Application of SiC Schottky Diode in PFC Circuit

[Introduction]At present, in the new technology revolution to realize “green energy”, many high-frequency switching power supplies have begun to realize high power factor correction technology (especially in communication power supplies), most of which use active power factor correction. Continuous conduction mode Boost converter is a widely used power factor correction converter in power system.

In the hard-switched continuous conduction mode Boost conversion, the reverse recovery of the boost diode will cause large reverse recovery loss and excessive di/dt, resulting in serious electromagnetic interference. While improving the power factor, it is particularly important to improve the thermal stability of the switching tube and the semiconductor tube, and to reduce electromagnetic interference (EMI), voltage stress and current stress.

At present, many soft-switching technologies and lossless absorption circuits are applied to PFC circuits, which have achieved good results, but the increased components increase the cost and reduce the reliability of the power supply.

This paper mentions a new material, silicon carbide (SiC). The Schottky barrier diode made of it has the characteristics of a positive temperature coefficient and a reverse recovery time close to zero, which reduces the turn-on loss of the MOSFET on the PFC and improves the efficiency. Taking it a step further, this argument can be tested by making a 500W AC/DC power supply.

1. Characteristics of silicon carbide diodes

In recent years, silicon carbide materials have made great progress in the application of Electronic device technology, and silicon carbide materials have more prominent advantages than general-purpose silicon. This is mainly because the silicon carbide material has a higher electric field breakdown voltage of 2.4 × 106V/cm, faster charge movement speed, and wider energy band gap than general-purpose materials, and the thermal conductivity of the material is 2 to 3 that of silicon. times.

These advantages make the Schottky barrier diode based on silicon carbide exhibit high temperature characteristics (allowing the maximum operating temperature to reach 300 ° C, which is twice that of silicon materials), high reverse withstand voltage, and low on-resistance. and high switching frequency. The above features can minimize the volume of the series switching device in the power system, and the increase of the switching frequency also further reduces the volume of the system.

2. Influence of steady-state and transient characteristics of SiC diodes on PFC

The basic topology of the continuous mode Boost converter is shown in Figure 1. It is widely used in power factor correction circuits, and the Inductor current is in continuous mode. In this circuit, the diode steady-state and transient characteristics have a great influence on the PFC circuit.

Figure 1 Boost converter

1) Steady state characteristics – forward voltage Uf

As shown in Figure 2(a), the forward voltage drop of the silicon ultrafast recovery diode (15A/600V) was tested at room temperature. At 2-5A, the forward voltage drop is basically unchanged and close to saturation. From another side, it shows that the forward voltage drop of the silicon diode becomes smaller when the temperature is high, and the diode has negative temperature characteristics.

As shown in Fig. 2(b), the forward voltage drop of the SiC Schottky diode (4A/600V) was tested at room temperature. When the load current changes from 0 to 4A, the forward voltage drop basically increases linearly. From another aspect, it shows that the forward voltage drop of the SiC Schottky diode increases linearly at high temperature, indicating that the SiC diode has positive temperature characteristics.

Figure 2 Load current and forward voltage drop

In high-power PFC circuits, diodes may need to be used in parallel to expand the capacity, and the problem of uniform current distribution of the device needs to be considered. The characteristics of the forward voltage and on-resistance of the diodes are the key. The positive temperature coefficient characteristic of SiC Schottky diodes can ensure the current sharing requirements when the devices are connected in parallel.

Assuming that due to some reasons, the two SiC diodes are in a state of uneven current, and one of the diodes distributes a larger current, then its on-resistance and forward voltage drop will increase accordingly, hindering the further increase of the current. , so as to promote the current distribution again and finally reach the current equilibrium state. Since the diode made of silicon material has negative temperature characteristics, the problem of current sharing of the device will be further aggravated, which is not conducive to the stability of operation. Therefore, SiC Schottky diodes are suitable for direct device paralleling.

2) Transient characteristics – reverse recovery current

There are many types of diodes, but only the Schottky barrier diode’s task of carrying current is completed by the majority carriers, there is no excess minority carrier recombination, and the recovery time is very small, about tens or hundreds of ps, the disadvantage is that its withstand voltage is very low. Other silicon diodes (such as ordinary diodes, fast diodes, ultra-fast recovery diodes) and other current-carrying tasks are performed by minority carriers, and there is a problem of reverse recovery time. The two ultra-fast recovery diodes used have Trr times of 30ns and 13ns respectively, but this problem of reverse current cannot be avoided.

Due to the characteristics of the material, the SiC Schottky diode has the advantages of both, not only the withstand voltage is very high, but also the reverse recovery characteristics and temperature characteristics are very good. The reverse current and reverse recovery time of the silicon rectifier will increase with the increase of temperature. The reverse recovery time and reverse current of SiC Schottky diodes are very small, and they have very good temperature characteristics, and their reverse recovery time will not change with temperature.

