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I'm trying to build a 12v to 24v boost converter. I have an output current requirement of 3A. I'm thinking of using two LM2587 5A Flyback Regulators in parallel like this: http://www.ti.com/lit/an/snva552/snva552.pdf

  1. Will this circuit work for me? (keeping in mind that LM2587 has an Input Current rating of 5A)

  2. How do I calculate the required inductance values?

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  • \$\begingroup\$ what variation in input supply voltage might you expect and what sort of output impedance does it have. What range of load currents can you expect and how much ripple voltage can you tolerate on the output? \$\endgroup\$ – Andy aka Oct 29 '13 at 20:07
  • \$\begingroup\$ I'm using an SMPS (computer power supply type). It supplies 12V and 25A(max). Im guessing it does have a very stable power output. Im planning to drive a stepper motor with this power supply on the output. The NEMA23. I have very low tolerance for ripple since i will be using the motor in a CNC. (4 axis CNC) (So, i require 24v 12A supply in total). Im low on budget so cannot afford a very expensive power supply and hence trying to build my own. Is there any other solution to this? (considered two SMPS in series but not too sure about it). Thanks! \$\endgroup\$ – Raghav Parwal Oct 29 '13 at 20:51
  • \$\begingroup\$ See my recommendation at the bottom of my answer. Let me know if you need further help. \$\endgroup\$ – Andy aka Oct 29 '13 at 21:38
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Take a look at this: -

enter image description here

This is from the data sheet and I'd say that it implies that one device would max-out if you tried to take more current than 1A. Two circuits would produce 2A at 24V using the technique shown in the other link so realistically you'd need three devices.

24 V at 3 A is 72 W and is probably asking too much of this type of device to provide half this power. You could still use three (tying the compensation pins together and shorting feedback to ground on two of them but it looks less attractive now.

I haven't looked into the details much but it seems that in boost mode it's better suited to provide higher output powers when using a larger input supply voltage - figure 53 shows a 36 V output at 2 A from a 24 Vnom supply for instance. Figure 54 is starting to hint at the device using a heatsink (48 V @ 1.5 A output from a 24 Vnom supply).

You just might get away with 24 V @1.5 A (3 A from 2 chips) if you used a heatsink but it'll be close!

I'd favour a more modern, higher frequency switcher from Linear Tech. They have a substantial range and an easy selection tool for finding devices that fit the bill. The LT3579 is about the best they offer with internal switching transistors so I think you are probably going to be looking at a chip with external transistors for switching.

However, the LTC3787 using external transistors and bi-phase drive for reduced ripple looks the beast for generating 24 V at 10 A - see figure 10.

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As Andy says, keep things simple. Paralleling converters isn't a trivial task.

I suggest a simple controller that uses an external MOSFET like the TI TPS40210. You can get a reference board from them and use it for your experimentation (put your own inductor and caps, etc.) or use it as the basis for your prototype.

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