge the input filter capacitors with the AC line removed.
This high-end process also integrates other features to help save several external devices; these include Power Save Mode (PSM), which reduces supply current to less than 70 μA. A typical approach used by conventional flyback control ICs is to use an active-close (acTIve-off) integration to enable low-power mode. This method requires additional bias current to pull down the optocoupler on the secondary side, reducing overall system energy efficiency. In comparison, the NCP1937 contains built-in circuitry that eliminates the need for such bias currents and thus increases system efficiency under no-load conditions.
The added digital integration capability and advanced 700 V high-voltage process also support the circuitry required for more advanced protection functions, such as fault detection, current sensing, and over-power compensation, reducing the reliance on external devices.
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