12V smps power supply. How it Works

smps, power, supply, works

Simple 12V, 1A SMPS Circuit

The following content explains two simple 12V, 1 Amp switch mode power supply (SMPS) circuit using the very reliable VIPerXX IC from ST microelectronics.

With the advent of modern ICs and circuits, the age old iron transformer type of power supply are surely becoming obsolete.

Today power supplies are much compact, smaller and efficient with their functioning. Here we discuss one outstanding switch mode power supply circuit which can be easily built at home for deriving clean, ripple free 12 V DC.

Thanks to ST Microelectronics IC, the VIPer22A, which has made the construction of truly efficient and compact SMPS power supply unit possible that too by using a very few number of electronic parts.

As can be seen in the picture, the circuit is indeed very small, compared to the power that is available from it. It’s just 50 by 40 mm in its dimensions.

The circuit diagram is very easy to understand, let’s study it with the following points:

) SMPS using VIPer22A

Looking at the figure we can easily see that the configuration does not involve too many stages or parts.

The input mains AC, as usual is first rectified using ordinary 1N4007 diodes which is fixed in the bridge network mode.

The rectified high voltage DC is filtered using the high voltage capacitor.

The next stage is the crucial one which incorporates the outstanding chip VIPer22A manufactured by ST Microelectronics.

The IC alone functions as the oscillator and induces a frequency of around 100 KHz into the primary winding of the ferrite E core transformer.

The IC is absolutely rugged and is internally protected from sudden voltage in rush and other voltage related component hazards.

The IC also incorporates built in over heat protection which makes the IC virtually indestructible.

The voltage induced at the input is effectively stepped down at the output winding, due to low eddy current losses, about 1 amp current becomes available from a relatively tiny ferrite transformer.

With the coil specs shown the voltage is around 12 and the current is around 1amp.

A special feedback circuitry is also included in the circuit for maintaining high degree of protection and power saving features.

The feedback loop is implemented via an opto-coupler which becomes active during abnormal circuit conditions.

When the output voltage tends to rise beyond the set threshold the feed back loop becomes operative and feeds an error signal to the IC FB input.

The IC instantly comes into an corrective mode and switches off the input to the primary winding until the output returns to the normal range.

You may also want to read this: 24watt, 12V, 2 amp SMPS using a single IC Most recommended for you.

2) Another 12V 1 amp simple SMPS using IC TNY267

The simple smps circuit shown above uses the popular tiny switch IC TNY267. It is a tiny mosfet based 120V to 220V switching oscillator IC which only requires configuring with a ferrite transformer and a stepped down Vdd operating voltage.

The design is so simple that a mere visualization of the schematic is enough to tell us the functioning details quickly.

The stepped down start voltage is acquired from stabilizing network using a 180V zener diodes and the fast recovery diode BA159 after rectifying the mains 220V through a 1N4007 diodes and the 10uF/400V filter capacitor.

smps, power, supply, works

As soon as this voltage is applied to the IC, it begins oscillating and its internal mosfet begins switching the ferrite transformer primary at the predetermined oscillating frequency.

Being a flyback design, the secondary also starts conducting during the OFF cycles of the primary through mutual induction and generates the required 12V voltage at the output side.

This voltage may not be stabilized, therefore an opto-coupler based feedback is used and the link is configured with the exclusive shut down pinout 4 of the IC.

This ensures that the output never exceeds, and remains fixed at 12V 1 amp proportion.

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V Power Supply Recommendations

There are some open frame 12V power supply recommendations that you should highly consider in your search for the right one. This information will give you a better idea of what’s out there and which one might be most suitable for your needs.

FSP065-P24-A12 : The FSP065-P24-A12 open frame power supply has AC-DC switching capabilities and is compact. The power supply is capable of delivering 65 watts of continuous power at convection cooling and 50℃ operation temperature. Some applications and uses include audio and video, display, information, and networking. A few of the most notable features are that it has high altitude 5000 meters operation, 3KVac isolated between RETURN and chassis, and has OTP or brown out protection.

FSP150-P24-A12 : Another recommendation when it comes to power supply 12v is the FSP150-P24-A12. It is an AC-DC switching power supply and is capable of delivering 150 watts of continuous power at 7 CFM forced air cooling. This type of power supply is designed for information, display, industrial and telecom applications. The features include 1.5KVac isolated between PE and RETURN, OTP, brown out protection, and fast-on ground tab.

