AN4242 datasheet(3/26 Pages) STMICROELECTRONICS

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AN4242

Features of the SiC diodes

1.1

Turn off behavior

1.1.1

Comparison with Si bipolar diode

The benefits brought by silicon-carbide diodes on the switching losses in the applications

working in continuous-conduction mode (such as PFC applications) are already well known.

The capacitive nature of the recovery current allows constant turn-off characteristics when

the temperature increases. In contrast the turn-off behavior of bipolar diodes is

characterized by a strong dependency on junction temperature, dI/dt slope and forward

current level (see Figure 1).

Thanks to their properties, SiC diodes allow significant reduction of power losses in the

associated MOSFETs when switched-on. They also permit new optimization options for the

power converter (for example, increasing the switching frequency and speed, lowering the

size of passive components, snubber-circuits and EMI filters).

Figure 1. Switching behavior comparison between Si and SiC diodes

for Tj=75 °C and Tj=125 °C

The capacitive recovery current is generated by the charge of the junction capacitance Cj

under a certain reverse voltage and corresponds to a quantity of stored charges Qc.

1.1.2

Capacitive charge (QC) measurement

Some confusion exists about the measurement conditions of Qc. A comparison between the

switch-off behavior and the integral of the current used to estimate Qc is shown in Figure 2.

Figure 2A and Figure 2B show measurements at low forward current (IF=1 A) and low dI/dt

slope (50 A/µs), with and without reverse voltage across the diode. A certain inaccuracy of

the measurement of Qc can be observed. It is linked to the probe, which features its own

equivalent capacitance. Figure 2C shows a measurement at IF=2 A and a high dI/dt slope

(200 A/µs) without any probe voltage. With such a high value of current-slope some

oscillations appear. Taking into account the total capacitive current until t0 when the reverse

voltage reaches VR, Qc measured by the integral of the current is similar to the one in

Figure 2B.

8 A bipolar

diode

8 A tandem (2 x

300 V diodes in

series)

8 A SiC

diode

VR=380 V, IF=8 A, dI/dt=200 A/μs, Tj=125 °C

20 ns/div

2 A/div

VR=380 V, IF=8 A, dI/dt=200 A/μs, Tj=75 °C

20 ns/div

2 A/div

8 A bipolar

diode

8 A tandem (2 x

300 V diodes in

series)

8 A SiC

diode

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