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Boost Converter information

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LAN_403

KLH

Thu Nov 20 2008, 06:58PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

Hello, everyone. This is my first post, so I will keep things simple. After reading a few of the posts on Boost Converters, I decided to present my knowledge to help clarify some things about it and for other people to make any corrections if needed.

NOTE: This post only covers capacitor chargers, not boost converters designed to have a continous load.

From what I have seen, most Boost Converter designs dedicated to charging capacitors rely on a standard timing device, such as 555 timer or a CD4047 with a 50% duty cycle. This may work for small, low power designs, but when designing high power, high effieciency converters meant to charge capacitors very quickly, pulse timing becomes a critcal factor that must be carefully controlled. Look below at the graph of inductor current over time (from Coilgun Systems).

1227207217 1819 FT0 Inductorcurrentgraph

As the graph shows, as the capacitor's voltage gets higher, its impedance increases. This causes the inductor's current to ramp down faster. With a fixed frequency and duty cycle, eventually the inductor's current will be at zero for part of the timing cycle. This mode is called discontinuous mode. Time at zero causes a waste of time that, instead, could be used to charge the inductor again. In a switching circuit, time at zero also adds core loss. Core loss will (literally) heat up the core in the inductor, and if it is a ferrite core, can permanently reduce the permeability of the core.

This makes the design of the timing circuit non-trivial, but it is a necessity if the boost converter is to have a rapid charging speed and high efficiency.

The inductor can be switched so that as soon as its current drops to zero or near zero, its switch will turn on again, bringing its current back to the planned maximum value. Another configuration can hold the inductor close to a specified current all the time until the charge is complete.

However, these methods warrant a current sensing circuit to sense inductor current. A current sense resistor can be placed in series with inductor just before the inductor's connection with V+, and the difference between V+ and the resistor's voltage can be used to find the current through the inductor. A schmitt trigger or a comparator can be used in conjunction witha gate driver to control the switch. Note that I am still working on the specfic control systems myself, and I will post as soon as I am finished.

I also saw a few questions on gate driving techniques. Driving the gate directly with a 555 timer, CD4047, or comparator / schmitt trigger will likely cause switching losses and destruction of components (not just the MOSFET / IGBT, but possibly even the "driver") unless the driven MOSFET is small. The reason is that these chips were not really designed to drive the capacitive load. The 555 timer has a maximum output current of 500mA, and this is not able to switch MOSFETs very quickly. The solution is a gate driver. I am using a UCC37321, and it has a peak output current of 9A, which should be able to switch large MOSFETs fast.

There were a few questions about the size of the inductor and what resistance it should have. Inductor resistance should be in the milliohms range. A good example is J.W. Miller's torodial inductors. However, it is not a good idea to use just the MOSFET and the inductors in series without some form of current limiting, or else the current draw could run off into the >250A range.

DON'T use a standard DMM to find the current draw in the circuit. The current throughout the circuit varies widely at a high frequency. USE AN OSCILLOSCOPE with a current sensing resistor.

Note that this is not a complete guide on how to design and build a boost converter. If you feel that this is too complicated, a better decision might to build your own push-pull converter, which (in my opinion) has less critical factors in the circuit design itself.

A design of a boost converter and a 1700W push-pull converter will be posted here soon.

If you have any questions, ask freely (with regards to the topic).
Comments are also welcome.

El_Roberto

Thu Nov 20 2008, 09:46PM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
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Posts: 135

Looks like some good info, cant wait to see your design as I need something to quickly charge my next coilgun (I am currently just using rectified mains 240v (charges up to 318v max), but I am using a current limiting resistor which can only handle about 140watts max, I would really like to use the full 2300watt mains power but I dont think 2300watt resistors exist and without this I have no way to limit current (and mains power isnt exactly portable))

KLH

Thu Nov 20 2008, 10:41PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

Does the converter you need have to be portable? Both of my designs are powered by a (few) lead-acid batteries, since they have been designed with portability in mind.

If you choose to use mains, however, it will be a lot easier in terms of charging time. A simple voltage multiplier circuit could be used to boost the mains up to the desired voltage, like the one on anothercoilgunsite.com.

I'll probably be able to post the push-pull converter design first, once I finish testing it.

El_Roberto

Thu Nov 20 2008, 10:48PM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

Yea I want it to be portable, the caps im charging are only 250v 3000uf (I have 14 of them (probably going to be a 10 stage coilgun so I only need to charge 10)) so a high power Boost converter should be fine I just dont know how to build one thats powerful enough. The only mosfet I have are rated at 7amps so ill have to get an igbt or something.

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