The Solution Of Interference Problem Of LED Power Supply.

- May 05, 2018-

For LED drive power supply interference problems is not good, the main reason is that the effects of interference is often small-scale operations, and unpredictable, how can the face of all the interference sizes do this problem well, just need to find the root cause of interference problem.

1. Cause of interference caused by LED power supply.

LED drive power supply will first ac rectifier dc, power frequency and high frequency inverter for, finally through the output rectifier filter circuit, a stable dc voltage, therefore itself contains a lot of harmonic interference.

At the same time, electromagnetic interference is formed due to the leakage inductance of the transformer and the peak caused by the reverse recovery current of the output diode.

The interference source in the switching power supply is mainly concentrated in the components with large voltage and current changes, which are prominently displayed on the switch tube, diode, and high-frequency transformer.

The electromagnetic interference produced by the switching circuit.

Switching circuit is one of the main interference sources of switching power supply.

The switching circuit is the core of the switching power supply (the same LED lamp power supply is the same as the LED tunnel light driver), mainly composed of the switch tube and the high-frequency transformer.

It produces du/dt with a large pulse, with a wide band and rich harmonics.

The main reason for this pulse interference is that the switching tube load is the primary coil of high-frequency transformer and is the inductive load.

At the moment of the switch tube, the primary coil produces a large flow and a high surge peak voltage appears at both ends of the primary coil.

In the switch tube disconnect instantly, as a result of the primary coil magnetic flux leakage, the part of the energy transmitted from a coil to the secondary coil, this part of the energy on the inductance of capacitance, resistance and the collector circuit is formed with a rush of attenuation of oscillation, superimposed on the cut-off voltage, forming the cut-off voltage spikes.

Supply voltage interruption will produce with the primary coil is the same magnetizing inrush current transients, the transient is a transmitting electromagnetic interference, affecting both transformer primary, will also make the conducted interference to return to power distribution system, cause electromagnetic interference of the harmonic wave in the power grid, which affect security and economic operation of other devices.

The electromagnetic interference produced by the rectifier circuit.

In rectifying circuit, there is a reverse current at the end of the output rectifier diode, which is related to the time of zero and the junction capacitance.

, among them can reverse the current rapid return to zero diode called hard recovery diode, the diode in the leakage inductance of the transformer and other distribution parameters will produce strong under the influence of high frequency interference, the frequency of a few MHz.

To the high-frequency rectifier circuits of rectifier diode is often larger forward current through the, in the reverse bias voltage and when to stop, because the more carrier accumulation in the PN junction, so before the carrier disappear a period of time, current will flow, the carrier of reverse recovery current decreased dramatically and produce large current changes.

High frequency transformer

The high frequency switching current loop of the primary coil, switch tube and filter capacitance of the high-frequency transformer may produce a large space radiation, forming the radiation interference.

If the capacitive filter capacity is insufficient or the high frequency characteristic is not good, the high frequency impedance of the capacitor will cause the high frequency current to be transmitted to the ac power source for conduction interference.

It is important to note that, in the electromagnetic interference generated by the diode rectifier circuit, di/dt of rectifier diodes is much larger than di/dt of the reverse recovery current of the continuous diode.

As an electromagnetic interference source, the interference intensity of rectifier diodes is large and bandwidth is wide.

However, the voltage jump generated by the rectifier diode is much less than the voltage jump generated when the power switch is connected and closed.

Therefore, also can be produced without rectifier diode dv/dt influence, the rectifier circuit as part of the electromagnetic interference coupling channels to study.

The interference caused by the distribution capacitance.

Switching power supply works in high frequency state, so its distribution capacitance cannot be ignored.

On the one hand, there is a large contact area between the heat sink and the switch tube collector, and the insulation film is thinner, so the distribution capacitance between the two can not be ignored at high frequency.

The high frequency current will flow through the distributed capacitance to the heat sink, and then flow to the enclosure to produce the common mode interference.

Pulse transformer, on the other hand, the first level between the distributed capacitance, the original side voltage can be directly coupled to the edge of the vice of dc output at the edge of the deputy two common-mode interference on the power cord.

