05-07-2012, 02:12 PM
Seminar on Snubber Circuits
Seminar on Snubber Circuits.pdf (Size: 127.29 KB / Downloads: 658)
What is a snubber circuit?
Snubber circuits are necessary to maintain the safe function and
longevity of power switches. These circuits provide greater
efficiency, they raise the possible switching speed and reduce
the EMI. For easy understanding it can be said that a snubber
circuit is a protection circuit of power switches.
Why should snubber circuits be installed?
In the real world a switched load appears as an impedance. So
when these loads are switched, there are unwanted voltage
transients or surge currents which have a negative effect on the
switching elements and the controlling circuit. These effects can
have immediate or time delayed consequences which can lead
to malfunction or even defects of the switching elements and the
controlling circuit. If snubber circuits are well designed, they can
reduce or even eliminate these effects. Therefore it is necessary
to use snubber circuits with relay contacts because electric arcs
can be minimized and a longer life of the contacts can be
achieved. In addition the number of switching cycles can be
increased which will allow savings in material and maintenance
costs. If SCRs (SemiConductor Relays), especially thyristors
and triacs, are used it snubbers are necessary to control the
rise of the voltage along the anode to cathode track. If this
voltage rises to fast it can lead to unwanted turn-on of the
device. If this happens, a faultless action of the device can’t be
guaranteed anymore and it could fail.
Functionality of a snubber circuit
The snubber circuit absorbs the energy which is stored in the
load. To keep the switching device within its safe operating
range, it is necessary to protect the switch. If the load is
inductive, the switch-off can be problematic because the selfinduction
wants to maintain the current flow which leads to
voltage transients which can be in the range of kilo- to
megawatts. These voltage transients should be avoided with
relay contacts because they induce an electric arc which causes
migration of the contact material and may lead to the destruction
of the contact.
Placement of the Snubber Circuit
For the placement of the snubber, the type of load must be
known. Because of the fact, that the protection circuit has to be
designed according to the load, it is impossible to design a
universal protection circuit.
For an inductive load the snubber must be connected either in
parallel to the switch, or in parallel to the load. A simple snubber
consists of a resistor and a capacitor. If the snubber is
connected in parallel to the switch, the current flows through the
protection circuit at switch-off and then decays. Because of the
connection in parallel to the switch, there is a constant current
flow through the snubber when the switch is open. To keep this
current flow to an acceptable level, a well designed circuit is
needed.
Design process of a relay contact
protection circuit with the template
With the provided Excel spreadsheet it should be easy and
simple for the designer to calculate the optimum values for a
particular application. The calculation is provided for inductive
loads. In the beginning the designer has to decide about the
type of placement. According to this the correct Excel
spreadsheet has to be chosen.
After choosing the type of placement the frequency and the
RMS value of the switched voltage have to be inserted in the
relevant fields.
The next step is to find out the nominal current of the load. This
current value can be found in the datasheet of the switched
device or it must be measured. The value is needed for the
calculation of the parameters of the protection circuit. The
calculation assumes that the load is a pure inductance, so all
parameters are calculated for the worst case.