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    High Voltage and Current Buck Converter

    Good Afternoon Everyone,

    This is my first post on the boards so tell me if I am doing anything wrong. I am looking for a way to step down DC voltages via a micro-controller from 48 Volts to any range between 48 Volts and 12 Volts. I have read some material on Buck converters, but it does not seem like they would be able to transform down any input voltages above 36 Volts. If anyone knows of a solution or has anymore questions please let me know.

    Thanks

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    Re: High Voltage and Current Buck Converter

    There's really no limit on what a buck converter topology can do, aside from the component ratings. I've built buck converters for 600V in/out in the past. But buck is not necessarily the best topology either.



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    Re: High Voltage and Current Buck Converter

    One problem that I am having is finding a system that will have an input voltage of 52 Vdc and output voltages at stages of 48V, 36V, 24V, and 12V. The buck converter must also be able to handle 40 Amps or less. I am having trouble finding anything that will support this high of amperage. I know that most semiconductors can only handle 1 amp at most. I was wondering if it would be impossible to construct a Buck Converter with such a high amperage or if I will have to design a system using a multi-tap transformer.



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    Re: High Voltage and Current Buck Converter

    Quote Originally Posted by turtlepokerman View Post
    I am having trouble finding anything that will support this high of amperage. I know that most semiconductors can only handle 1 amp at most.
    Where did you get this?
    There are many MOSFETs that can handle > 100A. There are MOSFETs that can handle > 200A. Take a look at the IXTK250N10.

    40A current handling would not be too big an issue. There are many many MOSFETs available that can handle far more than 40A. MOSFETs can also be paralleled to increase current handling capacity and lower conduction losses.

    Use the parametric search available on the websites of the various manufacturers or distributors to find suitable MOSFETs.

    I was wondering if it would be impossible to construct a Buck Converter with such a high amperage or if I will have to design a system using a multi-tap transformer.
    A buck converter even at this current rating is very possible. However, you should first construct a buck converter with much lower current/power capacity. Start with a 5A converter. Once you have successfully completed that, move on up.

    Hope this helps.
    Tahmid.


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    Re: High Voltage and Current Buck Converter

    I was wondering if their are any specific texts that expand on creating multiple voltage output and high current output buck converters. I asked one of my professors who specializes in the field and he said to wait till next year when he finishes his book. This is no help to me, because I need to have the project done by May of next year. Once again any help is greatly appreciated.



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    Re: High Voltage and Current Buck Converter

    To my knowledge there's no such thing as a multi output buck converter, at least not in the same way there are multi output flyback converters. If you want multiple outputs at such high output, then it makes more sense to just have several, independent buck converters operating from a common source.

    I sounds like you're new to SMPS design, and six months isn't much time. You should just try to get a single high power buck working, then just throw several of them together.


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    Re: High Voltage and Current Buck Converter

    I don't see a specific point of multiple outputs, presuming you want each output individually regulated.

    Synchronous buck would be a suitable topology for high current, there are several switched mode controllers from major manufacturers that cober the intended voltage range, you can take TPS40170 as an example. The drive current of the internal gate drivers is limited according to reasonable chip design parameters. You can either try to get along with it (possibly accepting slower switching) or supplement external gate drivers or boost transistors.

    You mentioned microprocessor control in your post, that would be an option of course. For fast switching, you would preferably use a dedicated digital SMPS controller processor, e.g. from Microchip.

    Multi-phase buck converter would be an option to reduce input ripple current. If the multiple outputs loads are somehow balanced, synchronizing the controllers with a phase shift would be another option.



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    Re: High Voltage and Current Buck Converter

    Maybe I should clarify, by what I mean when I say multiple output. I am not saying that I need multiple different voltages to operate at the same time. I am saying that I need the ability to switch between to switch one device from 12-48 Volts. The purpose being a lithium ion battery charging for those various voltages.



