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I made an H-bridge using this schematic: enter image description here

Although the circuit mostly works fine, there is noticeable shoot-through.Even with no load at 150-200hz the input voltage drops by a lot.I added dead time of 30us which seems to help, but does not solve the issue,even though the datasheets claim switching speeds of less than a micro-second.Are there any changes to be made to solve this without increasing the dead-time any more?

MicroservicesOnDDD
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Kostas Sarioglou
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  • start by not using 2N3906/2N3904? seriously, 40 V C-E is the absolute maximum rating, and considering your load might well be a bit inductive, that's absolutely the wrong choice of transistor. You'll want to go for a set of MOSFETs, here. – Marcus Müller Nov 12 '20 at 14:49
  • My load is not inductive, and im only using these transistors because i have them available.I am planning to get much higher voltage rated ones.I haven't pushed the circuit to its 40v limit, the shoot-through described is happening at way lower voltages as well. I looked at mosfets as well, but the fact that the input voltage will fluctuate from 5-40v means i will have to use gate drivers, something that im trying to avoid – Kostas Sarioglou Nov 12 '20 at 14:56
  • yeah well, you got unsuitable transistors that you'll exchange for better ones; your shoot-through happens because they're not switching fast enough, as you've suggested already. Switching speed depends on drive from your microcontroller and properties of your transistors. Changing them therefore changes your problem: no sense in fixing *this* circuit if the final one will have different transistors anyway. – Marcus Müller Nov 12 '20 at 14:58
  • Do you have a digital oscilloscope? Include a capture of the shoot-through, showing the 30us gap, both drive signals, and the collector nodes of Q1 and Q4. – Reinderien Nov 12 '20 at 15:01
  • MarcusMüller yes, i know that after changing the transistors this info will be irrelevant, however i still dont understand why this is happening with these ones.Im using an atmega microcontroller @5v, with the 1k resistor i believe that im able to saturate Q1 and Q4 – Kostas Sarioglou Nov 12 '20 at 15:08
  • 1k is a little high. The specsheet shows worst-case 0.95V Vbe on, and with three junctions and 5V, your worst-case Ib is 2.15mA. Given that some of the specsheet test base currents are over double this at 5mA, consider trying something like 470R. – Reinderien Nov 12 '20 at 15:11
  • Reinderien i will try later, i have a very cheap one and i doubt it can show anything useful.However i can confirm that the output of the microcontroller is correct.Are you referring to R1 and R2? – Kostas Sarioglou Nov 12 '20 at 15:13
  • No, R3/R4 - since these go through three junctions – Reinderien Nov 12 '20 at 15:15
  • The size of your load may have something to do with the problem. BJT's have an hFE rating (AKA "DC current gain"). If your load wants more current than your (base current * hFE), it won't "turn on" all the way. – Schuyler Horky Nov 12 '20 at 17:27

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