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Old 26th February 2009, 06:25 PM   #1
xyz9915 is offline xyz9915  Pakistan
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Default 50Hz UPS

Please view the attached schematic (for example only), which is a simple inverter circuit. I want to modify the circuit to make it Uninterruptible by adding relay for switching over from mains to battery & battery charging cut off function. My question is that can I use built-in body diode of FET for charging the battery?

My question is that can I use built-in body diode of FET for charging the battery?
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Old 26th February 2009, 09:31 PM   #2
star882 is offline star882  United States
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Looks like a poor design to me. The gate drive resistors are over an order of magnitude too big, resulting in high switching losses.

The cheapest approach is to modify a cheap inverter by adding a rectifier along with a small DC/DC converter and some control logic. The existing step up DC/DC converter should be disabled when on AC power (the rectifier keeps the internal B+ rail powered from the AC) and the small DC/DC converter enabled to act as a trickle charger. If the AC input is below a threshold, disable the small converter and enable the main one. If necessary, add some B+ capacitors to smooth the transition.
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"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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Old 27th February 2009, 04:35 PM   #3
xyz9915 is offline xyz9915  Pakistan
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This circuit is only for example. My question was that is it possible to use MOSFET integral (or body) diode for battery charging by adding series resistor to limit current in appropriate range.
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Old 27th February 2009, 05:32 PM   #4
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The reverse diode in a power MOSFET has the same power rating as that of the FET channel itself. In your case it is 50A, more than enough in terms of current rating for charging the batteries.

Out of curiosity, it looks like you are using the inverter transformer backwards to charge the battery when the mains supply is normal, but will that transformer really work at 50/60Hz?
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Old 28th February 2009, 08:04 PM   #5
xyz9915 is offline xyz9915  Pakistan
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Thanks for reply.
You are right, I want to use the inverter transformer backwards to charge the battery when the mains supply is normal. The circuit shown here is only for example, I will use my own design for this purpose but some clarification was required which you cleared.
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