MOSFET Driver for Boost Converter Design: A Look into IRF8707PBF and MAX4420

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Discussion Overview

The discussion revolves around the design of a boost converter utilizing the IRF8707PBF MOSFET and the MAX4420 driver IC. Participants explore the implications of operating at a switching frequency of 250kHz with varying input and output voltages, as well as considerations for efficiency and component selection.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning
  • Experimental/applied

Main Points Raised

  • One participant questions the variability of the output voltage and suggests it may be adjustable, leading to clarification that the design is for a solar cell charger with a maximum output voltage around 8.2V.
  • Another participant inquires about the choice of 250kHz for the switching frequency and discusses the trade-offs between switching frequency, efficiency, and component size.
  • Concerns are raised regarding the gate voltage required to fully turn on the MOSFET, noting that a source voltage of 3.5V may not suffice, potentially leading to power loss at low input voltages.
  • A participant mentions the use of an 82uH inductor to balance switching losses and PCB space, referencing a specific design guideline for inductor selection.
  • There is a discussion about the output resistance of different driver ICs, with one participant noting that the higher output resistance of the LM5112 may not pose significant issues at lower switching frequencies.
  • Some participants express uncertainty about the impact of using a microcontroller in the feedback loop for the MPPT algorithm, indicating a need for further exploration of this approach.

Areas of Agreement / Disagreement

Participants express varying opinions on the adequacy of the chosen components and design parameters, particularly regarding the gate drive voltage and switching frequency. No consensus is reached on the optimal configuration or potential issues.

Contextual Notes

Participants highlight limitations related to the assumptions about voltage levels and the impact of component choices on efficiency and performance. The discussion reflects a range of experiences and uncertainties regarding the design process.

Who May Find This Useful

Individuals interested in boost converter design, MOSFET driver selection, and power electronics, particularly in applications involving renewable energy sources and battery charging systems.

gnurf
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Dear Oracle,

I'm designing a boost converter where the input is a current limited (~500mA) voltage source that varies from 3.5V to 6V. The output (load) varies from 7V to 9V. I intend to drive a MOSFET, IRF8707PBF [1] from IR, with a 250kHz PWM control signal via Maxim's driver IC MAX4420 [2].

Do you see any unforeseen mis-happenings in your crystal ball, if I chose to mate the two devices at the given frequency and voltages?

Other comments/questions on driving MOSFET's or on the boost topology in general are welcomed.


[1] https://ec.irf.com/v6/en/US/adirect/ir?cmd=catSearchFrame&domSendTo=byID&domProductQueryName=irf7807apbf
[2] http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1486
 
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gnurf said:
Dear Oracle,

I'm designing a boost converter where the input is a current limited (~500mA) voltage source that varies from 3.5V to 6V. The output (load) varies from 7V to 9V. I intend to drive a MOSFET, IRF8707PBF [1] from IR, with a 250kHz PWM control signal via Maxim's driver IC MAX4420 [2].

Do you see any unforeseen mis-happenings in your crystal ball, if I chose to mate the two devices at the given frequency and voltages?

Other comments/questions on driving MOSFET's or on the boost topology in general are welcomed.


[1] https://ec.irf.com/v6/en/US/adirect/ir?cmd=catSearchFrame&domSendTo=byID&domProductQueryName=irf7807apbf
[2] http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1486

Why is your output voltage going to vary? Or do you mean that you are going to make the output adjustable?
 
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berkeman said:
Why is your output voltage going to vary? Or do you mean that you are going to make the output adjustable?

It's the power stage of a maximum power point tracking solar cell to lithium-ion battery charger -- i.e., the output is the battery voltage with a (float voltage) Vmax =~8.2V, (and not 9V as I said in the OP). I'll sense the output current from the converter and battery voltage into a uC ADC and implement a software MPPT algorithm that will generate the PWM output to the mosfet driver. I'm thinking a state machine with a MPPT constant (max) current state, and a constant voltage state where I'll move away from the MPP as necessary, as a start. Having said that, placing a uC in the feedback loop muddies the already murky waters for me, so I'm struck with bursts of confusion from time to time..

I was just looking to bounce some ideas of someone with a little more than my non-existing experience. Thanks.
 
What made you choose 250kHz for the switching frequency. Have you considered the trade-off's of going higher/lower?
 
If your source voltage is 3.5V, that won't be enough to turn your mosfet completely on. Looking at the datasheet, you need at least 4.5V on the gate to completely turn the mosfet on. It will work, but you will be loosing a lot of power at low input voltages, especially if your battery is low and drawing a lot of current.
 
uart said:
What made you choose 250kHz for the switching frequency. Have you considered the trade-off's of going higher/lower?

I need the system to be fairly small while still being efficient. Basically I've tried to squeeze the frequency down as much as possible to minimize switching losses, while at the same time not letting the inductor and capacitor occupy too much pcb real estate. By using the expression for Lmin (see page 11, http://focus.ti.com/lit/an/slva061/slva061.pdf" ) with fsw = 250kHz, I've decided to go with a 82uH (torodial for less EMI?) inductor in an attempt to gain some foothold somewhere from where I can proceed.

famousken said:
If your source voltage is 3.5V, that won't be enough to turn your mosfet completely on. Looking at the datasheet, you need at least 4.5V on the gate to completely turn the mosfet on. It will work, but you will be loosing a lot of power at low input voltages, especially if your battery is low and drawing a lot of current.

Yes, maybe http://www.national.com/pf/LM/LM5112.html#Overview" from National is a better choice. Similar switching speeds and current consumption at 250kHz from what I can see.


Thanks for the questions/comments. It helps, so don't be shy now! :)
 
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I just realized the Output Resistance is higher on the LM chip than the one from Maxim (30 ohm vs 1.5 ohm), but I was prepared to place (as a saftey measure) a small resistor on the gate input anyway to dampen any ringing oscillations that might occur.

As long as fsw is relatively low at 250kHz, the bigger Rout of LM5112 should not pose a big problem, right?