A freewheeling diode is a protective diode. We use this diode only for protecting the semiconductor switches. Such semiconductor switches are diodes, transistors (BJT/MOSFET/IGBT), thyristors, etc.
- In the above figure, we connect a diode across the load (at the left side in the picture). This diode is the Freewheeling Diode.
- We also call a freewheeling diode with other names like Flyback Diode, Snubber Diode, Commutating Diode etc.
Definition of Freewheeling Diode
A Freewheeling Diode is a protective diode. We connect it across an inductive load to protect a switch. It saves the switch from the sudden voltage-spike encountered due to the inductive load.
Let’s understand the Definition in pieces
- First of all, it is a protective device.
- What does it protect? The Semiconductor Switch.
- We normally connect a freewheeling diode across both the inductive loads as well as the switches.
- What is sudden voltage-spike? We know that the inductor has a property to oppose any change in the current through it.
But what happens if we open an inductive circuit, through which some current is flowing?
Then, the inductor induces a large voltage across its terminals which may be sufficient to damage a semiconductor switch easily. Mathematically, the voltage across an inductor relates the current by the equation;
If the inductor suddenly becomes open circuited then the current i decreases to zero suddenly. “Suddenly” means in a very short time interval i.e. dt is almost zero. Then from the above equation if dt tends to zero then V tends to infinity. That is the voltage across the inductor raises to a very high value. We technically refer to this phenomenon as Voltage-spike./li>
Now we will examine what may happen if we connect an inductive load directly through a semiconductor switch. When we close the switch this voltage-spike appears across the switch. The voltage spike may damage the switch. The freewheeling diode helps in protecting the switch from this voltage-spike.
Design of Freewheeling Diode
Characteristics of an Ideal Freewheeling Diode
- The diode must have very large forward current carrying capacity so that the freewheeling diode carry current without burning.
- It should have a low forward voltage drop nearly 0.2 V
- Also, it must undergo a quick reverse bias operation.
Based on the above characteristics, we generally prefer to use a Schottky Diode for the freewheeling diode.
How does the Freewheeling Diode work?
Consider the circuit as shown below (it is the same circuit as above). This is a step-down-chopper with thyristor as the switch and with an inductive RL load.
We can divide the operation of the chopper into two modes.
i) Mode I: When the switch (thyristor) is ON.
ii) Mode II: When the switch is OFF.
In Mode I, the freewheeling diode has no role i.e. it remains open circuited (i.e. reversed biased) as shown below.
In Mode II, the freewheeling diode comes into action. HOW?
- During Mode I, the inductor has stored magnetic energy. Now we suddenly open the switch (i.e. switched off). Hence, this can cause voltage-spike.
- Now suppose a freewheeling diode is in the circuit. Then after we make the switch off, the stored energy of the inductor makes the freewheeling diode forward biased. As a result, the current starts to flow through the freewheeling diode in Mode II.