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STK672-732AN-E

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STK672-732AN-E
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16
(2) I
D and
V
DSS
Operating Waveforms in Non-avalanche Mode
Although the waveforms during avalanche mode are given in Figure 1, sometimes an avalanche does not result during
actual operations.
Factors causing avalanche are listed below.
Poor coupling of the motor’s phase coils (electromagnetic coupling of A phase and AB phase, B phase and BB phase).
Increase in the lead inductance of the harness caused by the circuit pattern of the board and motor.
Increases in V
DSS
, tAVL, and IAVL in Figure 1 due to an increase in the supply voltage from 24 V to 36 V.
If the factors above are negligible, the waveforms shown in Figure 1 become waveforms without avalanche as shown
in Figure 2.
Under operations shown in Figure 2, avalanche does not occur and there is no need to consider the allowable loss
range of PAVL shown in Figure 3.
Figure 2. Output Current, I
D
, and Voltage, V
DS
, Waveforms 2 of the STK672-7** Series when Driving
a 2-Phase Stepper Motor with Constant Current Chopping
0
0.5
1
1.5
2
2.5
3
3.5
4
00.511.522.5
Average power loss PAVL during avalanche
PAVL W
Motor current, IOH A
Tc=105°C
Tc=80°C
Note :
The operating conditions given above represent a loss when driving a 2-phase stepper motor with constant current
chopping.
Because it is possible to apply 2.6 W or more at I
OH
= 0 A, be sure to avoid using the MOSFET body diode that is used
to drive the motor as a zener diode.
I
OH
: Motor current peak value
Figure 3. Allowable Loss Range, PAVL-I
OH
During STK672-732AN-E Avalanche Operations
PAVL-IOH
STK672-732AN-E
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17
4. Calculating STK672-732AN-E IPM Internal Power Loss
The average internal power loss in each excitation mode of the STK672-732AN-E can be calculated from the following
formulas.
Each excitation mode
2-phase excitation mode
2PdAVex = (Vsat+Vdf) (1/(t1+t2+t3)) IOHt2+(1/(t1+t2+t3)) IOH (Vsatt1+Vdft3)
1-2 Phase excitation mode
1-2PdAVex = (Vsat+Vdf) (1/(t1+t2+t3)) IOHt2+(1/(t1+t2+t3)) IOH (Vsatt1+Vdft3)
Motor hold mode
HoldPdAVex = (Vsat+Vdf) I
OH
Vsat : Combined voltage represented by the Ron voltage drop+shunt resistor
Vdf : Combined voltage represented by the MOSFET body diode+shunt resistor
t1, t2, and t3 represent the waveforms shown in the figure below.
t1 : Time required for the winding current to reach the set current (I
OH
)
t2 : Time in the constant current control (PWM) region
t3 : Time from end of phase input signal until inverse current regeneration is complete
Motor COM Current Waveform Model
t1 = (-L/(R+0.33)) ln (1-(((R+0.33)/V
CC
) I
OH
))
t3 = (-L/R) ln ((V
CC
+0.33)/(I
OH
R+V
CC
+0.33))
V
CC
: Motor supply voltage (V)
L : Motor inductance (H)
R : Motor winding resistance ()
I
OH
: Motor set output current crest value (A)
For the values of Vsat and Vdf, be sure to substitute from Vsat vs I
OH
and Vdf vs I
OH
at the setting current value I
OH
.
(See pages to follow)
Then, determine if a heat sink is necessary by comparing with the Tc vs Pd graph (see next page) based on the
calculated average output loss, IPM.
For heat sink design, be sure to see ‘5. Thermal Design’.
The IPM average power, PdAVex described above, represents loss when not in avalanche mode.
To add the loss in avalanche mode, be sure to add PAVL using the formula (for average power loss , PAVL, for
STK672-7** during avalanche mode, described below to PdAVex described above.)
When using this IC without a fin, always check for temperature increases in the set, because the HIC substrate
temperature, Tc, varies due to effects of convection around the IPM.
IOH
0 A
t1 t2
t3
STK672-732AN-E
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18
4-2. [Calculating the average power loss, PAVL, during avalanche mode]
The allowable avalanche energy, EAVL, during fixed current chopping operation is represented by Equation (3-2) used
to find the average power loss, PAVL, during avalanche mode that is calculated by multiplying Equation (3-1) by the
chopping frequency.
PAVL=V
DSS
IAVL0.5tAVLfc ············································································· (3-2)
fc : Hz units (fc is set to the PWM frequency of 50kHz.)
Be sure to actually operate an STK672-7** series and substitute values found when observing operations on an
oscilloscope for V
DSS
, IAVL, and tAVL.
The sum of PAVL values for each excitation mode is multiplied by the constants given below and added to the average
internal IPM loss equation, except in the case of 2-phase excitation.
1-2 excitation mode and higher : PAVL(1) = 0.7 PAVL ···················································· (4-1)
During2-phase excitation mode and motor hold : PAVL(1) = 1 PAVL ··································· (4-2)
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STK672-732AN-E

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Manufacturer:
ON Semiconductor
Description:
Motor / Motion / Ignition Controllers & Drivers STEPPING MOTOR DRIVER
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