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How to design a SMPS using op amp?

Every Electronic tool or product calls for a reliable strength deliver unit (PSU) to operate it. Almost all devices in our home, like TV, Printer, Music Player etc. includes a electricity supply unit built into it which converts the AC mains voltage to a appropriate stage of DC voltage for them to operate. The maximum typically used of type of power deliver circuit is the SMPS (Switching Mode Power Supply), you may easily find this form of circuits on your 12V adapter or Mobile/Laptop charger. In this tutorial we will learn how to build a 12v SMPS circuit that could convert AC mains power to 12V DC with a maximum cutting-edge score of 1.25A. This circuit can be used to power small masses or even be adapted right into a charger to price you lead-acid and lithium batteries. If this 12v 15watt strength supply circuit doesn’t match your requirement, you may test various power deliver circuit with special ratings.


12v SMPS Circuit – Design Considerations
Before intending with any kind of power deliver design, requirement analysis has to be executed based totally at the surroundings in which our Power supply could be used. Different kinds of strength deliver work in different environments and with specific enter-output boundaries.

Input Specification

Let's start with the enter. An input supply voltage is the first element so that you can be utilized by the SMPS and could be converted right into a useful value to feed the burden. As this design is certain for AC-DC conversion, the enter can be Alternating cutting-edge (AC). For India, the input AC is to be had in 220-230 volt, for the USA it's far rated for one hundred ten volts. There also are different countries which use specific voltage levels. Generally, SMPS works with popular enter voltage range. This way the enter voltage can range from the 85V AC to 265V AC. SMPS can be used in any united states and could provide a strong output of full load if the voltage is among 85-265V AC. The SMPS ought to also function generally under 50Hz and 60Hz frequency as well. This is the motive why we are capable of use our cellphone and computer chargers in any country.

Output Specification



On the output side, few masses are resistive, few are inductive. Depending at the load the construction of an SMPS can be unique. For this SMPS the load is assumed as a resistive load. However, there is not anything like a resistive load, each load includes at-least some amount of inductance and capacitance; right here it's far assumed that the inductance and capacitance of the weight are negligible.

The output specification of an SMPS is tremendously dependable on the Load, like how an awful lot voltage and modern can be required via load beneath all running conditions. For this task, the SMPS could provide 15W output. It is 12V and 1.25A. The centered output ripple is selected as less the 30mV pk-pk at 20000 Hz bandwidth.

Based on the output load, we additionally have to determine between the designing a Constant Voltage SMPS or Constant Current SMPS. Constant voltage means the voltage throughout the load can be steady and the current may be changed as a consequence with the modifications in load resistance. On the alternative hand, regular present day mode will allow the current to be constant however trade the voltage for that reason with the adjustments in load resistance. Also, each CV and CC can be available in an SMPS however they can not work in a unmarried time. When both alternatives exist in an SMPS, there needs to be a range while the SMPS will trade its output operation from CV to CC and vice versa. Normally CC and CV mode chargers are used to price lead acid or lithium batteries.

Input and Output Protection Features

There are various safety circuits that can be employed at the SMPS for more secure and reliable operations. The protection circuit protects the SMPS as well as the linked load. Depending on the location, the protection circuit can be related throughout enter or throughout the output. The maximum not unusual enter safety is Surge Protection and EMI filters. Surge protection protects the SMPS from enter surges or AC overvoltage. EMI filter out protects the SMPS from EMI generation throughout the enter line. In this ventureeach capabilities may be to be had. Output protection includes quick circuit safety, over-voltage safety and over modern protection. This SMPS design will additionally encompass all of these safety circuits.

