In any electronic circuit, the energy stored in an inductor must be properly channeled in the event of switching. This FAQ explains how a freewheeling diode can help achieve smooth inductor discharging and prevent voltage spikes.
What happens when no freewheeling diode is used?
The purpose of using a freewheeling diode can be best understood by starting with a step-down chopper without using a freewheeling diode, as shown in Figure 1. The simplest step-down chopper consists of a voltage source, a semiconductor switch, and a load (resistor and inductor). All these components are connected in series.
When the switch is closed, it leads to a closed circuit path, allowing the current flow. The current originating from the voltage source passes through the load and returns to the voltage source.
When the switch is opened, it cuts off the voltage source from the load, preventing any current flow. For a purely resistive load, the sudden opening and closure of switches are acceptable, as the purely resistive load does not store energy like an inductive load. Therefore, no stored energy needs to be dissipated when the chopper switch turns off. This means there is no risk of voltage spikes from the sudden current interruption in inductive loads.
In practical cases, most electrical loads have inductive components, making the situation tricky. Such loads have a finite amount of energy stored after the switch is turned off. Such a phenomenon leads to three critical problems:
- Without a closed path, this current has no path, leading to an abrupt interruption. This can cause the load voltage to drop to zero immediately, disrupting the power supply to the load.
- A sudden interruption of current can generate high voltage spikes due to the inductor’s tendency to oppose changes in current (Lenz’s Law). These spikes can damage sensitive circuit components, including the switch itself.
- The absence of a closed path results in inefficient energy transfer. The energy stored in the inductor cannot be utilized effectively, leading to increased losses and reduced overall efficiency of the chopper circuit.
Therefore, if a closed path for the load current to flow when the switch is turned off can be found, it will solve the above three problems.
How does a freewheeling diode solve the problems?
Understanding the problems above brings us to the concept of a freewheeling diode, as shown in Figure 2. The freewheeling diode is a component that completes the current path when the switch is turned off.
Figure 3 shows the operation of the step-down chopper when a freewheeling diode is used. When the switch is turned on, the current flow path is as expected in Figure 2. In this case, the freewheeling diode is reverse-biased and does not allow current to flow through it.
However, when the switch is turned off, the freewheeling diode is forward-biased and allows a current path. Thus, the energy stored in the inductor is released through the freewheeling diode, allowing it to dissipate safely rather than causing harmful voltage spikes.
By providing a discharge path for the inductor’s energy, freewheeling diodes help maintain stable circuit operation. They improve the input power factor and ensure that load voltage polarity remains stable, which is essential for applications like phase-controlled rectifiers and motor drivers.
Freewheeling diodes also prevent the switching components from experiencing excessive reverse voltage, which can lead to failure or damage.
Figure 4 shows the input voltage (Vs), output voltage (Vout), and output current (Iout) of a step-down chopper with a freewheeling diode. The output current waveform increases and decreases nearly linearly during switch turn-on and turn-off times. This smooth current waveform is important to avoid abrupt energy flow in circuits and prevent circuit heating and damage.
We have taken a very simple circuit to understand the workings of a freewheeling diode. Still, their use extends to advanced circuits such as half-bridge inverters, full-bridge inverters, and motor drives, as shown in Figure 5.
The ratings of the freewheeling diode will vary according to the applications required. It can be observed from the solution tree that the voltage rating of the freewheeling diode varies to support a blocking voltage of 3300 V to 8000 V, while the switching frequencies need to support a wide range of 50 to 10,000 Hz.
Summary
A freewheeling diode is important in power electronics, particularly circuits involving inductive loads such as motors and relays. Its primary purpose is to protect circuit components from voltage spikes when the current flowing through an inductor is suddenly interrupted. If the current flow through an inductor needs to be continuous and smooth, the freewheeling diode can be used with an existing circuit.
References
Principle of Step Down Chopper | Power Electronics | Lecture 67, YouTube
Types of Chopper Circuits or DC to DC Converters, Electronics Mind
Freewheeling diode family, Infineon Technologies
Step-down Chopper with R Load, Virtual Labs
Step Down Chopper, GeeksforGeeks
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In speaking about a purely resistive load, you conclude with the statement “This means there is no risk of voltage spikes from the sudden current interruption in inductive loads.” I believe you meant “in non-inductive loads” as I’m sure others have pointed out as well. Easy typo to make. Cheers.