Boost Converter MOSFET (LTC3872)

Thread Starter

ElectrAsh

Joined Feb 17, 2018
10
I am currently working on boost converter (3.3v input, 12v - 2A output ). I am using a LTC3872 chip (550kHz) and have done all the calculations following the datasheet.

However, I am confused over choosing the MOSFET. (Rds_on = 13mohms , Iin_peak = 8.9A). As the system gets powered by a Lithium-Ion battery I guess the Vgs should be around 2.5V?

I did simulate this circuit on LTspice however I'm not getting anymore than 6.6v output!


I attach my circuit diagram and chip datasheet.
Would appreciate it if you help me to pick an appropriate n- channel MOSFET, preferably non-SMD which i can test on breadboard.

LTC3872 Datasheet
 

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Thread Starter

ElectrAsh

Joined Feb 17, 2018
10
In your drawing the MOSFET gate is shorted to the source so it is running on only the internal switch.
My bad! Thanks for pointing out.:p
However the Proteus drawing is only for demonstration and I haven't simulated with it! My main question is the MOSFET selction.
It doesn't let me to edit it on the thread.
 

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ebp

Joined Feb 8, 2018
2,332
It requires are great deal of care to breadboard a switcher. Don't even consider plug-in type breadboards. Slew rates of voltages and currents are spectacular. The gate driver may slam around peak currents of amperes. Loop areas in the power path need to be carefully managed. ONE point in the circuit is "ground" and nothing else is. Switchers can be difficult on properly designed PCBs.

Except for SMD FETs, I select FETs for switchers based on ON resistance and gate charge and pretty much completely ignore the current rating. A boost converter switch sees the worst case input current and output voltage. You only need 12 volts, so a part rated at 15-20 V should be fine. There are some amazing low-voltage parts available, but almost exclusively in SMD. I haven't looked for such devices for some time, and there are new introductions all the time. DigiKey's selector for FETs has become pretty decent (from having been rather useless).

Your sim should have a load that can be varied rapidly, such as a switch in series with a resistor and an unswitched resistor of higher value in parallel - you need to put the thing through its paces.

The waveform plots on page 17 of the datasheet are mislabeled. The waveform in the middle is inductor current, not load current.
 

Thread Starter

ElectrAsh

Joined Feb 17, 2018
10
It requires are great deal of care to breadboard a switcher. Don't even consider plug-in type breadboards. Slew rates of voltages and currents are spectacular. The gate driver may slam around peak currents of amperes. Loop areas in the power path need to be carefully managed. ONE point in the circuit is "ground" and nothing else is. Switchers can be difficult on properly designed PCBs.

Except for SMD FETs, I select FETs for switchers based on ON resistance and gate charge and pretty much completely ignore the current rating. A boost converter switch sees the worst case input current and output voltage. You only need 12 volts, so a part rated at 15-20 V should be fine. There are some amazing low-voltage parts available, but almost exclusively in SMD. I haven't looked for such devices for some time, and there are new introductions all the time. DigiKey's selector for FETs has become pretty decent (from having been rather useless).

Your sim should have a load that can be varied rapidly, such as a switch in series with a resistor and an unswitched resistor of higher value in parallel - you need to put the thing through its paces.

The waveform plots on page 17 of the datasheet are mislabeled. The waveform in the middle is inductor current, not load current.
Thanks. Do you Do have any MOSFET recommendations in mind?
 
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