Schemes and Techniques of DC Current Detection Circuit

introduction

    It is generally believed that SiC MOSFETs can achieve very fast switching speeds and help to significantly reduce the energy loss in the power conversion process in the power electronics field. However, due to the limitations of traditional power semiconductor packaging, the full potential of SiC components is not always available in practical applications. In this article, we first discuss some of the limitations of traditional packaging, and then introduce the benefits of using better packaging. , Showing the improvement effect obtained after the package improvement of the 3.7kW single-phase PFC using the Totem-Pole topology.

    Switching performance limitations caused by traditional packaging of power components

    TO-247N (Figure 1) is one of the most widely used traditional packages for power transistors. As shown on the left side of Figure 1, each pin of the device has parasitic inductance components. The right side of Figure 1 is a very simple and typical example of a gate drive circuit. It can be seen from these figures that the inductance components of the drain pin and the source pin will be added to the main current switching circuit. These inductances will cause the device to generate an overvoltage when the device is turned off. Therefore, it is necessary to ensure that the overvoltage is If the value meets the technical specifications between the drain and the source, it is necessary to limit the switching speed of the device.

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    Figure 1: Traditional packaging of power components and their parasitic inductance

    The parasitic inductance of the gate pin and the source pin is a part of the gate drive circuit, so this part of inductance needs to be considered when driving the MOSFET. In addition, this part of the inductance may also oscillate with the parasitic capacitance in the gate drive circuit. When the MOSFET is turned on, ID increases, and an electromotive force (VLS) is generated in the inductance (Ls) of the source pin. On the other hand, a current (IG) flows into the gate pin, and a voltage drop occurs due to the gate resistance (RG). Since these voltages are included in the gate drive circuit, they will reduce the gate voltage required for MOSFET to turn on, resulting in a slower turn-on speed, as shown in Figure 2.

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    Figure 2: LS causes the VGS in the chip to decrease (when it is turned on)

    One of the ways to solve this problem is to use a power component package with a "driver source" pin. By equipping the driver source pin to separate the source pin and the gate drive loop, the influence of the source inductance (LS) on the gate voltage during turn-on can be eliminated, so the turn-on will not be reduced due to voltage drop Speed, which can greatly reduce the conduction loss.

    Improved switching performance brought by TO-263-7L

    In addition to the TO-247-4L package, ROHM has also developed a TO-263-7L surface mount package to enrich the discrete SiC MOSFET product lineup. The TO-263-7L package can realize the Kelvin connection of the source pins of the SiC MOSFET. The advantages of this package are shown in Figure 3. It can be seen from the figure that the parts related to the gate drive and the main current path no longer share the inductance LS on the main source side. Therefore, the turn-on speed of the device can be made faster and the loss is smaller.

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    Figure 3: TO-263-7L surface mount package and its parasitic inductance

    Another advantage of using the TO-263-7L package is that the inductance of the drain and source pins is much smaller than that of the TO-247N package. Due to the large junction area of the drain pin, and the source pin can be composed of multiple short leads connected in parallel, the inductance (LD or LS) of the package can be reduced. In order to quantify the degree of improvement in component performance brought about by the new package, we compared the switching actions of the same SiC MOSFET chip in two different packages during turn-on and turn-off (Figure 4).

    Figure 4: Comparison of switching actions of 1200V/40mΩ SiC MOSFET

    (TO-247N: SCT3040KL, TO-263-7L: SCT3040KW7, VDS=800V)

    The switching transient curve at turn-on shows that the switching speed of the "SCT3040KL" in the three-pin package (TO-247N) is limited. One of the reasons is that the electromotive force of the source pin reduces the effective gate voltage and causes the current to change The time becomes longer, which causes the conduction loss to increase. As for the "SCT3040KW7" in a surface mount package (TO-263-7L) with a driver source, the current change time becomes very short, so the conduction loss can be reduced. In addition, due to the reduced parasitic inductance, the dI/dt of the SiC MOSFET in the TO-263-7L package is much higher when it is turned off, so the turn-off loss is also less than that of the TO-247N package.

    The figure below shows the relationship between the switching loss and the switching current achieved by the two packages. Obviously, the increase in the conduction speed of TO-263-7L packaged devices helps reduce switching losses, especially in the high current area.

    Figure 5: Comparison of switching losses of 1200V/40mΩ SiC MOSFET in TO-247N package and TO-263-7L package

    [Gate drive circuit: Miller clamp (MC) and Schottky barrier diode (SBD) for surge clamp are used]

    As shown in the above comparison data, the package with the driver source pin that can be connected to the gate drive loop and the parasitic inductance can be reduced, and the device performance can be exerted, especially in the high current area. Therefore, the total loss of the device is smaller under the same switching frequency; in addition, if reducing the loss is not the main goal, you can also increase the switching frequency of the device.

