AN4191 datasheet(15/20 Pages) STMICROELECTRONICS

15 / 20 page

AN4191

Rg impact for MOSFETs in buck/buck-boost configurations

Doc ID 023815 Rev 1

15/20

In order to evaluate the Rg impact on the LS false turn-on, two identical FETs (same silicon)

but different Rg values (1.5 Ω vs. 4 Ω) are mounted in the converter shown in Figure 16.

Figure 18 and 19 illustrate the LS gate-source waveforms for the two LS FETs.

Comparing the two images, the “high Rg” MOSFET has a higher and larger (over the

minimum threshold voltage) spurious bouncing than the standard device. This causes a

“shoot-through” event (for each switching cycle), that increases the converter power losses.

So, higher device (MOSFETs and driver) temperatures are measured when “high Rg”

samples are mounted on the board in the low-side position, as shown in the thermal

captures of Figure 20 (standard part) and Figure 21 (high Rg).

3.3

Synchronous buck converter for HDD: LS false turn-on risk

enhancement

Figure 22 shows (inside the light blue rectangle) a single phase synchronous buck

converter, which provides 1.18 V as output voltage, with 1.1 MHz as switching frequency.

Figure 18.

High Rg (4 Ω) LS turn-off

Figure 19.

Low Rg (1.5 Ω) LS turn-off

AM16425V1

AM16426V1

Figure 20.

Thermal capture for standard part

Figure 21.

Thermal capture for high Rg

AM16427V1

AM16428V1

AN4191 Datasheet(PDF) 15 Page - STMicroelectronics