Its not 980kHz or 490kHz, its 980Hz or 490Hz!!!
yes, of course ,i have add a shottky diode, but with diode ,rpm does not goes very low. Motor starts from medium rpm until full
You cannot get linear speed control without using an H-bridge in fast decay mode
or synchronous rectification. In fact a 1/2 H-bridge is all that's needed for
single spin direction.
The free-wheel diode means the current falls very slowly during the off-duty parts of
the PWM cycle (slow decay mode), so it doesn't go to low rpm (except under large
A better way to control a motor with a single switch is perhaps to modulate the
time between pulses, ie varying the pulse frequency for constant-width pulses.
You can also add a zener diode in series with the free-wheel diode to increase the
voltage and thus the decay speed. However there can be significant power
dissipated in the zener, a heatsink might be needed, and its still not very linear
speed control curve.
A schottky diode is the worst choice, as it has the smallest forward voltage and
thus the slowest decay possible.
Perhaps several non-schottky diodes in series would fare a bit better?