Read the selection and function of freewheeling diodes

1. Overview of freewheeling diodes

A freewheeling diode is usually a diode that is connected in parallel to an energy storage element such as an inductor coil, a relay, or a thyristor, and protects other components in the circuit when a sudden change in voltage or current occurs in the circuit.

Taking an inductor coil as an example, when a current flows through the coil, an induced electromotive force is generated at both ends thereof. When the current disappears, its induced electromotive force produces a reverse voltage to the components in the circuit. When the reverse voltage is higher than the reverse breakdown voltage of the element, the element such as a triode or the like is burned out. If a diode is connected in parallel with the two 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 is consumed by the loop formed by the diode and the coil, thereby ensuring the circuit. The safety of other components in the middle.

For a relay, since the coil of the relay is a large inductor, it can store electrical energy in the form of a magnetic field, so it stores a large amount of magnetic field when it is attracted. When the triode of the control relay changes from conduction to cutoff, the coil will be de-energized, but at this time the magnetic field in the coil does not disappear immediately. The magnetic field will generate a back electromotive force, and its voltage can be as high as 1000v. Such a high voltage is easy to strike. Wear such as triodes or other circuit components. If we reverse-parallel a diode at both ends of the relay (for relays, a resistor is usually connected in series on the freewheeling diode to prevent the loop current from being too high), since the diode is connected exactly in the same direction as the back EMF, The back electromotive force can be consumed as a current to protect other circuit components.

For the thyristor circuit, since the thyristor is generally used as a contact switch, if the control is a large inductive load, the high voltage back electromotive force is generated, and the principle is the same as that of the relay. A freewheeling diode is also used on the display, typically on the coil of the degaussing relay.

2. Working principle of freewheeling diode

The figure above shows a typical application circuit for a freewheeling diode, where the resistor R is determined depending on the situation. When the VT element is turned on, the voltage is up and down, 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, and an induced potential is generated at this time, the direction of which is to keep the current constant, that is, always want to keep the current direction of the energy storage element from top to bottom. This induced potential is superimposed on the power supply voltage and applied to both ends of the VT, which easily causes the VT to break down. For this reason, VD can be added, so that the induced potential generated by the energy storage element can be short-circuited, thereby achieving the purpose of protecting the VT.

3, the role of freewheeling diode

Freewheeling diodes are often used with energy storage components to prevent sudden changes in voltage and current in the circuit and to provide a power path for the back electromotive force. The inductor coil can provide continuous current to the load, so as to avoid sudden changes in load current and smooth current! In the switching power supply, a freewheeling circuit consisting of a diode and a resistor connected in series can be seen. 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 break through the switch tube.

4, the selection of freewheeling diode

Generally, a fast recovery diode or a Schottky diode such as FR254, FR255, FR256, FR257, 1N5204, 1N5205, 1N5206, 1N5207, 1N5208, 1N5404, 1N5405, 5406, 5407, 5408, etc. are selected.

5, the application of the freewheeling diode considerations

Freewheeling diodes are commonly used in switching power supplies, relay circuits, thyristor circuits, IGBTs, etc., and are widely used. Pay attention to the following points when using:

(1) Freewheeling diode is an effective means to prevent damage caused by high voltage generated by self-inductive potential when the DC coil is de-energized.

(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 reversed to the DC voltage, that is, the negative terminal of the diode is connected to the positive terminal of the direct current.

(4) The freewheeling diode is in the forward conduction state, not the breakdown state or the high speed switching state.