High Power SMPS with MCU


New member
Hi, I'm new here and just found this forum about 2 weeks ago with lot of great threads about how to build custom SMPS. I'm currently developing an ON-LINE UPS for server rack and the goal is to make it in the most simplest way which I think is with a micro controller. The Inverter which would run from the power supply 24/7 is capable of outputting 3.5kW and requires DC24V for the operation. The problem is that on full load this means more than 150AMPs from the DC line which is in parallel with two 200Ah SLA battery which at the same time needs to be charged while the AC line is present. I have some components that could be useful to build the power supply and I used these to make the schematic that I posted in the attachments.

The idea is that there is an auxiliary power supply (I used the TOPSwitch277 and it's basic schema to make it) which provides DC12V which powers the system while the main power supply is not in use but the system also uses the main supply as secondary so the MCU is powered from the "redundant low power line". I used my basic knowledge of SMPS to design this thing so excuse my for my mistakes :D So the system looks like this, AC goes through the filtering stage then rectified by a full bridge rectifier, then fed to a capacitor bank and then into the IGBTs in Full Bridge configuration which are driven by two IR2106s and there would be a custom high frequency transformer to step down this high voltage dc. Then there are four schottky diodes to rectify this voltage at the output. To tell the MCU about the output current I used an ACS758 current sensor module and the output voltage would also be measured by the MCU on the DC line. The MCU provides the four PWM signals for the mosfet drivers.

What frequency and duty cycle would be ideal to use with this configuration and what should the MCU do when current rises or falls down to keep the ouput dc voltage from dropping?
What are the parameters of the transformer which is capable of delivering this much current constantly?
How can I eliminate the noise from the power supply both dc and ac line?
Also would this even work? :D
Excuse my for my English and my mistakes with this thing.
All helps are welcome and I'm open to any suggestion which helps me to improve this design.

I'm really new here, somehow the attachment is resized and could not be seen correctly so I uploaded it onto Mega.nz


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New member
Out of the many questions you have, I think you should try to make smps with lower power first then adjust and develop into bigger ones
You can try smps from silvio, ludo or other members.
It seems like you are experienced in using MCU, but SMPS is a different animal to be tamed
sorry if my answer makes you uncomfortable, but that's what I did first when learning smps


@ Scarface

I agree with my friend Res, Smps is no joke especially when they blow. Trying to create a brute for a first time is quite a task without experience.

Some other considerations for you, I think for a continuous 3.5Kw load I guess you better start at a higher voltage say 60 or 70v. I know the cost of batteries is considerable but at least your input current will be much less. Around 60A is better than 150A which is still quite high, stray inductances and capacitances are inevitable at these currents not to mention heat with copper losses, power factor losses etc.
Ready made inverters which are sine wave can be found working at these voltages. Usually they are used for solar panels.

I suggest you climb the ladder taking the first step not the last one.


New member
I remember this thread http://www.diysmps.com/forums/showthread.php?978-4000w-12v

@scarface, are you an engineer at an electronics manufacturing company?
Or if you are only going to use ups for your server (or your company's server) I suggest to just buy a finished product for example APC from Schneider Electric which has gone through a long process to ensure performance and reliability


New member

Totally agree with you about SMPS and the 48V is a good idea! The constant load would be 1.8-2kW, the 3.5kW is the maximum if the rack gets updates, but I think it is better to over size these units to be safe. Thank you for your reply!


It is a personal project. I have experience with MCUs and Linear Power Supplies and small SMPS like the TOPSwitch family or the TNY so far, but for this project I must use SMPS to reduce the costs as possible. I'm working on this project in my free time. Currently a "tuned" APC line-interactive UPS helps the servers with power failures but out of 10 at least 2 times the relays switch slowly so the whole rack powers off and this is why I decided to go with an ON-LINE Mode system which always powers the load from the inverter. As for the whole idea, I designed an inverter which is capable of supplying the load, it is also controlled by an MCU. I used an H or Full Bridge topology, and then I thought why couldn't I use the same principle for the SMPS unit only with minor modifications like higher voltage capacitors, gate drivers, IGBTs instead of mosfets and another transformer which instead of stepping up the voltage, steps it down to a desired level. The same MCU would also control the SMPS unit.
I will post the schematic of the SMPS what I designed. Thank you for your reply!


New member

Those Eltek systems look promising, but currently I don't want to spend for two of those units but the schema of it was useful!

So, here is what I achieved with the SMPS design so far:

This schema isn't contain the current sensor which is connected to the DCOUT+
In theory the MCU controls the ac relay to switch on or off the system, provides PWM signal for the mosfet drivers and monitoring the output voltage and current.
What do you think, could this be used in reality or we need a really complex system without an MCU?
Most of the components shown are available for me.
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New member
Dear scarface,

I can not say much about the MCU because only ATMEL AVR that I know. and I have never made a system with that much power

but if i have to choose i will choose to use a specific chip for PFC and fullbridge controller and use a microcontroller to control and monitor the whole system

IR2106 has a low current capability, maybe IR2110 or IR2113 can be considered or some drivers from fairchild (FAN739x)
consider using synchronous recifier for secondary, you can try to compare the total loss using a diode (Vf * I) vs mosfet (I^2*Rdson)