1

2

Voltage Current

Operating current Stall current

Power Rating Power (watts) = Voltage * Current

Power Spike Torque

Operating Torque Stall Torque

Velocity Operating Frequency T-W curve

3

design all control circuitry capable of handling this stall current

Model We know that an armature current can be calculated

as:I = V - E R

Where V is applied voltage, E is the internally generated voltage and R is armature resistance.we have,

E = K x magnetic flux x rpmthen,induced torque that is actually motor’s torque which we have on our output shaft. It is

motor torque = K x flux x IExplain graph b/w torque and w..

Consider a motor is running freely. Induced EMF exists... Apply load torque on motor shaft. Rpm decreases...E=k x flux x w...so emf decreases Which makes I=(V-E)/R to rise. Induced torque increases

T = K x flux x IFinally touque induced become equal to load torque at LOWER SPEED of rotation.

When E is zero..i.e.motor is static and is about to rotate...then currrent will be max. That makes induce torque max... This max torque is actually a stall torque and current will be stall current.

torque

speed

ke

V

R

kV

max speed

Linear mechanical power Pm = F v

Rotational version of Pm =

power output

speed vs. torque

stall torque

8

aa

aat edtdi

LiRv

+

ea

_

LaRa

ia+

Vt

_

at ikTe Electromagnetic torque

Ea ke Armature back e.m.f.

kke

Controlling speed with voltage

DC motor model

V e

R

• The back emf depends only on the motor speed.

• The motor’s torque depends only on the current, I. e = ke

= k I

• Consider this circuit’s V: V = IR + eIstall = V/R

current when motor is stalled

speed = 0torque = max

How is V related to

V = + ke R

k

= - + R

ke

V

Speed is proportional to voltage.

Jizhong Xiao

aaaa EIRV In steady state,

T

ae

T

a

k

VT

k

R 2

Therefore speed is given by,

Three possible methods of speed control:

Armature resistance RaArmature voltage Va

aa

aaa edt

diLiRV

Armature circuit:

Torque

Speed

MaximumloadTorque

No load speed

Separately excited DC motors have goodspeed regulation.

Full load speed

Torque

Speed

MaximumTorque

By Changing Ra

Ra increasing

• Power loss in Ra• Does not maintain maximum torque capability• Poor speed regulation

Torque

Speed

MaximumTorque

By Changing Armature voltage

Trated

Va increasing

• good speed regulation• maintain maximum torque capability

Model:GM8224D091 – R10

Characteristics: Continous torque 0.0185 Nm Stall torque 0.1186 Nm Stall current 3 A Speed 8130 – 9040 rpm Voltage 9.55 – 48V Armature Resistance 6.75 ohm

Torque_motor * rpm_motor = Torque_output * rpm_output

After gearing with the ratio of 30.9 :1 i.e.

Characteristics: Continous torque 0.57 Nm Stall torque 3.67 Nm Speed 350 rpm

Therefore,

Hence, the resultant mechanical constants are,

moment of inertia of motor

friction due to motor bearing

Translational (linear) motion:

dt

dJT

Rotational motion:

dt

dvMF

F : Force (Nm)M : Mass (Kg )v : velocity (m/s)

T : Torque (Nm)J : Moment of Inertia (Kgm2 ) : angular velocity ( rad/s )

dt

dJTTor

dt

dJTT LeLe

0 Le TT Acceleration

0 Le TT Deceleration

0 Le TT Constant speed

Te : motor torque (Nm) TL : Load torque (Nm)

Drive accelerates or decelerates depending on whether Te is greater or less than TL

During acceleration, motor must supply not onlythe load torque but also dynamic torque, ( Jd/dt ).

During deceleration, the dynamic torque, ( Jd/dt ), hasa negative sign. Therefore, it assists the motortorque, Te.

Te

Forwardrunning

Speed

Forwardbraking

Reverseacc.

Reverserunning

Reversebraking

Forwardacc.

4Q OPERATION

SPEED

TORQUE

I

III

II

IV

TeTe

Te Te

FMFB

RMRB

F: FORWARD R: REVERSE M : MOTORING B: BRAKING

23

Different TopologiesGate driver IC’sSwitch(MOSFET) selection

Datasheet Maximum Current Rating

H-bridge Operation Electronic Braking

Opto-IsolationDrive Logic Design

24

25

26

27

28

29

For an N-Mos device: Cutoff when VGS < Vt

Triode when VGS > Vt, and VDS < (VGS – Vt)

Saturation when VGS > Vt, and VDS > (VGS – Vt)

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For an P-Mos device:Cutoff when VGS > Vt

Triode when VGS < Vt, and VDS > (VGS – Vt)

Saturation when VGS < Vt, and VDS < (VGS – Vt)

VDG > Vt

VDG < Vt

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Why Gate Driver are needed??

