induction cooker emi filter design

induction cooker emi filter design

A good electronic product, in addition to its own functions, circuit design and electromagnetic compatibility (EMC) design level, product quality and technical performance indicators play a very key role.Modern electronic products, more and more powerful functions, electronic circuit is more and more complex, electromagnetic interference and electromagnetic compatibility has become an urgent need to pay attention to and solve an important technical problem, developed countries have electromagnetic compatibility as a non-tariff trade barrier to protect their market.



Design of EMI filter

The basic use of filtering technology is to select signals and suppress interference.Filter is a way to compress the spectrum signal circuit, when the spectrum of the composition is different from the useful signal frequency band, can use the filter to filter the interference of useless reduce to a certain degree, the interference of the efferent system not exceeding a given specification, interference from the incoming system cause misoperation of the system.The more fully the filter isolates the spectrum of the useful signal and the interference, the better it is at reducing the interference in the useful signal loop.Therefore, it is very important to design the filter properly to suppress the conducted interference.



Principles of EMI filter design

The design of the filter can be realized by either reactance or absorption components.The former will reflect unwanted interference signals back, the latter will absorb unwanted interference signals.A reflective filter usually consists of two reactance components, inductance or capacitance, so that

Low series impedance and high parallel impedance are provided in the passband, while high series impedance and low parallel impedance are provided in the stop band.Reflection filter is to use LC to establish a high series impedance and low parallel impedance to reflect the energy of the interference frequency components back to the signal source to achieve the purpose of interference suppression.

The effectiveness of the filter depends on the impedance of the network connected to the filter.In order to suppress EMI signal effectively, the source impedance and load impedance of EMI signal connected at both ends of filter must be rationally connected.


Parameter determination of EMI filter

  1. Value of residual voltage release resistance R:

If allowed, the smaller the resistance value is, the better. Consider the following factors according to the actual situation:

  • The resistance is required to use two-stage derating to ensure reliability.Derating coefficients were 0.75 and 0.6W.According to Ohm's law1 .
  • After lightning surge, the residual voltage, its instantaneous value is generally at 1000V;Its instantaneous power can not exceed 4 times the rated power, can be calculated2
  • The resistor should be used in conjunction with the capacitor so that the time constant is less than 2 seconds.When the capacitor charge and discharge time equals, the voltage at both ends of the capacitor rises or falls by 63% of the maximum value.

Generally, the resistance R ranges from 75K ω to 200K ω, and the power is 2W to 3W.


  1. Capacitor value

Capacitor refers to the failure of the capacitor, which will not cause electric shock to the staff and not endanger personal safety.Both ends of the capacitor must be connected in parallel with a safety resistance to prevent the power cable plug from being charged for a long time.Safety standards stipulate that when the power cord of a working machine is unplugged, within two seconds, the voltage (or ground potential) at both ends of the power cord plug must be 30% less than the original voltage.In addition to the rated voltage of the power supply, the capacitor is superimposed with the peak voltage of various EMI signals between L and N.Capacitors are classified into two safety classes, namely, and, according to the worst-case scenario of capacitor application and power disconnect conditions.


  1. Value of capacitor

These two capacitors will cause electric leakage of equipment or charging of the chassis, which is easy to endanger personal safety. They are safety capacitors, and their capacity should not be large, generally between 2200pF and 4700pF (for each electricity), and they require high voltage resistance, otherwise, the machine will leak electricity.The voltage withstand value of the capacitor must be set after the lightning surge. The instantaneous residual voltage value is not damaged at 1000V/s. The value is set to 275V based on the two-stage derating principle.

The frequency characteristic of the capacitor is related to the value of capacitance, the smaller the value, the better the frequency characteristic.Cx capacitors and Cy capacitors are generally connected in parallel with smaller capacitors to meet the capacity requirements, so that the high frequency characteristics of filters are good.

