Power supply is a core requirement for every electronic components. so designing a power supply should be very reliable and accurate.Before designing any power supply we must be clear about our requirements. so, the important things that must be considered before designing any power supply are:

  • Input-voltage (ac or dc) range
  • Output voltages (dc or ac) and tolerances
  • Output-current requirements
  • Ripple maximum
  • Estimated total power required
  • Efficiency requirements, if any
  • Electromagnetic-interference (EMI) considerations, if any

Here in our case input voltage is :

220v to 230v AC (that is our main supply )

Our output requirement is:

voltage: 5v dc and current : 2 amp



Before explaining each and every components of the circuit diagram the components used are respectively categorized into different functional blocks.

The circuit contains following blocks:

  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
  11. Feedback section.

Input surge or over voltage protection

In this section we include different component that protect our circuit. We have used Fuse F1 and varistor Rv1 for protecting our circuit.

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fig: varistor

A varistor is an electronics component with an electrical resistance that varies with the applied voltage. As the voltage increases across the varistor its resistance decreases.

What is a Varistor? Definition, construction, operation, characteristics,  advantages, disadvantages and applications of Varistor - Circuit Globe
fig 3: voltage resistance curve of varistor


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fig : fuse

A fuse is a electronics safety device which provide over current protection for electrical circuit.

Metal Oxide Varistor (MOV) Features, Working & Datasheet
fig4: fuse and varistor protection circuit

A varistor is always connected in parallel and fuse in series to the protection circuit. We have used fuse of 1amp and metal oxide varistor (MOV) of 275V .

In the normal condition when the input voltage is around 230 volt maximum current flows through the circuit as the resistance of varistor is very high it serves as a open circuit and there is almost no current flowing through the varistor as the voltage increases the resistance of the varistor decreases and the current starts flowing through the varistor when the input voltage reaches nearby 275 volt almost all the current flows through the varistor acting as a short circuit which will definitely blow our fuse .Once the fuse is blown our circuit part is totally isolated from the supply.

Varistor and the Metal Oxide Varistor Tutorial
fig:electrical spikes

sometime due to some issue we can receive a voltage spikes in our line voltage for a short period of time as shown in the figure. MOV can be very useful in such a case.

AUMOV® & LV UltraMOV™ Varistor Design Guide for DC & Automotive Applications

When we receive a sudden spikes then automatically voltage increases and the resistance of the MOV decreases and current flows through MOV which helps to clamp the over voltage as shown in the above graph. If there is high voltage for a long period of time then our MOV may get damaged due to flowing of high current and high heat dissipation.


AC-DC conversion

We have used full bridge rectifier for converting AC to DC. We can directly use full bridge rectifier module or we can make our own rectifier using four diode. Working of rectifier circuit is so simple so i have not described it here.

Bridge Rectifier Circuit - Design Details & Tips » Electronics Notes

PI filter

PI filter is a passive filter which is composed of two filter capacitor namely C1 and C2 connected in parallel and inductor L connected in series in between the two shunt capacitor as shown in the figure below.

Electronic Circuits - Filters - Tutorialspoint
fig: pi filter

The output of rectifier is fed to our first capacitor C1 as main filtering is itself done by C1 where as L and C2 help in ripple reduction.

What's the definition of electrical reactance? Do capacitors and inductors  have reactance in DC circuits? - Quora
reactance of inductor and capacitor

From the above equation we must be clear that , in case of inductor frequency and inductive reactance(XL) is directly proportional

In case of AC, there is high frequency and consequently high inductive reactance (XL) .Due to high reactance it acts as open circuit for AC components which also means it blocks AC components.

In case of DC, frequency is 0 .so the inductive reactance is also 0 that mean to say that it acts as open circuit for DC components also mean to say that it passes DC component.

From the above equation we must be clear that , in case of capacitor frequency and capacitive reactance are inversely proportional

In case of AC, there is high frequency and consequently Low capacitive reactance (Xc). Due to low reactance it acts like short circuit for AC components which mean it allows AC to pass through it.

In case of DC, frequency is 0 .so the capacitive reactance is also infinite that mean to say that it acts as open circuit for DC components also mean to say that DC components cannot pass through it.

capacitor “C1” allows a.c. component whereas the d.c. component maintains its journey toward the choke ‘L’

The choke “L” permits the d.c. component to supply through it, whereas the unbiased a.c. component can be blocked.

