Anti-electromagnetic interference design scheme for electronic ballast

Central topics:

• The composition of electronic ballast
• Sources and Impacts of Electromagnetic Interference in Electronic Ballasts
• Measures to Suppress Electromagnetic Interference in Electronic Ballasts

Solution:

• Conducted interference suppression measures
• Radiated interference suppression measures
• Harmonic interference suppression measures

1 Electronic ballast

Electronic ballast (Electricalballast) is a type of ballast, which refers to an electronic device that uses electronic technology to drive an electric light source to produce the required lighting. The corresponding one is an inductive ballast (or ballast). Modern fluorescent lamps are increasingly using electronic ballasts, which are light and compact, and can even be integrated with the lamp tube. At the same time, electronic ballasts can usually have the function of a starter, so a separate starter can be omitted. Electronic ballasts can also have more functions, such as improving or eliminating the flicker of fluorescent lamps by increasing the current frequency or current waveform (such as turning it into a square wave); it can also allow fluorescent lamps to use DC power through the power inversion process.

2 Composition of electronic ballast Electronic ballasts are composed of anti-interference filters, rectifier filter circuits, power factor adjusters, high-frequency conversion, resonant circuits, abnormal state protection circuits and fluorescent lamps. The functions of each part are as follows: 1) Anti-interference filter: Prevents the high-frequency interference signal generated by the electronic ballast from entering the power grid and causing radiation. 2) Rectification and filtering circuit: converts 220V industrial frequency (50Hz or 60Hz) AC power into 310V DC power as the power supply for the electronic ballast. 3) Power factor adjuster: adjusts and compensates the power factor of the machine. 4) High-frequency conversion circuit: the heart circuit of the electronic ballast, converts the DC power supply into a high-frequency power supply of about 20K~50KHz to drive the fluorescent lamp. This circuit is usually implemented by a self-excited oscillator composed of a pair of power tubes (transistors or field effect tubes). 5) Resonance circuit: used to replace the starter of ordinary fluorescent lamps. Before the fluorescent lamp is started, it can be equivalent to a series resonant circuit. Its oscillation frequency is consistent with the frequency of the high-frequency conversion circuit. When resonating, a very high voltage is generated on the capacitor C to ensure that the lamp tube ignites and lights up. When the lamp tube is lit, its equivalent resistance decreases. This resistance is connected in parallel with the capacitor C, which greatly reduces the Q value of the resonant circuit. The circuit becomes an RL series circuit, and L becomes a current limiter. 6) Abnormal state protection circuit: When the fluorescent lamp cannot light up normally, the high resonant voltage will cause the power device to burn out. The function of this circuit is to protect the power device from burning out in abnormal state. 7) Fluorescent lamp: Its function is to convert high-frequency electrical energy of about 20K~50KHz into light energy. 3 Sources and effects of electromagnetic interference in electronic ballasts 3.1 Sources and effects of conducted interference The electromagnetic noise generated by the electronic ballast during operation is transmitted to the power grid through the input power line, causing conducted interference, polluting the surrounding environment and affecting the normal operation of related electronic equipment or systems. The sources of conducted interference of electronic ballasts mainly come from the following aspects: (1) Intrinsic noise of components. Mainly thermal noise, shot noise, contact noise, etc. (2) Electromagnetic noise generated by semiconductor diodes during switching. When turning on and off quickly, the instantaneous change of voltage and current will form strong electromagnetic noise. (3) During the switching process, power semiconductor devices will generate very high transient voltage or current and cause oscillation. The faster the switching speed, the greater the switching current, and the greater the transient electromagnetic noise caused. Power semiconductor devices generate direct conducted interference on the AC power grid. This noise is divided into differential mode and common mode. (4) In the passive power factor correction structure using a high-frequency pump or dual-pump circuit, the high-frequency switching signal of the power switch tube is added to the input end through the feedback element and sent to the power grid through the power supply line, forming conducted interference. 3.2 Sources and effects of radiated interference When the electronic ballast is working, the magnetic field and electric field formed are radiated outward through the input and output wires and loads or certain components in the form of electromagnetic waves. The interference formed by the electromagnetic wave propagation between the surrounding electronic and electrical equipment is called radiated interference. Radiated interference mainly exists in the form of magnetic field. It is interference generated by the magnetic field and caused by the mutual inductance between conductors. When the current in the circuit changes suddenly, the magnetic flux linked to the circuit also changes accordingly, and then induces interference voltage. 3.3 Sources and effects of harmonic interference Since the load of the electronic ballast is a nonlinear load, harmonics will be generated during operation. In addition, the output current waveform of the electronic ballast with low power factor circuit or improper power factor correction circuit will produce serious harmonic distortion. The influence and harm of harmonics are mainly manifested in: increasing circuit loss, increasing temperature rise, reducing efficiency and service life; increasing dielectric loss and partial discharge in insulation, accelerating insulation aging; increasing noise, etc.

4 Measures to suppress electromagnetic interference of electronic ballasts

4.1 Measures to suppress conducted interference
1) Reasonable grounding
Conducted interference is generated by the common ground wire of the electronic ballast and the common impedance in the grounding network. The grounding should be arranged reasonably, the ground wire should be as short as possible, and the input ground and output ground should be separated. The grounding method of using independent ground wires in parallel can prevent the conduction coupling between the grounds and reduce the interference between the control signals. The shell grounding can shield the electric field and weaken the interference to the internal and external circuits with shielding. For example, the phase line and the neutral line of the power supply are connected to the shell and the earth through the Y capacitor, which can reduce the conducted interference noise of the system.

