G'day,

Amateur wants to learn.

My MOSFETs are getting hot and I'm wondering why.  Is my PWM frequency
too high?  Am I simply pushing them too hard?

I'm driving a pair of 21W 12V (1.75A) lamps using a MTP3055E MOSFET each
from port A bits zero and one using a pulse width modulation (PWM)
routine.  At an ambient temperature of 120C they reach 44.40C, with no
heatsink or fan.

Circuit is port to gate, source to ground, drain to lamp, then lamp to
the 13.6V 2A unregulated supply.  I have a GIF of the schematic I can
mail to anyone who asks.  It's a project for Technical Aid to the
Disabled ... a light box communication device for very young children.

The supply voltage stabilises under load to be about 14.4V, and a DVM
across the lamp shows about 10.5V at my chosen duty cycle of 200/256, or
71%.

The PWM routine "light" is called as frequently as possible; usually
with only two instruction cycles between each call.  The PWM is simple
and is derived from the Scott Edwards source book; repetitive addition
in an accumulator and taking the carry output as the PWM signal.

The code follows.  I calculate 15 cycles at 4MHz between each setting of
the output bits.  With a 71% bit pattern of 011101111011101111 that's
roughly 67 cycles between pulses, so a frequency of 15kHz.  I hope.

a_left  equ     0x12            ; duty cycle accumulators
a_right equ     0x13

d_left  equ     0x14            ; duty cycle settings (set to 200)
d_right equ     0x15

port    equ     PORTA

b_left  equ     0x00            ; gate of mosfet for left lamp
b_right equ     0x01            ; gate of mosfet for right lamp

[...]

light                           ; update lights using duty cycles
        movf    d_left,w
        addwf   a_left,f
        btfsc   STATUS,C
        goto    light_1
        bcf     port,b_left
        goto    light_2
light_1
        bsf     port,b_left
light_2
        movf    d_right,w
        addwf   a_right,f
        btfsc   STATUS,C
        goto    light_3
        bcf     port,b_right
        goto    light_4
light_3
        bsf     port,b_right
light_4
        return

[...]

--
James Cameron                              (james.cameron@digital.com)
Digital Equipment Corporation (Australia) Pty. Ltd. A.C.N. 000 446 800