In this era of battery-operated devices, a DC-DC converter is necessary to meet the different power requirements of electronic circuits. A battery is a type of DC source whose output voltage varies with use. This means that it cannot maintain a constant voltage at the output due to its discharge characteristics. For this reason, regulators are used to regulate the supply at constant voltage. To regulate the power supply and change the power supply to a constant voltage, step-up or step-down converters are used.
A step-down converter, also known as a buck converter, is used to decrease the voltage supply, while a step-up converter, also known as a boost converter, is used to increase the voltage supply.
In this experiment, we will design a step-up converter that will be powered by a lithium-ion battery and provide a constant 12V DC output. To do so, we will use the IC MT3608; we will test the converter for an output current of up to 250m A.
Figure 1. Pinout of IC MT3608
About MT3608
The MT3608 is an advanced DC-DC converter intended for small, low-power applications. It is manufactured by Aerosemi and has the following main features:
- High switching frequency (1.2 MHz). This allows the use of a low-cost inductor and capacitor.
- Wide input voltage range (2V to 24V). This makes it suitable for a 3.7V lithium-ion battery as input.
- Adjustable output voltage up to 28 V
- Up to 97% efficiency
- Internal current limit at 4A
MT3608 pinout
| Pin | Description |
| 1 | Power switch output. It is the drain of the internal MOSFET |
| two | Floor |
| 3 | Feedback input to regulate output voltage |
| 4 | Enable the pin to turn on/off the converter. High on this pin makes the converter turn on, low makes the converter turn off |
| 5 | Input voltage power pin |
| 6 | No connection |
Component selection
The IC needs external components to act as a DC-DC converter. The following bias components are required:
- Inductor
- Diode
- Capacitor
- Resistor
Figure 2. The Li-on to 12V DC converter circuit connection with MT3608
Inductor (L1)
The inductor is the energy storage element in a DC-DC converter. It stores energy when the MOSFET is turned on and provides the stored energy to the output when the MOSFET is turned off. The energy stored in the inductor causes it to act as a power source and increases the input power, which increases the output voltage relative to the input.
According to the datasheet, an inductor of 4.7uH to 22uH is suitable for the application. The rated current of the inductor must be greater than the maximum output current. We use an inductor with a value of 10uH/2.1A.
Diode (D1)
The diode is used to transfer the energy stored in the inductor. Power is transferred to the output capacitor when the MOSFET switch is off.
A Schottky diode is preferred because it offers low forward voltage drop. The rated current must be greater than the maximum output current. The diode must have a reverse breakdown voltage greater than the required output voltage. We use 30BQ100TR because it is more suitable for application.
Capacitor (C1, C2)
A capacitor is used to filter the DC input and output. It also ignores noise in the DC signal.
According to the datasheet, 22uF ceramic capacitors are recommended at the input and output. The voltage rating must be greater than the desired input and output voltage. We use 22uF/50V at input and output.
Resistors (R1, R2)
A resistor network is required to feed the output voltage to the IC. Feedback helps regulate output.
According to the datasheet, the formula for calculating the value of feedback resistors is given:
V OUT =V REFERENCE x (1 + R 1 /R 2 )
Where V REFERENCE = 0.6 V. If we consider R1 = 200k, then R2 = 10.47K. This resistor network provides a V OFF of 12.06V
Practical Observations
We carry out tests with the converter at different loads and input voltages to understand the efficiency and regulation of the output voltage.
|
Voltage (V)
(Prohibited) |
Current
(Prohibited) |
Power (W)
(Prohibited) |
Voltage (V)
(Exit) |
Current
(Exit) |
Power (W)
(Exit) |
Efficiency
(%) |
| 2.5 | 0.32 | 0.8 | 12.16 | 0.06 | 0.72 | 90 |
| 3.3 | 0.25 | 0.82 | 12.17 | 0.06 | 0.73 | 91.2 |
| 4.2 | 0.2 | 0.84 | 12.17 | 0.06 | 0.73 | 86.9 |
| 2.5 | 0.92 | 2.3 | 12.18 | 0.12 | 1.46 | 63.4 |
| 3.3 | 0.74 | 2.44 | 12.2 | 0.12 | 1.46 | 59.8 |
| 4.2 | 0.4 | 1.68 | 12.2 | 0.12 | 1.46 | 86.9 |
| 2.5 | 1.58 | 3.95 | 12/21 | 0.24 | 2.93 | 74.1 |
| 3.3 | 1.07 | 3.53 | 12.23 | 0.24 | 2.93 | 83 |
| 4.2 | 0.81 | 3.4 | 12.27 | 0.24 | 2.94 | 86.4 |
From observations, it can be seen that the efficiency is better at light loads. At higher loads, losses such as MOSFET conduction and switching loss, inductor loss, and diode loss decrease efficiency.
PCB Designs
To perform the experiments and test the IC, we designed a printed circuit board using KiCAD. The upper and lower signal layers are shown in Figures 3 and 4 .
Figure 3. Top PCB Layer
Figure 4. PCB Bottom Layer
Precautions and sources of error
- A high-power electromagnetic signal can disrupt the operation of the converter. The inductor will misbehave when it is interfered with by an external magnetic field.
- Ceramic capacitors must be used at the input and output for proper filtering. Ceramic capacitors offer low ESR (equivalent series resistance) due to which they are preferred over electrolytic capacitors.
- Tolerance in the resistors used for feedback may result in deviation in the desired output voltage. A low tolerance resistor can be used to minimize drift.
- Short-circuiting the converter output terminals must be avoided. This may cause permanent damage to the IC.
Figure 5. The real-time image of PCB connections for Li-on to 12V DC converter
