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full-brige problem

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dm927

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I am trying to build a full-bridge inverter circuit that will take 170VDC and give a 340 (peak-to-peak) or 120 VRMS output at 60Hz. In doing so, I am using IRF740 transistors [Vdss = 400V, Id = 10A] driven with IR2181 gate drivers at 18kHz. The voltage and current at the load (100-ohm, 225W) seem correct. The problem is when the DC rail voltage is increased to 100VDC, the current immediatly spikes. Then, the transistors instantaneously heat up and the multimeters that were set up to examine the input and output current read "Error". However, the 3A fuse that is inline with the source does not blow. Does anyone know what the problem could be?

I have incuded a schematic at:

http://www.electro-tech-online.com/custompdfs/2004/02/Power_Inverter_Schematic.pdf
 

Russlk

New Member
The page comes up blank. I assume you are simulating the circuit, otherwise you would have smoke. Do you have current limiting resistors in series with the fet gate? If not the fet may be shorting due to excessive dV/dT.
 

dm927

New Member
I have 10 ohm resistors in series with the gate to help prevent noise from entering the circuit.

Today, I found that the Vgs voltage on the top two transistors increases with an increase in rail voltage. The Vgs pulses should remain at about 12V to properly drive the MOSFETs. I would think this has to be something to do with the driver chips. Does anyone have any suggestions. Thanks in advance.
 

tavib

Member
:eek:
You must leave a small time between turning off a MOS and turning on the second MOS from one line. If will not exist a small time the both MOSs will be in ON state, for a short period when they switching, and a high current pulse will pass through both MOSs via Vcc - MOSs - GND.
To make this whorse the IR2181 have a propagation delays for OFF-ON transition =180ns and one for ON-OFF transition =220ns.
In IR2181 doc is not specified if the circuit have optocoupler between low voltage and high voltage stages, and is possible to have a capacitive separator, whitch can create some problems.
 
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