2) Add a decoupling circuit to the DC power supply circuit
Add an RC decoupling circuit to the switch tube power supply circuit, so that the current required for the switch tube to be turned on is no longer provided by the power supply, but the decoupling capacitor provides a current compensation source for the device to reduce the noise caused by the current fluctuation caused by the power supply and grounding system. In addition, in the half-bridge inverter circuit of the electronic ballast, the capacitor connected between the midpoint of the half-bridge and the ground can reduce △U/△t and △I/△t, which helps to suppress electromagnetic interference noise.

3) Use passive filters
Using passive EMI filters is the most effective way to suppress conducted interference. That is, insert a bandpass filter into the circuit to allow 50 Hz AC to pass smoothly and block signals of other frequencies. However, the parasitic parameters of the filter LC components must be strictly controlled. Their manufacturing process, installation location, and routing method will affect the EMI filtering effect. Adding the filter circuit shown in Figure 1 can control the noise level entering the power grid through conducted coupling.

Figure 1 EMI filter circuit

The output end of the filter is connected to the noise source, while the input end is connected to the power grid. The purpose is to prevent various high-frequency and transient noises from entering the power grid through conduction. The effect of the filter in suppressing electromagnetic noise can be measured by the insertion loss: the greater the insertion loss, the better the filtering effect and the greater the suppression of conducted interference.

Conducted interference mainly manifests as differential mode interference and common mode interference.

(1) Common mode interference suppression

Figure 2 Filter with common mode choke

The common mode choke is the inductor element that plays a leading role in the common mode insertion loss. According to the principle of electromagnetic induction, in Figure 2, since the common mode currents (I _cm and I' _cm ) have the same direction, the magnetic lines of force formed in the magnetic ring are superimposed on each other, that is, the magnetic flux is superimposed on each other. Since the magnetic flux Ф=LI, the total inductance of the common mode choke L=(Ф ₁ +Ф ₂ )/I _cm. If the common mode choke is connected in series in the circuit, it is equivalent to connecting a low-pass filter element in series in the circuit, which plays a common mode suppression role.

(2) Differential mode interference suppression
The differential mode choke is the inductor element that plays a leading role in the differential mode insertion loss. It is wound with a single winding structure, and the signal current on its line also generates a certain amount of magnetic flux in the magnetic ring, so it is easy to reach saturation. Therefore, the inductance value of the differential mode choke is small, and the order of magnitude is generally in μH. The common mode choke coil uses two identical windings on a magnetic core. The currents of the two windings are in opposite directions, and the magnetic fluxes generated by the signal currents in the magnetic ring cancel each other out, so there is no magnetic saturation. Therefore, its inductance value can be relatively large, and the order of magnitude of the common mode magnetic ring is generally in mH.

Figure 3 Filter with differential mode choke

According to the principle of electromagnetic induction, in Figure 3, due to the action of the differential mode current (I _dm and I' _dm ), magnetic flux is generated in the magnetic ring, thus generating inductance, so a low-pass filter element is connected in series in the circuit, thereby playing a differential mode suppression role. Of course, since Icm will also generate magnetic flux and then generate inductance, the differential mode choke also has a suppression effect on common mode interference, but suppressing common mode interference requires a larger inductance, and the inductance generated by the differential mode choke is small, so the suppression effect on common mode interference is small.

Similarly, according to the principle of electromagnetic induction, since the differential mode current (I _dm and I' _dm ) is in opposite directions, the magnetic lines of force formed in the magnetic ring cancel each other, that is, the magnetic flux cancels each other, so the common mode choke has a suppression effect on the differential mode current. In actual production, since the two lines (1 and 2) cannot be completely balanced (the lead length and leakage inductance are not completely symmetrical), there is an unbalanced inductance Le _, and the value of Le _is generally less than L/100. Therefore, the common-mode choke also works on differential-mode interference, but the effect is very small.

The above passive EMI filter is reciprocal, which can not only suppress the electromagnetic interference of the electronic ballast from being sent to the power grid, but also suppress the electromagnetic interference in the power grid from entering the electronic ballast.

The component parameters of the EMI filter with common-mode inductance cannot be calculated according to the formula obtained by the filter without mutual inductance. Usually, the circuit structure to be adopted should be determined first, and then the common-mode equivalent circuit and network analysis theory should be used to calculate its common-mode insertion loss.

4.2 Measures to suppress radiated interference
1) Shielding
Although the electronic ballast itself generates radiated interference, and the output wires and lamps also generate radiated electromagnetic interference, it can be solved by installing the electronic ballast in a metal shielding shell with a grounding point and reliably grounding the metal shell of the lamp. Shielding is the most effective way to reduce radiated interference.

2) Isolation
The radiated interference generated by the internal circuit of the electronic ballast is in the form of an electromagnetic field around the circuit, and interferes with other circuits through electromagnetic coupling. The simplest and most effective way to prevent this interference is to isolate the electronic ballast from other circuits and cut off or weaken the electromagnetic coupling between them. The principles and methods of isolation are:

(1) Interference circuits and other circuits should not be arranged in parallel as much as possible;
(2) If sensitive circuits and general circuits are arranged in parallel, the spacing between them should be greater than 50 mm;
(3) Power feeder and signal line should be isolated.

4.3 Measures to suppress harmonic interference
Suppressing the current harmonic content and reducing the lamp current crest factor are often contradictory. The contradiction between these two parameters can be solved by active power factor correction technology. However, when an electronic ballast using PFC is inserted into the EMI filter network, it may have a certain impact on technical indicators such as the total input current harmonics (THD), input power factor (PF) and lamp current crest ratio (CF).