Driving Transistors And Thyristors

DRIVING TRANSISTORS AND THYRISTORS

•

Functionality of Gate/Base Drive Turn power switch from off­state to on­state Circuits • Minimize turn­on time through active region where power dissipation is large • Provide adequate drive power to keep power switch in on­state

• Turn power switch from on­state to off­state • Minimize turn­off time through active region wherepower dissipation is large • Provide bias to insure that power switch remains off • Control power switch to protect it when overvoltages or overcurrents are sensed • Signal processing circuits which generate the logic control signals not considered part of the drive circuit • Drive circuit amplifies control signals to levels required to drive power switch  • Drive circuit has significant power capabilities compared to logic level signal processing circuits • Provide electrical isolation when needed between power switch and logic level signal processing/control circuits

Drive Circuit Design Considerations • Drive circuit topologies • Output signal polarity ­ unipolar or bipolar • AC or DC coupled • Connected in shunt or series with power switch • Output current magnitude • Large Ion shortens turn­on time but lengthens turn­off delay time • Large Ioff shortens turn­off time but lengthens turn­on delay time • Provisions for power switch protection • Overcurrents • Blanking times for bridge circuit drives

t

0

Unipolar

0

• Waveshaping to improve switch performance • Controlled diB/dt for BJT turn­off • Anti­saturation diodes for BJT drives • Speedup capacitors • Front­porch/backporch currents   • Component layout to minimize stray inductance and shielding from switching  noise

t

Bipolar

Unipolar DC­coupled Drive Circuit ­  BJT Example •

Circuit operation • V control > Vreference - BJ T at comparator output on which puts Qpnp and Qsw on •

•

V control < Vreference - BJ T at comparator output off which turns Qpnp off and thus Qsw off

V

V d

BB

Design procedure V BE,off

•

R2 = IB,off

•

Ipnp = IB,on +

•

value of Io. V BB = VCE,on(Qpnp) + R1 IC,pnp + VBE,on(Qsw) V BB = 8 to 10 V ; compromise between larger values which

•

; IB,off based on desired turn-off time. V BE,on R2

; IB,on value based on BJ T beta and

minimize effects of V BE variations and smaller values which minimize power dissipation in drive circuit

Comparator Vcontrol

Q pnp R1 R

Vreference

Io

2

Q sw

Unipolar DC­coupled Drive Circuits­ MOSFET  examples V

V d

BB

Comparator V control

Io

R1 R2

V control

> V reference

comparator output high and Q sw on • V control

Q sw

-

•

< V reference

comparator output low and Q sw off

+ V reference

V GG

V d Io

Comparator Vcontrol

R1

+

R

G

Q sw

• V control

comparator output high putting Q npn on and thus Q sw on • V control

< V reference

comparator output low putting Q pnp on and thus Q sw off

Vreference V

> Vreference

V

GG

d

Io V control

IC buffer amp with totem pole output DS0026 or UC1706/07

R

G

Q sw

Bipolar DC­coupled Drive Circuit­ BJT Example

V d

V BB+ Comparator

R

Df

B

Vcontrol

T

+ -

B+

CBB+ Qsw

T BC V

BB-

Io

BB-

• Vcontrol < Vreference ­ comparator output  low, TB­ on and  Qsw off. • Large reverse base current flows to  minimize turn­off time and base­emitter  of Qsw reversed biased to insure off­ state. • Vcontrol > Vreference ­ comparator output  high, TB+ on and Qsw on. • Large forward base current to minimize  turn­on time and to insure saturation of  Qsw for low on­state losses

Bipolar DC­coupled Drive Circuit­ MOSFET Example V d

VGG+ Comparator T

Vcontrol

Io

D f B+

C

+ R

V reference

Qsw

G

T BC V

GG-

GGV d

V GG+

Io

C GG+ V control

• Bipolar drive with substantial output  current capability

GG+

IC buffer amp with totem pole output DS0026 or UC1706/07 C GGV GG-

R

G

Q sw

• Simple bipolar drive circuit with  moderate (1 amp)output current  capability

Methods of Control Signal Isolation Input to remainder of isolated drive circuit

Logic level control ckts

Power switch reference node (BJT emitter, MOSFET source)

Logic ground Lightemitting diode

V

BB+

Signal from control logic Control logic ground

• Transformer isolation

Isolated dc supply

Input to remainder of isolated drive circuit

• Opto­coupler isolation

Power switch reference node Photo-transistor

V

BB+

AC power in

• Isolated dc power supplies  for drive circuits V BB-

GATE DRIVE DESIGN

Qg 3 − Qg 2 Vgg tdoff = Rg ⋅ n Vgg − Vg 2 Vg 2

Thyristor Gate Drive Circuit 1

4

3

2

Line Voltage

DC power supply for gate trigger circuit

zero crossing detection

Input Control Signal

Delay Angle Block

Control Logic Ground

gate pulse isolation transformers

Pulse Amplifier

gate pulse isolation transformers

Delay angle block is  commercially available  integrated circuit ­  TCA780 circuit family

Thyristor Gate Drive Circuit (cont.) Transformer Line Voltage

α

α

α

Ramp

Control voltage

Thyristor gate drive waveforms

Control of 1&2 Control of 3&4

D1 15 V D

f

Trigger signal

R

G

T

G

Gate pulse amplifier

GTO Gate Drive Circuit R

R

2 T

R

T

G2

G1

10 A pulse

1 R

4

R

5

V 6

GG+

2A Lσ 1 R

Control Circuit

3

L

Lσ 2

G V

R

GG-

7 T

G3 turn-off pulse

Auxilliary power supply for gate drive circuit

• Turn on TG1and TG2 to get  large front­porch current • Turn off TG1 after some  specified time to reduce total  gate current to back­porch  value.