FSP250-H24-A12 : The FSP250-H24-A12 12V open frame power supply features class-I design. It has safety construction and the PSU is capable of delivering 250 watts of continuous power at 14 CFM forced air cooling. As it has surge protection ±2 KV diff., ±4 KV com. You’ll want this power supply for uses such as audio, information, and networking. It also contains HVDC 128~310V input operation and high altitude 5000 meters operation.

FSP150-P35-A12 : You should also consider choosing the FSP150-P35-A12 power supply. This series AC-DC switching power supply is Class-I design and features 3 x 5 x 1.126 inches low profile and no-load input power less than 0.21W. PSU is capable of delivering 150 watts of continuous power at 7 CFM forced air cooling. It’s intended for information, display, industrial and telecom applications. Some features include high altitude 5000 meters operation, OTP, brown out protection, and ground pad terminal.

smps, power, supply, works

FSP200-P35-A12 : You can also add the FSP200-P35-A12 12V power supply to your list of options. It features 1.5KVac isolated between PE and RETURN and high altitude 5000 meters operation, as well as 12V fan driver. This PSU is capable of delivering 200 watts of continuous power at 7 CFM forced air cooling. Some uses and applications include audio and video, display, household (Europe), information, and networking.

About FSP

FSP Group is one of the global leading power supply manufacturers. Quality assurance is one of our promises to customers. FSP Group provides more competitive quality products and makes itself the most reliable partner for customers, consumers, and suppliers with the joint creation of maximized values. It’s where power never ends. Our vision is to be the global leading provider of green energy solutions, to touch people’s lives, and to contribute to a better environment.

We offer a wide range of products from adapters and open frame power supplies to TV power supply, PV Inverter, UPS, ESS, and battery chargers. All of our power supplies are designed with ultra-high efficiency, optimal product life, and high reliability. As far as the 12V power supplies and open frame power supply options go, 2 x 4, 3 x 5 standard form factors are available for selection. Modified standard and custom designs are always welcome too.

smps, power, supply, works

±15V or ±12V Output Switch-Mode Power Supply Has Wide Input-Voltage Range

This application note illustrates how to use the MAX668 boost circuit and the MAX1846 inverting circuit to implement a switch-mode power supply that provides a ±12V or ±15V at 0.5A output from a 4.5V to 12V input.


The switch-mode power-supply circuits illustrated in this application note provide a ±12V or ±15V at 0.5A output from a 4.5V to 12V input. This wide input-voltage range allows the device to be powered from a regulated DC voltage or even an unregulated DC voltage, such as the rectified output of an inexpensive AC wall wart step-down transformer. This design may be preferred to Maxim’s older MAX742 design, which requires a larger overall circuit.

The power supply consists of a MAX668 boost circuit or a MAX1846 inverting circuit. Each circuit operates at a 300kHz switching frequency, balancing cost, size, and performance. The circuits limit the maximum switch current, which ultimately limits output current for a given input voltage; however, maximum output current increases with increased input voltage.

The MAX668 Circuit

Figure 1 illustrates how to use the MAX668 boost circuit to implement a switch-mode power supply that provides a 15V at 0.5A output from a 4.5V to 12V input. This MAX668 circuit adds a few components to the minimum circuit implementation. C7 adds a pole to compensate for the ESR-zero of the output capacitor. R5 and C8 filter the current-sense voltage to prevent high-frequency switching noise from prematurely tripping the current limit. This functionality is in addition to the MAX668’s internal 60ns current-sense blanking time.

The MAX668 output voltage can be changed to 12V by changing the value of the R2 resistor to 86.6kΩ. This change reduces the minimum input voltage to approximately 10V. Since the MAX668 provides internal compensation, no other changes are required for 12V output.

The output ripple voltage due to switching can be reduced an order of magnitude with a secondary output filter set to one-tenth of the switching frequency. A 1Ω, 0.5W resistor in series with a 10µF, 25V capacitor with less than 100mΩ ESR introduces a 0.5V decrease in output voltage at a 0.5A load. The feedback voltage must be sensed before it reaches the secondary filter for the MAX668 to maintain stability.

The MAX1846 Circuit

Figure 2 uses the MAX1846 inverting circuit to implement a switch-mode power supply that provides a.15V at 0.5A output from a 4.5V to 12V input. This MAX1846 circuit also adds a few components to the minimum circuit implementation. C20 adds a pole to compensate for the ESR-zero of the output capacitor, while R16 and C22 filter the current-sense voltage to prevent high-frequency switching noise from tripping the current limit. This functionality is in addition to the MAX1846’s internal 100ns current-sense blanking time. The MAX1846 EXT pin implements controlled slew rate, which helps limit high-frequency switching noise.

