Your First Boost Converter: Learn the Basics

Boost converter circuit configuration

The basic circuit configuration of a boost converter is shown in Figure 1.

Figure 1 Boost converter (asynchronous rectification)

There are four main components:

• INDUCTORS (L)
• DIODES (D)
• FET (SWITCHES element)
• Output CAPACITORS (Cout)

DIODES is used in the asynchronous rectification method, but when synchronous rectification is adopted to improve efficiency, a FET is used instead of DIODES.

Boost Converter Operating Principle

A boost converter achieves voltage boost by repeatedly storing and releasing energy in INDUCTORS.
Here we will check the operation when the FET is ON and OFF.

• Operation when FET=ON

When the FET is turned on, the input voltage (Vin) is applied to INDUCTORS (L), and energy is accumulated as INDUCTORS current increases (see diagram A below).
During this time, current is supplied to the load from the output CAPACITORS (Cout) (see diagram B below).

Figure 2 Current path when FET is ON

• Operation when FET=OFF

When the FET is turned off, the energy stored in INDUCTORS (L) is released, and INDUCTORS current flows through DIODES (D) to the output CAPACITORS (Cout) and the load (see C in the diagram below).

Figure 3 Current path when FET is OFF

By repeatedly turning the FET on and off, an output voltage higher than the input voltage can be obtained.

Figure 4. Operating waveforms of the boost converter

Relationship between Duty and Output Voltage

Pulse width modulation (PWM), a typical control method for boost converters, controls the output voltage by adjusting the ON time (pulse width) of the FET while keeping SWITCHES frequency constant.
The ratio of the time that the FET is ON in one cycle is called Duty (D), and in boost operation the following approximate formula holds true:

• Duty = (Vout − Vin) / Vout
• Vout = Vin / (1 − Duty)

From this formula, we can see that the greater the difference between the input voltage and output voltage, the higher the duty cycle.

Calculation example
Input voltage: Vin = 5V
Output voltage: Vout = 12V
Duty = (12V − 5V) / 12V ≒ 58%

Generally, the maximum duty cycle of a boost converter is limited as part of its protection function.
Therefore, you must ensure that the desired output voltage is achievable within the input voltage range you are using.

INDUCTORS Current and Ripple Current

INDUCTORS ripple current

In a boost converter, INDUCTORS current is an important parameter that significantly affects component selection, efficiency, and noise.
The current that flows through INDUCTORS increases and decreases cyclically due to SWITCHES operation, and the amplitude of this increase is called INDUCTORS ripple current (ΔIL).
The ripple current ΔIL is calculated using the following formula:

• ΔIL = Vin × D / (L × Fsw)

Calculation example
Vin = 5V
Vout = 12V
Fsw = 500kHz
L = 6.8µH
ΔIL = 5V × 0.58 / (6.8µH × 500kHz) ≒ 0.85A

Average value of INDUCTORS current

The average INDUCTORS current in a boost converter is equal to the input current (IIN) and is given by:

• IIN = IL(Ave) = (Vout × Iout) / (Vin × η)

Calculation example
Vin = 5V
Vout = 12V
Iout = 1A
Efficiency η = 90%
IL(Ave) = (12V × 1A) / (5V × 0.9) ≒ 2.67A

INDUCTORS current peak value

INDUCTORS current increases or decreases by the amount of ripple around the average value.
The peak value can be calculated using the following formula:

• ILpeak = IL(Ave) + ΔIL / 2

Calculation example
ILpeak =2.67A + 0.85A / 2 ≒ 3.1A

INDUCTORS to be used must have a saturation current (Isat) that exceeds the peak INDUCTORS current. If the saturation current is insufficient, INDUCTORS will become magnetically saturated, leading to heat generation, reduced efficiency, unstable operation, and overcurrent.

Output Ripple Voltage

In a boost converter, the charging current to the output CAPACITORS (Cout) is intermittent, as shown in the operating waveforms in Figure 4.
Therefore, the output ripple voltage tends to become large.
On the other hand, the input current is continuous.
Selecting CAPACITORS capacitance and ESR is important for reducing output ripple voltage. For more details, please also refer to POWER SUPPLIES IC Technology Column "About Ripple Current and Ripple Voltage."

Ripple current and ripple voltage