DN82

DN82
Start up switches for switch mode power supplies
Andy Aspinall, Systems Engineer, Zetex Semiconductors
The use of Zetex medium voltage MOSFETS for switch mode power supplies
High voltage start up
Many topologies of switching power supplies power the control circuits from an auxiliary tap on
the main switching transformer or coil. This method avoids the need for costly house keeping
supplies. However the adoption of this scheme presents the designer with the challenge of
creating the initial voltage on the control circuit at start up. Any continuous delivery of current
from a high voltage line will cause unwanted dissipation and loss of efficiency. Such a scheme
may even take a system out of operational specification for low energy standby applications and
may even degrade reliability figures.
The main requirement for the design of this start up supply is to provide a high enough current
to raise the control IC supply and additionally charge any associated smoothing capacitance. At
a pre determined voltage the controller will initiate the first few pulses into the main power supply
switch transistor. Once this occurs the supply will become self sustaining and the start up path
can be switched off.
With start up currents of the order of tens of milliamps and high voltages across the device power
dissipations can be high for many milliseconds while start up occurs.
For AC line operated systems the typical requirement is 450V VDs devices and for 48V DC-DC
systems 100 to 200V is usually required.
D3
High voltage
Output
C2
R1
Q1
VGS
D2
VGS
D1
C1
VCC
Contol
IC
Q2
Figure 1
Issue 1 - September 2006
© Zetex Semiconductors plc 2006
High voltage MOSFET start up circuit
1
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DN82
Operation
Figure 1 shows the MOSFET start up circuit where the MOSFET is biased on by a low current in
R1 to form a resistive pass element. C1 charges up to the point where the control IC becomes
active, the maximum value of VCC in this phase are limited by Zener voltage D1 minus VGS.
Once the control IC becomes active drive pulses are issued to Q2 to form normal switching action
in the transformer and power is now fed to the control IC via D2. It is therefore important for the
designer to chose a value for D1 so that the supply via D2 is high is greater than D1 minus VGS
thus biasing off Q1 in normal operation.
D4
High voltage
Output
C2
R2
R1
Q1
Q3
D2
D3
Vcc
C3
D1
Figure 2
C1
Contol
IC
Q2
Improved high voltage MOSFET start up circuit
In addition to the basic requirements fault protection may be added such that if the control IC
supply can not be raised by the start up circuit a time out occurs to prevent excessive dissipation
in the start up MOSFET. By using an additional transistor a simple circuit could be constructed as
shown in Figure 2.
Figure 2 shows the modified circuit with a time out function to prevent Q1 remaining on and over
heating if the power supply can not start. Q3 is simply turned on after a delay from the time
constant of R2 and C3. This circuit provides additional protection under all operating conditions
and allows the use of smaller devices than if constant on time of Q1 in a fault mode had to be
designed for.
Zetex manufactures a range of medium voltage MOSFETS and bipolar transistors suitable for use
in this application contained in small outline packages with some offering additional pin spacing
for creep age distance compliancy.
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2
Issue 1 - September 2006
© Zetex Semiconductors plc 2006
DN82
Table 1
MOSFETS suitable for start up circuits
Device
Applications Type
Package
VDS (V)
ZXMN0545G4
Universal
N-channel
SOT223
450
140
0.60
2
ZVN2120G
Lower line
voltage
N-channel
SOT223
200
320
2.00
2
ZXMN10A07Z
Telecoms
N-channel
SOT89
100
1500
4.20
1.5
ZXMN10A07F
Telecoms
N-channel
SOT23
100
1500
3.50
0.625
ZXMN10A08E6
Telecoms
N-channel
SOT23-6
100
2500
8.6
1.7
Dual Nchannel
DN8
100
2600
9.00
1.25
ZXMN10A08DN8 Telecoms
ID (mA) IDPulse(A) P (W)(*)
D
NOTES:
(*) Based on 25 x 25mm FR4
S
Gap
D
N/C
Gap
2x
Gap
S
S
Gap
Gap
D
D
G
D
D
G
G
SOT223
(ZXMN0545G4)
SOT89
SOT223
(ZVN2120G)
Gap
Gap
S
D
G
SOT23
D1
G1
D1
Gap
S2
D2
S
G2
D2
D
D
D
G
SOT23-6
Figure 3
Issue 1 - September 2006
© Zetex Semiconductors plc 2006
S1
D
SO8
MOSFETS suitable for start up circuits
3
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DN82
The use of Zetex medium voltage bipolar transistors in switch mode power
supplies
D4
High voltage
R1
Output
C2
Q1
D2
D3
Vcc
D1
Contol
IC
C1
Q2
Figure 4
High voltage bipolar start up circuit
Operation
Figure 4 shows the Zetex bipolar start up circuit where the bipolar Q1 is biased on by a low current
in R1 to form a resistive pass element. C1 charges up to the point where the control IC becomes
active. The maximum value of VCC in this phase is limited by Zener voltage D1 minus VBE of Q1
and the VF of D2.
Once the control IC becomes active drive pulses are issued to Q2 to form normal switching action
in the transformer and power is now fed to the control IC via D3. It is therefore important for the
designer to chose a value for D1 so that the supply via D2 is high is greater than D1 minus VBE Q1
thus biasing off Q1 in normal operation.
D4
High voltage
C2
Q1
R1
R2
Output
Q3
D2
D3
Vcc
C3
D1
C1
Contol
IC
Q2
Figure 5
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Improved high voltage bipolar start up circuit
4
Issue 1 - September 2006
© Zetex Semiconductors plc 2006
DN82
In addition to the basic requirements fault protection may be added such that if the control IC
supply can not be raised by the start up circuit a time out occurs to prevent excessive dissipation
in the start up bipolar. By using a small outline Zetex transistor a simple circuit could be
constructed as shown in Figure 5.
Figure 5 shows the same circuits with a time out function to prevent Q1 remaining on and over
heating if the power supply can not start. Q3 is simply turned on after a delay from the time
constant of R2 and C3. This circuit provides additional protection under all operating conditions
and allows the use of smaller devices than if constant on time of Q1 had to be designed for.
Zetex manufactures a range of medium voltage bipolar transistors suitable for use in this
application contained in small outline packages.
Table 2
Device
Examples of bipolar transistors suitable for start up circuits
Applications Type
FMMT459 Telecoms
FZT493
Bipolar
Line voltage Bipolar
Package
VCEO (V)
IC (mA)
SOT23
500
150
50
0.625
SOT223
100
1000
2000
2
ICPulse(mA) P (W)(*)
D
NOTES:
(*) Based on 25 x 25mm FR4 board
Issue 1 - September 2006
© Zetex Semiconductors plc 2006
5
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DN82
Notes:
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© Zetex Semiconductors plc 2006
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