Snubber

Dating from the technical English word snubber ( damper to German ) describes a device for damping unwanted vibrations in technical applications.

Most often the term is used in the electronics. But snubber can also be mechanical vibration, such as shock absorbers, dampers or chain steering damper and hydraulic / pneumatic pulsation damper, muffler, in these applications, however, the term is less common.

Electrical snubber

As snubber element is called an electrical circuit to neutralize interfering radio waves or spikes, which usually occur when switching inductive loads when the power is cut off abruptly. It is used to achieve a better electromagnetic compatibility, to suppress sparks on switch contacts and limit the rate of voltage rise on semiconductor switches (thyristors, IGBT, bipolar transistors ).

The circuit is referred to in the German-speaking countries as Boucherot member, RC quenching combination, RC spark quenching unit or RC quenching element (contactor technology). In contrast, in the English language also protection diodes and other electronic measures for interference suppression are called snubber.

Components

An RC Snubberglied is a series circuit of a capacitor with a resistor. These components must be appropriately sized for the voltage and the current.

Among other things, the IGBT also capacitors are used without resistance. You will then be referred to as the snubber capacitor.

Use

Snubberglieder example, be used in contacts of power relays, contactors and other electrical contacts, if they have to switch inductive loads. The bouncing and opening of the contacts in a circuit which comprises inductances occur without steep surge capacitor, because the rate of change of the current is large. These spikes are intercepted and damped by a snubber, because the rate of current change is reduced: The current first flows in the switching off in the original amount instead of via the contacts to the condenser and then decreases with increasing charge on the capacitor corresponding to the decreasing magnetic energy. At the same time electrical energy is transformed into heat by the current flow in the resistor, which would otherwise lead to a resonant vibration. This makes it possible to avoid spark discharges and to increase contact life considerably. Furthermore, the emission of radio interference is greatly reduced. When switching the resistance ensures that the inrush current during discharging of the capacitor is limited.

A further object of Snubbergliedern is to limit the voltage rise speed of semiconductors on a non-critical for this value. Thyristors / triacs show otherwise undesirable overhead ignition. IGBTs and bipolar transistors may even be destroyed. In MOSFETs the Miller capacitance leads to rapid voltage rise to unclean switch-off and increased switching losses.

Have IGBTs or mechanical contacts ( eg for automotive ignition coil with a mechanical interrupter ) Snubber often only a capacitor without resistance. This snubber capacitor must be made very low inductance and pulse resistant.

On the speaker output of an audio amplifier circuit, an external Snubberglied often provides ( here usually called Boucherot link ) that the amplifier to the inductive- resistive load of a speaker works stably and does not perform so-called "wild" oscillations. The effect is partly due to the fact that the inductive impedance rise of the connected speakers is partially compensated capacitive through the opposing decrease in the impedance of the Snubbergliedes at high audio frequencies and the amplifier thus finds about the same real load resistance across its operating frequency range, then what overall stability ( = swing release ) of the NF- amplifier increases along with its negative feedback links. Above about 20 kHz, the Boucherot link in small selected Boucherot resistance acts increasingly as a short circuit for the output signal and thus acts like a frequency compensation ( = cut-off frequency limit) the operational amplifier, the amplitude drop at the output is stronger than the signal propagation time influence of the amplifier, the is expressed by the frequency-dependent phase angle between input and output signal. That No signal frequency within the operating frequency range and find something about it on the reinforcing defining external and internal counter couplings of the amplifier a Mitkopplungsbedingung before that could otherwise produce immediately the dreaded self-excitation ( "wild" oscillation).

Operation

When you open a switch (contact or semiconductor device ) to the inductive load is connected, arises without suitable wiring a steep voltage rise to breakthrough ( switching arc or avalanche breakdown ) because the inductance produces a high voltage, as it seeks to maintain the current flow.

The capacitor of the Snubbergliedes takes over when the contact opens temporarily the flow of current, so that the switch opens without sparks or with limited slew rate.

The resistor has two functions:

• Attenuation of the RF oscillations
• Limiting the discharge current of the capacitor when the switch

Disadvantage: In the opened state to flow in alternating current, due to the AC resistance (impedance ) of the capacitor, always a current through the snubber and the consumer. This can be avoided in some cases by the Snubberglied is switched directly via the (inductive) load. Since the impedance of the supply system in this case is in series ( the low voltage network is usually inductive! ), Reduces the interference suppression. This can be avoided by a further capacitor connected across the supply source (so-called "X- capacitor " often part of the input filter ).

Designs

Snubberglieder resistor and capacitor are either built using discrete components or housed together in a single housing. The magnitude of the resistance and its capacity must be matched to the capacitor and the switch. Common values ​​are 1 ... 4 ohms ( at the output of audio amplifiers ) to 10 ... 100 ohms ( switch contacts to the mains voltage ).

The size of the capacitor must be matched to the switching inductance, both of which form when switching off together form a resonant circuit whose peak voltage during turn-off can be much higher than the supply voltage. The capacitor must be very reliable, because its short to burn the resistance or a short circuit of the switching path leads. In audio amplifiers film capacitors are used with values ​​around 1 uF here usually.

In switching power supplies partly Snubberglieder be used consisting of a diode and an inductor, record the voltage spikes during the switching process and implement than active energy. Thereby, the overall efficiency is further increased.