Page 77 - CNC Robotics

CNC Robotics

AP PLICATION NOTE

RIPPLE CURRENT In the L6202 and L6203, the internal gate drive
Sinc e the rate of current change is related directly circuit works the same in response to either the
to the voltage applied across the coil by the equa- input or the enable so the switching losses are
lion: the same using enable or two phase chopping,
but would be lower using one phase chopping.
V =L .'!! However, the losses due to the voltage drops
dt
across the device are not the same. During en-
the ripple current will be determined primarily by able chopping all four of the output DMOS de-
the chopping frequency and the voltage across vices are turned off and the current recirculates
the coil. When the coil is driven on, the voltage through the body to drain diodes of the DMOS
across the coil is fixed by the powe r supply minus output transistors. When phase chopping the
the saturation voltages of the driver. On the other DMOS devices in the recirculation path are driven
hand the voltage across the coil during the recir- on and conduct current in the reverse direction.
culation time depends on the chopping mode Since the voltage drop across the DMOS device
chosen. is less than the forward voltage drop of the diode
When enable chopping or two phase chopping is for currents less than 2A, the DMOS take a sig-
selected, the voltage across the coil during recir- nificant amount of the current and the power dis-
culation is the supply voltage plus either the VF of sipation is much lower using phase chopping than
the diodes or the AI voltage of the DMOS devices enable choppi ng, as can be seen in the power
(when using the L6202 in two phase chopping). In cissoaton graphs in the data sheet.
this case the slope of the current rise and decay With these two devices, phase chopping will al-
are nearly the same and the ripple current can be ways provide lower dissipation in the device. For
large. discrete bridges the switching loss and saturation
losses should be evaluated to determine which is
When one phase chopping is used, the voltage lower.
across the coil during recirculation is Voo (Vsat for
Bipolar devices or I . ROSon for DMOS) of the tran-
sistor that remains on plus VF of one diode plus MINIMUM CURR ENT
the voltage drop across the sense resistor, if it is The mlnlmm current that can be regulated is im-
in the recirculation path. In this case the current portant when implementing microstepping, when
decays much slower than it rises and the ripple implementing multilevel current controls , or any-
current is much smaller than in the previous case . time when attempting to regulate a current that is
The effect will be much more noticeable at higher very small compared to the peak current that
supply voltages.
would flow if the motor were connected directly to
the supply voltage used.
MOTOR LOSSES With enable chopping or one phase chopping the
The losses in the motor include the resistive only problem is loss of regulation for current s be-
losses (12R) in the motor winding and parasitic Iow a minimum value. Figure 7 shows a typical re-
losses like eddie current losses. The latter group sponse curve for output current as a function of
of parasitic losses generally increases with in- the set reference. This mlnlmnn value is set by
creased ripple currents and frequency_Chopping the motor characteristics, primarily the motor re-
techniques that have a high ripple current wi. sistance, the supply voltage and the minirmm
have higher losses in the motor. Enable or two duty cycle achievabl e by the control circuit. The
phase chopping will cause higher losses in the minimim current that can be supplied is the cur-
motor with the effect of raisn g motor tempera- rent that flows through the winding when driven
ture. Generally lower motor losses are achieved by the minirmrn duty cyde. Below this value cur-
using phase chopping. rent regulation is not possible. With enable chop-
ping the current through the coil in response to
the minimum duty cycle can return completely to
POWE R DISSI PATION IN THE BRIDGE IC. zero during each cycle, as shown in figure 8.
In the l298N, the internal drive circuitry provides When using one phase chopping the current may
active turn off for the output devices when the or may not return completely to zero and there
outputs are switched in response to the 4 phase may be some residual DC compone nt.
inputs. However when the outputs are switched When using a constant frequency control like the
off in response to the enable inputs all base drive l297 or L6506, the minimum duty cycle is basi-
is removed from output devices but no active ele- cally the duty cycle of the oscillator (sync) since
ment is present to remove the stored charge in the set dominance of the flip-flop maintains the
the base. When enable chopping is used the fall output on during the time the sync is active. In
time of the current in the power devices will be constant off time regulators, like the PBL3717A,
longer and the device will have higher switching the minimum output time is set by the propaga-
losses than if phase chopping is used. tion delay through the circuit and it's ratio to the
selected off time.
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