12V 1A SMPS Power Supply Circuit Design

Hey guys!

Every Electronic device or product requires a reliable power supply unit (PSU) to operate it. Almost all devices in our home, like TV, Printer, Music Player, etc. consists of a power supply unit built into it which converts the AC mains voltage to a suitable level of DC voltage for them to operate. The most commonly used type of power supply circuit is the SMPS (Switching Mode Power Supply), you can easily find this type of circuits in your 12V adapter or Mobile/Laptop charger. In this tutorial, we will learn how to build a 12v SMPS circuit that would convert AC mains power to 12V DC with a maximum current rating of 1.25A. This circuit can be used to power small loads or even be adapted into a charger to charge you lead-acid and lithium batteries. If this 12v 15watt power supply circuit doesn’t match your requirement, you can check various power supply circuit with different ratings.

Before proceeding with any kind of power supply design, requirement analysis has to be done based on the environment in which our Power supply will be used. Different kinds of power supply work in different environments and with specific input-output boundaries.

Input Specification:

Let's start with the input. An input supply voltage is the first thing which will be used by the SMPS and will be transformed into a useful value to feed the load. As this design is specified for AC-DC conversion, the input will be Alternating current (AC). For India, the input AC is available in 220-230 volt, for the USA it is rated for 110 volts. There are also other nations which use different voltage levels. Generally, SMPS works with a universal input voltage range. This means the input voltage can differ from the 85V AC to 265V AC. SMPS can be used in any country and could provide a stable output of full load if the voltage is between 85-265V AC. The SMPS should also function normally under 50Hz and 60Hz frequency as well. This is the reason why we are able to use our phone and laptop chargers in any country.

Output Specification:

On the output side, few loads are resistive, few are inductive. Depending on the load the construction of an SMPS can be different. For this SMPS the load is assumed as a resistive load. However, there is nothing like a resistive load, each load consists of at least some amount of inductance and capacitance; here it is assumed that the inductance and capacitance of the load are negligible.

The output specification of an SMPS is highly dependable on the Load, like how much voltage and current will be required by load under all operating conditions. For this project, the SMPS could provide 15W output. It is 12V and 1.25A. The targeted output ripple is selected as less the 30mV pk-pk at 20000 Hz bandwidth.

Every SMPS circuit requires a Power Management IC also known as switching IC or SMPS IC or Drier IC. Let’s sum up the design considerations to select the ideal Power Management IC that will be suitable for our design. Our Design requirements are:

1. 15W output. 12V 1.25A with less than 30mV pk-pk ripple at full load.
2. Universal input rating.
3. Input surge protection.
4. Output short circuit, over-voltage and over-current protection.
5. Constant voltage operations.

From the above requirements there is a wide range of ICs to select from, but for this project, we have selected Power integration. Power integration is a semi-conductor company that has a wide range of power driver ICs in various power output ranges. Based on the requirements and availability we have decided to use the TNY268PN from tiny switch II families.

In the above image, the maximum power 15W is shown. However, we will make the SMPS in the open frame and for the universal input rating. In such a segment, TNY268PN could provide 15W output. Let’s see the pin diagram.

Before going straight into building the prototype part, let’s explore the 12v SMPS circuit diagram and its operation. The circuit has the following sections:

1. Input surge and SMPS fault protection
2. AC-DC conversion
3. PI filter
4. Driver circuitry or Switching circuit
5. Under-voltage lockout protection.
6. Clamp circuit
7. Magnetics and galvanic isolation
8. EMI filter
9. Secondary Rectifier and snubber circuit
10. Filter Section

Input surge and SMPS fault protection

This section consists of two components, F1 and RV1. F1 is a 1A 250VAC slow blow fuse and RV1 is a 7mm 275V MOV (Metal Oxide Varistor). During a high voltage surge (more than 275VAC), the MOV became dead short and blows the input Fuse. However, due to the slow blow feature, the fuse withstands inrush current through the SMPS.

AC-DC conversion

This section is governed by the diode bridge. These four diodes (inside DB107) make a full bridge rectifier. The diodes are 1N4006, but standard 1N4007 can do the job perfectly. In this project, these four diodes are replaced with a full bridge rectifier DB107.

PI filter

Different states have different EMI rejection standards. This design confirms EN61000-Class 3 standard and the PI filter is designed in such a way to reduce the common-mode EMI rejection. This section is created using C1, C2, and L1. C1 and C2 are 400V 18uF capacitors. It is an odd value so 22uF 400V is selected for this application. The L1 is a common mode choke that takes differential EMI signal to cancel both.

Driver circuitry or switching circuit

It is the heart of an SMPS. The transformer's primary side is controlled by the switching circuit TNY268PN. The switching frequency is 120-132khz. Due to this high switching frequency, smaller transformers can be used. The switching circuit has two components, U1, and the C3. U1 is the main driver of IC TNY268PN. The C3 is the bypass capacitor which is needed for the working of our driver IC.

Under-voltage lockout protection

Under-voltage lockout protection is done by the sense resistor R1 and R2. It is used when the SMPS goes into the auto-restart mode and sense the line voltage.

Clamp circuit

D1 and D2 are the clamp circuit. D1 is the TVS diode and D2 is an ultra-fast recovery diode. The transformer acts as a huge inductor across the power driver IC TNY268PN. Therefore during the switching off-cycle, the transformer creates high voltage spikes due to the leakage inductance of the transformer. These high-frequency voltage spikes are suppressed by the diode clamp across the transformer. UF4007 is selected due to the ultra-fast recovery and P6KE200A is selected for the TVS operation.

Magnetics and galvanic isolation

The transformer is a ferromagnetic transformer and it not only converts the high voltage AC to a low voltage ac but also provide galvanic isolation.

EMI filter

EMI filtering is done by the C4 capacitor. It increases the immunity of the circuit to reduce the high EMI interference.

Secondary Rectifier and Snubber circuit

The output from the transformer is rectified and converted to DC using D6, a Schottky rectifier diode. The snubber circuit across the D6 provides suppression of the voltage transient during switching operations. The snubber circuit consists of one resistor and one capacitor, R3, and C5.

Filter Section

The filter section consists of a filter capacitor C6. It is a Low ESR capacitor for better ripple rejection. Also, an LC filter using L2 and C7 provides better ripple rejection across the output.

You can draw the PCB Schematic with any software as per your convenience and send it to a PCB Manufacturer of your choice. I have a Gerber ready, I can share it.

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