External diode in L6390 for high switching frequency

[CGonz.2]

Hello. I have a custom made board with an L6390 used to drive two MOSFETs. My switching frequency is 100kHz. I noticed that at low duty cycles the bootstrap capacitor was not charging correctly. Per the datasheet, I discovered that due to the internal resistance of the driver there was not enough time for the capacitor to charge. I also noticed the part of the datasheet saying "if this drop is too high, or the circuit topology doesn’t allow a sufficient charging time, an external diode can be used".

Due to the previous information, I added a 1N4148 diode in between pins 4 and 16. When I do this, the driver does not work (PWM is correctly getting to the input legs, SD is on, yet nothing gets out). If I cut this diode while operating nothing happens, but if I turn off and on the power while the diode is removed it resumes working. However, if, while working without the diode, I force the diode between the two pins (4 and 16), it does not stop working and the problems with the low duties disappear. Is there something I am missing? I could add a switch in the design to introduce the diode after the power is on, but I doubt that is the intended motive from the design.

I would really appreciate the help since I've been stuck with this issue for several weeks now.

[Cristiana SCARAMEL · Accepted solution]

​Hi @Cristina González Moral​ welcome to ST Community.

To replace the internal bootstrap diode a fast recovery high voltage diode (e.g. STTH1L06) or Schottky diode with low forward voltage drop and low junction capacitance is recommended. Remember that the bootstrap diode must have a break−down voltage larger than high voltage bus rail.

Moreover a resistor in series is suggested to limit the peak currents at the bootstrap diode during start-up and limiting the dV/dt of VBO.

During the power-up there may be no charge in the bootstrap capacitors.

In such cases, the driver cannot start immediately with normal operation, but the bootstrap capacitor should be charged turning on the low side MOSFET.

The bootstrap capacitor is charged when the OUT voltage goes below the IC supply voltage.

In this situation the current flows from the IC supply (Vcc pin) to the bootstrap capacitor through the external diode.

Pay attention at the beginning of this procedure, a large amount of current could be required from Vcc power supply.

The problem with using diodes like the 1N4148 is that they are only rated for 75mA to 100mA current, and your peak currents can be very high.

[CGonz.2]

Thank you! I think this solves my issue. Just one thing, what would you recommend to limit the peak current?

[Cristiana SCARAMEL]

A small resistor in series with bootstrap diode, provides current limit only during a bootstrap charging period which represents when the VOUT goes below the IC supply voltage, VCC, or is pulled down to ground (the low−side switch is turned on and the high−side switch is turned off).

I recommended do not exceed the ohms (typically 5∼10 Ω) that would increase the VBO time constant, that depends on the values of bootstrap resistance, capacitance, and duty cycle of switching device,