ADVANCED POWER ELECTRONICS - ape.gastli.infoape.gastli.info/Chapter2/APE_CH2.pdf · Dr. Adel Gastli Firing Circuits 5 Time 2.6us 3.0us 3.5us 4.0us 4.5us 5.0us 5.4us ID(M1)*5 V(M1:d)

ADVANCED POWER ELECTRONICSADVANCED POWER ELECTRONICS

FIRING CIRCUITSFIRING CIRCUITS

Dr. Adel GastliEmail: [email protected]

http://adel.gastli.net

V2

TD = 0

TF = 0PW = 2.5uPER = 5u

V1 = 0

TR = 0

V2 = 15

M1IRF740

R1

100

I

Rs

0.1

0

C1

1n

RG1k

RD10

VDD

80Vdc

V

Dr. Adel Gastli Firing Circuits 2

CONTENTSCONTENTSCONTENTS

1. MOSFET Gate Drive2. BJT Base Drive3. Isolation of Gate & Base Drives4. Thyristor Firing Circuit5. Thyristor Converter Gating Circuit6. Summary

(Textbook: Sections 17.2, 17.3, 17.4, 17.5, 17.8)(Textbook: Sections 17.2, 17.3, 17.4, 17.5, 17.8)


MOSFET Gate DriveMOSFET Gate Drive

The turn on time of a MOSFET depends on the charging time of the input or gate capacitance.

The turn-on time can be reduced by connecting an RC circuit as shown in the figure to charge the gate capacitance faster


V2

TD = 0

TF = 0PW = 2.5uPER = 5u

V1 = 0

TR = 0

V2 = 15

M1IRF740

R1

100

I

Rs

0.1

0

C1

1n

RG1k

RD10

VDD

80Vdc

V

OrCADOrCAD circuit simulationcircuit simulation


Time

3.0us 3.5us 4.0us 4.5us 5.0us2.6us 5.4usID(M1)*5 V(M1:d)

0

25.0

50.0

75.0

88.4

C1=1pF


Time

3.00us 3.50us 4.00us 4.50us 5.00us2.51us 5.27usV(M1:d) ID(M1)*5

0

25

50

75

95

C1=1nF


Time

2.800us 3.200us 3.600us 4.000us 4.400us 4.800us2.475us 5.150usID(M1)*5 V(M1:d)

0

25.0

50.0

75.0

87.6

C1=100nF


IG

RGS

IG

When the gate voltage is turned on, the initial charging current of the capacitance is

GSS

GG RR

VI

+=

and the steady state value of gate voltage is

GS

GGGS RRR

VRV

++=

1

where the steady-state gate-source current is considered negligible.

Typically between 10 and 20V in on-state


In order to achieve switching speeds of the order of 100 ns or less, the gate-drive circuit should have:– a low output impedance and– the ability to sink and source

relatively large currents.


Time

0s 1.0us 2.0us 3.0us 4.0us 5.0us 6.0us-I(VDD)*5 V(M1:d) IG(M1)*500

-100

-50

0

50

100

Sink

Source

Gate currentGate current

Turn off Turn on


A totem-pole arrangement that is capable of sourcing and sinking a large current is shown in below.

The PNP- and NPN-transistors act as emitter followers and offer a low impedance.

These transistors operate in the linear region rather than in saturation mode, thereby minimizing the delay time.


Feedback via the capacitor C regulates the rate of rise and fall of the gate voltage, thereby controlling the rate of rise and fall of the MOSFET drain current.A diode across the capacitor C allows the gate voltage to change rapidly in one direction only.There are a number of integrated drive circuits on the market that are designed to drive transistors and are capable of sourcing and sinking large currents for most converters.

The gate signal for the power MOSFET may be generated by an op-amp.


V2TD = 0

TF = 0PW = 2.5usPER = 5us

V1 = 0

TR = 0

V2 = 10

0

DbreakD1

I

VG

-15Vdc

VDD

80VdcM1

IRF740

0

0

Q3Q2N5226

V

Q1

Q2N5223R1

1k

RD10


Time

3.00us 3.50us 4.00us 4.50us 5.00us2.54usID(M1)*5 V(M1:d)

0

25

50

75

87


Key PointsA MOSFET is a voltage-controlled device.

Applying a gate voltage turns it on and it draws negligible gate current.

The gate drive circuit should have low impedance for fast turn-on.


BJT BASE DRIVEBJT BASE DRIVE

The transistor turning-on time (ton) can be reduced by allowing base current peaking during turn-on, resulting in low forced gain (βF) at the beginning.

After turn on, βF can be increased to a sufficiently high value to maintain the transistor in quasi-saturation region.


The turn off time (toff) can be reduced by reversing base current and allowing base current peaking during turn-off.