The MAX1846 output voltage can be changed to.12V by changing the value of R13 to 97.6kΩ and R10 to 91kΩ. The maximum input voltage does not decrease, although duty cycle jitter increases somewhere between 10V and 12V input. Again, the output ripple voltage due to switching can be reduced an order of magnitude with the same secondary filter described in the MAX668 circuit above. As with the MAX668, the feedback voltage for the MAX1846 must be sensed before it reaches the secondary filter for the voltage inverter to work properly.

Soft start

Pin 4 of the TL494 is called the dead-time control input and can be used to implement a soft-start feature. C24 is initially discharged, so when power is applied, the DTC pin is held high. This inhibits the output. As C24 gradually charges up (via R19), pin 4 drops in voltage, which slowly decreases the dead time, bringing the output up to its operating level. Pin 4 settles at about 0.4V.

This part of the circuit had me stumped at first. I couldn’t see what it was supposed to do! It’s a very clever short-circuit protection.

Suppose that the supply is operating normally, with a 12V output. The base of Q5 is fed by a divider from the DC output voltage. Since the divided voltage produced by R38R31 (which would be about 2.2V) is well above the base-emitter drop of Q5 (0.7V), the transistor is held on, pulling the voltage on C30 low. Given the forward drop of D13, this will have no effect on the voltage at the DTC input. So, in normal operation, this circuit does nothing.

Suppose that the output is suddenly shorted. V drops to zero (or very close) which results in Q5 turning off. C30 will now charge up via R33 and ZD3 from the auxiliary supply. (I’m not sure about the purpose of ZD3). Once it reaches a voltage sufficient for D13 to conduct, it will pull up the DTC input and cause the TL494 to shut down.

If the output short is now removed, the output will remain shut down. Q5 remains off, so C30 is charged, holding the DTC pin high. You might wonder how there is still an auxiliary supply available, when the TL494 is shut down. remember the start-up behaviour, with the bridge transistors self-oscillating? The supply goes into this mode again, which is enough to provide an auxiliary supply of about 10V.

The only way to restore power is to switch the entire supply off, wait, and power on again. Which now begs the question, why doesn’t the short-circuit protection trigger at power-up? The short answer. thanks to the soft-start circuit, the DTC pin takes sufficiently long to go low that the output voltage has built up enough to keep Q5 conducting (stay tuned for some graphs of this happening).

Here’s some waveforms when the output is shorted during normal operation. Before shorting, Vcc is about 20V, the output (V) is 12V, DTC is about 0.4V, and Q5’s collector is near 0V. it’s being held on by the high output voltage. When the output is shorted, V falls to zero. Q5 turns off, and C30 starts to charge up so Q5’s collector voltage starts to rise, which in turn causes the DTC voltage to rise. As it rises, the TL494 begins to shut down (dead time increases), until finally the chip is completely disabled, with DTC reaching just under 3V. VCC drops to about 10V since the bridge is now operating in the self-excited mode, as it isn’t receiving any drive signals from the TL494.

Next, here’s the waveforms during startup with a normal load on the output (i.e. not shorted). On startup, the inverter goes into self-excited operation and VCC immediately goes up to 10-15V or so. DTC immediately jumps high because C24 is initially discharged, and then starts slowly dropping as it charges through R19. Since the output voltage is initially zero, C30 (at Q5’s collector) starts charging up via R33. However, as soon as the output voltage builds up to around 3 or 4V (again, thanks to self-excited operation), Q5 is switched on, discharging C30. After this, once DTC falls to a suitable level, normal operation commences. Note that, at all times during normal startup, Q5’s collector voltage never reaches DTC plus one diode drop (D13) so the short-circuit protection circuit can never affect the DTC level during normal startup operation.

Lastly, here’s the behaviour when the supply is started with the output shorted. The output voltage tries to increase, but can’t (since it’s shorted). Q5 is held permanently off, so C30 can charge up. Once it reaches a sufficient voltage (DTC one diode drop), it then holds the DTC pin high, preventing further operation until power is cycled.

