Other measures to reduce electromagnetic interference of PFC

Shenzhen Shenchuang Hi-tech Electronics Co., Ltd(SChitec) is a high tech enterprise which specialized in phone accessories production and sales. Our main products include travel chargers, car chargers, USB cables, power banks and other digital products.All products are safe and reliable, with unique styles.products pass certificates like CE,FCC,ROHS,UL,PSE,C-Tick,etc,  If you are interested in, you can contact ceo@schitec.com directly.

Stay Charging Safely with SChitec

Other measures to reduce electromagnetic interference of PFC

In order to further reduce the electromagnetic interference, it is necessary to improve the circuit to meet the requirements of the power adapter.

1. Selection of PFC topology

In order to achieve high power factor, high power efficiency and compact size, it is recommended to use a single-stage topology with high power factor when designing small and medium power PFC. Traditional two-stage topology (PFC boost + DC / DC converter) is difficult to meet the requirements.

Single stage topology can save PFC boost converter, reduce the number of components and reduce the total cost of the system. With a single-stage topology, the system will also be affected by some factors, such as no primary side high-voltage energy storage, short output voltage holding time. In addition, the output ripple is high, so more low-voltage output capacitors must be used to make the response to dynamic load slow. However, this does not pose a problem for LED lighting applications, because the power adapter generally does not require dynamic load response time, and even if the output current contains ripple, what the human eye observes is only the light output when the average current passes through the LED.

2. Cut off the transmission path of interference

(1) increase the distance between interference sources (such as motor and relay) and PFC, isolate them with ground wire or add shielding cover to PFC.

(2) the circuit board shall be divided reasonably, and the circuits of strong signal, weak signal, digital signal and analog signal shall be arranged reasonably.

(3) the ground wire of power device shall be grounded separately to reduce mutual interference. Place the power devices as close to the edge of the PCB as possible.

3. Frequency jitter technology of PFC

The so-called frequency jitter is to control the switch frequency to change at a certain low-frequency rate (for example, 250 times / s), which can limit the switch frequency ∫ to jitter in a very narrow band. Because the switching frequency is constantly changing near the rated value f, there is no correlation between it and the high-order harmonic interference of the fixed frequency f, so the conducted noise of PFC can be reduced by using the frequency jitter signal.

In the design of high-power PFC, the switching power IC with frequency jitter function can be selected, for example, TOPSwitch HX series single chip switching power IC with maximum output power of 33W, whose frequency jitter range is 5KHz (switching frequency is 132KHZ) or ± 2.5KHz (switching frequency is 66KHz). In contrast, tops switch GX is ± 4kHz (switching frequency is 132KHZ) or ± 2KHz (switching frequency is 66KHz). Increasing the frequency jitter range can reduce EMI and the cost of EMI filter.

4. Isolation technology

The so-called isolation technology refers to the technology that separates the noise source from the signal line. Optical couplers are often used in PFC to realize the isolation of primary side and secondary side. When isolating the transmitted analog signal, the linear optical coupler should be selected, and its current transfer ratio (CTR) is close to a constant. In addition, high frequency transformer isolation, relay isolation, wiring isolation and other technologies are often used

5. Correct selection of grounding point

There are five kinds of ground wires for power factor corrector: Analog ground, digital ground, power ground, AC ground (ground g) and shielding ground. When designing the circuit of the whole machine, we should decide whether floating grounding or grounding according to the technical conditions and the actual situation, which kind of ground wire should be adopted, and choose single point grounding or multi-point grounding.

6. Component selection

The EMI filter with better performance can further reduce the EMI. In order to reduce the noise of components, the metal film resistance and low noise active components should be used as far as possible. In order to reduce the temperature drift, all components need to undergo high and low temperature aging treatment. If necessary, the temperature compensation circuit can also be added.