Increasing the reverse base current IB2

decreases the storage time.

IB1

IBs

-IB2

t

iBCommonly used techniques for optimizing the base drive are:– Turn-on control

– Turn-off control

– Proportional base control

– Antisaturation control


TurnTurn--on controlon control C

E IE

ICIB

C1

R2R1

VC1++ _

_

RC

VCC

+_VB

t1 t2

vBV1

0t

1

1

R

VVI BE

B

−=

Limits base current when VB turns on (VC1=0)

21

1

RR

VVI BE

BS +−

= Final base current value (VC1=IBSR2)

21

211 RR

RVVC +

≅ Final C1 charge up voltage

21

1211 RR

CRR

+=τ Capacitor charging

time constant

vB=0

Base-emitter junction becomes reverse biased and C1 discharges through R2.


Time

0s 20us 40us 60us 80us 100us 120usI(R2) I(C1)

-2.0A

0A

2.0A

SEL>>

V(RC:2) IC(Q1)*5-100

0

100IB(Q1)

-2.0A

0A

2.0A

Base Current

Collector CurrentCollector-Emitter Voltage

Current in C1

Current in R2

Turn off Turn on


Time

50.00us 51.00us 52.00us 53.00usI(R2) I(C1)

-2.0A

-1.0A

0A

1.0A

V(RC:2) IC(Q1)*5-100

0

100IB(Q1)

-2.0A

0A

2.0A

SEL>>

Zooming on turn-off transition

Negative current in R2

Capacitor discharge current

Negative base current


Time

100us 101us 102us 103usI(R2) I(C1)

0A

1.0A

V(RC:2) IC(Q1)*5-100

0

100IB(Q1)

0.5A

1.0A

-0.1ASEL>>

Positive current in R2 (slow current increase)

Capacitor charge current (fast current increase)

Positive base current (fast current increase)

Zooming on turn-on transition


To allow sufficient charging and discharging times, the

width of the pulse must be and the off period

of the pulse must be

122 CR=τ Capacitor discharging time constant

11 5τ≥t

22 5τ≥t

The maximum switching frequency is:

2121minmax

2.011

ττ +=

+==

ttTfs

21

1211 RR

CRR

+=τ Capacitor charging time constant


I

I Q1

MRH1240N/125C

I

0

RC

10

C1

100n

I

VV2

TD = 0

TF = 0PW = 50usPER = 100us

V1 = 0

TR = 0

V2 = 15V

R1

5

R2

15

V

Vcc80Vdc

I

PSpice Simulation


Time

50.0us 52.0us 54.0us 56.0usIB(Q1) I(R2) I(C1)

-1.00A

0A

-1.56A

0.66A

Capacitor provides a negative current spike as the base charge is removed. Then, the capacitor continues discharging through R2.


TurnTurn--off controloff control C

E IE

ICIB

C1

R2R1

VC1++ _

_

RC

VCC

+_VB

t1 t2

vBV1

-V2

t

2VvB −=

Reverse voltage across transistor base-emitter junction

As C1 discharges, reverse voltage is reduced to steady-state value -V2

vB<0

Base-emitter junction becomes reverse biased and C1 discharges through R2.

)( 2VVV CBE +−=


t1 t2

vBV1

-V2

t

C

E IE

ICIB

C1

R3R1

+

_

RC

VCC

+_VB

R2

R4

C2

D1

Base current peaking during turn-on and turn-off

If different turn-on and turn-off characteristics are required, a turn-off circuit (using C2 R3, and R4), as shown in the above Figure, should be added.

The diode D1 isolates the forward base drive circuit from the reverse base drive circuit during turn-off.


Proportional controlProportional controlThis type of control has advantages over the constant drive circuit. If the collector current changes due to change in load demand, the base drive current is changed in proportion to the collectorcurrent.

When S1 turns on Pulse current flows through Q1 base.

Q1 turns on into saturation

Corresponding base current is induced due to transformer action.

IC starts flowing


Transistor would latch on itself, and S1 can be turned off.

Transformer turns ratio is

β==B

C

I

I

N

N

1

2

For proper operation of the circuit, the magnetizing current, which must be much smaller than the collector current, should be as small as possible.

Switch S1 can be implemented by a small-signal transistor, and an additional circuitry is necessary to discharge capacitor C1 and to reset the transformer core during turn-off of the power transistor, which makes the circuit complex and expensive.


Antisaturation controlAntisaturation controlIf the transistor is driven hard, the storage time, which is proportional to the base current, increases and the switching speed is reduced.

The storage time can be reduced by operating the transistor in soft saturation instead of hard saturation.

