STM32F303RETX synchronous Buck converter hint.

[rromano001]

Hello, I am designing a Buck converter to control two Peltier heating or cooling mode.

I found nothing available from market, so I designed mine.

Target stable temperature, logging and alarm. It control a vaccine VET fridge, stable control temperature, care if got out of range.

First prototype was not working, wrong old driver were chosen and @200KHz performed very bad. I found Infineon drivers can drive 85A mosfet to over 500KHz and more. Retrofitted prototype then I realized another issue is present I wish expose to community.

Synchronous PWM Buck work fine if just measure output voltage at output. It is stable and locked to PWM ratio from input voltage.

Looking close to it is fine when in continuous conduction mode, weird when in discontinuous.

First case current on Inductor is forever positive, it increase on active phase decrease on recirculate.

In discontinuous current increase to positive peak then decrease to zero. On commercial analog part when current cross zero line bottom mosfet open to next charge cycle.

On MCU PWM generated control Low side mosfet get closed till next charging, this is weird due negative peak is not zero but negative so it steal energy from output capacitor then discharge energy back to power line.

On an ideal inductor view no problem but we are using real one and Ohm law do the difference: Negative current can be large and inductor over heat wasting a lot of energy

One solution raise PWM frequency when load drop, decrease when load raise. Not a problem rise up to 1MHz, PWM ratio is still low, capacitive losses get in and irradiation too.

Can feature of PWM channel be used to break negative conduction opening low side cycle by cycle?

Read all document about but seems Break has different meaning. I try read again Timer chapter. Share idea and point of view can better solve.

Prototype with new driver is attached here, some last minute changes miss, also few esd devices on external lines. (One Wire, I2C)

 Edit:

Power conversion demo are based on 334 series, it carry HRTIM Timer with specialized sections.

[rromano001]

Solution was found:

303 reported from application notes as motor and power control. Good to non synchronous buck, is not to be used on synchronous converters.

HRTIM counterparts carry fast PWM controller: up to 5GHz counting rate and 170pS resolution. No need to this extreme performances but HRTIM is versatile and go close to power control.

xx4 parts also carry on specialized ultra fast comparator able to perform current limiting/zero cross detect cycle by cycle.

At last propend to STM32G474 instead of F334 or Fxx4 series.

HRTIM output can be assigned to GPIO on channel basis, generally one channel has two output can control Half Bridge. Lower or upper drive can be shut down by comparator output. G474 has one more DAC and internal dac sources to drive comparators. This way is more simple and intuitive to design a synchronous power controller.

On the way to order nucleo g474 to experiment on, I report here result from test when done.