Snubber
The word snubber, which comes from technical English, describes a device for damping undesired vibrations in technical applications.
The term is most commonly used in electronics . Snubber but can also mechanical vibration , such as shock absorbers , chain damper or steering damper , as well as hydraulic / pneumatic pulsation damper or silencer be, in these applications, however, the term is less common.
Electric snubbers
A snubber element is an electrical circuit that is supposed to neutralize interfering high frequencies or voltage peaks that usually occur when switching inductive loads when the current flow is interrupted abruptly. It is used to achieve better electromagnetic compatibility , to quench sparks on switch contacts and to limit the rate of voltage rise on semiconductor switches ( thyristors , IGBT , bipolar transistors ).
The circuit is also known in German-speaking countries as a Boucherot element , RC extinguishing combination, RC spark extinguishing combination or RC extinguishing element (contactor technology). In contrast, protective diodes and other electronic interference suppression measures are also referred to as snubbers in the English-speaking world .
Components
An RC snubber is a series connection of a capacitor with a resistor . These components must be dimensioned to match the voltage and current .
Capacitors without resistance are also used on IGBTs. They are then called a snubber capacitor .
use
Snubber links are used, for example, on contacts of power relays , contactors and other electrical contacts when they have to switch inductive loads. When bouncing and opening of the contacts in a circuit, the inductors contains arise without steep capacitor voltage spikes, because the rate of change of the current is large. These voltage peaks are intercepted and dampened by a snubber because the current rate of change is reduced: When the device is switched off, the current initially flows into the capacitor at its original level instead of through the contacts and then drops as the capacitor charges as the magnetic energy decreases. At the same time, the current flow in the resistor converts electrical energy into heat, which would otherwise lead to a resonance oscillation. This makes it possible to avoid spark discharges and significantly increase the contact life. Furthermore, the emission of radio interference is greatly reduced. When switching on again, the resistor ensures that the inrush current is limited when the capacitor is discharged.
Another task of snubber elements is to limit the rate of voltage rise in semiconductors to a value that is uncritical for them. Thyristors / triacs otherwise show undesirable overhead ignition. IGBTs and bipolar transistors can even be destroyed. In the case of MOSFETs , the Miller capacitance leads to unclean switch-off behavior and increased switching losses if the voltage rise is too rapid .
IGBTs or mechanical contacts (e.g. vehicle ignition coil with mechanical interrupter ) often only have a capacitor without a resistor as a snubber. This snubber capacitor must have a very low induction and pulse-proof design. Plastic film capacitors are mainly used as snubber capacitors .
At the loudspeaker output of an audio amplifier circuit, an often external snubber element (usually called a Boucherot element here) ensures that the amplifier works stably on the inductive-complex load of a loudspeaker box and does not carry out so-called "wild" oscillations. The effect is based, on the one hand, on the fact that the inductive increase in impedance of the connected loudspeakers at high AF frequencies is partially capacitively compensated by the opposing drop in impedance of the snubber element, and the amplifier thus has approximately the same real load resistance over its operating frequency range, which then results in overall stability (= Freedom from vibration) of the LF amplifier together with its negative feedback elements. Above about 20 kHz, the Boucherot element increasingly acts as a short circuit for the output signal when the Boucherot resistance is selected and thus acts similarly to frequency compensation (= limit frequency limitation) in operational amplifiers , with the amplitude drop at the output being greater than the signal propagation time influence of the amplifier is expressed by the frequency-dependent phase angle between the input and output signal. In other words, no signal frequency within the working frequency range and anything above that finds a positive feedback condition via the amplification-defining external and internal negative feedback of the amplifier, which otherwise could immediately generate the dreaded self-excitation ("wild" oscillation).
functionality
When a switch (contact or semiconductor component) to which an inductive consumer is connected is opened, there is a steep voltage rise up to the breakdown ( switching arc or avalanche breakdown ) without suitable wiring , because the inductance generates a high voltage as it strives to maintain the flow of current .
The capacitor of the snubber element temporarily takes over the current flow when the contact is opened, so that the switch opens without sparks or with a limited rate of voltage rise.
The resistance has two tasks:
- Damping of the HF oscillations
- Limitation of the charging current of the capacitor when closing the switch
Disadvantage: In the open state, with alternating current, due to the alternating current resistance ( impedance ) of the capacitor, a current always flows through the snubber and the consumer. This can be avoided in some cases by switching the snubber element directly via the (inductive) consumer. Since in this case the impedance of the supply network is in series (the low-voltage network is mostly inductive!), The interference suppression effect is reduced. This can be avoided by using an additional capacitor (so-called "X capacitor", often part of the line filter ) connected via the supply source .
Designs
Snubber elements made up of a resistor and a capacitor are either discreet or housed together in one housing. The size of the resistor and its load capacity must be matched to the capacitor and the switch. Common values are 1–4 Ω (at the output of audio amplifiers) to 10–100 Ω (switching contacts on mains voltage).
The size of the capacitor must be matched to the inductance to be switched. When switched off, both form an oscillating circuit, the peak voltage of which can be significantly higher than the supply voltage when switched off. The capacitor has to be very reliable because its short circuit leads to the burn-up of the resistor or to a short circuit of the switching path. Foil capacitors with values of around 1 µF are mostly used in audio amplifiers.
In switched-mode power supplies, snubber elements consisting of a diode and an inductance are sometimes used, which absorb voltage peaks during the switching process and convert them into active energy. This further increases the overall efficiency.