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Schematic (Current Limiter Circuit):

enter image description here

Question 1 :

I just want to understand what is the purpose of the capacitor when it is placed between the Base and Emitter of NPN or PNP transistor? In some designs I have noticed the values is in uF range and in other places, I have noticed the value is in nF range. In the above schematic, there are 3 capacitors placed across the base emitter of the BJTs. C2201, C2202 and C2203. All capacitors are of different values. So, on what basis is the value of the capacitors selected ?

Can someone tell me what is the purpose of those capacitors and how the capacitance value is arrived? If the answer is based on miller capacitance, can you please explain that concept in simple terms and somewhat practical or intuitive value of understanding?

All I understand about the Miller capacitance from what I have read is that, Miller capacitance is an inherent and undesirable property of BJT or MOSFET which cannot be avoided. Other than this, I am not sure how the miller capacitance actually affects during switching of BJT. Any simple example could be really helpful to me.

Question 2 :

Not in all circuit designs. In some designs, I have noticed zener diodes placed between the gate and source of the MOSFETs. The zener diode breakdown voltage is less than the maximum gate source voltage. Can someone tell me the reasons for placing the Zener diode between the gate and source of the MOSFET?

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  • \$\begingroup\$ You need to provide an example. \$\endgroup\$
    – Andy aka
    Oct 7 '20 at 8:19
  • \$\begingroup\$ @Andyaka, added a schematic example and explained my question with little more details. \$\endgroup\$
    – Newbie
    Oct 7 '20 at 10:44
  • \$\begingroup\$ Can someone provide a clear answer in simple terms to this question \$\endgroup\$
    – Newbie
    Oct 8 '20 at 3:49
  • \$\begingroup\$ The trouble is that you say all those transistors are operating as "switches only" but, my gut feeling is that is untrue. So, from a personal point of view, making an answer also involves rewinding your beliefs about the circuit and that makes it doubly hard and quite possibly, you might be reluctant to agree. \$\endgroup\$
    – Andy aka
    Oct 8 '20 at 6:47
  • \$\begingroup\$ Oh, I was under the assumption that the transistors would act as switches only. Sorry, I was not sure. Could you please provide an answer? \$\endgroup\$
    – Newbie
    Oct 8 '20 at 6:59
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My gut feeling is this: -

  • C2202 de-glitches surge currents flowing through the 1.87 ohm (net) current sense resistor.
  • C2201 does pretty much the same thing but on a subsequent stage of the current limit monitoring circuit.

Both might have been added as afterthoughts to the original design i.e. the design evolved to add these capacitors to fix problems found during testing of prototypes

  • C2203 might be there to allow the circuit to "start" from cold as quickly as possible i.e. it keeps Q2204 inactive for some several milliseconds while power stabilizes to the load then gradually allows Q2204 to become active. What Q2204 does is a little difficult to say - it looks like it protects the MOSFET from having too much voltage across it when the current through it is at the full-load level i.e. it turns off the MOSFET (and load) if the MOSFET's power dissipation is construed to be too high. I can't adequately say more on this.

All the capacitor values look to me like they were chosen as an afterthought and possibly chosen based on what was seen to fix the problem during prototype testing. They have nothing to do with miller capacitance.

The zener diode protects the MOSFET's gate-source region from over-voltage damage.

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  1. Adding a capacitor between the gate and emitter will influence the switching behavior of the IGBT. Its job is to take up additional charge originating from the Miller capacitance. This increases the required driver power and the IGBT exhibits higher switching losses for the same gate resistor.

By increasing the gate-emitter capacitance directly at the IGBT, the peak fault current is reduced. In other words, the transistor can not turn on by iteslf.

https://www.controleng.com/articles/tutorial-mitigating-parasitic-turn-on-effect-in-igbt-output-drives-to-improve-drive-performance/

https://www.researchgate.net/figure/By-increasing-the-gate-emitter-capacitance-directly-at-the-IGBT-the-peak-fault-current_fig6_228894736

  1. The zener diode sustains the voltage on the gate stable(approximately), which is necessary to control the MOS. Its a standard circuit, it has the same purpose as a resistor divider, except that the voltage through the zener will always be the same, even when the voltage of the source changes. As long as the voltage of the source does not fall to much, in which case the voltage on the gate will be the same as the source and the zener will not turn on.
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  • \$\begingroup\$ Could you please explain more briefly on your first point? I am not able to get clarity. Please also provide some intuitive explanation about Miller capacitance \$\endgroup\$
    – Newbie
    Oct 7 '20 at 5:44
  • \$\begingroup\$ If you put any capacitance on the gate, this will change how the transistor works. The parasitic capacicance can come from various places. It may cause oscillation at radio frequencies. This forms a lowpass filter, with a corner frequency determined by the source resistance of the previous stage and the input capacitance. You increase this capacitance in order to change which frequencies it blocks or oscilates. Or to attempt to remove the oscilation. \$\endgroup\$ Oct 7 '20 at 5:52
  • \$\begingroup\$ Gate and emitter? IGBTs? The zener is there for MOSFET protection. The MOSFET isn't switching - it's a linear current limit circuit. There is no evidence that miller capacitance is at all a dominant consideration here. \$\endgroup\$
    – Andy aka
    Oct 8 '20 at 8:21

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