Induction heating principle and IGBT application topology analysis (Part 1)

Latest update time：2023-12-08

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I often encounter many colleagues and friends asking: Why do induction cookers and rice cookers use IGBTs? What characteristics do IGBTs need for this application? Today we will give you a detailed explanation of the principle of induction heating and the topological analysis of induction heating.

Before understanding the principle of electromagnetic induction heating, ask yourself a question. If there were no induction cookers in the world, what would you do if you wanted to boil a pot of water?

The most common way is to light the stove and put the pot on the fire.

Figure 1. Traditional gas heating

However, this method is an indirect heating method. The heat source first heats the pot, and then conducts the heat to the food in the pot through the pot. In this series of processes, energy loss will inevitably occur.

So, is there a way to turn the pot itself into a heat source and let the pot boil water directly?

This requires induction heating.

The principle of induction heating is shown in Figure 2:

The control board generates high-frequency AC current flowing through the copper coil through the resonant conversion circuit;

An induced magnetic field is generated on the working coil, and the induced magnetic field generates eddy currents at the bottom of the metal pot;

Eddy currents cause Joule heating through the metal's resistance through the skin effect, while hysteresis losses associated with the material's permeability also generate heat.

Figure 2. Eddy current effect

The equivalent circuit is shown below:

Figure 3. Equivalent circuit

Induction heating speed has the following characteristics:

The eddy current at the bottom of the cooking vessel is proportional to the magnitude of the current flowing through the induction coil, meaning that increasing the current to the induction coil will cause an increase in eddy current; the cooking vessel will heat up faster.

Higher frequencies will concentrate the eddy current density closer to the surface, which in turn will greatly reduce the cross-sectional area of the active current flow, thereby indirectly increasing resistance and increasing heating efficiency, so increasing the operating frequency will also speed up the heating of the vessel. speed.

Therefore, induction heating requires a resonant controller to generate alternating current on the coil, and in order to achieve a certain power, the current and switching frequency of the resonant controller are required to be relatively high, so the switching device must be selected that can conduct large currents and switch quickly. IGBT.

Figure 4. Effect of frequency on standard penetration depth of materials

The current mainstream topologies for induction heating include single-ended resonance and half-bridge resonance. The following figure provides a detailed comparison of the characteristics of the two topologies:

Figure 5. Induction heating application range and characteristics of resonant topology

Single-ended resonance is relatively simple to control and has advantages in overall cost;

As shown in the figure below, the disadvantage of single-ended resonance is that the power range is obviously restricted. For low power (<600W), there will be a hard turn-on that generates a large current and increases the temperature rise. For high power (>2.3KW), the temperature rise of a single tube is difficult to control. ;

Figure 6. Correspondence between power output and temperature rise of two topologies

In the next article, we will conduct a detailed analysis of the equivalent circuit currents of the two topologies based on the loop paths of the topologies.