This can be accomplished by clamping the collector-emitter voltage to a pre-determined level and the collector current is given by

C

cmCCC R

VVI

−=

Clamping voltage

)(satCEcm VV >

Cut-off ActiveSaturation

Soft Hard

VCE

IB


BC II β=After IC rises, transistor turns on, and clamping takes place (due to the fact that diode D2 gets forward biased and conducts)

21 ddBECE VVVV −+=C

ddBECC

C

CECCL R

VVVV

R

VVI 21 +−−

=−

=

( )LLCBC IIIIIII ++

=+−== 11 1)(

ββββ

B

BEdBB R

VVVII

−−== 1

1

Without clamping, the base current is adequate to drive transistor hard.

C

E IE

ICIBRB

+

_

RC

VCC

+_VB

C2

Collector clamping circuit (Baker’s Clamp)

Vd2+ _

Vd1+ _

I2=IC-IL IL

VBE

VCEI1


For clamping, Vd1>Vd2 and this can be accomplished by connecting two or more diodes in place of D1.

Load resistance RC should satisfy LB II >β

C

ddBECCL R

VVVVI 21 +−−=Since 21 ddBECCCB VVVVRI +−−>β

Clamping action results in a reduced collector current and almost elimination of storage time.

At the same time, a fast turn-on is accomplished.

However, due to increased VCE, the on-state power loss is increased, whereas the switching power loss is decreased.


Example 17.1: Finding the transistor voltage and current with clamping

The base drive has VCC=100V, RC=1.5Ω, Vd1=2.1V, Vd2=0.9V, VBE=15V, RB=2.5Ω, and β=16.

Calculate:– Collector current without clamping

– Collector-emitter voltage VCE

– Collector current with clamping


Solution

AR

VVVII

B

BEdBB 88.4

5.2

7.01.21511 =

−−=

−−==

Without clamping

AII BC 08.7888.416 =×== βClamping voltage

VVVVV ddBECE 9.19.01.27.021 =−+=−+=

With clamping

AR

VVI

C

CECCL 4.65

5.1

9.1100=

−=

−=

( ) ( ) AIII LC 15.664.6588.4116

16

1 1 =++

=++

=β

β


Key pointsKey points

A BJT is a current controlled device

Base current peaking can reduce the turn-on time and reversing the base current can reduce the turn-off time

The storage time of a BJT increases with the amount of base drive current, and overdrive should be avoided.


ISOLATION OF GATE & BASE ISOLATION OF GATE & BASE DRIVESDRIVES

For power transistors, the control voltage should be applied between the gate and the source terminals or between the base and emitter terminals.

Power converters generally require multiple transistors and each transistor must be gated individually.


Ground terminal

Generates 4 pulses

Common terminal for pulses


Terminal g1, which has a voltage of Vg1 with respect to Ccannot be connected directly to gate terminal G1.Vg1 should be applied between gate terminal G1 and source terminal S1 of transistor M1.There is a need for isolation and interfacing circuits between the logic circuit and power transistors.However, M2 and M4 can be gated directly without isolation or interfacing circuits if logic signals are compatible with gate drive requirements of the transistors.


The importance of gating a transistor between its gate and source rather than applying gating voltage between the gate and common ground can be demonstrated with the following figure.

Load resistance

)( GSDLGGS VIRVV −=

ID(VGS) varies with VGS

VGS when transistor turns on and reaches a steady-state value (required to balance the load drain current)


The effective value of VGS is thus unpredictable and such an arrangement is not suitable.

There are basically two ways of floating or isolating the control or gate signal with respect to ground:– Pulse transformers

– Optocouplers



Key point

The low-level gate circuit must be isolated from the high level power circuit through isolation devices or techniques such as optocouplers and pulse transformers.


THYRISTOR FIRING CIRCUITTHYRISTOR FIRING CIRCUIT

In thyristor converters, different potentials exist at various terminals. The power circuit is subjected to a high voltage, usually greater than 100 V, and the gate circuit is held at a low voltage, typically 12 to 30 V. An isolation circuit is required between an individual thyristor and its gate-pulse generating circuit.


The isolation can he accomplished by either pulse transformers or optocouplers.

An optocoupler could be a phototransistor or photo-silicon-controlled rectifier (SCR).

A short pulse lo the input of a LED, D1 turns on the photo-SCR T1; and the power thyristor TL is triggered.

This type of isolation requires a separate power supply Vcc and increases the cost and weight of the firing circuit.


ShortShort--Pulse:Pulse:When a pulse of adequate voltage is applied to the base of Q1,the transistor saturates and the dc voltage Vcc appears across the transformer primary, inducing a pulsed voltage on the transformer secondary.