  1. Value the inductor

The selection of inductance material is considered from the following aspects: first, the magnetic core material frequency range is wide, to ensure that the highest frequency in 1GHz, that is, in a very wide frequency range there is a relatively stable permeability;Second, the permeability is high, which is difficult to meet this requirement in practice. Therefore, the permeability is usually considered in sections.Common mode coil magnetic core materials generally choose ferrite ring core, ring core is suitable for large current and small inductance, its magnetic circuit is longer than E and U shape, no gap, with less winding number can obtain a larger inductance, because of these characteristics it has better frequency characteristics.The estimate of inductance should consider impedance and frequency.The common mode coil is usually between 1.5mh and 5mH, and the differential mode choke is between 10μH and 50μH.

When designing EMI filters for power supply, because they work in high voltage, high current and harsh electromagnetic environment, the safety performance of inductors and capacitors must be considered first.For inductance coil, its magnetic core, winding material, insulation material and insulation distance, coil temperature rise should be paid attention to.For capacitors, the type of capacitance, voltage resistance, safety grade, capacity, leakage current and so on should be given priority, especially the products certified by international safety organizations.


Installation of EMI filter

The suppression effect of power EMI filter on electromagnetic interference depends not only on its design and working conditions, but also on the actual installation of the filter.The installation quality of EMI filter of power supply has a great influence on the filtering effect. Only when the installation position is appropriate and the installation method is correct, can it play the expected filtering effect on the interference.Pay attention to the following points during installation:

  • .The installation position should be determined according to the intrusion path of interference.When only one or a small number of interference sources affect multiple sensitive devices, the filter should be connected to the interference source side. In this way, the number of filters can be reduced, and the interference can be confined to the vicinity of the interference source, reducing the requirement on the low level line.Conversely, if there is only one sensitive device but multiple interference sources, the filter should be installed on one side of the sensitive device.If the interference comes from power line radiation, a filter should be installed at the outlet of the power supply, otherwise the radiation interference will enter the sensitive equipment by various means.
  • The input and output wiring of the filter should be far away as far as possible, and shielded isolation, in order to minimize the coupling capacitance between the input and output, reduce the conduction interference and radiation interference introduced by the bypass effect.

(3)Good high frequency grounding, otherwise, when the filter capacitor resonates with the ground impedance, it will generate strong interference, reducing the high frequency filter

Wave effects.For this reason, the filter adopts shell direct grounding.

  • All the wires of the filter, especially the ground wire, should be as short as possible and arranged in order.



EMI filter test results

EMI filter can be divided into integrated type and separate type according to shape.Integrated type is the inductance coil, capacitor and other packaging in a metal or plastic shell;Separation is installed on the printed board inductor coils, capacitors, etc., constitute EMI filter.The test results of the designed discrete EMI filter are shown in Figure 1.


Figure 1


EMI filter design, mainly according to the characteristics of electromagnetic interference and electromagnetic compatibility requirements of the system, in the understanding of electromagnetic interference frequency range, estimate the approximate magnitude of interference on the basis of.Firstly, it is necessary to understand the application environment of the filter and consider its safety performance parameters.According to the requirements of the access circuit, the network structure and parameters of the filter are designed according to the principle of maximum impedance mismatch, so that the filter can produce the best suppression effect on EMI noise, so as to achieve EMC design.



Why Use an EMI Filter?

In the broadest terms, EMI total up to disturbances of an electrical or magnetic nature that can break down exactly how well a piece of electronic equipment executes or the high quality of the signal it receives-- which is in charge of its nickname "sound".

From narrowband synthetic EMI, such as the hum typically spoken with power lines, to natural broadband interference like solar flares and also lightning strikes, our globe is full of electro-magnetic interferences that can range from random and also sporadic to constant as well as constant.

Along with exterior types of EMI, there are a variety of ways digital tools can create their own EMI via activities such as producing insusceptibility, or a resistance to the existing moving through a given tool. Besides the electronic sound present on signals and also power lines, a bargain of high-frequency noise is created by devices such as electronic appliances, controls, motors, inverters, power supplies and microprocessors.

Sensitive gadgets, such as computer systems, clinical devices as well as armed forces tools, require to operate devoid of disturbance for objectives that are frequently of a mission-critical nature.


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