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In the typical connection diagram for the IRS2007 Half-Bridge MOSFET driver taken from here and shown below, what is the function of the capacitor in parallel with the two MOSFET's (is it just to stabilise the power rails) and if critical how would an appropriate size be calculated? Typical circuit for IRS2007

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    \$\begingroup\$ It depends on the power feed and the load. \$\endgroup\$
    – Andy aka
    Jul 23, 2020 at 9:10
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    \$\begingroup\$ Decoupling capacitor. \$\endgroup\$
    – winny
    Jul 23, 2020 at 9:13
  • \$\begingroup\$ I have studied more than 5 H-bridges (l293x, l298x, DRV3xxx, TB6xx, BTS/BTN79xx, all for 40V or lower), but I have never seen any such capacitor inside the chip. Is ti because the high 200VDC power supply needs an internal bypass cap? \$\endgroup\$
    – tlfong01
    Jul 23, 2020 at 9:19
  • \$\begingroup\$ I am reading App Note 978: infineon.com/dgdl/…. Section 6 6. says the following: "To eliminate the effects of the inductance of the wiring between the power supply and the test circuit, a 100 uF/250 V electrolytic capacitor was connected between Q1D and Q2S terminals, as shown in Figures 6 and 7. This virtually eliminates any stray inductance in the dc path.. So I think the "by pass" cap Cf 250V 100uF is to eliminate stray inductance, not simply bypassing. \$\endgroup\$
    – tlfong01
    Jul 23, 2020 at 11:13

2 Answers 2

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It seems to be bypass capacitor to stabilise the power rails

This application note gives the same schematic enter image description here

enter image description here

the appropriate size will depend on the power rails, the value depends on the noise frequency

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I see you've already accepted an answer but that answer misses an important point about the power feed and the load. For instance, if the power feed is from a voltage regulator then energy transferred back to the power rails when switching an inductive load or a load that can produce energy (a motor for instance), will try and raise the "regulated" supply voltage. This happens quite easily with a series voltage regulator because there is no mechanism inside it to "shunt" away excess energy.

This means that a capacitor (of decent value) is required to be across the voltage rails to "soak" up the excess energy being returned by the load through the MOSFET parasitic diodes. This isn't needed with resistive loads but inductive loads (or motors that can generate when being mechanically rotated) will (or can) force the supply voltage higher than the supposed regulated value.

Path of current flow when a motor is connected as a load and allowed to free-wheel: -

enter image description here

You should be able to see that without a capacitor, there is nothing to prevent the voltage rails rising to an unacceptable level.

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