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schmitt trigger dead-time?

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Roff

Well-Known Member
Schmitt triggers don't create dead time. They create dead voltage, as it were. Perhaps you could elaborate on what you are trying to accomplish.
 

dm927

New Member
Our pulses full-bridge inverter pulses look like figure 1 in the diagram at the following link:

https://www.electro-tech-online.com/custompdfs/2004/02/deadtimecircuit.pdf

We are placing these circuits between the PIC and driver chips of the full-bridge inverter shown at the following link:

https://www.electro-tech-online.com/custompdfs/2004/02/Power_Inverter_Schematic-1.pdf

The problem when the schmitt trigger circuits are added is that the power supply that is feeding the DC voltage to the rails of the inverter overloads (current exceeds max value of .5A).
 

Roff

Well-Known Member
I could be totally off base here, but it appears to me that since the Schmitt trigger inverts, you are going to wind up with overlap instead of underlap. If you turn the diode around in the underlap circuit, it should do what you want. You may have to swap the outputs from the PIC or invert them with your code or ... to deal with the inversion of the Schmitt trigger.

On the other hand, I may have misinterpreted your power inverter schematic. :cry:
 

dm927

New Member
The inverting is ok because each of the two ouput signals that were inverted coming out of the PIC are inverted again. Also, the schmitt trigger setup we are using gave some dead-time. I just don't understand why the circuit shorts out.
 

Roff

Well-Known Member
Are you sure you're getting dead time? If you run complementary square waves into two of these circuits, you get the waveforms shown below. This looks like overlap to me. Am I wrong?
 

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Roff

Well-Known Member
John Sorensen said:
They don't overlap if you look at the low parts.

j.
True, but according to the IR2181 datasheet, the H-bridge drivers are non-inverting, so whenever both inputs are high (overlap), the H-bridge will crowbar the supply to ground on one side or the other, causing excessive dissipation in the IRF740s.
 
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