Safety Certified Capacitors Circuit Applications
Omar Al Taher, Applications Engineer
April 18, 2006 |
Safety Certified capacitors are designed to withstand high impulse voltages in
applications where human beings might be exposed to voltage surges. These capacitors
will shunt the energy from the impulse to ground, protecting the circuit from the surge.
Johanson Dielectrics offers Safety Capacitors certified to IEC384-14, EN132400 and IEC
60950.
Safety Capacitors are identified by their place in the circuit as either X capacitors or Y
capacitors.
X Capacitor: A capacitor for use in situations where failure would not lead to danger of
electric shock. This is also known as a line to line application.
Y Capacitor: A capacitor for use in situations where failure could lead to danger of
electric shock. This is also known as a line to ground application.
The impulse capability of the part is indicated using a number from 1 to 3 as follows: |
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| The impulse waveform is as follows: |
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The capacitor is subjected to 10 impulses of alternating polarity. The time T1 is known as
the rise time, this is the time it takes the pulse to go from 10% to 90% of the peak
voltage.
The T2 is known as time to half value and is the time it takes for the voltage to go from
50% of the peak back to same level.
In telecom applications the T1 = 10 µs and T2 = 700 µs (per IEC 60950)
In power mains applications the T1 = 1.2 µs and T2 = 50 µs (per IEC 60384-14)
Johanson Dielectrics Safety Capacitors meet both requirements
The impulse voltage is applied using the circuit shown in Figure 2. |
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| Safety Capacitors also have to pass an AC voltage endurance test for 1000 hours. The
waveform is shown below in Figure 3. |
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Safety Capacitors are primarily used two applications: 1) EMI filtering 2) Insulation
EMI Filtering:
In this application the capacitors are used for filtering EMI on the input of a circuit, so the
main function of the capacitor is EMI filtering. However, these capacitors bridge the
isolation barrier so they have to be able to withstand high impulse voltages in case of a
power surge.
Examples of this application:
- Capacitors connecting the Tip or Ring lines to ground in telecom applications to
meet EN 55024 (using X2Y3 or X1Y2 ratings)
- Capacitors used to filter the input side of an AC to DC power supply (using X1Y2
ratings).
Note: Some power supplies require a Y1 rated capacitor, which is not available on the
market in a surface mount package. Two Y2 capacitors in series are considered an
equivalent to a Y1 capacitor and are the only surface mount solution available. |
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Insulation:
The capacitors are used for insulation between circuitry connected to the Telecom
Network Voltage and any circuitry connected to protective earth. Johanson Dielectrics
Safety Certified Capacitors fulfill Basic Insulation and Supplementary Insulation
requirements of IEC 384-14 and IEC 60950. See Figure 6. |
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| Notice: Specifications are subject to change without notice. Contact your nearest Johanson Dielectrics
Sales Office for the latest specifications. All statements, information and data given herein are believed
to be accurate and reliable, but are presented without guarantee, warranty, or responsibility of any kind,
expressed or implied. Statements or suggestions concerning possible use of our products are made
without representation or warranty that any such use is free of patent infringement and are not
recommendations to infringe any patents. The user should not assume that all safety measures are
indicated or that other measures may not be required. Specifications are typical and may not apply to all
applications. |
