Re: Chopper max voltage?

Hi,

Here's my 2 cent's worth on the great chopper frequency debate. The
choice of switching frequency is based on the best balance between
two conflicting goals, motor losses vs. drive losses:

1) High frequency = low motor losses.
2) Low frequency = low drive losses.

1) Motor iron heating is caused by AC currents. Chopping contributes
to this by causing a "ripple current" AC component on top of the DC
current passing thru the winding. The higher the amplitude of
this "ripple current" the hotter the motor will get.

The amplitude of this current is equal to V / 2fL. "V" is the power
supply voltage, "L" is the motor inductance and "f" is the chopping
frequency.

The higher the chopping frequency "f", the lower the motor heating.

2) Transistors in a chopping drive get very hot when there is
simultaneous high current and high voltage across them. This happens
breifly everytime the output changes from high to low or visa versa.

This are called "switching losses", and the occur on every output
change edge.

Higher frequencies mean more edges per second, and thus more heating
in the drive transistors.

The L297 uses Darlington output transistors. They have high gain but
are very slow in switching. They also have a high "on" voltage. Both
contribute to heating of the drive.

Mariss



P.S. Here's how an L298 compares to a MOSFET (IRF530) bridge at 24V,
2A and 20kHz:

IRF530:

Conductance losses (I squared R)= 4 X (.15 + .15) = 1.200W
Switching losses 2f(IV)t = (2 X 20kHz X 24 X 2)100nS = .192W
------
Total heating (spread out over 2 125W packages) 1.372W

L298:

Conductance losses (Von times Icc) = (2.7 + 2.3)X 2 = 10.000W
Switching losses 2f(IV)t = (2 X 20kHz X 24 X 2)1uS = 1.920W
------
Total heating (in one 15W package) 11.920W