The CrM PFC+QR Flyback topology is a widely used architecture in the output power range of 65W to 150W. With the trend of miniaturization and integrated circuits, QR combo control chips have emerged. In addition, for consumer electronic products, not only does energy efficiency need to comply with regulatory requirements, but their standby loss is also an important evaluation indicator. The SO20 packaging not only integrates the functions of PFC and QR controllers, but also integrates high-voltage startup and X2 cap discharge mechanisms. Of course, the IC must also consider the distance between insulation pins, so that some pin positions have composite functions, such as HV/X2, BO/X2, PCS/PZCD... In this regard, small signal detection of PCS/PZCD is particularly sensitive, avoiding users from making inappropriate PCB layout designs and triggering abnormal action protection when they should pay attention. 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 to achieve high energy efficiency and compact switching power supplies, such as PD fast charging, industrial communication power supply, electric tool fast charging, and other solutions. This is an AC-DC device that adopts a hybrid digital kernel architecture, which can provide higher energy efficiency, enhance flexibility, and simplify system design applications. This 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 to build a robust and compact PFC stage, while minimizing the number of external devices. 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 clamping exceeding the fourth valley to eliminate audible noise. The skip cycle mode operation allows for excellent efficiency under light load conditions while maintaining very low standby power consumption.

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 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 and represents them as PGNDx. Meanwhile, in order to distinguish between analog or signal grounding points, it is represented as AGNDx. Star grounding is well-known in the industry as a good practical layout method. Figure 2 is an example of the application line of NCP1937 with a star shaped grounding configuration on the primary side.

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

1） Provide separate paths for the switching current of PFC and flyback converter respectively. 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, with their respective current circuits. This will avoid overlap between the switching current and the driving current from the two converters at the gate.

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 for 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 grounding terminal. AGND2 and AGND3 should intersect at this point.

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

3） AGND2 should be connected separately to AGND1.

Figure 1: Application circuit of NCP1937 and differences in various grounding points

Table 1: Explanation of each grounding point in Figure 1

Figure 2: Example of using star shaped grounding for NCP1937

Use case: PCB layout of 90W power adapter application circuit

Figure 3 shows the display board of the Ansemi 90W power adapter. Next, we will further illustrate the practical PCB layout method through the application circuit (Figure 4).

1） When placing NCP1937, the IC can approach the PFC current sensing resistor.

2） The PFC current sensing resistor should be as close as possible to the ground terminal of the Bulk cap.

3） It is recommended to prioritize wiring for PCS/PZCD and QCS loops, with the path being as short as possible. Any high-frequency driving signals and high dv/dt signals are prohibited from crossing or approaching the PCS/PZCD and QCS signal return paths

4） PFC power current must be separately returned to Bulk cap GND (Red power grounding)

5） Flyback power current must return to Bulk cap GND separately and cannot pass through the PFC power current path before returning to Bulk cap GND

6） The grounding terminal of the PFC choke auxiliary winding must be directly connected to the grounding terminal of the VCC SMT capacitor

7） VCC SMT capacitors and PCS/PZCD filtering capacitors must be close to the GND of the IC

8） All small signal grounding must first be connected to the VCC SMT capacitor. That is to say, the grounding terminal of the VCC SMT capacitor will appear as a star shaped dispersed connection to all small signal grounding (blue grounding)

9） QR Aux winding GND must first be connected to the electrolytic capacitor of VCC, and then split into two paths from the electrolytic capacitor, each connected to VCC SMT capacitor GND and Bulk cap GND (green ground)

10） The RC filter of PCS/PZCD must be close to the IC pin (blue circle 1)

11） The RC filtering of QCS must be close to the IC pin (blue circle 2)

12） QZCD high low line compensation resistor close to IC pin

13） HV/X2 and HV/BO pins can reserve ground high-voltage filtering capacitors (~470pF)

14） The Y cap on the primary and secondary sides should be separately looped to the Bulk cap GND and output cap GND. Cannot first import power loop or small signal grounding loop

Figure 3: Display board of Ansemy 90W power adapter

Figure 4: Layout of application lines and grounding

Optimizing ESD through PCB layout to avoid accidental triggering of protection mechanisms

On the other hand, in order to pass ESD testing, the path of ESD energy will be optimized through PCB layout to avoid accidentally triggering the protection mechanism of the IC. Figure 5 shows the grounding method before optimization. The ESD energy will pass through the Y cap to the primary side and return to the Bulk cap GND through an independent trace. However, another path will be coupled to the primary side through a transformer winding. If the AUX winding grounding is first connected to the power trace of the Current Senses, it will cause a disturbance in the CS signal due to the ESD injection energy, leading to the false triggering of the OCP protection mechanism. However, for layouts with better ESD performance, as shown in Figure 6, it can be seen that not only will the Y cap go back to the Bulk cap GND through an independent Trace to the primary side, but the other path will also be coupled to the Bulk cap GND through a transformer winding. When the AUX winding grounding is connected to the Bulk cap GND first, it will not cause the CS signal to be disturbed by the ESD injection energy and trigger the OCP protection mechanism incorrectly.

In short, for Combo IC controllers to operate two power converters, PCB layout is a key factor for the efficient and stable operation of the power converter. Following the above grounding recommendations will effectively reduce the noise coupling of one converter to the sensitive control signals of other converters.

Figure 5: Before optimization, the GND of the auxiliary power supply winding is connected to the negative end of the PFC current sensing resistor

Figure 6: After optimization, the GND of the auxiliary power supply winding is connected to the negative terminal of the PFC bulk capacitor