I'm dealing with an application in which a DC-DC converter is used to transform an input voltage of 15V DC into lower voltages. The current configuration makes use of an LM27222 with two mosfets in push-pull configuration. The output voltage is linearized into an LC filter.
Here's a picture of the general schematic used.
The LM27222 is driven by a PWM signal with a carrier frequency of 65 KHz. The mosfets used are of type FDMS86101DC. The logic level voltage is 5V. The output current to design the system is 10A. The load is resistive. The system works fine, but some concerns have been raised to avoid collisions between the two mosfets (both transistors ON), especially during start-up/turn-off sequence. This happened a couple of times during testing of new firmwares, but, most probably, the problem was related to the testing engineer which not so carefully "touched" the output leads generating a destructive shortcut.
According to one of the new engineers we have been working with, we should:
- Substitute the LM27222 with another driver, the L6388E, which should be much more stable than the LM27222 and avoid any change of collision, even in transition sequences.
- Add a diode in series with the low-side mosfet. This diode should add additional protection during transient
- Add a complex filter to the Vin line. The filter should have a common-mode and a differential mode inductor, together with a large electrolytic capacitor and several small ceramic capacitors. According to the engineer this filter should limit the demand of pulsed power from the power supply, and result in a more "compliance-friendly" design to meet the EMC (electromagnetic compatibility) regulations. The Vin comes from a AC-DC desktop power adapter and he is concerned that the connection cable from the power supply to the electronic card could radiate frequencies because of the required pulsed power. Besides, since several of these power supplies got broken during tests, the engineer is also suspecting that we are throwing some important noise back into the power supply, leading to failures.
Here's a picture of the L6388E internal schematic.
I would like to ask two questions:
- Is it necessary to carry out all the steps suggested by the engineer? The change of LM27222 is particularly painful and I would avoid that if it does not give clear advantages.
- If we decide to keep the current configuration with the LM27222, how can we drive the LEN signal to avoid problems at start-up and turn-off of the system?