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PFC Circuits: Examination of Appropriate Gate Driving Voltages

2021.08.18

Points of this article

・When a SiC MOSFET is used as a switching element, because the change in R_on with changes in the gate driving voltage V_GS is considerable, the V_GS setting is important.

・When studying the gate driving voltage V_GS of a SiC MOSFET, a balance between efficiency and safety should be considered.

PFC Circuits: Examination of Appropriate Gate Driving Voltages

Here, appropriate values for the gate driving voltage V_GS of a SiC MOSFET that is to be used as a switching element in a PFC circuit are examined.

Example Circuit

As an example circuit, the simulation circuit “A-2. PFC BCM Diode-Bridge-Less V_in＝200V I_in＝2.5A” in the Power Device Solution Circuit/AC-DC PFC list is used (see Fig. 9). Details of the circuit diagram can also be seen here.

In this example, simulations are used to examine an appropriate V_GS value to drive the SCT2450KE SiC MOSFET shown in Fig. 9, which is the low-side switching element.

Fig.9 : “A-2. PFC BCM Diode-Bridge-Less V_in＝200V I_in＝2.5A” PFC simulation circuit

Relation between On Resistance and Gate Driving Voltage

As shown in Fig. 10, the on-resistance R_on of a conventional Si (silicon) MOSFET is essentially constant with respect to V_GS while in the on state. In contrast, in the case of a SiC MOSFET R_on changes considerably depending on V_GS, as shown in Fig. 11, and so the V_GS setting is even more important than for a Si MOSFET. In other words, if the V_GS value of a SiC MOSFET is too low, conduction losses increase and efficiency is worsened. Conversely, if V_GS is set too high with the intention of improving efficiency, there is the concern that ratings may be exceeded. Hence it is extremely important that V_GS be set appropriately.

Fig. 10: Relation between R_on and V_GS for a Si MOSFET

Fig. 11: Relation between R_on and V_GS for a SiC MOSFET

Examination of Appropriate Gate Driving Voltages

Fig. 12 shows the results of simulation of the efficiency when the value of V_GS for the SiC MOSFET in the PFC circuit of Fig. 9 is changed.

Fig. 12: Efficiency simulation results when the V_GS of a SiC MOSFET is changed

When V_GS is 14 V or lower, an increase in R_on causes the efficiency to drop sharply. This phenomenon is more prominent at lower temperatures; in this region there is a strong possibility of device failure, making use impossible. On the other hand, the higher the value of V_GS, the better is the efficiency, but if the rated value (V_GS = 22 V) is exceeded, use is not possible. Hence when considering a balance between efficiency and safety given these simulation results, we can say that a V_GS of around 18 V is appropriate. And in actuality, it is recommended that ROHM SiC MOSFETs normally be used at a V_GS of around 18 V.

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