What is a freewheeling diode?

What is a freewheeling diode? What does it do? Since freewheeling diodes are connected in parallel at both ends of the coil, when the coil passes current, it will generate an induced electromotive force at both ends. When the current disappears, its induced electromotive force will produce a reverse voltage on the original components in the circuit. When the reverse voltage is higher than the reverse breakdown voltage of the original component, the original component such as a transistor will be damaged. Freewheeling diodes are connected in parallel at both ends of the line. When the current flowing through the coil disappears, the induced electromotive force generated by the coil is consumed by doing work through the loop formed by the diode and the coil. This protects the safety of other components in the circuit.

It is connected in reverse parallel at both ends of the relay or inductor coil in the circuit. When the inductor coil is powered off, the electromotive force at both ends does not disappear immediately. At this time, the residual electromotive force is released through a diode. The diode that plays this role is called a freewheeling diode. In fact, it is still a diode, but it plays a role in freewheeling. For example, the diode connected in reverse at both ends of the relay coil or the two ends of the unidirectional thyristor are connected in reverse. Why should a diode be connected in reverse? Woolen cloth?

Because the coil of the relay is a large inductor, it can store electrical energy in the form of a magnetic field, so when it pulls in, it stores a large amount of magnetic field. When the transistor controlling the relay changes from conduction to cut-off, the coil is powered off, but there is The magnetic field will generate a reverse electromotive force at this time, and the voltage can be as high as 1000V or more, which can easily break down the transistor or other circuit components. This is because the access of the diode is exactly in the same direction as the reverse electromotive force, and the reverse electromotive force passes through the freewheeling diode in the form of current. It neutralizes and protects other circuit components. Therefore, it is generally a diode with a relatively fast switching speed. Like a thyristor circuit, the thyristor is generally used as a contact switch. If a large inductance load is controlled, it will The principle of generating high voltage back electromotive force is the same as that of a relay. It is also used in monitors and is generally used in coils of degaussing relays.

Often used together with energy storage components to prevent sudden changes in voltage and current and provide a path. The inductor can provide a continuous current to the load through it to avoid sudden changes in the load current and play a role in smoothing the current! In a switching power supply, you can see a freewheeling circuit composed of a diode and a resistor connected in series. This circuit is connected in parallel with the primary side of the transformer. When the switch tube is turned off, the freewheeling circuit can release the energy stored in the transformer coil to prevent the induced voltage from being too high and breakdown of the switch tube.

Generally, you can choose a fast recovery diode or a Schottky diode to release the reverse potential generated by the coil!

In Figure 3, when KR is turned on at VT, the upper voltage is positive and negative, and the current direction is from top to bottom. When VT is turned off, the current in KR is suddenly interrupted, which will generate an induced potential. Its direction is to try to keep the current unchanged, that is, it always wants to keep the direction of the KR current from bottom to bottom. This induced potential is superimposed with the power supply voltage and is added to both ends of VT, which can easily cause VT to break through. Add VD to this to short-circuit the induced potential generated by KR. The electric injection is what you call "flowing clockwise in the small circuit of the diode and relay", thereby protecting VT. R and C in Figure 2 also use the principle that the voltage on C cannot change suddenly to absorb the induced potential. It can be seen that the "freewheeling diode" is not an actual component, it just plays a role in the circuit called "freewheeling".

What is the role of freewheeling diode in forward switching power supply?

In a forward switching power supply, when the MOS is turned off, the secondary side of the transformer relies on the energy stored in the inductor to provide current to the outside world. In order for the inductor to play this role when there is a load, a freewheeling diode is added to the secondary side of the transformer. When the MOS is turned off, the inductor, load and freewheeling diode will create a path to transfer the energy in the inductor to the outside.

Only when there is an external load, the current flows through the freewheeling diode

In converter technology, what role does the freewheeling diode play in the circuit?

Among electronic converter circuits, the single-phase bridge rectifier in the rectifier part is the most commonly used single-phase rectifier circuit. The three-phase bridge rectifier is the most commonly used method in power systems, especially generator excitation systems. Both circuits must be connected to freewheeling diodes. Its function is roughly the same. Taking a single-phase bridge circuit as an example: when the controllable rectifier bridge is connected to an inductive load, since the inductor current cannot change suddenly, during the shutdown period of the thyristor, it must be connected at both ends of the load. The freewheeling diode maintains the path of the inductor current to prevent dangerous overvoltage at both ends of the inductive load when the thyristor is turned off and prevents the thyristor from commutating and conducting.

However, the three-phase bridge rectifier circuit that is widely used in generator excitation systems can be divided into three-phase half-controlled bridge circuit and three-phase fully controlled bridge circuit. Therefore, in order to ensure reliable commutation of the rectifier components, the half-controlled bridge needs to connect freewheeling diodes in parallel at both ends of the inductive load, while the fully controlled bridge does not need to do this. When the conduction angle changes, the average voltage and line current of the half-controlled bridge change more slowly than that of the fully controlled bridge.

