How to properly drive MOSFET in MPP tracking duty cycle above 50% for a synchronous buck regulator at 50-100V input?

Question

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:

1. 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).
2. 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?

Answer 1

It's odd what you say about the bootstrap circuit not performing well above D=0.5. I'm using the same driver chip for a hefty 200 watt power supply where the duty range is quite wide and I've never seen this happening.

I'm switching at 100 kHz - maybe you are using a switch frequency that is too low? Too low a frequency will cause bootstrap power to be a problem.

Another alternative is to use something like a little 2 watt Traco-Power or XP or Murata mini-power module to generate a floating 12 volt for your top FET.

Answer 2

You should be able to run a bootstrapped high side gate drive well above 50%. 90+ is typical even at 100’s of khz or more (pulse widths are what matters, not duty cycle directly).

My first guess is that you put a pull-down resistor on the gate with a value too low. If you must have one (it’s not usually needed) think 100k or more (and do the math against your bootstrap cap).

Another guess is that at very light loads the switch node may not be returning to zero. Even synchronous converters sometimes switch to a “diode emulation” scheme which can make this possible.

Answer 3

This shouldn’t be a problem unless your switch frequency is extremely low (low 10’s of kHz or less).

My first guess is that you put a pull-down resistor on the gate with a value too low. If you must have one (it’s not usually needed) think 100k or more (and do the math against your bootstrap cap. Also factor in the driver quiescent current of 450uA).

Another guess is that at very light loads the switch node may not be returning to zero. Even synchronous converters sometimes switch to a “diode emulation” scheme which can make this possible.