H bridge short circuit problem

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mariogomez1

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I am trying to control a single-phase motor using the H-bridge circuit. I have used two different power supplies and two different ground wires. There is no problem in the logic part, but there is a short circuit in the 18 volt part. What should I do to solve this?
 

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Where is the 18V part?
You have GND, 30V, 12V, 5V and VCC but it isn't clear how they are related. You probably also have a problem with the voltage needed to turn off the top MOSFETS.

Brian.
 

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betwixt said:
Where is the 18V part?
You have GND, 30V, 12V, 5V and VCC but it isn't clear how they are related. You probably also have a problem with the voltage needed to turn off the top MOSFETS.

Brian.
Click to expand...
The place written as 30v was changed to 18v. I did not update the drawing.
 

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What is VCC? As Brian points out, it’s probably not high enough to turn off the upper mosfets. And those 10K resistors are probably too large.
 

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According to the schematic Vcc should be 12V (LM7812), and if this is the case, I agree with Brian, the upper P-MOSFETs will not turn off.
--- Updated ---

... additional problem I see, is if the photocouple input is LOW, the output is HIGH-Z. This leads to times 10k in series dividing the 18V (former 30V) down to 9V at the gate of the upper and lower MOSFET. This will turn on the N-MOSFET and the P-MOSFET at the same time.
--- Updated ---

... ok the photocouple should pull its output low. Sorry for the confusion.
 
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That is a really bad way to drive the fets - as for most of the transition both fets in a leg are ON = shoot thru = poof ! ! !
--- Updated ---

Use N fets in the top and a proper totem pole driver - e.g. IR2110 - cheap and plentiful.
 

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The obvious has been stated, in addition you must implement controlled deadband generation ?



Such that vertical leg MOSFETs never on at same time. Eg. in above example PH1 and PH2 never
on at same time, the drive to the top and bottom MOSFETs in a vertical leg.


Regards, Dana.
 

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