IXMS150PSI Ver la hoja de datos (PDF) - IXYS CORPORATION
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IXMS150PSI Datasheet PDF : 10 Pages
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IXMS 150
Fig. 10b Input Offset Adjust Circuit
Fig. 10c Gain Adjust Circuit
Ge/a(s) = (1 + sRC)/(sR2C)
(15)
Kpwm = 2 VHV/VA
(16)
Gm(S) = 1/(sLm + Rm +Rsw +Rs)
(17)
Gi(s) = 2 Rs
(ignoring sampling effects)
(18)
where:
R, C = external compensation
components
R2 = internal input resistor,
typically 20 kΩ
VHV = motor high voltage power supply
VA = oscillator amplitude, typically 7 V
Lm = motor inductance
Rm = motor winding resistance
Rsw = power switch resistance
Rs = sense resistor
It is very important that the motor induc-
tance value used in the analysis is not
the value on the manufacturers data
sheet but rather the value observed in
actual operation. The PWM action
causes high frequency effects that can
change the apparent small signal
inductance significantly. These effects
are dependent upon voltage as well as
current and frequency. It is best to
measure the observed current ripple at
the motor supply voltage and switching
frequency you expect to use and
calculate the actual motor inductance
using:
Lm = VHV/((2 Fosc)(Imax-Imin))
(19)
It is also important to note that both Rm
and Rsw are temperature dependent.
The motor winding resistance can
increase by as much as 30 % at high
temperatures, and if FETs are used as
power devices, Rsw can increase to 2.2
times its value at room temperature.
Substituting equations 15 through 18
into equation 14 gives the expanded
loop gain equation (eq. 20):
(1+sRC) 2Vhv 1 2Rs
Gloop(s) =
sR2C VA (sLm+Rm+Rs+Rsw)
which can be written as (eq.21):
4 VHV Rs
Gloop(s) =
VA(Rm+Rs Rsw)
(1+RC)
(sR2 C) [1+sLm/Rm+Rs+Rsw)]
Therefore the poles and zeros of the
system are:
pole at DC, with a 0dB intercept of:
4VHVRs/[VAR2C(Rm + Rs + Rsw)]
zero at 1/(R C)
pole at (Rm + Rs + Rsw) /Lm
A simple Bode analysis can be per-
formed to provide the necessary infor-
mation to guarantee the stability of the
loop. A stable system will result when
the gain crossover occurs at a point
where the loop phase shift is less than -
180 degrees. The gain crossover point
is defined as the frequency where the
magnitude of Gloop(s) = 1 (0dB).
The Bode plot will show two figures of
merit that give an indication of the
behavior of the closed loop system,
gain margin and phase margin. Gain
margin is the amount of loop signal at-
tenuation at the point where the loop
phase has reached -180 degrees. It is a
qualitative measure of how susceptible
the loop is to noise outside its band-
width. Phase margin is the amount of
Fig. 11a Loop Compensation Block Diagram
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