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I have been trying to implement this H-bridge circuit to design a Class D amplifier. So far I have been able to correct a few little errors in the design.

However, I am unsure whether the circuit is ready to be printed on a PCB yet. Can anyone see anything that stands out in the schematic below as an issue I may need to re-evaluate?

N-Mosfets are: BSZ065N06L

enter image description here

Many thanks.

EDIT:

  1. Replaced 47R gate resistors with 1.8R gate resistors to decrease the rise time from 75ns to 2.88ns
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  • \$\begingroup\$ Why have you got flyback diodes? Data sheet links to the parts please and maybe you should consider squidging your circuit (left and right) so that it is better presented in the image above. \$\endgroup\$
    – Andy aka
    Jun 7, 2021 at 13:46

1 Answer 1

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Redrawn to make clearer: -

enter image description here

If you need the flyback diodes inside the purple boxes (and I question that you do given the bulk diodes inside each MOSFET) then you've missed a node connection where I've made a red circle.

The MOSFETs are good for current pulses up to over 200 amps so why have you got a 0.1 ohm resistor (R3) in series with the bridge on the lower supply end?

The MOSFETs have 1.6 nF gate-source capacitance and you have 47R gate resistors. Together this makes a rise time of around 1x CR or 75 ns yet, those MOSFETs are good for rise times that are significantly lower than this. Is there a reason? Maybe it's because the HIP4081A driver can only deliver 2.5 amps to the MOSFET gates? If so, use a better driver.

Are you happy with the 75 ns anti-shoot-through characteristic of the MOSFET driver given that the MOSFETs could be driven more quickly and could therefore dissipate lower power?

You haven't said what Vcc can be so I'm guessing it's below the 100 volt rating of the RF05VYM1SFHTR diodes used in the bootstrap circuits.

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  • \$\begingroup\$ Thank you for your comment. I didn't realize the MOSFETs I had chosen already had diodes within them. I have since removed the other flyback diodes in the schematic. Apologies I just took that resistance from the data sheet for HIP4081a as I saw two resistors in series with the low supply end. How did you know that the gat-source capacitance is 1.6nF? I could not find that anywhere in the datasheet. If I could get the rise time significantly lower than 75ns I will definitely try. Yeah I'm definitely wanting the Vcc to be well below 100V. \$\endgroup\$
    – Turei
    Jun 7, 2021 at 21:28
  • \$\begingroup\$ Using MOSFET body diodes for flyback purposes can lead to trouble in some cases. Excessive reverse recovery current could damage the devices if the body diodes are not designed for fast recovery. \$\endgroup\$
    – w_hile
    Jun 7, 2021 at 22:12
  • \$\begingroup\$ @Turei As the BSZ065N06LS5 appears to be recovery optimized, the body diodes should work for flyback. If you use other MOSFETs, you may want to make sure the datasheet mentions "optimized for SMPS" or shows a reverse recovery time in the ns range (see Table 7 for the BSZ065N06LS5). \$\endgroup\$
    – w_hile
    Jun 7, 2021 at 22:50
  • \$\begingroup\$ I was just looking at that table too, I wasn't to sure if the trr was good or not until I compared it to other fast recovery diodes. Thanks for the tip. I am a little bit concerned about the bootstrap and decoupling caps tho. I'm not sure if they are suitable for the switching frequency of 40-42kHz. \$\endgroup\$
    – Turei
    Jun 7, 2021 at 22:56
  • \$\begingroup\$ @Turei the capacitance is stated in table 5 of this data sheet. Nominal is 1.4 nF and max is 1.8 nF so, I split the difference. The reverse recovery time of the bulk diode is typically 18 ns as per table 7 so, it's pretty good for handling the back-emfs without causing a problem. \$\endgroup\$
    – Andy aka
    Jun 8, 2021 at 7:06

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