I'm currently building a 10W high voltage (~50V-100V) Maximum Power Point Tracking (MPPT) buck regulator on the side for educational purposes. For the time being, the output is tied to a load. Things are working well, and the circuit properly tracks the maximum power point as expected. So far so good.
I'm using the LTC4444-5 High Voltage Synchronous N-Channel MOSFET Driver to drive the top and bottom NMOS MOSFETs controlling the buck power stage (switching at 100kHz), which introduces an issue. On many occasions, my MPPT algorithm wants to set the duty cycle above 50%. This is problematic for a synchronous buck stage since it starts lowering the voltage on the bootstrap capacitor charged by the synchronous MOSFET driver, to the point where the top MOSFET barely turns on.
In order to have the top MOSFET turn on for duty cycles >50%, I have two options:
- Find a high side MOSFET driver that uses a charge pump rather than a bootstrap cap. The best candidate I found was the MIC5011 from Micrel. Ignoring the fact that the maximum input voltage is too low, it's biggest drawback is that it turns on too slowly: 25us absolute best case (40kHz), more common case is 50us(20kHz).
- Have an isolated MOSFET driver with an isolated power supply, but that's starting to get pricey.
Is isolation the only way to properly drive these MOSFETs? Are there high-side MOSFET drivers that are high voltage (say >50V), switch quickly (easily support 100kHz) and can handle >50% duty cycle?
Better yet, am I going down the wrong path? Is there a simpler way to solve this issue?