Application Note: AN-400 IX2127 Design Considerations

Application Note: AN-400
INTEGRATED CIRCUITS DIVISION
IX2127
Design Considerations
AN-400-R03
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AN-400
INTEGRATED CIRCUITS DIVISION
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High-Side Gate Driver Using the IX2127
This application note provides general guidelines for
designing a desaturation detection circuit, and
selecting bootstrap components for use with IXYS
Integrated Circuits Division’s IX2127.
The IX2127 is a high-voltage, high-speed driver
optimized to drive IXYS high voltage power MOSFETs
and IGBTs. The IXYS high voltage SOI process and
the IX2127 high voltage level shifters allow this driver to
Figure 1
VCC
operate at up to 600V. In addition, proprietary common
mode techniques insure stable operation in high dV/dt
noise environments.
The IX2127 features an on-board comparator that
detects an over-current condition in the external power
switch, and then terminates the drive to that switch. An
open drain output, FAULT, indicates to the system
controller that an over-current shutdown has occurred.
IX2127 Block Diagram
Low Side
VCC
VB
High Side
Undervoltage Lockout
HO
Buffer
Data Latch
Transmitter
IN
Low-High
Level Shift
Receiver
VS
Enable
Blanking
Signal
Delay
FAULT
Q
Enable
R
Receiver
COM
S
High-Low
Level Shift
Transmitter
+
Data Latch
CS
_
Comparator
2
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AN-400
INTEGRATED CIRCUITS DIVISION
Figure 2
Application Circuit Diagram
VCC
IN
FAULT
2
Rbs
Dbs
1 VCC
VB
8
2
IN
HO
7
3 FAULT
CS
6
4 COM
VS
5
Cbs
Simple Bootstrap Circuit Operation
The bootstrap circuit is a simple and inexpensive way
to provide power to the high side driver circuitry. It
consists of resistor, diode and capacitor. The sequence
of bootstrap charging is as follows. When Vs is pulled
below VCC or is pulled down to ground by the load, the
bootstrap capacitor begins to charge through the
resistor and bootstrap diode from the VCC supply. This
charge continues until VS is pulled up to a higher
voltage than VCC by the external high side power
MOSFET. VBS (the difference voltage between VB and
VS) starts to float, and the bootstrap diode begins to
reverse bias and block the high rail voltage.
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Bootstrap Capacitor
The bootstrap capacitor is selected based on the
maximum allowable voltage drop when the high side
switch is on. This voltage is determined by the power
MOSFET’s minimum VGS voltage which is required to
keep the high-side switch turned on, a parameter
usually given in the MOSFET datasheet. The capacitor
voltage drop can be expressed by:
V boot  V CC –  V fbs + V GS  min  
Where:
• VCC = Supply voltage
• Vfbs = Bootstrap Diode Forward Voltage Drop
• VGS(min) = Minimum Gate to Source Voltage of
Power MOSFET
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The minimum value of the bootstrap capacitor is given
by the formula:
C bs = Q total  V boot
Where Qtotal is the total amount of charge supplied by
the capacitor.
The total charge can be expressed as follows:
Q total = Q g +  I lkcap + I lkgs + I qbs + I lk + I Cbs  leak    t on + Q ls
Where:
• Qg = Total gate charge of power MOSFET or
IGBT
• Ilkgs = MOSFET gate to source leakage current
(given in MOSFET datasheet as IGSS)
• Ilkcap = Bootstrap capacitor leakage current (can
be ignored if a ceramic capacitor is used)
• Iqbs = Bootstrap circuit quiescent current
• Ilk = Bootstrap circuit leakage current
• Qls = Charge of IX2127 internal level shifter (5nC)
• ton = High-side switch-on time
• ICbs(leak) = Bootstrap leakage current
The bootstrap capacitor voltage is determined by the
VCC power supply, and in this case a 25V to 50V
capacitor should be adequate.
Note that the leakage current in a ceramic capacitor is
very low, and can be ignored in most calculations. The
use of electrolytic capacitors is strongly discouraged
due to their contribution of excessive leakage currents
at higher temperatures.
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AN-400
INTEGRATED CIRCUITS DIVISION
3.1
•
•
•
•
•
•
Example:
Calculate the bootstrap capacitor value based on the
following circuit components, an operating frequency of
20 kHz, and at a Duty Cycle of 50%:
•
•
•
•
•
IXTA5N60P - Power MOSFET
MURS160-13-F- Bootstrap Diode
VCC = 12V
Qgate = 14.2nC maximum
Ilkgs = 100nA maximum
–9
Ilk cap = 0
Iqbs = 1000uA
Ilk = 50uA
Qls = 5nC
ton = 25uS
ICbs(leak) = 10nA
If the maximum allowable voltage drop on the capacitor
is 1V during the high-side switch-on state, then the
minimum capacitor value can be calculated:
–9
Q total =  14.2 10  +  100 10
–3
–6
+ 1 10 + 50 10
= 31.25nC
–9
–9
For instance, if Rbs = 3, Cbs = 0.1uF, and D = 10%,
then the time constant can be evaluated:
The value of the bootstrap capacitor is:
Cbs = Qtotal / Vboot = 31.25nC / 1V = 31 nF
 =  3  0.1   0.1
= 3s
Note that the voltage drop of the bootstrap diode
contributes about 0.8V to the total voltage drop
The value obtained from the above equation is the
absolute minimum required. In some cases, though, a
low value capacitor can cause overcharging and ripple
on Vbs so the value obtained from the equation should
be evaluated in an application circuit, and increased if
required.