Selection of the Power Management IC
Every SMPS circuit requires a Power Management IC additionally called switching IC or SMPS IC or Drier IC. Let’s sum up the design concerns to choose the precise Power Management IC as a way to be appropriate for our design. Our Design requirements are

15W output. 12V 1.25A with less than 30mV pk-pk ripple at full load.
Universal enter score.
Input surge safety.
Output short circuit, over voltage and over modern safety.
Constant voltage operations.
From the above necessities there's wide variety of ICs to select from, but for this challenge we've decided on Power integration. Power integration is a semi-conductor organization that has a wide variety of power driver ICs in various power output ranges. Based on the necessities and availability we've got determined to apply the TNY268PN from tiny transfer II families.


In the above image, the maximum power 15W is shown. However, we can make the SMPS in the open body and for the time-honored enter rating. In any such segment, TNY268PN could provide 15W output. Let’s see the pin diagram.


Designing the 12v 1Amp SMPS Circuit
The fine manner to build the circuit is to use Power integration’s PI professional software program. It is incredible electricity deliver layout software. The Circuit is constructed using Power Integration IC. The design manner is explained below, alternatively you may also scroll down for the video explaining the same.

Step -1: Select the Tiny switch II and also choose the preferred package. We selected the DIP package. Select the Enclosure type, Adapter or Open Frame. Here Open Frame is selected.

Then choose the Feedback type. It is vital as Flyback topology is used. TL431 is an top notch preference for remarks. TL431 is a shunt regulator and it's going to offer first-rate over-voltage safety and accurate output voltage.


Step-2: Select the enter voltage variety. As it'll be a regularly occurring input SMPS, the enter voltage is selected as 85-265V AC. Line Frequency is 50 Hz.


Step- 3:


Select the output voltage, modern, and wattage. The SMPS rating will be 12V 1.25A. The wattage is showing 15W. The operation mode is likewise decided on as CV, way constant voltage operation mode. Finally, the whole thing is accomplished in three clean steps, and the schematic is generated.

12V SMPS Circuit Diagram and Explanation
The beneath circuit is barely modified to in shape our venture.




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

Input surge and SMPS fault safety
AC-DC conversion
PI filter out
Driver circuitry or Switching circuit
Under-voltage lockout safety.
Clamp circuit
Magnetics and galvanic isolation
EMI filter out
Secondary Rectifier and snubber circuit
Filter Section
Feedback section.
Input surge and SMPS fault safety

This phase consists of two additives, F1 and RV1. F1 is a 1A 250VAC gradual blow fuse and RV1 is a 7mm 275V MOV (Metal Oxide Varistor). During a high voltage surge (extra than 275VAC), the MOV became dead short and blows the enter Fuse. However, because of the slow blow feature, the fuse withstands inrush current via the SMPS.

AC-DC conversion

This section is governed by using the diode bridge. These 4 diodes (internal DB107) make a complete bridge rectifier. The diodes are 1N4006, but preferred 1N4007 can do the job flawlessly. In this task, these four diodes are changed with a complete bridge rectifier DB107.

PI clear out

Different states have distinctive EMI rejection widespread. This design confirms EN61000-Class 3 trendy and the PI clear out is designed in this type of way to reduce the commonplace-mode EMI rejection. This segment is created using C1, C2, and L1. C1 and C2 are 400V 18uF capacitors. It is an odd cost so 22uF 400V is chosen for this application. The L1 is a common mode choke that takes differential EMI signal to cancel each.

Driver circuitry or switching circuit

It is the coronary heart of an SMPS. The transformer's primary facet is managed with the aid of the switching circuit TNY268PN. The switching frequency is 120-132khz. Due to this excessive switching frequency, smaller transformers can be used. The switching circuit has two additives, U1, and the C3. U1 is the principle motive force IC TNY268PN. The C3 is the pass capacitor which is needed for the operating of our motive force IC.

Under-voltage lockout safety

Under-voltage lockout protection is performed via the sense resistor R1 and R2. It is used while the SMPS is going into the auto-restart mode and feel the line voltage.

Clamp circuit

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

Magnetics and galvanic isolation

The transformer is a ferromagnetic transformer and it not handiest converts the excessive voltage AC to a low voltage ac but also offer galvanic isolation.

