In the application range of 65W~150W output power, the CrM PFC+QR Flyback topology is a widely used architecture. With the trend of miniaturized integrated circuits, QR combo control chips have emerged. In addition, for consumer electronics products, not only does energy efficiency need to comply with regulatory requirements, but their standby loss is also a very important evaluation indicator. The SO20 package not only integrates the functions of PFC and QR controllers, but also integrates high-voltage start-up and X2 cap discharge mechanisms. Of course, the IC must also consider the distance between the insulation empty pins, so some pin positions have composite functions, such as HV/X2, BO/X2, PCS/PZCD... Among them, small signal detection of PCS/PZCD is particularly sensitive to avoid users making inappropriate PCB layout designs without paying attention, which may cause abnormal action protection triggering. The following will introduce the application experience and precautions related to NCP1937.

NCP1937 integrates power factor correction (PFC) and quasi resonant (QR) flyback controllers, designed for use in power adapters and achieving high energy efficiency, compact switching power supplies, such as PD fast charging, industrial communication power supplies, and power tool fast charging solutions. This is an AC-DC device that adopts a hybrid digital core architecture, which can provide higher energy efficiency, enhance flexibility, and simplify system design applications. The PFC stage exhibits a power factor close to 1 when operating in critical conduction mode (CrM) with maximum frequency clamping. This circuit combines all the necessary functions required to build a robust and compact PFC stage while minimizing the number of external components. The quasi resonant current mode flyback stage has a proprietary valley locking circuit to ensure stable valley switching. The system operates to the fourth valley and switches to a frequency reversal mode, with the minimum frequency clamp exceeding the fourth valley to eliminate audible noise. The skip cycle mode operation allows for excellent efficiency under light load conditions, while the standby power consumption is very low.

The influence of current path and grounding point on noise

In any power converter, PCB layout and wiring need to consider minimizing noise generation and ensuring stable operation. As a combination IC, NCP1937 controls two variable switching frequency converters and operates independently of each other. In fact, the PFC and QR gate drivers can be turned on and off at any point. Therefore, it is necessary to pay special attention to the current path and grounding point to avoid the interaction of noise between the two converters. Before arranging the PCB for NCP1937, it is recommended to distinguish and annotate various grounding points (as shown in Figure 1). Table 1 below illustrates the grounding points for different current paths, denoted as PGNDx. Meanwhile, to distinguish between analog or signal grounding points, they are represented as AGNDx. Star grounding is well-known in the industry and is a good practical layout method. Figure 2 shows an example of the primary side star grounding configuration for the application circuit of NCP1937.

The following is an explanation of the PCB layout and grounding point for the primary side current path:

1） Provide separate paths for the switching currents of PFC and flyback converters. From Figure 1, it can be seen that the current path from PGND3 to PGND4 (PFC current path) and the current path from PGND4 to PGND5 (flyback converter current path) are completely separated and have their own current circuits. This will avoid the overlap of switch current and gate drive current from two converters.

2） The path between PGND6 and PGND4 can improve the tolerance to surges. Suggest using separate wiring and sufficient line width to connect PGND6 back to PGND4.

3） It is recommended that the wiring between PGND4 and PGND5 be as short as possible.

4） PGND4 will be the star shaped connection center point of the entire analog signal ground. The connection between PGND4 and AGND 1 should be as short and wide as possible.

5） PGND1, PGND2, and PGND3 can be continuous paths, meaning there is no need to isolate these paths.

Explanation of PCB layout and grounding points for primary side analog signals:

1） AGND1 is the star shaped center point of the analog signal ground terminal. AGND2 and AGND3 should intersect at this point.

2） AGND3 comes from the PFC choke auxiliary winding and should be connected separately to AGND1.

3） AGND2 should be connected separately to AGND1.

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