Flyback diode

Protection diodes are semiconductor diodes that are used to protect against surges and unauthorized voltages. A distinction is made between different protection diodes.

Freewheeling diode

Freewheeling diodes (English flyback diode) are used to protect against over- voltage when switching off an inductive DC load (eg electric motor, relay coil, pull magnet ). These are semiconductor diodes ( in the diagram: freewheeling diode ) so parallel to inductive DC loads: connected ( in the diagram L with resistance component RL ) that they are claimed by the supply voltage in the reverse direction.

After switching off the supply voltage, the inductance of the coil causes the first current continues to flow in the original direction. Without free-wheeling diode which results in a voltage spike which adds to the operating voltage, and can damage or destroy the switching path. With a freewheeling diode, the spike voltage is limited to the forward voltage of the diode ( about 0.7 V in case of silicon ). This protects the electronic components ( such as semiconductors such as transistors ), but also switch contacts very effective against overvoltage (pictured at maximum 12.6 V). The current flows through the diode, and the energy of the magnetic field which corresponds to the green shaded area, is mostly converted into heat in the ohmic resistance of the coil and to a small extent in the diode.

The green surface area is determined by the stored magnetic energy. If the magnitude of this voltage -time area is limited, increases to compensate for the time to alleviation, which leads to a fall delay of relay and pull magnet. The diode reverse voltage and reverse voltage of the switching element must be only as high as the switching voltage to be measured ( in the picture 12 V) this type of circuit. In addition, the permissible peak current of the diode must match the current across the inductor at the time of shutdown.

To shorten the turn-off delay, you have the green area to concede more height. There are several ways to accomplish this: One can

• Use a Boucherot member instead of the freewheeling diode, or
• A suitable resistor or a Zener diode in series with the rectifier switch. The product of the voltage and the thus formed over time is constant.
• Parallel with the switching path and the switching transistor insert a Zener diode or a varistor. They must have a higher blocking voltage than the operating voltage and to limit the switching voltage at its breakdown voltage.

In these above-mentioned variants, the switching section has a much higher switching voltage than the operating voltage to bear the load.

The switching diodes of flyback converters and switching regulators are referred to as free-wheeling diodes. Again, they ensure that the magnetic energy of an inductor ( choke or transformer ) derivative. Here, however, this energy is desired, and produces the output.

Protection Diodes on IC

Protection diodes in parallel to the input of integrated circuits ( IC) are poled for allowing an input voltage in the reverse direction. Voltages above the supply voltage or below the ground potential to be discharged to this potential, and can not destroy the actual internal circuit. They are part of the input or the output protection circuit of many IC and make them robust against ESD and overload switch-off. In many cases it is useful to be wired to the inputs and outputs of the circuits with a resistor. This limits caused by the discharge current, thereby preventing overloading of the protection diodes.

Is dispensed in an actual circuit, a current limitation of the ESD current, the discharging of the charges to the supply voltage can jeopardize the entire board. If the introduced through the discharge loads from the circuit is not fast enough "consumed" or buffered in the existing suppression capacitors, it can result in significant voltage spikes on the supply voltage. This danger can be met, for example, with suitable Zener or suppressor diodes that are switched from the supply voltage to the circuit ground.

In order to react quickly and to limit the voltage to a value below the so-called latch-up effect ( the so-called break-through body diode and opening a parasitic transistor in the circuit ), Schottky diodes are often used.

In English they are called clipping diode, called the circuit as clipping circuit. Is AC coupled, it is a clamping diode of the diode.

Special diodes for overvoltage limitation

To protect electronic circuits special avalanche surge protection diodes have been developed such as the suppressor, which differ from Zener diodes in that they have a more rectangular characteristic even at high currents. You can absorb very high energies. Terms are in addition to suppressor depending on the manufacturer also Transzorb, transil or TVS diode.

There are, in contrast to Zener diodes in bipolar execution that consists of two counter- suppression diodes connected in series. This suppressor can also be used for surge protection in AC circuits. Bipolar Suppressors respond within nanoseconds and are therefore also suitable for ESD protection.

More special protection diodes found in telecommunication networks use: they have no square wave, but also cause a short circuit with overvoltage by as ignites a switching path in a thyristor. They behave like a gas discharge tube, but can be made ​​in contrast to those for smaller response voltages.

Varistors are also suitable as a rapid protection member, but not referred to as a diode.

The aforementioned components are known in English as a surge protector.

At the Overvoltage protection circuits with thyristors see clamp circuit (English crowbar ).