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Old 16th February 2012, 09:32 PM   #1
snakub is offline snakub  United States
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Default Hall Effect Over Current Protection

I am not new to electronics but I don't know Jack Schmidt about SMPS's. I am looking to build a step down dc to dc converter from 250 volt dc to 5 to 12 volts at 100 amps. For over current I am thinking of using cycle by cycle protection using this hall effect sensor http://http://www.lem.com/docs/products/lf%20306-s%20e.pdf and a nand gate. Pwm is sent through the nand gate as normal until a particular current is reached and a positive voltage is sent to the other lead of the nand gate shutting off current for sometime. My questions about this current sensor is on the data sheet what is meant by with 12 volts and with 15 volts on the first page? Is this the required supply voltage to the sensor or can a lower voltage like 5 volts be used? If voltage out of the sensor is proportional to the amount of current in the sensor how do I make sure that the nand gate shuts off at the desired voltage say if I am using 5 volts to supply the sensor but I want the nand to shut off at 2.5 volts or 150 amps of current? And can a toroid withstand 100 amps? How big would it and the wire and core have to be?
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Old 16th February 2012, 10:47 PM   #2
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The link doesn't work, but typically these current sensors are not fast enough to be used for cycle by cycle current limiting.
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Old 17th February 2012, 02:11 AM   #3
snakub is offline snakub  United States
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What sensors are fast enough? Shunts? Are you certain of this I have seen plenty of motor controller and charger projects that use hall effects for current protection from 16 to 32 khertz.

Last edited by snakub; 17th February 2012 at 02:25 AM.
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Old 17th February 2012, 05:53 AM   #4
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the sensor has its max rms current rating, which is the thermal limit of a toroid.
16 to 25 sq. mm cable will eaily carry 100A.
The supply rating of a sensor is a supply for its internal amplifiers, so it should be as specified. If you need 5V, try HTFS400 or similar.
Cucly be cycle limiting at 32kHz is feasable but may be challanging, even if it is within sensor's frequency response (HTFS400 is even slower), you'd typically like to filter it some further, they are often a bit noisy. Anyway you can use an average current protection of the output.

Last edited by darkfenriz; 17th February 2012 at 06:03 AM.
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Old 17th February 2012, 06:02 AM   #5
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Quote:
Originally Posted by snakub View Post
If voltage out of the sensor is proportional to the amount of current in the sensor how do I make sure that the nand gate shuts off at the desired voltage say if I am using 5 volts to supply the sensor but I want the nand to shut off at 2.5 volts or 150 amps of current?
Comparator.
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Old 17th February 2012, 09:45 PM   #6
snakub is offline snakub  United States
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Quote:
Comparator.
Thank you I knew there was something I was forgetting
Is there anywhere I can buy a toroid rated specifically for what I am trying to do or will I probably have to wire it my self? Should I go ferrite or powdered iron core? Should I go step down or one to one and limit the duty cycle to 5 volts and if I do limit duty cycle would that make it more difficult to get 100 amps?
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Old 17th February 2012, 11:16 PM   #7
gmarsh is offline gmarsh  Canada
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I'd say find a existing design for a AC line input, 12VDC output supply and copy as much of it as you can - go with a current mode supply and you'll have overcurrent protection built in. Unitrode and Intersil appnotes are a gold mine for this kind of stuff.

I'd keep the supply isolated. Grounds tend to not become grounds anymore when you start running 100A through them.
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Old 21st February 2012, 02:54 AM   #8
snakub is offline snakub  United States
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Default Toroid Help

Here's a question you may be able to answer for me: Can a toroid transformer work with a square wave just pwm or does it have to have a true + - inversion on the leads of the primary? If so how would I accomplish this?
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