In electronic products, various disturbances are often transmitted to electrical devices through the power supply, thereby causing harm to these devices. According to the failure probability statistics of the microcomputer system, 90 of the 100 failures of the microcomputer system come from the power supply, and 10 of them are from the microcomputer, so the reliability of the power supply is the most important. Power supply with good resistance to dry interference. It can help users in product design without considering the anti-interference problem caused by the power supply, greatly shorten the user's product development cycle and save development costs.

1, the type of power interference

Power interference can exist in either "common mode" or "differential mode" mode. Interference types can vary from short duration spike interference to complete loss of power. These also include voltage changes (such as voltage drops, surges and interruptions), frequency changes, waveform distortion (voltage or current), persistent noise or clutter, and transients. The disturbances listed in Table 4, which can be transmitted through the power supply and cause damage to the equipment or affect its work, are mainly electric fast transient pulse groups and surge shock waves, while electrostatic discharge and other disturbances will not cause the influence on the electrical equipment caused by the power supply as long as the power supply equipment does not produce the phenomenon of vibration stop and output voltage drop.

Interference type	Typical cause
fall	Lightning strike: Heavy load on: The power grid voltage is low
Loss of power	Bad weather: Transformer failure: failure for other reasons
Frequency shift	Generator instability: regional power grid failure
Electrical noise	Radar: Radio signals: flarcs for power companies and industrial equipment: converters and inverters
surge	Sudden load reduction: Improper tapping of the transformer
Harmonic distortion	Rectifier: switching load: switching power supply: speed control drive
Instant delivery	Lightning strike: power line load device switching: power factor compensation capacitor switching: no-load motor disconnection

Table 1 Types of power interference

2. Methods to suppress interference

a. Add a line filter to the power input

A typical power line filter is shown in Figure 1. Among them, the L1 and L2 coils are wound in the same direction on the same magnetic core, and the two inductors cancel each other for the differential mode current and the magnetic flux generated by the main current, so it will not cause the saturation of the magnetic core: and for the common mode current, it can be reflected as a large inductance in order to obtain the maximum filtering effect, so it is also called "tyli inductance";.

Figure 1 Typical power line filter

The CX capacitor is used to reduce differential mode interference, the CY capacitor is used to attenuate common mode interference, and the R is used to eliminate electrostatic accumulation that may occur in the filter.

The power filter is mainly used to suppress noise in the frequency range below 30 MHz, and the harmonic frequency of the opponent pulse interference is often as high as hundreds of MHz, and the actual use effect is often not obvious. A research institute has tested the ability of 20 power filters to suppress surge waves, four of which exceed 20 dB, and some of which produce oscillations at the output end.

b. Use transformer with shielding layer

Because common-mode interference is a relative ground disturbance, it is mainly transmitted through the coupling capacitance between the transformer windings. If the screen shielding layer is inserted between the primary and secondary and is well grounded, the interference voltage can be bypassed through the shielding layer to reduce the interference voltage at the output end. The shielding layer has no adverse effect on the energy transmission of the transformer, but it affects the coupling capacitance between the windings. Figure 2 illustrates the common mode interference path of an isolated transformer with a shielding layer. Where C1 is the distributed capacitance between the primary winding and the shield layer; C2 is the distributed capacitance between the secondary winding and the shield layer; Z1 is the ground impedance of the shield layer, and Z2 is the load to ground impedance. e1 is the primary interference (common model) voltage; e2 is the secondary interference (common model) voltage.

Figure 2 Isolation transformer with shielding layer

In Figure 2, to increase the common-mode loss, it is only necessary to reduce the grounding impedance of the transformer shield. In theory, the crossbooster with shielding layer can make the mourning reduction reach 60dB, but it is found that the peak interference is suppressed after practical use, and its effect is not very obvious.

c. Add a multistage line filter to the input end of the power supply

In general, a differential mode filter is added to the circuit shown in Figure 1 to attenuate differential mode interference. In general, the design of a differential mode filter is shown in Figure 3.

