PWM DC/DC Converter Diodes

There are two types of diodes used in PWM DC/DC converters in switching power supplies: fast recovery diodes and Schottky diodes.

1 Fast recovery diode

In a switching power supply, in addition to the power frequency rectifier, most diodes also work in a high-frequency switching state. Therefore, the dynamic switching characteristics of the diode are very important, mainly the forward characteristics and reverse recovery characteristics. In use, the diode is required to have a small forward transient voltage drop, a short reverse recovery time, a small reverse recovery charge, and a fast recovery characteristic.

(1) Turn-on characteristics. The turn-on time characteristics of the diode are shown in Figure (a). A higher forward peak voltage Ufp appears at the beginning of the turn-on. When the voltage drops to 2V, it is the forward recovery time tfr, and then continues to drop to the steady-state voltage drop (such as about 1V). diF/dt is the current change rate, which is related to factors such as the internal mechanism of the device, the length of the lead, and the presence or absence of magnetic materials. As the junction temperature increases, the forward recovery time tfr and the peak voltage Ufp also increase.

(2) Turn-off characteristics. When a diode is passing a large forward current and a reverse voltage is suddenly applied, the recovery process of its reverse blocking capability is shown in Figure (b).

As shown in Figure 1, the current and voltage waveforms during the diode switching process

① At the instant tf, a reverse voltage is applied (the circuit voltage becomes negative), and the forward current IF decreases at a rate of diF/dt. The diF/dt is determined by the reverse voltage U and the distributed inductance L.

② At the instant t0, the current passes through zero. Before the carrier disappears, the diode has not restored its blocking ability, and the reverse recovery current IR flows. The current increases in the reverse direction, and the forward voltage drop (determined by the carrier) decreases slightly.

③ At the moment t1, the charge Q1 has been drawn away, the current reaches the maximum reverse recovery current IRM, the diode begins to restore its blocking ability and begins to withstand reverse voltage.

④ After the moment t1, the ability to withstand reverse voltage increases rapidly, and the reverse recovery current drops rapidly according to |dirr/dt|. The voltage with higher inductance in the circuit inductance plus the power supply voltage obtains the maximum reverse voltage URM.

⑤ At the moment t2, the reverse current is reduced to IRM/4, and Q2 has been removed.

⑥ During t0~t2: the charge drawn away for the reverse recovery time trr is the reverse recovery charge QR=Q1+Q2. At this point, the definition stage of reverse recovery ends, and the diode is basically in the stage of bearing static reverse voltage.

Reverse recovery charge QR is an important parameter. When the forward current is constant, the smaller the QR, the shorter the reverse recovery time trr. QR is related to the diode forward current diF/dt and the junction temperature. When the forward current and the current change rate increase, QR increases. When the junction temperature increases, the carrier lifetime increases and QR also increases. During the fast recovery of the diode, the energy consumed in the diode is determined by the following formula:

In the formula, the first term is related to the reverse recovery charge QR. In order to reduce losses, a diode with a small QR and a small reverse recovery time trr=t2-t0 should be selected; the second term is the energy stored in the inductor L, and the inductance (including distributed inductance) in the circuit should be reduced.

The falling speed of the reverse recovery current dirr/dt is also an important parameter. If dirr/dt is too large, due to the presence of inductance L and capacitance C in the circuit, strong oscillation occurs, parasitic interference is generated, and the reverse peak voltage URM is too high, and even the diode is damaged. The concept of soft characteristics and hard characteristics can be used to represent the impact of dirr/dt on reverse characteristics. The softness and hardness of the characteristics are measured by the "soft factor", and the soft factor Sr is defined as

Figure (a) shows the current curve of a hard characteristic device when Sr=0.3, and Figure (b) shows the current curve of a soft characteristic device when Sr=0.8. In practical applications, a diode with a large Sr should be selected. If Sr is large, the reverse voltage peak URM that the diode withstands is small.

As shown in Figure 2, the current waveform of the diode reverse recovery process

At present, there are two types of fast recovery rectifier diodes: PN type and PIN type. Under the same capacity, the PIN type has the advantages of lower turn-on voltage drop and good reverse fast recovery performance. However, the PIN type diode has hard recovery characteristics, while the PN type has soft recovery characteristics. In actual use, you can choose according to the application conditions.

The comparison of the three high-speed rectifiers is shown in the table below.

Comparison of three high-speed rectifiers as shown in the table