Introduction

​SiC MOSFETs are increasingly replacing silicon MOSFETs and IGBTs in high-voltage switching applications.

However, SiC MOSFETs have specific requirements for gate drive, and unless the optimum gate driver product is selected, the characteristics of SiC MOSFETs cannot be fully utilized.

This article provides an overview of SiC MOSFET characteristics and discusses key design considerations related to optimal gate drive design.

In order to compare the characteristics of silicon MOSFETs and SiC MOSFETs, we will compare and explain the characteristics of the following two products.

・Silicon MOSFET: NTH4L040N65S3F
(onsemi super junction MOSFET SUPERFET® III, FRFET®, 650 V, 65 A, 40 mΩ, TO-247-4L)

・SiC MOSFET: NTH4L045N065SC1
(onsemi SiC MOSFET, 650 V, 55 A, 33 mΩ, TO-247-4L)

Comparison of silicon MOSFET and SiC MOSFET characteristics

Transconductance (gFS)

The transconductance is MOSFET gate voltage change (V _GS-V _TH) drain current to (I _D.) at the rate of change, MOSFET defines the gain of the output with respect to the input of .

I _D = _gFS ×(_VGS-_VTH)

The transconductance of SiC MOSFET is lower than that of silicon MOSFET.

・Silicon MOSFET

・SiC MOSFETs

Analog Devices LTspice® Using the silicon used in the comparison this time MOSFET When SiC MOSFET of g _FS was simulated.

(Blue: Silicon MOSFET, Green: SiC MOSFET)

From this result, it can be confirmed that the silicon MOSFET has a steep rise in ​_ID due to its high ​_gFS, but the current saturates above a certain ​_VGS.

but, SiC MOSFET is low g _FS by I _D. rises gently, but the current does not saturate, so V. _GS the higher SiC MOSFET It can be seen that the benefits of low on-resistance can be obtained.

The horizontal axis indicates V _GS and the vertical axis indicates ​_ID. The drain voltage (V _DS) is assumed to be 400V.

On resistance (RDS(on))

Silicon MOSFET on-resistance always has a positive temperature coefficient when V _GS >V _TH,

SiC MOSFETs are dominated by the negative temperature coefficient channel resistance (R _CH) when ​_V​​ ​​_GS is low, and the positive temperature coefficient JFET resistance (R _J) and drift region resistance (R _DRIFT) becomes dominant.

Due to the influence of the resistance component with these two temperature coefficients, the relationship between the junction temperature and on-resistance of the SiC MOSFET becomes a parabolic characteristic as shown in the figure.

(Left: Silicon MOSFET, Right: SiC MOSFET)

From the results of simulating the temperature characteristics of the on-resistance,

SiC MOSFET of V. _GS It can be seen that if is increased, the characteristics will have a positive temperature coefficient without being affected by this negative temperature coefficient.

(green: V. _GS =15V / Pink: V. _GS =18V / black: V. _GS =20V)

Horizontal axis: Junction temperature * The unit [V] in the graph is replaced with [°C].
Vertical axis: ON resistance
I _D =100A.

Input capacitance (CISS)

SiC MOSFET dies are much smaller than silicon MOSFET dies.

This makes SiC MOSFET input capacitance of C. _ISS is silicon MOSFET, thereby reducing the gate charge Q. _G. and SiC MOSFET silicon is better MOSFET is smaller than

・Silicon MOSFET

・SiC MOSFETs

To limit the gate drive current, the time constant of the gate signal can be adjusted by increasing or decreasing an external high series gate resistor to control the dV/dt transition of VDS with a greater degree of freedom. I can do it.

Gate charge QG

Ueno”Input capacitance (C _ISS)” As explained in SiC MOSFET gate charge of Q. _G. is silicon MOSFET is smaller than

However, in the V _GS vs gate charge (Q _G) curve, silicon MOSFETs start at V _GS =0V, while SiC MOSFETs start at V _GS =-5V.

this is SiC MOSFET To fully discharge the gate of V. _GS of-5V means that you must pull down to

(left:silicon MOSFET,right: SiC MOSFET)

again, SiC MOSFET of V. _TH is low, and at the minimum value 1V There are many things that come close.

for that reason, V. _GS But 0V to VDD switching in the range of , unexpected turn-on due to spurious gate noise, V. _DS of dV/dt lead to the problem of not being able to ensure margin for turn-on causing

Therefore, SiC MOSFETs require a negative voltage ​_VGS.

What is required for SiC MOSFET gate drivers

From what has been explained so far, SiC MOSFET gate drivers are required to have a switching capability that can drive V _GS from a negative voltage to a high positive voltage of about 20V.

This wide turn-on transition requires a peak source current that can quickly charge the SiC internal gate capacitance to limit switching losses in SiC MOSFETs.

Assuming that the entire turn-on event occurs within ⊿ t < 10ns, the total swing of ​_V GS is ⊿ V _GS =30V, and C _ISS =1870 pF, then the required peak current is ​_IG(SRC) =5.61A.

When turning off the SiC MOSFET, it must sink a large peak current to discharge the SiC MOSFET 's CISS capacitance as quickly as possible.

In addition, low V. _TH have SiC MOSFET must be pulled below ground and held there, the gate driver's sink capability must be much higher than it sources current.

onsemi SiC MOSFET and gate driver lineup

In addition to 650V​ ​SiC MOSFETs, onsemi has released a large lineup of 900V and 1200V​ ​SiC MOSFETs and 900V and 1200V half-bridge and full-bridge SiC MOSFET modules.

Onsemi 's SiC products feature a patented termination structure that provides superior robustness against harsh environmental conditions.

We have also released a gate driver lineup with large current drive capability suitable for SiC MOSFET gate drive.

We have a lineup of non-isolated type, galvanic isolated type, single output type and dual output type.

You can choose the product that matches your usage conditions.

At the end

This article was created with reference to onsemi Technical Note TND-6237 “SiC MOSFETs: Gate Drive Optimization”.

Inquiry

If you are interested in the contents and products introduced this time, please contact us.