Figure 3. Differential mode filter

d. Use wave absorbing devices

Varistor, transient voltage control tube (TVS tube) and other wave absorbing devices have a common characteristic, that is, high impedance below the national voltage, and once the threshold voltage is exceeded, the impedance drops sharply, so the peak voltage has a certain inhibition effect, but also has its own limitations. For example, the current absorption capacity of the varistor is not large enough, and the threshold voltage of the TVS tube is generally only 300 to 400 V. Varistor and TVS tube should be used in parallel in the circuit.

3. Parameter selection

For different circuits, the selection of power line filter parameters should be comprehensively considered according to current, voltage, frequency range and other factors, and sometimes should be determined by experiment. This section only describes the parameter selection of varistor and TVS tubes.

varistor

(1) The role of varistor
A varistor is a metallide rheostat. Its voltage and current do not obey Ohm's law, and form a special nonlinear relationship. When the voltage applied at both ends is lower than the nominal rated voltage value,
The resistance value of a varistor approaches infinity. Internal few current flow; Slightly above the nominal rated voltage value of inches, the varistor will quickly break down conduction and change from a high resistance state to a low resistance
State, the working current is also sharply increased: lower than the nominal rated voltage value, the varistor is restored to a high resistance state, exceeds the maximum limit voltage value, the varistor will be completely broken down and damaged, can no longer recover by itself.
Varistor is widely used in household appliances and other electronic products, to play overvoltage protection, lightning protection, suppression of surge current, absorption of peak pulse, limiting amplitude, high voltage arc extinguishing, elimination
Noise, protection of semiconductor components, etc.

(2) Selection and calculation of varistor
In one strand, a varistor is used in parallel with a protected device or device. Under normal circumstances, the DC or AC voltage at both ends of the varistor should be lower than the nominal voltage, even in the worst case of power supply fluctuations, it should not be higher than the rated value of the most human continuous operating voltage selected, the nominal voltage value corresponding to the maximum continuous operating voltage value is the selected value. For overvoltage protection applications, the pressure sensitive voltage value VmA should be greater than the actual circuit voltage value, generally use the following formula for selection:

Where: a is the circuit voltage fluctuation coefficient, generally 1.2; v is the DC working voltage of the circuit (effective value in AC), b is the pressure-sensitive voltage error, generally 0.85, c is the aging coefficient of the part, generally 0.9; The actual value of the VmA obtained by this calculation is 1.5 times that of the DC operating voltage. In the AC state, the peak value should also be considered, so the calculation result should be expanded by 1.414 times. In addition, the selection must also pay attention to:

It must be ensured that the continuous operating voltage will not exceed the maximum allowable when the voltage fluctuation is maximumValue, otherwise it will shorten the service life of the varistor.
When a varistor is used between the power line and the earth, sometimes the voltage between the line and the ground rises due to poor grounding, so a varistor with a higher nominal voltage than that used between the line and the line is usually used.
③ The surge current absorbed by the varistor should be less than the maximum current of the product.

TVS tube

(1) The role of TVS tube
TVS tube is a kind of high performance protection device in the form of diode. When the poles of the TVS diode are subjected to reverse transient high energy impact, it can change the high impedance between the poles into low impedance at a speed of 10-12 seconds, absorb the surge power of up to several dry watts, so that the voltage clamp between the poles is located at a predetermined value, and effectively protect the precision components in the electronic circuit from damage by various surge pulses. Because it has the advantages of fast response time, large transient power, low leakage current, small breakdown voltage deviation, easy control of clamp voltage, no damage limit, small size, etc. At present, it has been widely used in computer systems, communication equipment, AC/DC power supplies, automobiles, electronic ballasts, household appliances, instruments, RS232/422/423/485, 1/O, LAN, ISDN, ADSL, USB, MP3, PDAS, GPS, CDMA, GSM, digital camera protection, common mode/differential mode protection, RF coupling /IC driver reception protection, electromagnetic wave interference suppression, audio/video input, sensor/transmission, industrial control loop, relay, contact noise suppression, etc .
(2) TVS tube selection

