Vivanco UNIVERSAL SWITCHMODE AC TO DC MAINS ADAPTERS WITH USB CHARGING FUNCTION Podręcznik Użytkownika Pobierz pdf (Strona 45)

Hardware Design

2.3 SINGLE-PHASE SYNCHRONOUS BUCK CONVERTER

The Single-Phase Synchronous Buck converter uses the same basic topology as the

standard step-down Buck converter, but replaces the free-wheel diode with a MOSFET.

Figure 2-11 shows the main power circuitry. The two switches are operated as a

complementary pair with a dead-time inserted by the PWM controller to avoid

shoot-through. The low-side MOSFET is operated in the third quadrant with current

flowing from source to drain when the current is required to free-wheel, and, due to the

very lower ON state resistance of the MOSFET, higher efficiency is achieved compared

with a conventional Schottky diode.

FIGURE 2-11: SINGLE-PHASE SYNCHRONOUS BUCK CONVERTER

2.3.1 Single-Phase Buck Converter Power-Train Design

The target specification for the Single-Phase Buck converter is as follows:

• Input voltage, V

IN = 12 VDC

• Switching frequency, f

= 500 kHz

• Output voltage, V

OUT = 5 VDC

• Output current, IOUT = 23A

• Voltage ripple < 2%

• Output slew rate > 5 A/μs

2.3.1.1 MOSFETS AND GATE DRIVE

The equation governing the duty cycle of a Buck converter is shown in Equation 2-26.

EQUATION 2-26:

The rms currents in the high-side and low-side MOSFETs, assuming a low inductor

ripple current is as follows in Equation 2-27 and Equation 2-28.

EQUATION 2-27:

out

bus

high o

iID

1 2 ... 40 41 42 43 44 45 46 47 48 49 50 ... 117 118

Brak uwag

Vivanco UNIVERSAL SWITCHMODE AC TO DC MAINS ADAPTERS WITH USB CHARGING FUNCTION Podręcznik Użytkownika Strona 45