Comparison of several schemes for reverse recovery

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Comparison of several schemes to suppress the reverse recovery of power diodes

0 introduction

high frequency power diodes are widely used in power electronic devices. However, in the process of changing the PN junction power diode from on to off state, there are the following common photo reverse recovery phenomena of several special engineering plastics. This will cause problems such as increased diode loss, reduced circuit efficiency and increased EMI. This problem is more prominent in high-power power supply. RC absorption, series saturation reactor absorption, soft switching circuit and other switching softening methods are commonly used to solve this problem, but there are few research reports on the comparison of their effects. Taking buck circuit as an example, this paper compares these schemes, and draws useful conclusions through experiments and simulations

1 diode reverse recovery principle

taking the ordinary PN junction diode as an example, the carrier in the PN junction has diffusion movement due to the concentration gradient, and at the same time, there is drift movement due to the action of the electric field. After the two are balanced, a space charge region is formed in the PN junction. When there is a forward bias at both ends of the diode, the space charge region is reduced, and when there is a reverse bias at both ends of the diode, the space charge region is widened. When the diode suddenly applies the reverse voltage in the on state, the leakage of the storage tank will soon catch fire, and the stored charge will return to its own area or be compounded under the action of the electric field, thus generating a reverse current

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2 several methods to solve the reverse recovery of power diodes

there have been many schemes to solve the reverse recovery of power diodes. One idea is to start from the device itself and find new materials to fundamentally solve this problem. For example, the emergence of silicon carbide diodes has brought the dawn of the device revolution, and there is almost no reverse recovery problem. Another idea is to soften the reverse recovery of power diodes by adding some devices or auxiliary circuits from the perspective of topology. At present, silicon carbide diodes have not been put into practice in large numbers, and their high cost restricts their popularization and application. The softening circuit under the second idea is widely used. Taking a 36V input, 30v/30a output circuit with a switching frequency of 62.5khz (as shown in Figure 1) as an example, this paper compares several switching softening methods

Figure 1 buck circuit

2.1 RC absorption

this is a common method to solve the reverse recovery problem of power diodes. Parasitic parameters should be considered for power diodes operating at high frequencies. Figure 2 (a) shows the circuit model, where D is the ideal diode, LP is the lead inductance, CJ is the junction capacitance, RP is the parallel resistance (high resistance value), and RS is the lead resistance. RC absorption circuit is shown in Figure 2 (b), C1 and R1 are connected in series and connected in parallel to power diode d0. When the diode is turned off in reverse, the energy in the parasitic inductance charges the parasitic capacitance, and at the same time, it also charges the absorption capacitance C1 through the absorption resistance R1. In the case of absorbing the same energy, the larger the absorption capacitance, the smaller the voltage on it; When the diode turns on rapidly in the forward direction, C1 discharges through R1, and most of the energy will be consumed on R1

(a) power diode circuit model (b) RC absorption circuit

(c) series saturated reactor (d) diode reverse recovery softening circuit

Figure 2 common scheme to solve the reverse recovery problem of power diode

2.2 series saturated reactor

this is another common method to solve this problem, as shown in Figure 2 (C). Generally, ferrite magnetic rings and amorphous alloy magnetic rings can be used as saturated reactors. According to literature [1], when using saturated reactor to solve the problem of diode reverse recovery, the commonly used manganese zinc ferrite is effective, but the energy loss is greater than that of amorphous materials. With the development of material technology, the properties of amorphous saturated magnetic materials have been greatly improved in recent years. In this paper, the magnetic ring (model: mt1284.5w) made of amorphous material of Toshiba company is wound around 2 turns as a saturated reactor

corresponding to figure 3 (a) and figure 3 (b), the current passing through d0 in stage I is large, the reactor LS is saturated, and the inductance value is very small; In the second stage, when the diode current begins to drop, LS is still very small; In the third stage, the diode current is reversed, and the reverse recovery process begins (TRR is the reverse recovery time). The LS value increases quickly, which suppresses the increase of the reverse recovery current. In this way, the current becomes a soft recovery with a small di/dt, reduces the diode loss, and suppresses an important noise source at the same time; Stage IV diode reverse recovery ends; In the fifth stage, the diode is turned on again. As the current increases, LS soon saturates

(a) reverse recovery current waveform

(b) saturation reactor magnetization curve

Figure 3 Schematic diagram of saturation reactor inhibiting diode reverse recovery

2.3 soft switching circuit

Figure 2 (d) is an effective diode reverse recovery softening circuit [2]. LK is the leakage inductance of transformer. N is the transformer turn ratio, where n=3 is taken, and its working process is shown in Figure 4

(a) stage 1

(b) stage 2

(c) stage 3

(d) stage 4

(E) stage 5

Figure 4 working principle of soft switch

stage 1 as shown in Figure 4 (a), the switch s has been turned on, d0 is in the reverse cut-off state, and the excitation inductance LM and leakage inductance LK are linearly charged. Phase 2 switch S is turned off, and the parasitic capacitance CP of S is charged. This process is very short and can be approximately regarded as linear, as shown in Figure 4 (b). Stage 3D0 and DB are on, as shown in Figure 4 (c). The current in diode d0 of stage 4 gradually decreases to 0 under the action of leakage inductance LK, as shown in Figure 4 (d). Phase 5 switch S is on, as shown in Figure 4 (E), and the current in branch diode DB continues to drop, falling to 0 before s is turned off

in Figure 4 (c), d0 is on, ud0 is 0. When in Figure 4 (d), ud0= - u2=u0/(1 + n), this is verified by the test waveform in Figure 5 (d)

3 experimental results

Figure 5 shows the terminal voltage waveform of diode d0 under various conditions

(a) d0 end voltage without reverse recovery inhibition measures

(b) d0 end voltage after parallel RC absorption

(c) d0 end voltage after series connection into saturated reactor LS

(d) d0 end voltage after softening circuit

Fig. 5 experimental waveform

it can be seen from the waveform in Fig. 5 that the voltage burr of diode reverse recovery is reduced, indicating that the three schemes have inhibitory effects on diode reverse recovery. Although the reverse recovery of diode is suppressed by RC absorption circuit, the voltage burr and oscillation of reverse recovery are still obvious. After using the softening circuit, as previously analyzed, theoretically, the reverse recovery current should be reduced to zero, but due to the influence of stray parameters in the circuit, the voltage waveform still oscillates during diode shutdown. The series saturation reactor has the best inhibition effect on diode reverse recovery

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4 conclusion

the popularization and application of silicon carbide may be the fundamental solution to the problem of diode reverse recovery. At present, RC absorption circuit is mainly used. Series saturated reactor and softening circuit are also effective schemes to suppress diode reverse recovery. Theoretical analysis and experiments show that series amorphous saturated reactor is the most simple and effective, which is expected to be further popularized

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