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    Re: High Voltage and Current Buck Converter

    Oh, so you mean variable output voltage, not multiple output voltages. A variable output buck is not difficult, though if you want to use it for charging lithium batteries then that's a tricky control issue. But that's mainly a challenge with software and battery monitoring; the buck converter power electronics themselves aren't much different.



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    Re: High Voltage and Current Buck Converter

    Adjusting the duty cycle will allow you to adjust the output voltage. For variable output voltage, the feedback circuitry should have a variable voltage reference to adjust output voltage. Select the inductance and capacitance so that you get clean DC output voltage throughout the entire range.

    For charging the batteries, you should implement a charge controller that monitors and/or controls the charging voltage, current, temperature, etc.

    Hope this helps.
    Tahmid.



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    Re: High Voltage and Current Buck Converter

    I think you want the microcontroller to turn the switching mosfet on and off? It will need to detect when current has ramped up sufficiently. This might be done by detecting voltage on a sense resistor, or on a capacitor in parallel with the battery.

    The microcontroller might not be able to switch a device which is hooked up to 52V. It will need to have its signal stepped up, etc.

    The most important thing will be to use a proper battery charging waveform. You will have to make sure current shuts off before it exceeds a safe level. Then your microcontroller will wait until coil current declines before switching on the mosfet again. It will repeat this many times a second.

    Your microcontroller will need to halt charging at some point. Ideally you will program it to deliver a certain number of amp-hours. This means it must keep track of both the amount of current and the elapsed time. When sufficient amp-hours have been delivered, it will shut off the mosfet.

    Knowing the battery volt level is important. The microcontroller will need to detect this with its ADC, across a resistive divider. It will compare it to a reference table in memory, in order to tell when the battery is getting overcharged. This will have to be programmed by you.



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    Re: High Voltage and Current Buck Converter

    Okay if everyone is ready for a good laugh, here is what I have so far. A couple of things to note. I want to end up using a 120Vac source and a step-down transformer to tie into the full-wave rectifier. I also do not want to use the diodes being used in the circuit. As it turns out PSPICE does not have any of the templates for the diodes I want to use. Also the voltage source I am using as my PWM does not send multiple Pulses. These are issues with PSPICE that I still need to fix. If you could please examine what I have for the rectifier part of the circuit and provide some helpful tips on what I could do better. Thanks for all of the help so far.

    Click image for larger version. 

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    Re: High Voltage and Current Buck Converter

    U1 is configured as a high-side switch. You can't drive it as shown in the diagram. You need to use a high-side MOSFET driver. You may use a high-side driver chip such as IR2117, or you can use a gate-drive transformer or you can use an isolated power supply and drive the MOSFET from there. Before proceeding, please read up on high-side MOSFET and high-side MOSFET driver.

    You may want to go through this: https://www.edaboard.com/thread272781.html#post1169879

    Hope this helps.
    Tahmid.



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    Re: High Voltage and Current Buck Converter

    I want to end up using a 120Vac source and a step-down transformer to tie into the full-wave rectifier.
    That will be the correct way to use the diode bridge, instead of the way it is shown in your schematic after a battery.

    Your life will be easier if you start out with a diode in the place where you put the lower mosfet. (At least during these early development steps anyway.)

    Right now you might want to experiment with an interactive animated simulation of a buck converter.
    It will show you how coil action takes place in two stages. It will portray how current flows alternately through the mosfet, then through the diode.

    Click the link below. It will open the website www.falstad.com/circuit.
    It will load the schematic and run it on your computer. (Click Allow to load the Java applet.)

    http://tinyurl.com/cv2xchj

    Click the switch to close it, turning on the mosfet. Then let up the switch.

    As you will see, there is no particular voltage being applied to the battery. (I used 24 V in this case, but it could be any of the figures you mention). There are only pulses of current going to the battery. You must install the control circuitry which will limit the intensity of the pulses.

    The simulated flow goes up to 18 Amps. I suppose the limit is due to a parameter in the mosfet model. With the right component you will be able to obtain 40 Amps as you planned.

    Below is a screenshot only. It is not animated, nor will it interact.




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