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–6
+ 10 10    25 10  +  5 10 
Bootstrap Resistor
The bootstrap resistor, Rbs , can be used to control
charging current into the bootstrap capacitor. The value
of this resistor has to be selected very carefully so that
the bootstrap capacitor is able to charge fully under all
operating conditions.
This resistor will introduce a voltage drop that can be
approximated with this formula:
V drop =  I chg – R bs   t chg
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Bootstrap Diode
The bootstrap diode provides a voltage-charging path
for the bootstrap capacitor, and also serves to block the
full power rail voltage. This diode must have a very fast
recovery time in order to minimize the amount of
charge injected back from the bootstrap capacitor to
VCC. Here are some parameters that should be looked
at when selecting this diode:
• Vrrm = Power Rail Voltage
• trr = 100nS or less
• If = Qtotal x Frequency
Note that reverse recovery time (trr) is a function of the
forward diode current.
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Desaturation Detection Circuit
The voltage across the MOSFET will increase during
an over-current condition; hence, we can use a basic
circuit consisting of R1, D2, R2 and R3 to detect this
overload condition.
Where,
• Ichg = Bootstrap capacitor charging current
• Rbs = Bootstrap resistor
• tchg =Bootstrap capacitor charging time
In practice Rbs will be a small value of up to 10. Let's
look closer at the time constant that depends on Rbs ,
Cbs , and the duty cycle, D, of the switching power
switch.
R1 is selected to be 10k. This value minimizes Miller
capacitance from D2, and ensures that there is no
significant current being drawn from the high-side HO
output pin. Due to the presence of the high voltage rail,
D2 can be the same as DBS.
 =  R bs  C bs   D
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AN-400
INTEGRATED CIRCUITS DIVISION
6.1
Example:
Let R2=33k, and Solve for Vx:
VX = VA   R3   R2 + R3  
Calculate component values in the event of an 8V
overload condition (use the circuit in Figure 3):
0.26V = 9.2V   R 3   33k + R 3  
This calculation is based on a D2 maximum voltage
drop of 1.2V. The voltage at VA can be expressed as:
0.26V   33k + R 3  = 9.2V  R 3
VA = Vdiode + VDS
8580 + 0.25V  R 3 = 9.2V  R 3
VA = 1.2V + 8V
8580 = 8.94  R 3
R 3 = 960
VA = 9.2V
The CS threshold is 260mV; hence, we need to divide
VA so that when VA=9.2V, then VX=260mV.
Figure 3
Saturation Detection Circuit
R
D
bs
bs
V
A
IX2127
1
VCC
V
B
R
2
8
C
PWM
2
IN
HO
7
µController
3
FAULT
CS
6
4
COM
V
S
5
R
HV BUS
2
1
bs
V
R
g
X
R
7
D
3
To Load
Gate Resistor
Rg is the gate resistor, which is chosen to optimize
switching speed and losses. Let's select a gate resistor
that will provide100nS switching time at 12VCC . In this
example case, the IXTA5N60P MOSFET is selected.
From the IXTA5N60P and IX2127 datasheets we get
the following parameters:
•
•
•
•
•
R total =  V CC – V gs  th     I gate  avg  
R total = 12V CC – 5.5V gs  th   0.096A
= 68
The driver-on resistance can be approximated:
R driver  on  = V CC  I OH+
VCC=12V
Qgs=4.8nC
Qgd=4.8nC
Vgs(th)=5.5V
tSW=100ns
Typical IOH+ is listed as 250mA in the IX2127 data
sheet.
R driver  on  = 12V CC  250mA
= 48
We define average gate current as:
I gate  avg  =  Q gs + Q gd   t SW
I gate  avg  =  4.8nC + 4.8nC    100ns 
= 0.096A
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The total resistance can be expressed:
The difference of Rtotal and Rdriver(on) will yield a 20
resistor that will provide 100ns switching time at
12VCC.
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AN-400
INTEGRATED CIRCUITS DIVISION
8
Layout Considerations
Proper layout techniques are important when designing
high-speed gate drivers. The decoupling capacitors,
the bootstrap resistor, and the gate resistor, Rg, should
be close to the gate driver.
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Specification: AN-400-R03
©Copyright 2013, IXYS Integrated Circuits Division
All rights reserved. Printed in USA.
6/24/2013
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