Searching \ for 'Re[4]: Clarifying PIC control of DC-DC converter' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/microchip/devices.htm?key=pic
Search entire site for: 'Clarifying PIC control of DC-DC converter'.

Truncated match.
PICList Thread
'Re[4]: Clarifying PIC control of DC-DC converter'
1996\05\17@091849 by Mike Riendeau

flavicon
face
Scott Dattalo wrote:


>Again, in my experience with switching power supplies of this type,
>you want to have very hard MOSFET on/off transitions. If you don't,
>you will have a less-than-efficient design. This extra power loss
>winds up mostly in the MOSFET.

I'm not trying to knock your apparent knowledge in this area, but
I assumed you were implying that the PIC's port pin was inadequate
to drive a MOSFET with a large capacitance. As for size, I picked an
IRLL014 device out of the IRF Hexfet data book with the
following characteristics:

    Vdss  = 60V
    Rdson = 0.2ohm
    Id    = 2.7A
    Ciss  = 400pF

    Package = SOT223 (small indeed)

    Under ambient temperatures this device will dissipate 2W
    into a 1" sq. area of the PCB.


IRF has a table of turn-on and turn-off times for standard CMOS
and TTL gate drives for logic level MOSFETS in thier APP note AN971.
I picked a worst case approximation for the above device
compared to a device with similar gate and miller
capacitance. I rounded up both times to the nearest 100nS.

    ton  = 200nS
    toff = 100nS

I assumed a boost converter topology for simplicity and convenience:

    Vin  = 10V
    Vout = 16V
    Fsw  = 20KHZ
    L    = 100uH

Energy of the inductor storage cycle:

    Ipk = 2A
    ton = 20uS
    Rdson = 0.2Ohm

           /20uS
          |                 2
    Els = |     (Ipk/ton * t) Rdson dt  = 5.33uJ
          |
          /0

Energy of the turn-on was approximated with a linear transition from
10V to 0V with a load current on the inductor as an initial condition
which is approximated as a constant for the turn on switch time. This
should give a VERY converative estimate.

    Iload = 0.8A
    Von   = 10V
    tson  = 200nS


            /200nS
           |
    Eson = |     ( Von - (Von/tson)*t )(Iload) dt = 0.8uJ
           |
           /0

Energy of the turn-off was approximated with a linear transition
of Voltage from 0 to 16V and current from Ipk (2A) to 0A during
the turn-off.

    Von   = 16V
    Ipk   = 2A
    tsoff = 100nS

             /100nS
            |
    Esoff = |   (Von/tsoff * t)(Ipk - (Ipk/tsoff)*t) dt = 0.53uJ
            |
            /0

Total Enery per cycle times the frequency gives the power:


    Pdiss = (5.33uJ + 0.8uJ + 0.53uJ) * 20KHZ = 133mW


No heat sink required. No special gate drive.


                                        Mike

1996\05\17@130817 by Jonathan King

flavicon
face
At 09:19 AM 5/17/96 -0400, you wrote:

This is straying pretty far from a straight PIC discussion, but
I felt a few more comments may help those trying to interface
FETS to PICs sucessfully



{Quote hidden}

Neither am I,as by your email address, you have a better chance than many of
having some, but

The basic problem I saw with your approach was the assumption about
drive capability.  A PIC is NOT going to produce the rise and fall times
you gave.

The reason you need high drive capability to drive a FET is to quickly
charge/discharge the miller capacitance when the drain is falling.
Without the ability to quickly transition out of that region your
power dissipation can get out of hand.  That is one of the things that
dedicated hardware will get you.

Much can be done with the PIC output, but if you are looking for a high
switching frequency to allow better response/smaller inductor, you will
have real problems with a PIC



In the example you gave, the power dissipation is 65% higher by my
calculation but still doesn't need a heat sink.

However someone trying to the mythical max of 2W dissipation in the
transistor will find themselves in for an unplesant surprise
based on your assumptions.

>
>IRF has a table of turn-on and turn-off times for standard CMOS
>and TTL gate drives for logic level MOSFETS in thier APP note AN971.
>I picked a worst case approximation for the above device
>compared to a device with similar gate and miller
>capacitance. I rounded up both times to the nearest 100nS.
>
>     ton  = 200nS
>     toff = 100nS

Try using the gate charge graphs  in the IR datasheet, adjusted for the
voltage and current conditions you are using.

Capacitance during switching varies widely as the miller effect multiplies
the gate-drain capacitance, and that capacitance changes during the
transition.  It is more accurate to use gate charge and the equations

       I= C*dV/dt

       Q = C*V




Using Voh/Ioh values from the 16C74 datasheet to get output resistances,
I calculated

   ton = 1000ns
   toff = 350ns

This changes the total power dissipation from 133mW to 220 mW.

Trying to switch extra current will quickly cause problems with
these switching times.


I hope this discussion helps some folks,



Jonathan




*************************************************************************
Jonathan King        *  Unitrode Corp.                * spam_OUTkingTakeThisOuTspamuicc.com
Phone 603 424-2410   *  7 Continental Boulevard       * http://www.unitrode.com
FAX   603 424-3460   *  Merrimack, NH   03054-0399    *
                    *                                *
*************************************************************************

1996\05\17@172411 by Scott Dattalo

face
flavicon
face
Thanks Jonathan, you saved me a little time.

At the risk of being flamed, I might add that the thermal resistance
of the device Mike quoted is 15 degrees C/watt for junction to case and
42 degrees C/Watt from junction to ambient. I'm not exactly sure of the
thermal resitance of the 1 inch square copper foil heat sink. However,
I'll buy a six pack for the first person that can keep their pinky on
that puppy and sing one verse of "Row-Row Your Boat" while the converter
supplies 15 watts!

Well, I'm not being exactly fair. The junction is rated for 150 degrees
Centigrade, while your finger is rated for only about 70-80 degrees. But
you get the point, it will be _very_ hot if you do not use an appropriate
driver.

Scott

More... (looser matching)
- Last day of these posts
- In 1996 , 1997 only
- Today
- New search...