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I am asked to design a dc-dc converter with the following specifications: Vin=24V(2 lead-acid batteries in series), Vout=300V and Iout=1A.

My initial choice of topology was the boost converter. Problem is the gate drive duty cycle can go up as high as 1 - 24/300 = 92%, and that's assuming perfect efficiency.

A tapped-inductor boost would solve the duty cycle issue, but at the cost of added complexity.

I am also not sure if the boost family is the way to go at the 300W level.

What dc-dc topology would you recommend for my application?

EDIT(to clarify):

Okay so my main problem is that the duty cycle at the specified load reaches well above 90%. Research tells me that it's not a good idea to push the duty limits of dc-dc IC controllers. Such a very high duty also means high current stresses on the switches. At 92% (ideally), the average switch current goes around 1A / (1-0.92) = 12.5A. That's a lot of current to handle. Same trend goes for the rest of the components.

One idea to fix this is to use a tapped-inductor boost topology such as this one tapped-inductor http://www.edn.com/ContentEETimes/Images/power_tip_62_fig1.jpg! This way, the needed duty cycle goes down thanks to the added voltage boost from the transformer, and so component stresses are relaxed. However, the transformer adds to the complexity.

I have not explored, and am not knowledgeable, with other dc-dc topologies that can step-up the voltage such as the buck-boost, and forward converter topology. Maybe other such topologies can step-up the power more efficiently compared to the boost and tapped-boost based ideas I gave - I am not sure. My only other concern is the complexity of implementation, as this only an undergraduate project :)

Should I push through with the boost-based idea? If not, what topology would you recommend?

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  • \$\begingroup\$ Definitely something inductor-/transformer-based. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 24 '13 at 16:09
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    \$\begingroup\$ Welcome to EE.SE! As asked, your question is very broad and answers may be based largely on the reader's opinion. Can you try to ask a more specific question? \$\endgroup\$ – Joe Hass Dec 24 '13 at 16:39
  • \$\begingroup\$ Hi I have clarified (hopefully) my post \$\endgroup\$ – John Patrick Dec 24 '13 at 17:23
  • \$\begingroup\$ Biggest issue with 300W flyback is that energy must be stored in magnetic field so inductor tends to be larger than some version of a forward converter. Usually people will use more complex topologies BUT the tapped boost converter can be made partially isolated by taking the output winding to ground (so DCin never "flows to output when PWM is off and HV has less chance of flowing to input when D! dies short-circuit) and fully isolated by isolating the HV winding and using eg opto feedback. As you will wind even a tapped winding as two separate windings the inductor is no more complex.... \$\endgroup\$ – Russell McMahon Dec 24 '13 at 17:48
  • \$\begingroup\$ ... Semi-isolated with HV winding to ground is potentially slightly less efficient than straight 1:N flyback but not by much. \$\endgroup\$ – Russell McMahon Dec 24 '13 at 17:48
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I'd go for a full H-bridge driving a transformer.

With a 24V supply you can drive nearly 48 Vp-p onto the primary and this reduces the secondary winding turns to a ratio that produces 600 Vp-p - then I'd use a fast bridge rectifier and smoothing to give a 300 V dc output.

Step up ratio would probably be about 14:1 and if you wanted to control the amplitude I'd consider feeding the H bridge from a decent efficiency Buck convertor. Maybe make the turns ratio 15:1 so that a 20V output from the buck would do the business - leave a little overhead for loads etc..

My main consideration is avoiding the secondary winding having too much self-capacitance and acting like a parallel tuned circuit. It should be quite a way off with this type of topology.

It's not going to be a small lump of ferrite and I'd use the best I can get my hands on such as 3F4 material from Ferroxcube - you should be able to get 300 watts from an E68 planar transformer. I'm getting 200 watts from a slightly smaller E58 - using two half cores rather than a half core and a plate.

Mine's running at about 600 kHz so I can use PCB tracks for the coils - a sandwich of PCBs does the trick for the primary (4 turns in total) - mine is 1:1 so it's the same PCBs on the output - yours will need thinking about as to whether you can get 60 turns at 1A from PCB. I think you should just about squeeze them in.

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