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When driving a high-side MOSFET, we have the option of either bootstrapping or using a charge-pump. Assume that my operating frequency is very high. Which method I should go for to drive the high-side MOSFET?

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    \$\begingroup\$ You can use transformer coupling too. \$\endgroup\$ – Andy aka Sep 5 '13 at 18:50
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A charge pump will give you more flexibility.

Bootstrapping is essentially a charge pump operated by the main switching devices.

The advantage of bootstrapping is low cost - usually a single external capacitor and a diode. The diodes is internal to the driving IC if an integrated driver IC is used.

A separate charge pump requires a clock and typically two diodes plus s pump capacitor. The components (usually with caps external) my be contained in a custom charge pump IC. This takes more room and may cost slightly more than bootstrapping.

A charge pump has the following advantages.:

  • Still works when duty cycle approaches 0% or 100% (unlike bootstrapping).

  • Works with load switching as slow as you want.

  • Not forced to comply with constraints imposed by the main system - eg clock speed.

In your case the last point is probably major. You can design the charge pump to work optimally without having to deal with a high clock rate or load driving behaviour. However, many people use bootstrapping with acceptable results.

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The advantages of the bootstrap circuit are its simplicity and the fast switching times, but to keep the bootstrap capacitor charged the MOS-gated device has to periodically be turned off. This applies to both Mosfets and IGBT's.

With the addition of a simple charge pump, both the fast switching of the bootstrap circuit and unlimited on-time of the charge pump circuit can be achieved.

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Quite the old topic, but one important thing is missing :

Half bridge voltage may force you NOT to use charge pumps.

There is no practical charge pumps that allow to supply 20V to a high side switch with 400V ground offset. Charge pumps are more of a low voltage thing (I've seen them up to 60V bus voltage with few tricks to avoid reverse currents) but honestly I would not use them without reason above 24V bus voltage. By reason I mean, if you don't know just default to bootstrapping. Charge pump have current limitation that are independent of the main switching frequency. With increasing main frequency, the gate driver load increase and may exceed the charge pump capability. Bootstraps are working with the same frequency as the switches thus have a current capability directly proportional it - increasing frequency increase both gate drive load current and gate drive supply current. At least until diode reverse recovery get predominant.

A good reason for using charge pump (as said above) is low duty cycle where the low side no longer turns ON long enough for the bootstrapping to works. This is seldom the case in high voltage system that have more voltage headroom and also explain why all AC/DC power supply use bootstrapping.

Note that at some point, even bootstrapping becomes unpractical (over 600V bus voltage usualy or too high frequency for the diode to properly block). Those systems use a transformer to supply the high side.

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