StepperMotorDriver.pdf

APPLICATION NOTE

STEPPER MOTOR DRIVER CONSIDERATIONS

COMMON PROBLEMS & SOLUTIONS

by Thomas L. Hopkins

This note explains how to avoid same of the more common pitfalls in motor drive design. It is

basedontheauthor’sexperiencein respondingtoenquiriesfromthe field.

Bipolar driven motors. In the past unipolar motors

were common and preferred for their simple drive

configurations. However, with the advent of cost

effective integrated drivers, bipolar motors are

now more common. These bipolar motors typi-

cally produce a higher torque in a given form fac-

tor [1].

INTRODUCTION

Over the years while working with stepper motor

users, many of the same questions keep occur-

ring from novice as well as experienced users of

stepper motors. This application note is intended

as a collection of answers to commonly asked

questions about stepper motors and driver de-

sign. In addition the reference list contains a num-

ber of other application notes, books and articles

that a designer may find useful in applying step-

per motors.

Throughout the course of this discussion the

reader will find references to the L6201, L6202

and L6203. Since these devices are the same die

and differ only in package, any reference to one

of the devices should be considered to mean any

of the three devices.

Drive Topology Selection

Depending on the torque and speed required

from a stepper motor there are several motor

drive topologies available [5, chapter3]. At low

speeds a simple direct voltage drive, giving the

motor just sufficient voltage so that the internal re-

sistance of the motor limits the current to the al-

lowed value as shown in Figure 1A, may be suffi-

cient. However at higher rotational speeds there

is a significant fall off of torque since the winding

inductance limits the rate of change of the current

and the current can no longer reach it’s full value

in each step, as shown in Figure 2.

Motor Selection (Unipolar vs Bipolar)

Stepper motors in common use can be divided

into general classes, Unipolar driven motors and

Figure 1: Simple direct voltage unipolar motors drive.

1/12

AN460/0392

APPLICATION NOTE

higher voltage is used and the current limit is set

by an external resistor in series with the motor

winding such that the sum of the external resis-

tance and the internal winding resistance limits

the current to the allowed value. This drive tech-

nique increases the current slew rate and typically

provides better torque at high rotational speed.

However there is a significant penalty paid in ad-

ditional dissipation in the external resistances.

To avoid the additional dissipation a chopping

controlled current drive may be employed, as

shown in Figure 3. In this technique the current

through the motor is sensed and controlled by a

chopping control circuit so that it is maintained

within the rated level. Devices like the L297,

L6506 and PBL3717A implement this type of con-

trol. This technique improves the current rise time

in the motor and improves the torque at high

speeds while maintaining a high efficiency in the

drive [2]. Figure 4 shows a comparison between

the winding current wave forms for the same mo-

tor driven in these three techniques.

Figure 2: Direct voltage drive.

A - low speed;

B - too high speed generates fall of

torque.

One solution is to use what is commonly referred

to as an L/nR drive (Fig. 1B). In this topology a

Figure 3: Chopper drive provides better performance.

Figure 4: Motor current using L/R, L/5R and chopper constant current drive.

2/12

APPLICATION NOTE

bridge transistors will be forward biased by the

transformer action of the motor windings, provid-

ing an effective short circuit across the supply.

Secondly the L298N, even though it has split sup-

ply voltages, may not be used without a high volt-

age supply on the chip since a portion of the drive

current for the output bridge is derived from this

supply.

In general the best performance, in terms of

torque, is achieved using the chopping current

control technique [2]. This technique also allows

easy implementation of multiple current level

drive techniques to improve the motor perform-

ance. [1]

Driving a Unipolar Motor with the L298N or

L6202

Although it is not the optimal solution, design con-

straints sometimes limit the motor selection. In

the case where the designer is looking for a

highly integrated drive stage with improved per-

formance over previous designs but is con-

strained to drive a unipolar wound (6 leaded) mo-

tor it is possible to drive the motor with H-Bridge

drivers like the L298N or L6202. To drive such a

motor the center tap of the motor should be left

unconnected and the two ends of the common

windings are connected to the bridge outputs, as

shown in Figure 5. In this configuration the user

should notice a marked improvement in torque for

the same coil current, or put another way, the

same torque output will be achieved with a lower

coil current.

A solution where the L298N or L6202 is used to

drive a unipolar motor while keeping the center

connection of each coil connected to the supply

will not work. First, the protection diodes needed

from

Selecting Enable or Phase chopping

When implementing chopping control of the cur-

rent in a stepper motor, there are several ways in

which the current control can be implemented. A

bridge output, like the L6202 or L298N, may be

driven in enable chopping, one phase chopping or

two phase chopping, as shown in Figure 6. The

L297 implements enable chopping or one phase

chopping, selected by the control input. The

L6506 implements one phase chopping, with the

recirculation path around the lower half of the

bridge, if the four outputs are connected to the 4

inputs of the bridge or enable chopping if the odd

numbered outputs are connected to the enable

inputs of the bridge. Selecting the correct chop-

ping mode is an important consideration that af-

fects the stability of the system as well as the dis-

sipation. Table 1 shows a relative comparison of

the different chopping modes, for a fixed chop-

ping frequency, motor current and motor induc-

tance.

collector to emitter (drain to source) of the

Figure 5: Driving a unipolar wound motor with a bipolar drive

3/12

APPLICATION NOTE

Table 1: Comparative advantages of chopping modes

Chopping Mode

Ripple Current

Motor Dissipation

Bridge Dissipation *

Minimum Current

ENABLE

HIGH

LOWER

ONE PHASE

LOW

LOWEST

LOW

TWO PHASE

HIGH

LOW

Ipp/2

(*) As related to L298N, L6203 or L6202.