The second filter “C2” allows the a.c. the component which the choke has unsuccessful to block. Thus, simply d.c. component shows across the load.

PI filter produce a high o/p voltage on small current drains.

Driver circuitry or Switching circuit

Switching circuit is heart of any SMPS circuit. We have used TNY2860 as our switching regulator. 0.1uf C3 is used as boost capacitor. The switching frequency is nearly about 120-132khz. Due to this high frequency switching, smaller transformers can be used.

If you want to know detail about switching action you can visit


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This pin has dual functions: Enable input and line under voltage sense. During normal operation, switching of the power MOSFET is controlled by this pin. MOSFET switching is terminated when a current greater than a threshold current is drawn from this pin. Switching resumes when the current being pulled from the pin drops to less than a threshold current. A modulation of the threshold current reduces group pulsing.

The ENABLE/UNDERVOLTAGE pin signal is generated on the
secondary by comparing the power supply output voltage with a
reference voltage. The ENABLE/UNDERVOLTAGE pin signal is high
when the power supply output voltage is less than the reference
voltage. In a typical implementation, the ENABLE/UNDERVOLTAGE
pin is driven by an optocoupler. The collector of the optocoupler
transistor is connected to the ENABLE/UNDERVOLTAGE pin and the
emitter is connected to the SOURCE pin. The optocoupler LED is
connected in series with a TL341 across the DC output voltage
to be regulated. When the output voltage exceeds the target
regulation voltage level (optocoupler LED voltage drop plus TL341), the optocoupler LED will start to conduct, pulling the


In the circuit D1 is a tvs diode and D2 is ultra fast recovery diode.


The transformer acts a huge inductor . Therefore during the switching off-cycle, the transformer creates high voltage spikes . TVS diode is used to protect the circuit from such voltage spikes.

V-I Charectertics of Unidirectional and Bidirectioncal TVS Diode | Diodes,  Transients, Power electronics

From this above figure we can say that in a normal voltage diode will be in reverse stand off region but when there is sudden spikes it goes to the breakdown reason and slowly current starts flowing and it reaches to clamping region where the diode behave some how like short circuit.

Driving Loads with High Power | Acroname

1 – When the coil is active, a current flows through the coil. No current passes through the diode because it is inversely polarized.

2 – When the coil is deactivated, the coil tries to maintain the current flow. As there is no way for the current to circulate, a diode is placed parallel to the coil. In this way, the current circulates through the diode and voltage peaks are prevented from damaging other components of the circuit.

Magnetics and galvanic isolation

We have used ferromagnetic transformer which converts high voltage AC to low voltage AC. It also provides galvanic isolation.


Capacitor C4 of 2.2nf ,250ac is used as EMI filter. It increases the immunity of the circuit to reduce the high EMI interference.


The out put from the transformer is rectified and converted into DC using diode D4


A simple RC snubber uses a small Resistor (R) in series with a small capacitor(C).

How Snubber Circuit works ? - Instrumentation Tools

The basic principle of snubber circuit is to regulate the dynamic voltage across the SCR.

The capacitor opposes the change of voltage across it by charging or discharging current.

Snubber circuit limits the rate of rise of voltage or rate of fall of voltage across SCR.

In order to limit the magnitude of the discharge current resistance should be connected in series with the capacitor.


In order to maintain the galvanic isolation between the primary and secondary side we have used Optocoupler. The Optocoupler has a transistor and a LED inside of it. By controlling the LED, the transistor is controlled. Since the communication is done by optically, it has no direct electrical connection, therefore satisfying the galvanic isolation on the feedback circuit too. The output sensed in secondary side is transferred to primary side with opto coupler to maintain the output voltage.This control system is employed by the TL431

As the shunt regulator has a resistor divider across it reference pin, it can control the Optocoupler led which is connected across it. The feedback pin has a reference voltage of 2.5V. Therefore, the TL431 can be active only if the voltage across the divider is sufficient. In our case, the voltage divider is set to a value of 5V. Therefore, when the output reaches 5V the TL431 gets 2.5V across the reference pin and thus activate the Optocoupler’s LED which controls the transistor of the Optocoupler and indirectly controls the TNY268PN. If the voltage is not sufficient across the output the switching cycle is immediately suspended.

R6 and R7 is a simple voltage divider calculated by the formula TL431 REF voltage = (Vout x R7) / R6 + R7. The reference voltage is 2.5V and the Vout is 5V. By selecting the value of R6 4.7k, the R7 became 4.7k approximately.


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