32

Boot-strap suppliesSeparate floating supplies

MOSFET needs VGS > VGSsat to

turn completely on

Source of High-Side device “floats” with motor phase

High-Side Gate Drive powered by bootstrap capacitor

Capacitor charged through diode when low-side device is ON

Can’t run at 100% PWM duty cycle indefinitely

Need some low-side ON-time to charge bootstrap capacitor

Inexpensive

Some considerations for sizing bootstrap components Minimum Vboot

voltage Gate driver

quiescent current Mosfet Gate charge High-side On-time

The minimum bootstrap capacitor value can be calculated from the following equation:

where: Qg = Gate charge of high-side FET f = frequency of operation ICbs (leak) = bootstrap capacitor leakage current Iqbs (max) = Maximum VBS quiescent current VCC = Logic section voltage source Vf = Forward voltage drop across the bootstrap diode VLS = Voltage drop across the low-side FET or load VMin = Minimum voltage between VB and VS. Qls = level shift charge required per cycle

How much current that MOSFET can safely carry??

Maximum Current:

NOTE:id max given in datasheet is current rating

that is achieved by keeping case temperature at 25’c .

42

The first parameter (and the one everyone goes for first) is maximum current through the drain Id(max). For the IR1405 that is:

Id @ 25 °C 79A max

Current and temperature are linked so the next parameter we need for the model is the Max Junction Temperature. That is the limit of how hot the silicon can get before it decrystalizes (a fancy way of saying it melts :-)

Relationship between current and temperatureThat relationship is defined further on in the data sheet where there thermal resistance of the TO-220 package is specified by the term, Rtheta(jc). The data sheet lists it as: Rtheta(jc)    1.32 °C/W

Given the junction melts at 175, the case is at 25 degrees (its part of the Id(max) spec), and Rtheta(jc) is .45 dC/Watt. How many watts would we be dissipating if the case was at 25 degrees and the junction was at 175? Answer :

(175 - 25) =1.32* Watts113 = Watts

This matches another value in the datasheet which is the PD @ TC 25 °C. (Power Dissipated at a case temperature of 25 degrees) which is 330 watts.  Now, what is the maximum current we can carry in this FET ? Again by the data sheet 169A. So now we can use the formula P = I2R to compute what the internal resistance is, if we're dissipating 333 watts.

113 = (79)^2 * R0.0182= R

But wait, isn't Rds(on) on this datasheet 7.1 milliOhms not 18.2 milliOhms? Why yes it is, at 25 °C, but looking into the data sheet reveals a table that shows Rds on) increases with temperature.

Looking further into the data sheet we see that the Rtheta(ja) which is junction to ambient air. Is 62 °C/Watt. This tells us that in still air, the junction is 62 degrees warmer than the temperature of the ambient air for each watt of power we're dissipating.

Again we know the max junction temperature is 175 degrees. Assuming the FET is sitting in air that is at room temperature or 25 degrees there is 150 degrees difference between the ambient air and the junction. Using that number and plugging in our 62 degree constant we get:

P = 150 °C / Rtheta(ja) (°C/W)P = 150 °C/ 62  (°C/W)P = 2.42 Watts

This means that the FET can be dissipating  2.42 Watts and continue working. As we know that the Rds(on) value at 175 degrees will be 11.3 milliOhms from our previous exercise so we use that to calculate our current using Ohm's law again as follows:

2.42 W = I2 *18.2mOhmsI= 12.1 Amps 

Vdc

Load

A+ B+

A– B–

Va Vb

+ Vdc −

46

Vdc

Load

A+ B+

A– B–

Va Vb

+ Vdc −

V = L * dI/dT - What does that mean???

• Current rises indefinitely based upon inductance and voltage.

Real inductors have resistance Current * resistance = voltage Eventually current levels out Strength of magnetic field + level of stored

energy are proportional to the current.

What happens when switch is opened?

• Current dissipates quickly in the ARC

Diodes used to suppress arcing Recirculating currents dissipate slower

• Note: current continues to flow in inductor• Power is dissipated across diode & inductor internal

resistance.

•This represents traditional PWM motor control•Note low average current flow

•Use switches instead of diodes•Much more efficient, regenerative braking

V = IR V = V+

V = 0(V = -IR)V = IR

Electronic BrakingMechanical Braking

54

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InterfaceInterface between control (low power electronics) and (high between control (low power electronics) and (high power) switchpower) switch.

