I made some strange experience with a new design using LM5176PWPR but today I see a behavior I can't explain myself. First of all, I control the output conditions of the DCDC converter with an external (low speed) regulation loop at the SS pin. As an example, it can provide a changeable CC/CV characteristics controlled by a microcontroller. It’s a 55V/500W output design. Input is 48V.
The system is working in buck mode. The HS MOSFET of the boost side is constantly active. In the past I noticed, that the bootstrap circuit switched the boost half bridge one time at a gate voltage of about 3.5V to bootstrap the drive circuit. This behavior us not working today. I see no switching, BUT the gate voltage is constantly on at about 7V. Yesterday I notice the same behavior on the buck MOSFET. Is there an internal circuit that provides power for the bootstrap path? I don’t think so, but the voltage is also available and without ripple using an additional “bleeding resistor” at the gate?
The next thing I can’t explain is, that if the same system switches to boost more (constantly voltage instead of constantly current), the gate of the HS MOSFET stops switching. There is no gate-source voltage and the MOSFET works as a simple Diode. Is there a reason the HS-MOSFET is not switching? Are there safety features I don’t know?
Last but not least, I have some troubles finding the right MOSFETs. I need 80V types to have a good margin but most of them have a threshold voltage higher than the minimum high-side gate voltage of 3.5V. I found one with a minimum of 1.1V but with this MOSFET I have to add a bjt gate discharge circuit. Without it, I see a high shoot-trough current (hiccup triggers as well when active). The 80V TI MOSFETs have a threshold that is critical with the actual bootstrap circuit (3.5V min.)
Regards,