AN4191 datasheet(12/20 Pages) STMICROELECTRONICS

12 / 20 page

Rg impact for MOSFETs in buck/buck-boost configurations

AN4191

12/20

Doc ID 023815 Rev 1

Rg impact for MOSFETs in buck/buck-boost

configurations

Let's consider now the Rg impact on MOSFET switching behavior and thermal management

in a real customer application. In particular, the MOSFET is used as a control or

synchronous switch in a single-phase synchronous buck or buck-boost converter. These

topologies are widely used in the computer and peripherals sector, when a fixed output

voltage is generated from a regulated input voltage by adjusting properly the device turn-on

time and therefore the converter duty cycle (D = VOUT / VIN). The output voltage feeds

various system blocks (i.e. CPU, DDR, etc.…).

Too high Rg values can affect the MOSFET performance in the application, particularly:

Rising/falling edges slowing down.

LS false turn-on risk enhancement.

Driver/MOSFET temperature increase.

3.1

Rising/falling edges slowing down

Initially, let's consider a buck-boost converter (Figure 12), that lowers the input voltage (5 V)

generating - 5 V as VOUT. Ch2 (drain voltage) and Ch3 (gate voltage) indicate the probe

placement. Two equal FETs, with different Rg values, are compared as the control switch.

Figure 12.

Buck-boost converter

In Figure 13 and Figure 14 respectively, the waveforms for a “good” part (standard Rg value)

and “bad” part (high Rg value) are illustrated.

CH3

CH2

5Vin

-5Vout

CH3

CH2

5Vin

-5Vout

AM16419V1

AN4191 Datasheet(PDF) 12 Page - STMicroelectronics