Functions:– amplifies control signal to a level required to drive power amplifies control signal to a level required to drive power switchswitch– provides electrical isolation between power switch and provides electrical isolation between power switch and logic levellogic level

isolation

56

Isolation is required to prevent damages on the high power required to prevent damages on the high power switchswitch to propagate back to low power electronics.

The basic steps in designing a simple digital circuit are: Step 1: Define the problem

▪ Truth tables

Step 2: Translate truth tables into combinatorial logic circuit▪ Boolean Algebra▪ Minterms▪ Sum of Products (or Product of Sums)

Step 3: Optimization▪ Boolean Identities▪ DeMorgan’s Law▪ Karnaugh Maps (K-Maps)

Step 4: Build It!▪ Protoboard and Integrated Circuits.

▪ Warning: This is a lot of information if it is your first exposure to circuits!

Digital logic circuits can contain multiple inputs and outputs.

The combinations of inputs and outputs can be represented in a table form (called truth tables).

Truth tables should list ALL the combinations of inputs and outputs.

Example: Inverter

Input Output

0 1

1 0

We want to build a device called an H-Bridge. An H-bridge is a simple motor controller that is used to

provide 4 functions to an electric motor: Forward, Reverse, Brake, and Coast. The functions are selected with 2 input lines.

The H-bridge is built with 4 switches, and allows voltage

to be applied across the motor in either direction.

H-Bridge input table H-Bridge output table

H-Bridge full combinatorial logic full truth table

IN 2

IN 1

Function

0 0 Coast

0 1 Forward

1 0 Reverse

1 1 Brake

Function

SW1 SW2 SW3 SW4

Coast 0 0 0 0

Forward 1 0 0 1

Reverse 0 1 1 0

Brake 1 1 0 0

IN2 IN1 SW1 SW2 SW3 SW4 Function

0 0 0 0 0 0 Coast

0 1 1 0 0 1 Forward

1 0 0 1 1 0 Reverse

1 1 1 1 0 0 Brake

Select the rows that generate a TRUE output, and then combine the terms with an OR gate.

You do this separately for each output value (sw1. . .sw4).IN2 IN1 SW1 SW2 SW3 SW4 Functio

n

0 0 0 0 0 0 Coast

0 1 1 0 0 1 Forward

1 0 0 1 1 0 Reverse

1 1 1 1 0 0 BrakeIN2 IN

1SW1

0 0 0

0 1 1

1 0 0

1 1 1

IN2 IN1 SW2

0 0 0

0 1 0

1 0 1

1 1 1

IN2 IN1 SW3

0 0 0

0 1 0

1 0 1

1 1 0

IN2 IN1 SW4

0 0 0

0 1 1

1 0 0

1 1 0SW1= SW2= SW3= SW4=

IN2*IN1 + IN2*IN1 IN2*IN1 + IN2*IN1 IN2*IN1 IN2*IN1

IN2

IN1

IN2

IN1

SW1

IN2

IN1

IN2

IN1

SW2

SW1=IN2*IN1 + IN2*IN1

SW2=IN2*IN1 + IN2*IN1

IN2

IN1

IN2

IN1SW3 SW4

SW3=IN2*IN1 SW4=IN2*IN1

Do you see any unnecessary gates?

Using Boolean identities, the circuit can be simplified.

M

Power

GND

IN2

IN1

Final Circuit

Shunt resistor Current is measured as voltage drop

across a current sense resistor

Questions??

67

PCB construction, wire loops creates stray inductance, wire loops creates stray inductance, LLss..Using KVL,Using KVL,

From previous equation, the voltage across the the voltage across the switch is bigger than the supply (for a short switch is bigger than the supply (for a short moment).moment).

The spike may exceed the switch rated blocking spike may exceed the switch rated blocking voltage and causes damagevoltage and causes damage due to overvoltage.

To preventTo prevent such occurrence, a snubber is put a snubber is put across the switchacross the switch. An example of a snubber is an RCD circuit shown below.

Snubber circuit “smoothened” the transition and Snubber circuit “smoothened” the transition and make the switch voltage rise more “slowly”.make the switch voltage rise more “slowly”. In effect it dampens the high voltage spike to a safe value.

Switches and diodes requires snubbersSwitches and diodes requires snubbers. However, new generation of IGBT, MOSFET and GCT do not require it.

In general, snubbers are used for:

– turn-on:turn-on: to minimize large overcurrents through the device to minimize large overcurrents through the device at turn-onat turn-on

– turn-off:turn-off: to minimize large overvoltages across the device to minimize large overvoltages across the device during turn- during turn-

off.off.

– Stress reductionStress reduction: to shape the device switching waveform to shape the device switching waveform such that the voltage and current associated with the device such that the voltage and current associated with the device are not high simultaneously.are not high simultaneously.