As shown in Figure 3, at room temperature of 25 °C, the reverse recovery time of the ultrafast recovery diode is 3 times that of the SiC Schottky diode, and the reverse current is 4 times that of the SiC Schottky diode. At a high temperature of 150°C, the reverse recovery time of the ultrafast recovery diode is 6 times that of the SiC Schottky diode, and the reverse current is 12 times that of the SiC Schottky diode.

Figure 3 Comparison of reverse recovery characteristics of SiC diodes and ultrafast recovery diodes at different temperatures

In general, we all want the reverse recovery time of diode D1 in a single-phase PFC circuit to be as short as possible. The reverse recovery current will bring us many problems, such as diode reverse recovery losses, and the resulting severe MOSFET turn-on losses. Many soft-switching or lossless absorption techniques are applied to PFC circuits. Figure 4 is a typical lossless absorption application. The purpose is to overcome the problem caused by the diode’s reverse recovery time. It can realize close to zero-current turn-on and zero-voltage turn-off of the main switch tube, and at the same time, the boost diode is turned off at zero current, which improves the efficiency of the PFC. However, in this kind of circuit, the resonant voltage of the diode will be relatively high, even reaching the rated voltage of the diode, and at the same time, many components are used, which increases the cost and reduces the reliability of the system.

Figure 4 PFC lossless absorption circuit

In order to verify that SiC Schottky diodes can bring new improvements to PFC circuits, we fabricated a 500W AC/DC power supply and compared it with an ultrafast recovery diode (DSEP15-06A). The circuit parameters shown in Figure 4 are as follows:

Output: 535W (53.5V/10A);

Input: 90VAC;

Q1: IRF460A (500V/22A);

D1: 650V/4A SiC Schottky diode/DSEP15-06A;

L1: 400μH;

C0: 440μF/450V;

Frequency f: 70 kHz.

At room temperature of 25°C, under full load, the ultrafast recovery diode and the SiC Schottky diode are used as D1 for comparison. The reverse recovery characteristics of the ultrafast recovery diode at room temperature of 25°C are shown in Figure 5. The forward current IF is 7.5A, the reverse current is 6.5A maximum, the reverse recovery time is 40ns, and the reverse recovery voltage of the diode is the highest It reaches 460V and stabilizes after 5 oscillations.

The reverse recovery characteristics of SiC Schottky diodes are shown in Figure 6. The forward current is the same, the maximum reverse current is 0.7A (89% less than the ultrafast recovery diode), and the reverse recovery time is 12ns (70 %), the diode reverse recovery voltage is 380V (18% reduction), and there is no subsequent oscillation, and the turn-off loss is also reduced accordingly.

Figure 5 Ultrafast recovery diode turn-off current, voltage waveform

Figure 6 SiC diode turn-off current, voltage waveform

When the diode is turned off, there is a problem of reverse recovery time, which causes the turn-on current of the MOSFET to increase when it is turned on in this interval. The larger the reverse barrier capacitance of the diode, the larger the turn-on peak current of the MOSFET.

The MOSFET turn-on current and voltage waveforms are shown in Fig. 7(b) when the ultrafast recovery diode is used, and the peak value of the MOSFET turn-on current is as high as 11.4 A. The turn-on current, voltage and turn-on loss waveforms of the MOSFET when the SiC Schottky diode is used are shown in Figure 7(a), and the peak value of the MOSFET turn-on current is only 6.5A. The turn-on loss (area) of the latter is nearly 2/3 smaller than the turn-on loss (area) of the former.

Figure 7 Full load, MOSFET turn-on waveform

Through the above analysis, the forward voltage of silicon carbide is 2.00 V at the rated current value, which is higher than the forward voltage (1.30 V) of the ultrafast recovery diode. Therefore, the conduction loss of silicon carbide is higher than that of the ultrafast recovery diode, but the conduction loss only accounts for a small part of the total power loss. The key is to reduce the switching loss of semiconductor devices. The effect of reducing the turn-on losses of the MOSFET is particularly pronounced with the use of SiC Schottky diodes.

In the 90V AC input test, the efficiency of the whole machine increased from 85% to 86%, and the loss was reduced by about 6W: when the 220V AC input was used, the efficiency of the whole machine was over 90%. Therefore, the heat sink can be appropriately reduced, and the frequency can be appropriately increased, thereby saving costs.

3. Summary

There are many benefits to using SiC Schottky diodes in power PFC circuits. First of all, the efficiency of the power supply has been significantly improved. When other conditions remain unchanged, the loss can be reduced by simply replacing the diode. At the same time, since soft switching or lossless absorption technology no longer needs to be considered, the development cycle of the power supply is further shortened and the number of components is reduced. The circuit structure is further simplified; more importantly, it reduces the electromagnetic interference to the surrounding circuits, improves the reliability of the power supply, and makes the product more competitive.

(This article is reproduced from the Power Electronics Technology and Application WeChat public account)