Example of measuring volt-second capacity of switching transformer

Example 1: The line scan flyback switching transformer used in televisions is referred to as a high-voltage package. Its working principle also belongs to the flyback switching transformer. The inductance of its primary coil is 6 millihenries, the working voltage is generally 120V, the forward scanning time (pulse width) is 52, and the reverse scanning time is 12. Check whether its volt-second capacity is designed reasonably or whether it works in the best working state.

To this end, we can first calculate the maximum current Im flowing through the primary coil of the high-voltage package according to formula (2-150), and then find the value of its limiting current Imax, that is: the value of the superimposed current is selected during the test.

Substitute the known parameters into formula (2-150):

According to the above analysis, as well as Figures 2-55 and 2-56, during normal operation, the maximum current Im flowing through the primary coil of the high-voltage package should not exceed 70% of the limiting current value Imax. Therefore, the limiting current Imax flowing through the primary coil of the high-voltage package can be obtained to be 1.49 A.

The limit current Imax value calculated above is the value used to test the superimposed current of the primary coil of the high-voltage package. According to Figure 2-54, the current of the current source is set to 1.49 A, that is, the superimposed current of the primary coil of the high-voltage package is set to 1.49 A, and then the inductance of the primary coil of the high-voltage package is tested; if the value of the test result Lx is equal to or greater than 90% of the initial inductance L0, it means that the volt-second capacity design of the primary coil of the high-voltage package is qualified, that is, the magnetic induction intensity of the iron core of the high-voltage package basically works within the optimal state range; if the test result Lx is less than 90% of the initial inductance L0, it means that the volt-second capacity margin of the primary coil of the high-voltage package is too small and unqualified, that is, the magnetic induction intensity of the iron core of the high-voltage package works within the range close to the saturation zone, the hysteresis loss and eddy current loss are relatively large, and the switching transformer is prone to magnetic saturation.

For high-voltage transformers or switching transformers, in addition to testing the volt-second capacity, the leakage inductance of the primary coil of the high-voltage transformer or switching transformer should also be tested. The normal leakage inductance value is generally less than 2% of the primary coil inductance. If it is too large, it means that the air gap length left in the transformer core is too large, or the structure or winding method of the primary and secondary coils of the switching transformer is unreasonable.

Here, we will explain what you should pay attention to when testing with Figure 2-54. In Figure 2-54, the size of the isolation inductor LT is required to be more than 3 times the value of the test inductor Lx, and when measuring the initial inductance value L0 of the primary coil of the high-voltage package, it is best to connect it to the circuit. Here, the isolation inductor LT can be selected as an inductor with a silicon steel core of more than 20 millihenries, and the core of the inductor must have a certain air gap; the current source can be replaced by a voltage-stabilized power supply connected in series with a high-power resistor, as shown in Figure 2-57; or a voltage-stabilized power supply connected in series with a high-power crystal amplifier, as shown in Figure 2-58.

In Figure 2-57, E is a voltage-stabilized power supply, and R is a high-power resistor. A resistance range of 1 to 10 ohms is more appropriate. If the resistance is too large, the power loss will be very large. By adjusting the voltage output of the voltage-stabilized power supply, the size of the superimposed current can be adjusted.

In Figure 2-58, E is a voltage-stabilized power supply, Rx is an adjustable resistor, and Q is a transistor high-power amplifier (must have a heat sink); by adjusting the voltage output of the voltage-stabilized power supply or changing the resistance of the variable resistor, the magnitude of the superimposed current can be changed, but the voltage drop between the collector and emitter of the transistor high-power amplifier should not be greater than 10V, otherwise, the loss of the transistor high-power amplifier will be very large. Generally, the voltage-stabilized power supply has a current output indication, so there is no need to install an ammeter in the test circuit.

It is particularly pointed out here that when testing the initial inductance L0 of the high-voltage package or the primary coil of the switching transformer, the iron core of the high-voltage package must be demagnetized, otherwise, the test result will be inaccurate. Usually, the inductance of the primary coil of the switching transformer with magnetism is slightly larger than the inductance of the primary coil of the switching transformer without magnetism. For the method of demagnetizing the high-voltage package, please refer to the content of the following section "2-1-22-4. Demagnetization method of switching transformer".

In addition, the maximum value Imax of the superimposed current used to test the high-voltage package is generally several times the current (average value or effective value) flowing through the primary coil when the high-voltage package is working normally. For example: the high-voltage package tested in the above example has an average current IA of only about 0.42 A when working normally, but the value of the superimposed current Imax is 1.49 A; it can be seen that the value of the superimposed current Imax is 3.5 times the average current when working normally. Generally, the current density of the primary coil enameled wire of the high-voltage package is about 3A/mm2, so the maximum current density of the superimposed current flowing through the primary coil enameled wire of the high-voltage package is about 10.5 A/mm2.

Usually, when the temperature rise of enameled wire is 40 degrees, its maximum current density is about 13A/mm2 (DC). Therefore, by measuring the temperature rise of the high-voltage coil, we can also know whether the selection of the enameled wire of the high-voltage coil is reasonable.

Here we will briefly introduce the method of calculating the average current IA and its relationship with the maximum current Im and the limiting current Imax. Figure 2-59 is a diagram showing the relationship between the average current IA and the maximum current Im and the limiting current Imax.

In Figure 2-59, IA is the average current flowing through the primary coil of the high-voltage package, IAτ1 is the average current flowing through the primary coil of the high-voltage package during the forward scan; Im is the maximum current flowing through the primary coil of the high-voltage package during the forward scan; Imax is the limit value of the current flowing through the primary coil of the high-voltage package during the forward scan; τ is the TV forward scan time (52us), τ2 is the TV reverse scan time (12us), and τx is the limit value of the forward scan time.