STK672-442BN-E

No.A2249-18/28

3. Allowable Avalanche Energy Value

(1) Allowable Range in Avalanche Mode

When driving a 2-phase stepping motor with constant current chopping using an STK672-4** Series hybrid IC, the

waveforms shown in Figure 1 below result for the output current, ID, and voltage, VDS.

Figure 1 Output Current, ID, and Voltage, VDS, Waveforms 1 of the STK672-4** Series when Driving a

2-Phase Stepper Motor with Constant Current Chopping

When operations of the MOSFET built into STK672-4** Series ICs is turned off for constant current chopping, the ID

signal falls like the waveform shown in the figure above. At this time, the output voltage, VDS, suddenly rises due to

electromagnetic induction generated by the motor coil.

In the case of voltage that rises suddenly, voltage is restricted by the MOSFET VDSS. Voltage restriction by VDSS

results in a MOSFET avalanche. During avalanche operations, ID flows and the instantaneous energy at this time, EAVL1,

is represented by Equation (3-1).

EAVL1=VDSSIAVL0.5tAVL ------------------------------------------- (3-1)

VDSS: V units, IAVL: A units, tAVL: sec units

The coefficient 0.5 in Equation (3-1) is a constant required to convert the IAVL triangle wave to a square wave.

During STK672-4** Series operations, the waveforms in the figure above repeat due to the constant current chopping

operation. The allowable avalanche energy, EAVL, is therefore represented by Equation (3-2) used to find the average

power loss, PAVL, during avalanche mode multiplied by the chopping frequency in Equation (3-1).

PAVL=VDSSIAVL0.5tAVLfc ------------------------------------------- (3-2)

fc: Hz units (fc is set to the PWM frequency of 50kHz.)

For VDSS, IAVL, and tAVL, be sure to actually operate the STK672-4** Series and substitute values when operations

are observed using an oscilloscope.

Ex. If VDSS=110V, IAVL=1A, tAVL=0.2s, the result is:

PAVL=110

VDSS=110V is a value actually measured using an oscilloscope.

The allowable loss range for the allowable avalanche energy value, PAVL, is shown in the graph in Figure 3.

When examining the avalanche energy, be sure to actually drive a motor and observe the ID, VDSS, and tAVL waveforms

during operation, and then check that the result of calculating Equation (3-2) falls within the allowable range for

avalanche operations.

IOH: Motor current peak value

tAVL: Time of avalanche operations

IAVL: Current during avalanche operations

VDSS: Voltage during avalanche operations