The following points should be noted when calculating selection:
① Tvs rated reverse turn-off voltage V _WM It should be greater than or equal to the maximum operating voltage of the protected circuit.
② Minimum impact voltage _VBR=_VWM/_KBR(where _KBR=0.8~0.9).
③ The maximum clamping voltage VC of TVS should be less than the damage voltage of the protected circuit, that is, VC=_KCxVBR (where, KC=1.3).

④ In the specified pulse duration, the maximum peak pulse power consumption PM of TVS must be greater than the peak pulse power that may occur in the protected circuit. After the maximum clamping voltage is determined, the peak pulse current should be greater than the instantaneous surge current.

Comparison of TVS tubes and varistors

At present, many devices requiring surge protection in China use varistors. TVS tube is generally used for the protection of electric fast transient pulse groups, and its characteristics are much superior to varistors, and the comparison of specific characteristic parameters is listed in Table 5.

Key parameters or limit values	TVS tube	varistor
Reaction time /s	10-12	5x10-8
Whether there is aging phenomenon	no	There are
Maximum operating temperature /℃	175	115
Element polarity	Unipolar (DC) Bipolar (AC)	unipolarity
Typical reverse leakage current /μA	5	200
Clamping factor (Vc/VBR)	1.5 or less	P. 7 ~ 8
Sealing property	airtight	ventilate
Price	More expensive	cheap

Table 5 Comparison of TVS tubes and varistors

4. Application examples

Alternating current circuit

Figure 4 is the principle diagram of microcomputer power supply using TVS tube as line protection.

Figure 4 Schematic diagram of microcomputer power supply

The line protection in Figure 4 is illustrated below.
① Add a two-way TVS tube D1 at the AC 220 V of the incoming line to suppress 220 V ACSpike interference in power grid. Two-way TVS tubes D1
_VWM= 220V X 1.4= 308 V
_VBR= 308V / 0.8= 385V
VC ₌ 1.3 X 385V = 500.5V
If D1 is selected, you can obtain the value by looking up the table based on the preceding parameters.
② Add anti-interference power line filter at the transformer inlet line to eliminate small spike interference.
③ Add two-way TVS tube D2 at AC 20 V at the output end of the transformer. Once again, suppress the noise. Two-way TVS tubes for D2
_VWM=20V x 1.4 = 28V
_VBR=28V / 0.8=35V
VC₌1.3 x 35V=45.5V

You can obtain the value by looking up the table based on the preceding parameters when selecting D2.
④ When the rectifier filter output DC 10 V, plus a one-way TVS tube D3 to suppress interference. One-way TVS tube D3
_VWM = 10 V
_VBR = 10 V/0.8= 12.5 V
VC₌1-3× 12.5v =16.25.x
When selecting D3, it can be obtained by looking up the table according to the above parameters.
Through the above four times of inhibition, the so-called "purified power supply" is obtained. In order to prevent surge voltage such as lightning strikes, a varistor can also be added to the AC 220 V inlet line end to more effectively prevent interference from entering the CPU and memory of the computer, thus further improving the reliability of the system.
DCDC circuit
Figure 5 is a pre-suppression interference schematic diagram of a DC power supply.

Figure 5 Schematic diagram of DC power supply

In Figure 5, KZ and KF external 24V DC power supply: YR1 is a varistor, VmA=1.5×24 = 36V, for surge impact resistance: L1, L2, C1 and C2 constitute a balanced LC filter, to suppress differential mode interference: L3 is a common mode inductor, to suppress common mode interference: TVS1 is used to suppress electrical fast transient pulse group interference. Note that if GND cannot be grounded through the housing, TVS2 must be added according to KF, in order to "guide" the transient pulse kilodisturbance signal to the ground.