    Lineup of new surface mount package products

    In addition to the 1200V/40mΩ products mentioned above, ROHM’s product lineup also includes TO-263-7L packaged SiC MOSFET products with rated voltages of 650V and 1200V (Table 1). In addition, automotive-grade products that meet the reliability standards of automotive electronics are also planned.

    P/N

    30mΩ

    40mΩ

    60mΩ

    80mΩ

    105mΩ

    120mΩ

    160mΩ

    650 V

    SCT30xxAW7

    1200 V

    SCT30xxKW7

    Table 1: Trench SiC MOSFET lineup in TO-263-7L package

    Applicability of surface mount package SiC MOSFET in on-board charger (OBC)

    This article will use a 3.7kW single-phase PFC circuit as an application to illustrate the performance that a surface-mount package SiC MOSFET can achieve. This power-level single-phase PFC can be used as the input stage of a single-phase 3.7kW on-board charger, or as a component of an 11kW on-board charging system. In the latter case, combining three single-phase PFCs through a switch matrix can achieve single-phase drive or 11kW three-phase drive. See Figure 6 for the application block diagram.

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    Figure 6: Block diagram of 11kW OBC composed of multiple 3.7kW PFCs

    Figure 7 includes several applicable PFC circuit topologies. There is a diode rectifier circuit at the input end of the traditional boost PFC, so its efficiency improvement is limited. Two-phase bridgeless PFC and totem pole PFC can reduce the diode rectifier circuit, which can reduce the total conduction loss. However, it should be noted that although the two-phase bridgeless PFC can achieve high efficiency, it has the disadvantage that each bridge arm is only used within half the input cycle. Therefore, the ratio of the peak current of each device to the effective value of the current (the so-called "Crest factor") increases, so that the power cycle pressure on the power semiconductor is very high.

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    Figure 7: Conceptual diagram of single-phase PFC

    There are two different types of Totem Pole PFC. The simple type contains only two MOSFETs and two diodes. Since the diode switches at low frequencies, a device with a low forward voltage drop is selected. On the other hand, since the body diode in the MOSFET is used for commutation, it is very important to choose a device with excellent body diode characteristics. In addition, new wide-bandgap semiconductors (such as SiC MOSFETs) have body diodes that support hard switching, so they are very suitable for such applications. If you want to get better performance as much as possible, you can use active switches (such as SJ MOSFET) to replace low-frequency switching diodes to further reduce losses.

    In order to demonstrate the several performances that can be achieved with totem pole PFC, we implemented simulations. In the simulation, we verified the switching loss measurement value of the 650V/60mΩ SiC MOSFET in the TO-263-7L package. Assuming that the switching frequency is 100 kHz, we have modeled the semiconductor losses of both the high-frequency side bridge arm and the low-frequency side bridge arm. For the low-frequency bridge arm, because the influence of the switching loss is very small, only the conduction loss of the 60mΩ product is considered.

    The simulation result is shown in Figure 8. It can be seen from the figure that the efficiency is 98.7%, which appears near 60% of the nominal output power. The other losses at this stage are not modeled. Of course, in order to conduct a comprehensive analysis, not only the control circuit and the gate drive circuit, but also the loss of inductors and other passive components need to be considered. However, it is obvious that a high-performance PFC circuit can be realized in a totem pole PFC using a 650V SiC MOSFET.

    Figure 8: Estimated efficiency of totem pole PFC considering only semiconductor losses

    (Vin = 230V, Vout = 400V, fSW = 100 kHz, high-frequency side arm: SCT3060AW7, low-frequency side arm: 60m? product)

    Conclusion

    In this article, we confirmed the performance advantages of SiC MOSFETs in low-inductance surface-mount packages with driver source pins. The research results show that, especially under high current conditions, since the gate loop is not affected by the voltage drop caused by dI/dt and the source pin inductance, the conduction loss of the surface mount package product is greatly reduced. The overall reduction in package inductance also speeds up the turn-off speed of the SiC MOSFET. These two advantages significantly reduce the switching losses when the device is turned on and off. In terms of system, we have seen that the conversion efficiency of totem pole PFC using 650V SiC MOSFET with RDS(ON) of 60mΩ exceeds 98%, which will facilitate the realization of a very compact design. Therefore, it can be said that this is useful for on-board charging. It is a very important key point for the development of in-vehicle applications such as devices.