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ADuM5230 for driving AC switch

Hi,

I am currently doing a project of resonant converter which involves AC switching in some of the tuning components.

The AC switch that I am using is consisted of two N-channel MOSFETs with a back-to-back configuration to control the current flow of a capacitor.

Since the design has a cost and size constraint (pretty much like every other consumer electronics), ADuM5230 seems like a pretty good choice to us as it has an isolated supply power.

However, the problem is that both the high-side and low-side MOSFET have floating ground so I am not quite sure how exactly we can use this driver to switch this AC switch.

The attached schematic is something that I think it might work for my application, could you please comment on it?

Description of schematic:

The input of the resonant tank is driven by a current of square waveform, and the tank voltage is around +/-30V.

ADuM5230 is intended to drive Q5 and Q6. The Viso is connected to Vddb to provide an isolated supply power for output B. Since the source of the two MOSFETs can be regarded as a common ground, I've connected it to GNDiso and GNDb.

Kind regards,

William

attachments.zip
  • William:

    The schematic connections for the ADuM5230 to drive the back to back MOSFETs look fine.  The bypass capacitors for VISO and VDD1 are undersized at only 100pF. The datasheet PC Board Layout section recommends 0.1uF and 10uF values in parallel, but 0.1uF and 1uF would be good, with the 0.1uF a ceramic capacitor with low ESR and the 1uF could be ceramic or other type to get the charge needed for the DC to DC converter which drives the MOSFETs.  What is the PWM frequency and gate charge of the MOSFETs?

    Regards, Brian

  • Hi Brian,

    Thanks for your help!

    The PWM frequency is going to be around 100 to 200 kHz.

    The datasheet of MOSFET that I am using is attached.

    There is one thing that concerns me which is whether ADuM5230 will be able to supply sufficient power for driving Q5 and Q6?

    Regards,

    William

  • William:

    For the frequency of 100 - 200KHz and the MOSFET you selected, which has about a 7nC gate charge at 8V (highest voltage shown), the continuous current is 2mA, which the ADuM5230 can easily provide. The peak current of the ADuM5230 is limited to 0.1A source and 0.3A sink, and with this MOSFET would have slow rise of about 70ns, and may cause too much switching losses. To improve the output drive current and switching losses, you could add output buffer transistors as shown in figure 25 of the datasheet. You can use bipolar transistors BC807 and BC817 and an input resistor of about 220ohms and gate resistor of 10 ohms, for a peak output current of about 1.5A at 15V. The MOSFET you selected has a max spec of 30V, so you may want to consider one with a larger voltage spec of 40V or more to allow for peaks.  The MOSFET can have a larger VGS threshold voltage of 10V since the ADuM5230 can output 12V to 15V.

    Feel free to ask if you have more questions.

    Regards, Brian

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