SEPIC DC-DC Converter Using the MT3608 Boost

Modern electronic design must prioritize effective and adaptable power management, particularly for battery-operated devices where the input voltage varies greatly throughout discharge cycles. The design and execution of a Single-Ended Primary Inductor Converter (SEPIC) based on the MT3608, a low-cost, high-frequency boost converter integrated circuit, are shown in this project. By adding a linked inductor and a Schottky diode, the MT3608 may be set up as a SEPIC even though it is not its native architecture. This creates a converter that can generate a steady output voltage that can be adjusted to be greater, lower, or equal to the input voltage.

For Full Project:

https://electronicsworkshops.com/sepic-dc-dc-converter-using-the-mt3608-boost/

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A SEPIC (Single-Ended Primary Inductor Converter) functions by utilizing two inductors (L1 and L2) and a coupling capacitor (C_couple) to move energy from the input to the output in two separate phases. When the internal switch (e.g., in the MT3608) is ON, the input voltage charges L1, capturing energy in its magnetic field, while at the same time the coupling capacitor discharges, transferring its accumulated energy into L2. When the switch is OFF, the collapsing magnetic fields of the two inductors work together, forward-biasing the output diode and enabling the combined energy from L1 and L2 to charge the output capacitor and supply the load. This distinctive push-pull mechanism via the coupling capacitor allows for the output voltage to be accurately controlled at a level that can be either higher or lower than the input voltage, while ensuring a non-inverting output and shared ground with the input.

Why the MT3608 alone is not a SEPIC controller?

The MT3608 is not capable of functioning as a SEPIC controller because it is specifically designed to operate as a boost (step-up) converter; its internal control circuitry and power switch configuration are exclusively built for elevating voltage from the input to the output. A SEPIC (Single-Ended Primary Inductor Converter) requires the use of two inductors (or a coupled inductor) along with a coupling capacitor to facilitate both step-up and step-down capabilities, with energy transfer being managed in a manner distinct from that of a boost converter. The MT3608 does not possess the necessary control logic and feedback compensation to effectively regulate voltage when the input voltage is higher than that of the output — it can only function properly when the output voltage exceeds the input. Attempting to configure it in a SEPIC manner would lead to instability, inadequate regulation, or potential circuit failure.

Circuit overview

This circuit uses the MT3608 IC (U3) to boost (step up) a DC input voltage to a higher output voltage.

1. The input voltage (VIN) is applied at the right side through a slide switch (PWR1) that turns the converter on or off.
2. The output voltage (VOUT) appears on the left side.
3. Key components: L1, D2, C1–C4, R1–R2, and the MT3608 itself form the boost converter’s power and feedback network.

a. Inductor L1 (22 µH, 3 A)

1. L1 is the main boost inductor.
2. When the MT3608’s internal switch (connected to pin 1, SW) turns on, current builds up in L1.
3. When the switch turns off, the inductor’s energy is released through the diode D2 (SS34) into the output capacitor C4, raising the output voltage above VIN.

b. Diode D2 (SS34)

1. Acts as a rectifier, directing current from L1 to the output when the switch turns off.
2. Prevents current from flowing backward from the output to the switch node.

c. Capacitors (C1, C2, C3, C4)

1. C3 (22 µF): Input filter capacitor. It smooths out VIN and reduces input ripple.
2. C1 and C2 (4.7 µF each): These are placed near the switching node to filter out switching spikes and stabilize the converter.
3. C4 (22 µF): Output capacitor. It stores charge and filters ripple at VOUT for a smooth DC output.

d. Inductor L2 (22 µH, 3 A)

1. Although named separately, in this schematic L2 is not actually part of the MT3608’s boost path. It might be left from an experimental SEPIC-style layout, but in this connection it’s tied to ground and not transferring energy between input and output.
2. In other words, L2 acts as an additional filter or a placeholder, not as a second SEPIC inductor.

Feedback network (R1, R2)

1. he MT3608 regulates the output voltage based on the feedback pin (FB), which maintains 0.6 V reference voltage internally.
2. The divider formed by R1 (2 kΩ) and R2 (9.1 kΩ) sets the output voltage using the formula:

So this circuit as drawn will output roughly 3.3 V, assuming VIN is below that.

If you want higher voltage (e.g., 12 V), you would increase R2 accordingly

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Conclusion

This comprehensive guide provides all necessary information to successfully design, build, and test a robust SEPIC converter using the MT3608 IC, suitable for various power supply applications requiring wide input voltage range capability