Figure 6a: Two Phase Chopping.

Figure 6b: One Phase Chopping.

Figure 6c: Enable Chopping.

4/12

APPLICATION NOTE

RIPPLE CURRENT

Since the rate of current change is related directly

to the voltage applied across the coil by the equa-

tion:

In the L6202 and L6203, the internal gate drive

circuit works the same in response to either the

input or the enable so the switching losses are

the same using enable or two phase chopping,

but would be lower using one phase chopping.

However, the losses due to the voltage drops

across the device are not the same. During en-

able chopping all four of the output DMOS de-

vices are turned off and the current recirculates

through the body to drain diodes of the DMOS

output transistors. When phase chopping the

DMOS devices in the recirculation path are driven

on and conduct current in the reverse direction.

Since the voltage drop across the DMOS device

is less than the forward voltage drop of the diode

for currents less than 2A, the DMOS take a sig-

nificant amount of the current and the power dis-

sipation is much lower using phase chopping than

enable chopping, as can be seen in the power

dissipation graphs in the data sheet.

With these two devices, phase chopping will al-

ways provide lower dissipation in the device. For

discrete bridges the switching loss and saturation

losses should be evaluated to determine which is

lower.

L di

the ripple current will be determined primarily by

the chopping frequency and the voltage across

the coil. When the coil is driven on, the voltage

across the coil is fixed by the power supply minus

the saturation voltages of the driver. On the other

hand the voltage across the coil during the recir-

culation time depends on the chopping mode

chosen.

When enable chopping or two phase chopping is

selected, the voltage across the coil during recir-

culation is the supply voltage plus either the V F of

the diodes or the RI voltage of the DMOS devices

(when using the L6202 in two phase chopping). In

this case the slope of the current rise and decay

are nearly the same and the ripple current can be

large.

When one phase chopping is used, the voltage

across the coil during recirculation is V on (V sat for

Bipolar devices or I V R DSon for DMOS) of the tran-

sistor that remains on plus V F of one diode plus

the voltage drop across the sense resistor, if it is

in the recirculation path. In this case the current

decays much slower than it rises and the ripple

current is much smaller than in the previous case.

The effect will be much more noticeable at higher

supply voltages.

MINIMUM CURRENT

The minimum current that can be regulated is im-

portant when implementing microstepping, when

implementing multilevel current controls, or any-

time when attempting to regulate a current that is

very small compared to the peak current that

would flow if the motor were connected directly to

the supply voltage used.

With enable chopping or one phase chopping the

only problem is loss of regulation for currents be-

low a minimum value. Figure 7 shows a typical re-

sponse curve for output current as a function of

the set reference. This minimum value is set by

the motor characteristics, primarily the motor re-

sistance, the supply voltage and the minimum

duty cycle achievable by the control circuit. The

minimum current that can be supplied is the cur-

rent that flows through the winding when driven

by the minimum duty cycle. Below this value cur-

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

zero during each cycle, as shown in figure 8.

When using one phase chopping the current may

or may not return completely to zero and there

may be some residual DC component.

When using a constant frequency control like the

L297 or L6506, the minimum duty cycle is basi-

cally the duty cycle of the oscillator (sync) since

the set dominance of the flip-flop maintains the

output on during the time the sync is active. In

constant off time regulators, like the PBL3717A,

the minimum output time is set by the propaga-

tion delay through the circuit and it’s ratio to the

selected off time.

MOTOR LOSSES

The losses in the motor include the resistive

losses (I 2 R) in the motor winding and parasitic

losses like eddie current losses. The latter group

of parasitic losses generally increases with in-

creased ripple currents and frequency. Chopping

techniques that have a high ripple current will

have higher losses in the motor. Enable or two

phase chopping will cause higher losses in the

motor with the effect of raising motor tempera-

ture. Generally lower motor losses are achieved

using phase chopping.

POWER DISSIPATION IN THE BRIDGE IC.

In the L298N, the internal drive circuitry provides

active turn off for the output devices when the

outputs are switched in response to the 4 phase

inputs. However when the outputs are switched

off in response to the enable inputs all base drive

is removed from output devices but no active ele-

ment is present to remove the stored charge in

the base. When enable chopping is used the fall

time of the current in the power devices will be

longer and the device will have higher switching

losses than if phase chopping is used.

5/12

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