Operating principles of step-down DC-DC converters

Step-down DC-DC converter circuit

Fig. 1 shows the circuit of a step-down DC-DC converter with synchronous rectification.
The circuit configuration generally consists of a MOSFET (Q1) responsible for switching, followed by a MOSFET (Q2) for rectification and an output smoothing capacitor (C2) in parallel, and an inductor coil in series with the MOSFET (Q1).
Q1 and Q2 are controlled by control signals output from the IC.

Fig. 1. Step-down DC-DC Converter with Synchronous Rectification

When MOSFET (Q1) is ON

Fig. 2 shows the state when the switching element Q1 is ON.
When Q1 is ON, current flows from input Vin through coil L, charging output smoothing capacitor C2, which supplies output current Iout.
The current flowing in the coil L generates a magnetic field, and electrical energy is converted into magnetic energy and stored.

Fig. 2. When Q1 is ON

The voltage of input voltage Vin and output voltage Vout is applied to coil L during the ton period when Q1 is ON, and the ripple current centered on the output current Iout is as follows.

ΔIL: Amount of change in current flowing to L (ripple current) [A]
Vin: Input voltage [V]
Vout: Output voltage [V]
L: Inductance of coil [H]
ton: ON time for Q1 [sec]

When MOSFET (Q2) is ON

Fig. 3 shows the state when the switching element Q2 is ON.
When Q1 is set to OFF, Q2 turns ON and the energy stored in L is released to the output side.

Fig. 3. When Q2 is ON

During the toff period when Q1 is OFF, current is released to the load via Q2 to maintain the current flowing to coil L.
The ripple current during this toff period is as follows.

ΔIL: Amount of change in current flowing to L (ripple current) [A]
Vout: Output voltage [V]
L: Inductance of coil [H]
toff: OFF time for Q1 [sec]

Duty Cycle

In the steady state, the ΔIL in equation (1), where Q1 is ON, and in equation (2), where Q2 is ON, become equal, and so the relationship between the input and output voltages is as follows.

ton/(ton + toff) is the ratio of the time that Q1 is ON and is called the duty cycle.

Vout is determined by the duty cycle, and Vout is controlled by controlling the duty cycle.

Let (ton + toff) be the switching period T [sec] and the switching frequency at that time be fsw [Hz]. Rearranging equations (1) and (3) with respect to fsw, we get

When Vin and Vout are fixed, the output ripple current is determined by L and fsw.

A large ripple current causes a large output ripple voltage.
Also, the coil tends to saturate easily, resulting in increased core loss and other effects.
To counter these problems, it is necessary to increase the switching frequency or increase the number of turns of the coil to increase the inductance value.