Protection diode
Protective diodes are semiconductor diodes which are used to protect against overvoltages and impermissible voltages. A distinction is made between different protection diodes.
Freewheeling diode
Freewheeling diodes (engl. Flyback diode ) serve for protection from an overvoltage when switching off an inductive load DC voltage (for. Example electric motor , the relay coil , pull magnet ). For this purpose, semiconductor diodes (referred to as freewheeling diodes in the circuit diagram ) are connected in parallel to inductive direct current consumers (in the circuit diagram: L with resistance component R L ) in such a way that they are loaded by the supply voltage in the reverse direction .
After switching off the supply voltage, the self-induction of the coil ensures that the current initially continues to flow in the original direction. Without a freewheeling diode, this leads to a voltage spike which is added to the operating voltage and can damage or destroy the switching path. With a freewheeling diode, however, the voltage peak is limited to the forward voltage of the diode (around 0.7 V for silicon). This protects the electronic components (for example semiconductors such as transistors ), but also switch contacts very effectively against overvoltage (in the picture to a maximum of 12.6 V). The current flows through the diode and the energy of the magnetic field, which corresponds to the area marked green, is largely converted into heat in the ohmic resistance of the coil and a small part in the diode.
The green area is determined by the stored magnetic energy. If the height of this voltage-time area is limited, the time to decay increases to compensate, which leads to a drop-out delay of the relay and pull magnet. The diode reverse voltage and the reverse voltage of the switching element only needs to be as high as the switching voltage (12 V in the image) in this type of circuit dimensioned to be. In addition, the permissible peak current of the diode must correspond to the current at the inductance at the time of switching off.
In order to shorten the switch-off delay, the green area must be allowed more height. There are several ways to do this: You can
- use a Boucherot element instead of the free-wheeling diode or
- Connect a suitable resistor or a Zener diode in series with the rectifier. The product of the resulting overvoltage and the duration is constant.
- Connect a Zener diode or a varistor parallel to the switching path or the switching transistor . These must have a higher reverse voltage than the operating voltage and limit the switching voltage to its breakdown voltage.
With these variants mentioned, the switching path must withstand a significantly higher switching voltage than the operating voltage of the consumer.
The switching diodes of flyback converters and switching regulators are also called freewheeling diodes. Here, too, they ensure that the magnetic energy of an inductance (storage choke or transformer) is diverted. Here, however, this energy is desired and generates the output power.
Protection diodes on IC
Protection diodes parallel to the input of integrated circuits (IC) are polarized in the reverse direction for permitted input voltages. Voltages above the operating voltage or below the ground potential are diverted against these potentials and thus cannot destroy the actual internal circuit. They are part of the input or output protection circuit of many ICs and make them robust against ESD and load shutdown peaks. In many cases it makes sense to wire the inputs and outputs of the circuits with a resistor. This limits the currents caused by the discharge and thus prevents overloading of the protective diodes.
If the ESD currents are not limited in a specific circuit, the discharge of the charges against the supply voltage can endanger the entire board. If the charges brought in by the discharge are not "used up" quickly enough by the circuit or are temporarily stored in the existing interference suppression capacitors, there can be considerable voltage peaks on the supply voltage. This risk can be countered, for example, with suitable Zener diodes or suppressor diodes , which are switched from the supply voltage to the circuit ground.
Schottky diodes are often used in order to react particularly quickly and to limit the voltage to a value below the so-called latch-up effect (breakdown of the so-called body diode and opening of a parasitic transistor in the circuit) .
In English they are called clipping diodes and the circuit is called clipping circuit . If there is an alternating voltage coupling, the diode becomes a clamping diode .
Special diodes for overvoltage limitation
To protect electronic circuits, special avalanche overvoltage protection diodes such as the suppressor diodes have been developed, which differ from Zener diodes in that they have a more rectangular characteristic, even with high currents. They can absorb very high energies. In addition to suppressor diodes, names are also Transzorb , Transil or TVS diode , depending on the manufacturer .
In contrast to Zener diodes, they are also available in a bipolar version, which consists of two suppressor diodes connected in series in opposite directions. These suppressor diodes can also be used for overvoltage protection in alternating voltage circuits. Bipolar suppressor diodes react within nanoseconds and are therefore also suitable as ESD protection.
Other special protective diodes are used in telecommunication networks : they do not have a square wave characteristic, but cause a short circuit in the event of overvoltage by triggering a switching path like a thyristor . They behave in a similar way to a gas discharge tube , but, in contrast to them, can also be manufactured for lower response voltages.
Varistors are also suitable as a fast protective element, but are not referred to as diodes.
The aforementioned components are called surge protectors in English .
For overvoltage protection circuits with thyristors, see clamp circuit ( crowbar ).
See also
- Swingback diode (protective diode in converters)
- Protective circuit
literature
- Ulrich Nicolai, Tobias Reimann, Jürgen Petzoldt, Josef Lutz: Application manual IGBT and MOSFET power modules . 2nd Edition. ISLE Verlag, 2015, ISBN 978-3-938843-85-7 ( PDF version ).
- Ralph Weißel, Franz Schubert: Digital circuit technology . 2nd Edition. Springer-Verlag, 1995, ISBN 978-3-540-57012-7 .
Individual evidence
- ↑ Waveform clipping with Schottky Diodes , Application Bulletin 14, Hewlett-Packard
- ↑ Waveform clamping with Schottky Diodes , Application Bulletin 15, Hewlett-Packard