When the pulse is removed, Q1 turns off and a voltage of opposite polarity is induced across the primary and the freewheeling diode Dm conducts. The current due to the transformer magnetic energy decays through Dm to zero. During this transient decay a corresponding reverse voltage is induced in the secondary.


LongLong--Pulse:Pulse:The pulse width can be made longer by connecting a capacitor C cross the resistor R.

The transformer carries unidirectional current and the magnetic core can saturate, thereby limiting the pulse width. This type of pulse isolation is suitable for pulses of typically 50 to 100 μs.


Pulse train:Pulse train:

In many power converters with inductive loads, the conduction period of a thyristor depends on the load power factor (PF); therefore, the beginning of thyristor conduction is not well defined.

In this situation, it is often necessary to trigger the thyristors continuously.

However, a continuous gating increases thyristor losses.

A pulse train that is preferable can be obtained with an auxiliary winding


When transistor Q1 is turned on, a voltage is also induced in the auxiliary winding N3 at the base of transistor Q1, such that diode D1 is reverse biased and Q1 turns off.

In the meantime, capacitor C1 charges up through R1 and turns on Q1 again.

This process of turn-on and turn-off continuous as long as there is an input signal v1 to the isolator.


In-stead of using the auxiliary winding as a blocking oscillator, an AND-logic gale with an oscillator (or a timer) could generate a pulse train. In practice, the AND gate cannot drive transistor Q1 directly, and a buffer stage is normally connected before the transistor.


The output of gate circuits is normally connected between the gate and cathode along with other gate-protecting components.

The resistor Rg in (a) increases the dvldt capability of the thyristor, reduces the turn-off time, and increases the holding and latching currents.

The capacitor Cg in (b) removes high-frequency noise components and increases dv/dt capability and gate delay lime.


The diode D1 (c) protects the gate from negative voltage. However, for asymmetric silicon-controlled rectifiers, SCRs, it is desirable to have some amount of negative gate voltage to improve the dvldtcapability and also to reduce the turn-off time.

All these features can be combined as shown in (d), where diode D1 allows only the positive pulses and R1 damps out any transient oscillation and limits the gate current.


Key points

Applying a pulse signal turns on a thyristor

The low-level gate circuit must be isolated from the high-level power circuit through isolation techniques

The gate should be protected from triggering by a high frequency or interference signal.


THYRISTOR CONVERTER GATING THYRISTOR CONVERTER GATING CIRCUITSCIRCUITS

The generation of gating signals for thyristors of ac-dc converters requires:– Detecting zero crossing of the input voltage

– Appropriate phase shifting of signals

– Pulse shaping to generate pulses of short duration

– Pulse isolation through pulse transformer or optocouplers



DRIVE IC FOR CONVERTERSDRIVE IC FOR CONVERTERSThere are numerous IC gate drives that are commercially available for gating power converters.These include – Pulse-width-modulation (PWM) control, – Power factor correction (PFC) control, – Combined PWM and PFC control– Current mode control– Bridge driver – Servo driver, half-bridge drivers, stepper motor

driver, thyristor gate driver– Etc…


These ICs can be used for applications such as: – Buck converters for battery chargers– Dual forward converters for switched

reluctance motor drives – Full-bridge inverter with current-mode control– Three-phase inverter for brushless and

induction motor drives– Push-pull bridge converter for power

supplies– Synchronous PWM control of switched-mode

power supplies (SMPSs)


An IC gate drive integrates most of the control functions including some protection functions to operate under overload and fault conditions.The especial purpose ICs for motor drives include many features such as – gate driving with protection, – soft start charging of DC bus, – linear current sensing of motor phase

current, and – control algorithms for V/Hz to sensorless

vector or servo control.


SUMMARYSUMMARY

MOSFETs are voltage-controlled devices requiring low gating power.

Gate signals can be isolated from the power circuit by pulse transformers or optocouplers.

BJTs are current-controlled devices requiring reverse base current during turn-off to reduce storage time, but they have low on state or saturation voltage.

A mean of isolation between power circuit and gate circuit is necessary.


The pulse transformers are simple, but leakage inductance should be very small. The transformers may be saturated at low frequency and long pulse. Optocouplers require separate power supply. For inductive loads, a pulse train reduces thyristor loss and is normally used for gating thyristors, instead of a continuous pulse.There are numerous drive ICs for drives that are commercially available for gating power converters. These ICs integrate logic, gate isolation, protection, and control functions. As a result, discrete gate circuits have become obsolete.

Documents

ADVANCED POWER ELECTRONICS - ape.gastli.infoape.gastli.info/Chapter2/APE_CH2.pdf · Dr. Adel Gastli Firing Circuits 5 Time 2.6us 3.0us 3.5us 4.0us 4.5us 5.0us 5.4us ID(M1)*5 V(M1:d)