Wish to design a DC-DC buck converter

Easy peasy said:

Really you should think about the max size and heatsinking and efficiency that you wish to arrive at (?) - and the max freq of operation (?) - is this for a volume production or a one off for your own amusement ?

+85 deg C at full power is a big ask - do you really need this ?

-40 deg C is a big ask too - you really need expensive electro's and quality SMT caps to perform over this range.

Will the input voltage ever go over 48VDC? even at hard switch on ? - this needs considering..!

How about radiated RF noise and conducted noise down the input & output wires (?) does this factor in ?

What is the output ripple to be 250mV rms ? the input current ripple, 500mA rms ?

Obviously a buck converter will work - 2 phase would be better here, only 5A chokes, and reduced input current ripple.

What is the dynamic performance required on the output - for load steps ?

This is really a big job to get it totally right - have you considered hiring a power electronics engineer who has done this before ? - there is a lot to go wrong here and it is easy to overlook things

for e.g. in a +85 ambient - all the parts will be sitting at 100 deg C or hotter - so the parts had better be pretty good.

Best of luck ...!

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• Application: comercer/industrial/automotive
• Efficiency: 90%-95%-98%
• Size
• EMI
• Startup time
• Step load respond
• Output ripple
• Working temperature
• Reliability
• Life time
• Input ripple current/ voltage
• BOM cost
• Assembly cost
• Leadtime for buying

cupoftea said:

Here is a Buck of that Spec in the free LTspice to help you get into it.
Give me a shout if you want it made synchronous.
Or Interleaved.
Analog.com do some good synchronous interleaved buck controllers.

For overcurrent protection, just add a current sensor to output and turn off if overcurrent...or go into hiccup mode.
OV protection is tricky with a buck since of the main fet fails then input and output are connected...so you might need say some transformer isolated SMPS.
But to prevent overvoltage due to open cct feedback say, just add OV comparator and divider and shut down if OV.

Actually for your high temp spec, i would use say a UC1845, and use pulse transformer high side fet drive, and current transformer hi side current sense. I dont think UC1845 is officially rated for your temp, but may survive it for a while. Maybe not use synch FETs at your high temp...just use >1 interleaved as Easy Peasy says. Wind your own inductor using offtheshelf gapped TDK or ferroxcube cores.

Here is a free SMPS course to help
SMPS course_Big folders
https://drive.google.com/file/d/17ij_pQLkAzzYeL_TRNci86sXHMirKHmD/view?usp=sharing

SMPS course_little folders
https://drive.google.com/file/d/1WvElhDNfHI0EBei4rouSNEdWWTO1oCtJ/view?usp=sharing

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Taihung said:

Hi
This application is often use for automotive.
Last project, I used LTC3810 with same paramters. Competitive IC is LM5116. But I strongly suggest using IC with current directly sensing if reliable request higher. Like: LTC7801.
Fault pin is not clear, what is fault? Normally IC support PG(Power Good) pin to indicate output voltage OK or not OK.
Many spec/parameters you need to list down before select solution:

• Application: comercer/industrial/automotive
• Efficiency: 90%-95%-98%
• Size
• EMI
• Startup time
• Step load respond
• Output ripple
• Working temperature
• Reliability
• Life time
• Input ripple current/ voltage
• BOM cost
• Assembly cost
• Leadtime for buying

Brg!

Click to expand...

Hasan2017 said:

Thank you.
Your simulation file works well.

--- Updated Jul 29, 2022 ---

Fault might be for OVR or OTR

Click to expand...

• OVR - Over voltage protection? -> Use low power comparator power from VCC of buck controller, often 5V to compare with reference devided from Vcc. With Smith trigger, you will hold off fault until output release under defined voltage or cut off output by using P-mos. Or using a latching circuit to block output until have reset command from other hardware. Simple way usin IC solution like e-fuse IC/ over voltage clamp LT4356 IC.
• OTR - Over temperature ? -> Use Smith trigger cmparator to set up high temp & reset level.

• Use ADC to monitor all parameters like input&output voltage, output current, temperature, then turn on/off enable pin of buck controller. Or dimming output current/voltage. Delay on/off, power sequence, I2C/BM bus interface. MCU use power from VCC of Buck with few mA. If can, use a small buck with input is 12V from Main power buck 12V and "ORing" with power from VCC - 5V to supply MCU. You will have a flexible/smartter dc/dc power module and higher efficiency.