EMI filter out

EMI filtering is completed by means of the C4 capacitor. It will increase the immunity of the circuit to lessen the excessive 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 at some stage in switching operations. The snubber circuit consist 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 clear out using L2 and C7 provide higher ripple rejection throughout the output.

Feedback segment

The output voltage is sensed by means of the U3 TL431 and R6 and R7. After sensing the line, U2, the optocoupler is controlled and galvanically separating the secondary feedback sensing portion with the primary side controller. The Optocoupler has a transistor and a LED interior of it. By controlling the LED, the transistor is controlled. Since the verbal exchange is accomplished by optically, it has no direct electrical connection, therefore satisfying the galvanic isolation at the comments circuit too.

Now, because the LED without delay controls the transistor, by using providing sufficient bias throughout the optocoupler LED, one can manipulate the optocoupler transistor, greater particularly driving force circuit. This manipulate gadget is employed by means of the TL431.As the shunt regulator has a resistor divider throughout its reference pin, it could manage the optocoupler led that's linked throughout it. The remarks pin has a reference voltage of 2.5V. Therefore, the TL431 may be active only if the voltage throughout the divider is enough. In our case, the voltage divider set to a fee of 12V. Therefore, while the output reaches 12V the TL431 gets 2.5V throughout the reference pin and thus spark off the optocoupler's LED which controls the transistor of the optocoupler and circuitously controls the TNY268PN. If the voltage is not enough throughout the output the switching cycle is right now suspended.

First, the TNY268PN turns on the first cycle of switching and then experience it's EN pin. If everything is alright, it'll retain the switching, if not, it'll strive over again after sometimes. This loop receives continued until everything gets normal, thus stopping shortcircuit or overvoltage issues. This is why it is called flyback topology, as the output voltage is flown back to the motive force for sensing related operations. Also, the attempting loop is known as a hiccup mode of operation on the failure circumstance.

The D3 is a Schottky barrier diode. This diode converts the excessive frequency AC output to a DC. 3A 60V Schottky Diode is chosen for dependable operation. R4 and R5 is selected and calculated by means of the PI Expert. It creates a voltage divider and passes the current to the Optocoupler LED from the TL431.

R6 and R7 is a simple voltage divider calculated through the method TL431 REF voltage = (Vout x R7) / R6 + R7. The reference voltage is 2.5V and the Vout is 12V. By choosing the cost of R6 23.7k, the R7 became 9.09k approximately.

Fabricating PCB for 12v 1A SMPS Circuit
Now that we apprehend how the schematics paintingswe can proceed with building the PCB for our SMPS. Since this is an SMPS circuit a PCB is usually recommended because it could cope with noise and isolation trouble. The PCB format for the above circuit is also to be had for down load as Gerber from the link

Download Gerber record for 15W SMPS circuit
Now, that our Design is ready it's time to get them fabricated the use of the Gerber file. To get the PCB completed is pretty smoothabsolutely comply with the steps below

Step 1: Get into www.Pcbgogo.Com, sign on if that is your first time. Then, within the PCB Prototype tab enter the size of your PCB, the range of layers and the range of PCB you require. Assuming the PCB is 80cm×80cm you could set the scale as shown under.


Step 2: Proceed via clicking on the Quote Now button. You may be taken to a web page wherein to set few extra parameters if required just like the fabric used song spacing etc. But basically the default values will work fine. The handiest thing that we've got to consider here is the price and time. As you may see the Build Time is only 2-three days and it just costs only $5 for our PSB. You can then pick a preferred shipping technique based to your requirement.


Step three: The final step is to upload the Gerber file and continue with the payment. To ensure the process is clean PCBGOGO verifies if your Gerber record is valid before proceeding with the payment. This way you can certain that your PCB is fabrication friendly and will attain you as committed.<br style="box-sizing: b

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