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| Speed up BJT switching |
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| npelov:
First of all thanks to the forum for being my rubber duck. I solved 80% of the problems while describing them here. I'm making li-ion battery balancer (schematic and LT spice files attached). The first battery is easy to balance with NPN transistor. Because it's ON hard for lower voltage drop I speedup the turning off using capacitor which applies negative voltage on the base when the MCU sends 0V. I do the same with Q1 and Q2. However a capacitor in parallel with R12 and R13 won't work because I'm driving it with open collector (not push-pull). So I to add R2 and R3 to not allow the transistor to turn on that hard. But that's not helping much because now the voltage drop on the transistor is higher and it heats up more. Question is: How do I speed up the turn off of Q4 and Q5 when they are turned on hard? P.S. I didn't use MOSFETs because it would be hard to generate the gate voltages for the upper transistors. I did try to use all low side switches, but I don't remember what was the problem there. |
| xavier60:
You could try a diode between Collector and Base to stop the transistors going into saturation. A small signal diode might have some effect. A Schottky diode will have the most effect. There is no need for R2 and R3 to be different values. |
| Zero999:
I doubt R2 and R3 are reducing the base drive that much. The reason why the transistors are turning off quicker with Re and R3, is because they're helping to discharge the base's capacitance. How about using logic level MOSFETs and associated gate drivers, rather than BJTs? |
| xavier60:
Another option is to try some 2SB1010 transistors, they turn off super fast. |
| Prithul0218:
--- Quote from: npelov on October 02, 2018, 12:25:19 pm ---First of all thanks to the forum for being my rubber duck. I solved 80% of the problems while describing them here. I'm making li-ion battery balancer (schematic and LT spice files attached). The first battery is easy to balance with NPN transistor. Because it's ON hard for lower voltage drop I speedup the turning off using capacitor which applies negative voltage on the base when the MCU sends 0V. I do the same with Q1 and Q2. However a capacitor in parallel with R12 and R13 won't work because I'm driving it with open collector (not push-pull). So I to add R2 and R3 to not allow the transistor to turn on that hard. But that's not helping much because now the voltage drop on the transistor is higher and it heats up more. Question is: How do I speed up the turn off of Q4 and Q5 when they are turned on hard? P.S. I didn't use MOSFETs because it would be hard to generate the gate voltages for the upper transistors. I did try to use all low side switches, but I don't remember what was the problem there. --- End quote --- Hi. Just wanted to say I am also making a battery balancer. I only got started, so haven't designed any schematic yet. I will post in the forum once I am done. You should too! |
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