DN30

Design Note 30
Issue 1 February 1996
High Speed Turn-off Circuit for PNP Pass Transistors
Cost-Effective replacement of P-Channel MOSFETs
L2
ZTX788B
Input
+Bat
Q3
150µH
Q1
1K
2N3904
Figure 1
Active Turn-off Circuit for High
Current, Low Vce(sat) PNP.
1N5820
L1
10µH
10µF
1N4148
Q2
2N3904
Drive
Feedback
68
The turn-off circuit shown in Figure 1 can
be used with an efficient PNP transistor
as a cost effective alternative to
P-Channel MOSFETs in step-down
(Buck) converters.
It is well appreciated that optimised
Bipolar transistor technologies can
display lower on-state loss than similar
sized (or indeed much larger) MOSFETs,
thereby leading to lower cost switch
options. [This is essentially due to the
pattern of current flow within the
structure, and allows relatively small die
geometries to display current
capabilities far in excess of like packaged
MOSFET options.]
However at high switching frequencies,
the turn-off times of the Bipolar device,
and particularly the storage time portion
of the turn-off transition, can lead to
significant switching power losses,
limiting the product’s operation at
frequencies much above 70kHz. The
MOS FET (being a majority carrier
device) does not exhibit storage time
effects, and so simpler passive turn-off
networks can be employed at
low-medium switching frequencies
(~100kHz) providing turn-on and turn-off
times in tens of nano-seconds. (Note :
Though for optimum performance, and
espec ially at higher s witching
frequencies, attention must be given to
the control of gate charge, and the gate
drive circuitry must be capable of
sourcing/sinking high charge/discharge
currents.)
DN 30 - 1
Design Note 30
Issue 1 February 1996
The ZTX788B is one member of the
ZTX788B - 796A Super-β PNP family,
which are also available in the SOT223
surface mount package.
The addition of a few inexpensive
components to effect this turn-off circuit
allows low loss operation to 150kHz and
beyond. Comparison of turn-off times
with passive turn-off circuitry, and with
this modification show a dramatic
improvement. Storage time particularly
being reduced from typically 500ns (see
Note 1 below) to several nano-seconds,
and the entire turn-off event being
measured at 50ns.
Note 1: Measured on a simple PWM IC
controlled, 12 to 5V,100kHz converter,
biased for 1A output, and the PNP
operated with a forced gain of 50.
When a cost analysis is performed, it will
be evident that this option has much in
it’s favour - a cost saving of x2 to x4
being possible.
PARAMETER
SYMBOL
VALUE
UNIT
Collector-Base Voltage
V CBO
-15
V
Collector-Emitter Voltage
V CEO
-15
V
Emitter-Base Voltage
VEBO
-5
V
Peak Pulse Current
ICM
-8
A
Continuous Collector Current
IC
-3
A
Practical Power Dissipation*
P totp
1.5
W
1
5.7
W
mW/°C
-55 to +200
°C
Power Dissipation at
Ptot
Tamb=25°C derate above 25°C
Operating and Storage
Temperature Range
tj:tstg
Collector-Emitter Saturation
Voltage
VCE(sat)
Static Forward Current
Transfer Ratio
hFE
MIN.
MAX.
-0.15
-0.25
-0.45
500
400
300
150
1500
CONDITIONS.
V
V
V
IC=-0.5A, IB=-2.5mA*
IC=-1A, IB=-5mA*
IC=-2A, IB=-10mA*
IC=-10mA, V CE=-2V*
IC=-1A, VCE=-2V*
IC=-2A, VCE=-2V*
IC=-6A, VCE=-2V*
Table 1
Maximum Ratings and Sample Parametric Data for ZTX788B Transistor - (For full details
please refer to the datasheet).
DN 30 - 2
Design Note 30
Issue 1 February 1996
High Speed Turn-off Circuit for PNP Pass Transistors
Cost-Effective replacement of P-Channel MOSFETs
L2
ZTX788B
Input
+Bat
Q3
150µH
Q1
1K
2N3904
Figure 1
Active Turn-off Circuit for High
Current, Low Vce(sat) PNP.
1N5820
L1
10µH
10µF
1N4148
Q2
2N3904
Drive
Feedback
68
The turn-off circuit shown in Figure 1 can
be used with an efficient PNP transistor
as a cost effective alternative to
P-Channel MOSFETs in step-down
(Buck) converters.
It is well appreciated that optimised
Bipolar transistor technologies can
display lower on-state loss than similar
sized (or indeed much larger) MOSFETs,
thereby leading to lower cost switch
options. [This is essentially due to the
pattern of current flow within the
structure, and allows relatively small die
geometries to display current
capabilities far in excess of like packaged
MOSFET options.]
However at high switching frequencies,
the turn-off times of the Bipolar device,
and particularly the storage time portion
of the turn-off transition, can lead to
significant switching power losses,
limiting the product’s operation at
frequencies much above 70kHz. The
MOS FET (being a majority carrier
device) does not exhibit storage time
effects, and so simpler passive turn-off
networks can be employed at
low-medium switching frequencies
(~100kHz) providing turn-on and turn-off
times in tens of nano-seconds. (Note :
Though for optimum performance, and
espec ially at higher s witching
frequencies, attention must be given to
the control of gate charge, and the gate
drive circuitry must be capable of
sourcing/sinking high charge/discharge
currents.)
DN 30 - 1
Design Note 30
Issue 1 February 1996
The ZTX788B is one member of the
ZTX788B - 796A Super-β PNP family,
which are also available in the SOT223
surface mount package.
The addition of a few inexpensive
components to effect this turn-off circuit
allows low loss operation to 150kHz and
beyond. Comparison of turn-off times
with passive turn-off circuitry, and with
this modification show a dramatic
improvement. Storage time particularly
being reduced from typically 500ns (see
Note 1 below) to several nano-seconds,
and the entire turn-off event being
measured at 50ns.
Note 1: Measured on a simple PWM IC
controlled, 12 to 5V,100kHz converter,
biased for 1A output, and the PNP
operated with a forced gain of 50.
When a cost analysis is performed, it will
be evident that this option has much in
it’s favour - a cost saving of x2 to x4
being possible.
PARAMETER
SYMBOL
VALUE
UNIT
Collector-Base Voltage
V CBO
-15
V
Collector-Emitter Voltage
V CEO
-15
V
Emitter-Base Voltage
VEBO
-5
V
Peak Pulse Current
ICM
-8
A
Continuous Collector Current
IC
-3
A
Practical Power Dissipation*
P totp
1.5
W
1
5.7
W
mW/°C
-55 to +200
°C
Power Dissipation at
Ptot
Tamb=25°C derate above 25°C
Operating and Storage
Temperature Range
tj:tstg
Collector-Emitter Saturation
Voltage
VCE(sat)
Static Forward Current
Transfer Ratio
hFE
MIN.
MAX.
-0.15
-0.25
-0.45
500
400
300
150
1500
CONDITIONS.
V
V
V
IC=-0.5A, IB=-2.5mA*
IC=-1A, IB=-5mA*
IC=-2A, IB=-10mA*
IC=-10mA, V CE=-2V*
IC=-1A, VCE=-2V*
IC=-2A, VCE=-2V*
IC=-6A, VCE=-2V*
Table 1
Maximum Ratings and Sample Parametric Data for ZTX788B Transistor - (For full details
please refer to the datasheet).
DN 30 - 2
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