When we’re here, an important note regarding the short-circuit protection. Although I’ve given examples of it tripping in when there’s a direct short across the output, it will actually operate whenever the output voltage is insufficient to keep Q5 on. this occurs below about 4V. This means that, when the supply is modified to give a variable output voltage, it isn’t possible to reduce the output to below 4V, because the short-circuit protection would kick in. To enable output below 4V, you’d have to disable the short-circuit protection. simplest is to remove D13. However, you then run into another problem. the voltage at pin 2 of the TL494 is held at 2.5V by the R30R34 divider and it therefore isn’t possible to adjust the output below 2.5V. Unless, of course, you changed the values of the divider resistors to produce a different (lower) reference voltage at pin 2, but that’s getting more and more involved.

Designing a new feedback divider

Here is the new feedback divider I’ve cooked up. this replaces the contents of the dotted box marked Voltage sense in the schematic further up the page.

[Note: there is no earthly reason for the two 1kΩ resistors in series. I simply didn’t have any 2kΩ resistors in stock!]

There’s one important difference between this and the original divider. The original had a very nonlinear adjustment, because VR1 was simply used as a variable resistor between the feedback pin and ground. The new divider has a linear adjustment, thanks to the grounded-wiper configuration. With the values shown, the adjustment is around 4.8-15V; note that I deliberately avoided going too low, to prevent the short-circuit protection kicking in (see earlier). For more details on the advantages of a grounded-wiper feedback configuration, please see this page.

What’s with the capacitors? Remember that the original divider had a couple of capacitors in it to provide loop compensation. Now, I really don’t know what I’m doing regarding loop compensation, but I reckoned that it would be best to try and get the gain/phase response of the new divider as close as possible to that of the old one, to reduce the chance of any instabilities. I determined the correct component values by trial-and-error in LTSpice. Here’s the gain/phase vs. frequency plots for both old and new feedback networks over the full adjustment range. note how, although the range of values is wider for the new divider (thanks to the increased adjustment range), the various corner frequencies are about the same. The boost around 100Hz-10kHz is from C1R39 coupling more of the output voltage through to the feedback pin, and the drop at high frequencies is from the decreasing impedance of C26.

Hardware modifications

First, remove some of the original components from the PCB. Remove C31, R32, R40, and VR1. Here’s a before and after view:

We’ll use some of the existing tracks and pads to connect up the components for the new feedback divider. Watch out for the correct orientation for the 10kΩ potentiometer. Here’s the layout (viewed from above, looking through the PCB):

And that, as they say, is that! The new feedback divider is the only modification needed to give the wider adjustment range. I measured a range of 4.8V to 15V, but it might vary slightly depending on component tolerances. Even at the lowest output voltage of 4.8V, there was no sign of the short-circuit protection kicking in.

In addition to the voltage divider modifications, I also decided to add a little digital voltmeter module to display the current output voltage. I had bought some meter modules a while back and hadn’t found a use for them yet.

Search either AliExpress for TK0600 voltmeter 0-30V or EBay for New 1pcs Digital Voltmeter DC 0-30V Useful LED Panel Meter Red. That’s the likeliest search terms to bring up results, but you might have to use a bit of imagination to search for other terms. These particular modules use separate connections for the power supply and the sense, so can measure right down to 0V. Other modules actually run from the sensed voltage, so are limited in how low they can measure. They’re neat little modules. 3 digits, automatic decimal point, 0-30V range, and have an onboard STM800S3F3 microcontroller. There’s even several I/O pins broken out to a header, so doubtless it could be reprogrammed. Here’s a couple of people who have analysed the circuit:

The power supply for the voltmeter module is derived from a couple of extra diodes 100µF capacitor 220µH series filter inductor, tacked on to the anodes of D11 D12 (see photo below). This provides about 20V to the module. According to the EEVBlog posting, the module uses a Holtek 7130 voltage regulator, which has a maximum input voltage of 24V, so this is well within limits. I didn’t use the existing auxiliary supply because I found it was a little bit unstable when the supply is operated in a low-load/self-excited mode. The voltmeter module’s sense connection goes to one of the various big wire jumpers that are used on the output side to increase the current-handling capability of the PCB.

I mounted both the adjustment potentiometer and voltmeter module on the casing of the supply, just above the output terminals. Bit of a squeeze, but there was just enough space to fit them in. I also added a piece of red filter plastic in front of the module to make the display a bit clearer to see.


The power supply is now adjustable from 4.8V to 15V and appears to work well over the full range. Set to 7.4V, it happily runs the brushless motor; there’s a slight drop in voltage at maximum speed, but that’s to be expected. I’m using a servo tester to provide the adjustable PWM signal to the ESC.

Here’s a video overview covering most aspects of the modification:

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