I need to design a step up converter with the following requierements:

Vin = 6V - 8.4V
Vout = 35 - 40V, Iout = 2.5A

Could anyone give me some insights on how to start it? I have found some step up schematics but could not find any that could deliver 40V and 2.5A. Any reference is well apreciated.


  • \$\begingroup\$ The hardest bit of about a boost converter is what control method you are going to use. There are lots of controller ICs out there for boost converters. Texas Instruments, Analogue Devices, Maxim and many others do them. Or you could do your own controller with a suitable processor core. What work have you done so far? What typology were you planning to use? What current ripple can you live with? What voltage ripple? \$\endgroup\$ – Puffafish Jun 26 '19 at 9:35
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    \$\begingroup\$ This is a broad question. But you can start using supplier-provided tools to design this, for example Texas Instrument's Webench Power Designer, Analog Design's LTpowerCAD, and/or use simulators (e.g. LTSpice) to check and understand how a specific design works. \$\endgroup\$ – dim Jun 26 '19 at 9:38
  • \$\begingroup\$ @Puffafish, the purpose of the step up is to charge a big capacitor that moves a solenoid. Using TI Webench Power Designer, I created this circuit, but is not working properly and behaves in a strange way. Here you can see what I have done so far: drive.google.com/open?id=1260GChzKZc64UZXIM4EQjB1xFFMnanWo \$\endgroup\$ – alex_unio Jun 26 '19 at 9:49
  • \$\begingroup\$ Just in case you are thingking to make just one or two you are likely better served by purchasing a ready unit. Also I like to see what others have done and then modify existing working designs to get what I want. Sometimes simple changes can make a tacky product kind of nice, possible by hacking a ready unit or altering the design for your own production run. Commercial vendors have flogged the switching power supply horse pretty well these last 30 years. \$\endgroup\$ – KalleMP Jun 26 '19 at 10:26

I would start with a selection site where you input the supply voltages and output currents and it makes recommendations based on that. The former Linear technology site is good for that (now run by Analog Devices): -

enter image description here

In the above picture I have pre-selected "External Power Switch Boost" (inside the red box) and added some values inside the purple box. The 2nd part in the list I chose based on what the front page of the data sheet indicated: -

enter image description here

As you can see, the input voltage can go down to 6 volts and the output can be up to 80 volts at 7 amps. All those numbers exceed your requirements so that design would work provided that your battery (6 volts to 8.4 volts) can supply the 100 watts of power needed.

Look at the graph on the right that shows efficiency versus load versus supply voltage. With a 6 volt supply you'll get 80 volts at about 1.7 amps. If you altered the FB voltage resistor divider, you'll get 40 volts and a similar power hence you should easily be able to achieve 2.4 amps at 40 volts.

But, don't stop there; check out other offered parts and see if the data sheet might give something that works down to maybe 5.8 volts. Cover the options. I chose non-synchronous converters in the above but try looking for synchronous converters too.

There are similar engines from TI but I find the one above to be easiest. Fly-back designs are also capable of giving you 100 watts but have maybe an efficiency of only 85%.

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  • \$\begingroup\$ Thanks Andy! Really helpful the Analog Devices tool. I am trying to solve the voltatge divider part to get 40v instead of 80V, with the schematic you posted. Could you please help me with that? What resistors values must I change to get 40V? \$\endgroup\$ – alex_unio Jun 26 '19 at 14:31
  • \$\begingroup\$ Look in the data sheet to see what the FB voltage should be. It's probably around 1.25 volts. Then ask yourself what the 796 kohm resistor value needs to reduce to when 40 volts is applied to give 1.25 volts at FB. There'll be a formula in the DS for sure. \$\endgroup\$ – Andy aka Jun 26 '19 at 14:44
  • \$\begingroup\$ Thanks for your support, Andy. I couldn't figure out the 1.25V you proposed in your last mail. I readed the datasheet but could not find a formula to calculate it. \$\endgroup\$ – alex_unio Jun 28 '19 at 10:40
  • \$\begingroup\$ Did you find the reference voltage in the data sheet on page 3? Do you know how that voltage is "made" via the potential divider (796 k and 12.4 k) from 80 volts? Do you know how to recalculate the value of the 796 k resistor to obtain "reference voltage" from 40 volts? \$\endgroup\$ – Andy aka Jun 28 '19 at 10:44

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