The characteristics of the coupling channel are influenced by the stray parameters.

In the conduction interference frequency band (<30MHz), the coupling channel of most switching power supply interference can be described by circuit network.

However, switching power supply any of the actual components, such as resistor, capacitor, inductor and switch tube, diode, contains a stray parameters, and study the wide frequency band, equivalent circuit of the higher order time.

Therefore, the equivalent circuit of switching power supply, including the stray parameters of each component and the coupling between components, will be much more complicated.

In high frequency, stray parameters have great influence on the characteristics of the coupling channel, and the existence of distributed capacitance becomes the channel of electromagnetic interference.

In addition, when the switch tube power is larger, the collector is generally need to add heat sink, heat sink and the distributed capacitance between the switch tube can't be ignored in high frequency, it can form for space radiation interference and the power cord conduction common-mode interference.

2. Control technology of electromagnetic interference of switching power supply.

To solve the electromagnetic interference problem of switching power supply, we can start from 3 aspects: 1) reduce interference signal generated by the interference source;

2) cut off the transmission channel of interference signal;

3) enhance the anti-interference ability of the disturbed body.

As a result, the main control technologies include circuit measures, EMI filtering, components selection, shielding and anti-interference design of printed circuit boards.

Reduce the interference of the switching power supply itself.

Soft switch technology: the increase in the original hard switching circuit inductance and capacitance component, using the resonant inductance and capacitance, reduce switch du/dt and di/dt, in the process of the switch device is opened when the voltage drop before current rise, or shut off the current decline in prior to the rise in voltage to eliminate the overlap of voltage and current.

Switching frequency modulation technology: through modulation switching frequency fc, focus on fc and its harmonics 2fc, 3fc...

The energy is dispersed to the band around them to reduce the EMI amplitude at each frequency point.

This method can not reduce the total amount of interference, but the energy is dispersed to the base band of the frequency point, so that each frequency point does not exceed the limit set by EMI.

In order to reduce the peak of noise spectrum, there are usually two methods: random frequency method and modulation frequency method.

Selection of components: components that are not easy to produce noise, not easy to transmit and radiate noise.

It is usually notable that the selection of diode and transformer and other winding components.

The quick recovery diode is the ideal device for the high frequency rectifying part of the switching power supply.

Active common-mode interference suppression techniques: try to removed from the main circuit of a main switch and cause electromagnetic interference voltage waveform completely reversed phase compensation EMI noise voltage, and use it to balance the original switch voltage.

Filtering: one of the main purpose of the EMI filter, is to 150 KHZ to 30 MHZ frequency range to obtain high insertion loss, but the frequency of 50 hz power frequency signals does not produce attenuation, the rated voltage, electric current pass, at the same time, you have to meet the requirements of a certain size.

Any conduction interference signal on any power line can be expressed by differential mode and common mode signal.

In general, differential mode interference is small and the frequency is low.

The common mode interference amplitude is large, the frequency is high, also can produce the radiation through the wire, the interference is bigger.

To weaken the conducted interference, therefore, the EMI control signal in the EMC standard under the limit of the level, the most effective way is in the input and output circuit with the switch power supply EMI filter.

PCB design: PCB anti-interference design mainly includes PCB layout, wiring and grounding. The purpose of PCB is to reduce the crosstalk between PCB's electromagnetic radiation and PCB circuit.

The best method for switching power supply is similar to electrical design.

After determining the size shape of PCB, the location of special components (such as various generators, crystal oscillator, etc.) is determined.

Finally, according to the functional unit of the circuit, all components of the circuit are laid out.

Reduce the electromagnetic interference of the buffer circuit: it is composed of linear impedance stable network, is used to eliminate the interference within the power supply electric potential, including power line interference, electrical transient, transient, and power line harmonic voltage levels change.

These interferences are not very important to the general regulated power supply, but the effect on the high frequency switching power supply is significant.

The transmission of the interference signal - common - mode and differential - mode power line filter design.

Power cord interference can be filtered by power line filter.

A reasonable and effective EMI filter should have a strong inhibitory effect on the difference mode and common mode interference on the power line.

To enhance the anti-interference ability of the sensitive circuit.