What are the factors that increase the frequency of high-frequency switching power supplies?

Under the guidance of market applications, switching power supply products are increasingly demanding small, lightweight, high efficiency, low radiation, low cost and other characteristics to meet a variety of electronic terminal equipment. In order to meet the current portable electronic terminal equipment, the switching power supply must be small. The light weight features, therefore, increasing the operating frequency of the switching power supply has become a problem that designers are paying more and more attention to. However, what are the factors that limit the frequency increase of the switching power supply?

First, the switching frequency is increased, the loss of the power device is increased.

1. What are the factors that limit the switching frequency of the switch tube?

a, switching speed

The loss of the MOS transistor is composed of switching loss and driving loss, as shown in Fig. 1: the turn-on delay time td(on), the rise time tr, the turn-off delay time td(off), and the fall time tf.

figure 1

Take FAIRCHILD's MOS as an example, as shown in Table 1: FDD8880 switching time characteristics table.

Table 1

For this MOS tube, its limit switching frequency is: fs = 1/(td(on)+ tr+ td(off)+ tf) Hz=1/(8ns+91ns+38ns+32ns) =5.9MHz, in actual design In the control switch duty cycle to achieve voltage regulation, the conduction and turn-off of the switch tube can not be completed instantaneously, that is, the actual limit switch frequency of the switch is much less than 5.9MHz, so the switching speed of the switch tube itself limits the switching frequency.

b, switching loss

The corresponding waveform diagram when the switch is turned on is shown in Fig. 2(A). The corresponding waveform diagram when the switch is turned off is shown in Fig. 2(B). It can be seen that the VDS voltage of the switching transistor and the flow through the switch tube are turned on and off each time the switch tube is turned on and off. The current ID has overlap time (yellow shaded position in the figure), resulting in loss P1, then the total loss PS = P1 * fs at the switching frequency fs operating state, that is, the number of times the switch is turned on and off when the switching frequency is increased The more, the greater the loss.

figure 2

Summary: Switching speed and switching loss are two factors that limit the switching frequency.

1. The iron loss of the transformer limits the frequency increase.

The iron loss of the transformer is mainly caused by the eddy current loss of the transformer. As shown in Fig. 3, when the high-frequency current is applied to the coil, a changing magnetic field is generated in the conductor and outside the conductor perpendicular to the current direction (1→2→3 and 4 in the figure). →5→6). According to the law of electromagnetic induction, a changing magnetic field generates an induced electromotive force inside the conductor, and this electromotive force generates eddy currents in the entire length direction (L side and N side) of the conductor (a→b→c→a and d→e→f→d). Then, the main current and the eddy current are strengthened on the surface of the conductor, and the current tends to the surface. Then, the effective AC cross-sectional area of ​​the wire is reduced, resulting in an increase in the AC resistance (eddy current loss coefficient) of the conductor and an increase in loss.

image 3

As shown in Fig. 4, the transformer iron loss is proportional to the kf power of the switching frequency, and is related to the magnetic temperature limitation. Therefore, as the switching frequency increases, the high-frequency current flows in the coil to generate a severe high-frequency effect. Thereby reducing the conversion efficiency of the transformer, resulting in an increase in the temperature of the transformer, thereby limiting the increase in the switching frequency.

Figure 4

Second, the switching frequency is increased, EMI design, PCB layout is more difficult

Assuming that the power device loss described above is solved, the real high frequency also needs to solve a series of engineering problems, because at high frequencies, the inductor is not the inductor we are familiar with, the capacitor is not the capacitor we know, all the parasitic parameters. Corresponding parasitic effects will be generated, which will seriously affect the performance of the power supply, such as the parasitic capacitance of the transformer's primary side, the leakage inductance of the transformer, the parasitic inductance and parasitic capacitance between the PCB wiring, which will cause a series of voltage and current waveform oscillations and EMI problems. The voltage stress of the switch tube is also a test.

To improve the power density of switching power supply products, the first consideration is to increase the switching frequency, which can effectively reduce the volume of transformers, filter inductors and capacitors, but it is faced with losses caused by switching frequency, which makes the design of temperature rise heat dissipation difficult. The increase in frequency will also lead to a series of engineering problems such as driving and EMI.