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Using PIC to drive FET gates for boost

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ACharnley

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Hi,

I have a requirement to generate a boost from as little voltage as possible. Max amperage will be circa 500mA but a lot less when the input voltage is further away from the ideal output voltage. The closest I came to an all in one is the MP1642, however in bypass mode it only supports 800mA (ideally I need 1.5A).

So I came to the conclusion using a more expensive PIC I can do away with several buck/boosts and roll my own. They are to be simply voltage feedback with no current loop. I'm knowledgeable in deadzone and high-side driving.

For the circuit below I've opted to keep it as simple as possible using the PIC's built in emitter-follower drive to toggle the fets. One potential issue is the drive strength of these pins which are rated for 10mA. Considering the FET's have low gate capacitance do you think I'll get away with it? Frequency choice is at my discretion and I have the option of bumping up the inductance if I need to lower it.


Screenshot_2018-12-25_10-05-22.png
 

ACharnley

Member
PS) Alternatively if anyone knows of a chip that will do the above containing two FET's and drivers for up to 5V (so no oscilliator, logic etc) please let me know!
 

JonSea

Well-Known Member
Most Helpful Member
Eagle? You could "smash" each part and move the labels so they aren't all on top of each other.
 

ronsimpson

Well-Known Member
Most Helpful Member
Q31 is being used as a diode. Its Gate drive is not easy. I would use a diode. Also the inductor is small so keep the PWM frequency high.
 

ACharnley

Member
I'm using sync for efficiency. Looking at the MCP1642, it works at 2MHz and a 4.7uH inductor. I reckon both of its fet's are driven using an emitter follower, the Rds for both is relatively high for low voltage fets so probably the gate capacitance is small on both (I've determined from viewing many datasheets there is a direct link).

As I could only improve on the MCP1642 by using an N fet top-side and a boosted drive voltage, and because all of this adds to the footprint space, I've decided to abort using the PIC here and go with the MCP1642.
 

Pommie

Well-Known Member
Most Helpful Member
I built a nixie 200V power supply using a baseline pic and it worked well. Search should find it. Only thing I'd change would be to swap the 2n7000 for a mosfet driver ic.

Mike.
 
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