Nowadays, equipment such as frequency converters that are widely used contains converter circuits such as rectifiers and inverters. The freewheeling diodes used in them are generally added to the DC bus inside the frequency converter. That is Because if the load is an inductive component and the large-capacity inverter on the bus fails, huge reverse surge energy will be generated on the DC bus. At this time, we need to provide a discharge channel for this energy, otherwise huge The energy will breakdown or burn out the small inverter. This channel requires a diode, so it should be a freewheeling diode.

When a unidirectional half-wave controlled rectifier circuit has a large inductive load, why is it necessary to add a freewheeling diode? The unidirectional half-wave controlled rectifier circuit has a large inductive load. When the thyristor is cut off in the negative half cycle, the inductive load will produce a very high reaction. This reverse electromotive force is enough to cause the thyristor to break down and burn. After adding a freewheeling diode, the reverse electromotive force can be discharged into the forward voltage drop of the diode (about 0.7v), thereby effectively protecting the thyristor.

Freewheeling diode The diode usually refers to the inductor coil, relay supplier, thyristor supplier and other energy storage components connected in reverse parallel. When there is a sudden change in the voltage or current in the circuit, the Diodes that protect other components in the circuit. Freewheeling diodes are named because they play a role in freewheeling in the circuit. Generally, fast recovery diodes or Schottky diodes are selected as freewheeling diodes.

Introduction to freewheeling diodes

Taking an inductor coil as an example, when a current passes through the coil, an induced electromotive force will be generated at both ends of the coil. When the current disappears, its induced electromotive force creates a reverse voltage on the components in the circuit. When the reverse voltage is higher than the reverse breakdown voltage of the component, components such as transistors will be burned out. If a diode is connected in anti-parallel at both ends of the coil (sometimes a resistor is connected in series), when the current flowing through the coil disappears, the induced electromotive force generated by the coil will be consumed through the loop formed by the diode and coil, thereby ensuring that the circuit the safety of other components in it.

For relays, since the coil of the relay is a large inductor, it can store electrical energy in the form of a magnetic field, so it will store a large amount of magnetic field when it attracts. When the transistor controlling the relay changes from on to off, the coil will be powered off, but the magnetic field in the coil will not disappear immediately. The magnetic field will generate a reverse electromotive force, and its voltage can be as high as 1000v. Such high voltage can easily cause damage. through transistors or other circuit components. If we connect a diode in reverse parallel to both ends of the relay (for relays, a resistor is usually connected in series with the freewheeling diode to prevent the loop current from being too high), since the connection of the diode is exactly in the same direction as the reverse electromotive force, this way The reverse electromotive force can be consumed in the form of current to achieve the purpose of protecting other circuit components.

For thyristor circuits, since thyristors are generally used as a supplier of contact switches, if a large inductance load is controlled, high-voltage counter electromotive force will also be generated, and its principle is the same as that of a relay. Freewheeling diodes are also used in displays, usually on the coils of degaussing relays.

How does a freewheeling diode work?

The above figure shows a typical application circuit of a freewheeling diode, in which the resistor R depends on the situation to determine whether it is needed. When the energy storage element is turned on at VT, the voltage is positive and negative, and the current direction is from top to bottom. When the VT is turned off, the current in the energy storage element is suddenly interrupted. At this time, an induced potential is generated, and its direction is to try to keep the current unchanged, that is, it always wants to keep the current direction of the energy storage element from top to bottom. This induced potential is superimposed with the supplier voltage of the power supply and then added to both ends of VT, which can easily cause VT to breakdown. For this reason, VD can be added to short-circuit the induced potential generated by the energy storage element to protect VT. the goal of.

The function of freewheeling diode

Freewheeling diodes are usually used together with energy storage components. Their function is to prevent sudden changes in voltage and current in the circuit and provide a power-consuming path for reverse electromotive force. The inductor coil can provide a continuous current to the load through it to avoid sudden changes in the load current and play a role in smoothing the current! Among the suppliers of switching power supplies, you can see a continuous circuit consisting of a diode and a resistor connected in series. flow circuit. This circuit is connected in parallel with the primary side of the transformer.

When the switch tube is turned off, the freewheeling circuit can release the energy stored in the transformer coil to prevent the induced voltage from being too high and breakdown of the switch tube.

Selection of freewheeling diodes

Generally choose fast recovery diode or Schottky diode

Precautions for freewheeling diodes

Freewheeling diodes are usually used in switching power supplies, relay circuits, thyristor circuits, IGBT suppliers and other circuits, and their applications are very wide. You should pay attention to the following points when using it:

(1) The freewheeling diode is an effective means to prevent the high voltage generated by the self-induced potential from damaging related components when the DC coil is powered off!

(2) The polarity of the freewheeling diode cannot be connected incorrectly, otherwise it will cause a short circuit accident;

(3) The freewheeling diode is always connected inversely to the DC voltage, that is, the negative electrode of the diode is connected to the positive terminal of the DC voltage;

(4) The freewheeling diode works in the forward conduction state, not in the breakdown state or high-speed switching state. The above is an analysis of the working principle of freewheeling diodes. I hope it can help you.