Switching Power Supply Principle and Design (Series 80)

2-1-22-3. The significance of the volt-second capacity of the switching transformer

The limiting volt-second capacity VTmax parameter of the switching transformer or energy storage inductor is actually as important as the maximum collector voltage BVceo parameter of the transistor. In a transistor amplifier circuit, when the voltage across the collector and emitter of the transistor exceeds the maximum collector voltage BVceo, the transistor will be broken down and damaged. Similarly, in a switching power supply, when the volt-second capacity (voltage amplitude and time length) applied to the switching transformer exceeds the limiting volt-second capacity VTmax, the switching transformer will also be damaged, and the power switch tube and other circuit components will also be damaged.

The meaning of the volt-second capacity of the switching transformer is equivalent to the area of ​​the rectangle bounded by the oblique line in Figure 2-56. The two sides that determine the size of the area are respectively composed of the product of the operating voltage (DC pulse amplitude) V of the switching transformer and the continuous power-on time T (pulse width). Its limit volt-second capacity VTmax is equivalent to the area formed by Vmax as one side and τmax as the other side. The gray area is equivalent to the area of ​​the volt-second capacity of the switching transformer when it is working normally.

However, it should be emphasized here that as long as the area of ​​the volt-second capacity does not exceed the area of ​​the limiting volt-second capacity, either side of V or T can exceed the length of Vmax and τmax on the V or T side shown in Figure 2-56.

Combining Figure 2-55 and Figure 2-56, we can see that when using a switching transformer, it is best to make the maximum operating current flowing through the switching transformer coil approximately equal to that in Figure 2-55, or to make the width of the switching pulse approximately equal to.

Figure 2-56 Significance of the volt-second capacity of a switching transformer

When the maximum operating current flowing through the switching transformer coil is equal to Ib in Figure 2-55, the inductance of the switching transformer coil is the maximum value Lmax; in this case, the switching transformer has the highest working efficiency because the product of the switching transformer core loss and the switching transformer coil loss is the smallest (hysteresis loss is proportional to the magnitude of the excitation current and to the square of the increase in magnetic induction intensity; eddy current loss is proportional to the square of the increase in magnetic induction intensity; copper resistance loss is proportional to the length of the wire); and the switching transformer's volt-second capacity VTb is still a long way from the limit volt-second capacity VTmax.

At present, most switching transformers use ferrite cores as core materials. The saturation magnetic flux density Bs of such ferrite cores is generally 4500 to 5000 gauss. Therefore, as can be seen from Figure 2-55, the optimal magnetic flux density Bb of the switching transformer core is about half of the saturation magnetic flux density Bs, that is, Bb = 2300 to 2500 gauss. Therefore, when using formula (4) to calculate the primary coil of the switching transformer, the value of the maximum magnetic flux density Bm in the formula should not exceed 2500 gauss.

Due to the dispersion of the magnetic saturation magnetic flux density Bs parameter of the switching transformer core, how can we know that the switching transformer core is working at the position of the optimal magnetic flux density Bb? Or when we get a switching transformer, what pulse width, duty cycle, or operating frequency should we choose to be reasonable?

This must be determined by measuring the volt-second capacity of the switching transformer. At the same time, it can also be checked whether the air gap length of the switching transformer core is appropriate.

Below we further analyze the practical significance of volt-second capacity through the example of measuring the volt-second capacity of a switching transformer.