Small Capacity Inverter using IGBT.pdf
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Application Note 9017
June, 2001
Manufacturing Technology of a Small Capacity Inverter
Using a Fairchild IGBT
by Kee Ju Um, Yeong Joo Kim
CONTENTS
1.
Introduction..........................................................................................2
2.
How to choose gate resistance ...........................................................2
3.
Design technique of protection circuit ................................................8
4.
Usage of the gate drive IC with a boot strap circuit...........................19
5.
Example of an inverter design with SGP5N60RUFD ........................22
6.
Conclusion .........................................................................................27
1/28
Rev. A, May 2001
1. Introduction
The IGBT, previously used only in large power circuits for industrial use, is now being increas-
ingly used in general products. This is especially true in the household electronics arena
where mid-to-small size motors are used, and where high-quality and high efficiency in power
consumption is required. The IGBTs features make it ideal for this market. Fairchild Semicon-
ductors’ IGBTs are superior in many aspects. The fact that low tail current at turn off and low
saturation voltage in the on-state can reduce IGBT loss is an unprecedented and big advan-
tage of many Fairchild IGBTs.
Because of the low tail current feature the Fairchild IGBTs can turn off quickly. This reduces
the switching loss in high speed operations. This enables IGBT designs with fast switching
speeds and without the need for a separate cooling apparatus. The low saturation voltage
reduces conduction loss, resulting in the reduction of overall power loss. Furthermore, a short
circuit rated IGBT can be used easily in a variety of application circuits because it can with-
stand at least 10[
sec] under any short-circuit situations. This application note describes the
technology for producing a small capacity inverter using the superior features of the Fairchild
IGBT. This application note comprehensively covers all information needed to design a small
capacity inverter. Because this inverter is for household electronic applications, this applica-
tion note focuses on producing a low cost inverter. Section 2 details the items which must be
considered when deciding on the gate resistance. Section 3 deals with general items in the
small capacity inverter related to driving the motor. It describes the over-current protection
design in the inverter appropriate for driving the motor, and the short circuit protection circuit to
protect the IGBT. Section 4 introduces the boot strap used in the inverter gate drive. It also
describes the method for deciding on the rating of each component. These values can be
used in other application circuits. Lastly, Section 5 describes the design of an actual 3[kVA]
class inverter based on the information discussed in the previous sections
2. How to choose a gate resistance
The following figure is the output circuitry of the three phase inverter to be designed in this
application note.
VDD
Dbs1
Dbs2
Dbs3
+
VCC
VCC
VCC
Ron1
Ron3
Ron5
VCC
VB
HO
VCC
VB
HO
VCC
VB
HO
DC
Input
Supply
VDD
D1
D3
D5
Roff1
Roff3
Roff5
HIN3
HIN1
HIN2
Q3
Q1
Q5
HIN
HIN
HIN
C1
C2
C3
Ua
Ub
Uc
VS
VS
VS
Ron4
Ron6
Ron2
LIN1
LIN2
LIN3
LIN
LO
LIN
LO
LIN
LO
D4
D6
D2
Roff4
Roff6
Roff2
Q6
Q2
Q4
VSS
COM
VSS
COM
VSS
COM
-
Rsense
Figure 2.1 Output Circuitry of a three phase Inverter
Normally, the inverter gate resistance is designed by separating it into the turn-on resistance
(R
ON
) and the turn-off resistance (R
OFF
), as shown in Figure 2.1. The selection methods for
each resistance are in respect to the gate drive IC and to the IGBT, respectively. The value of
the gate resistance can be selected from the intersection of the possible gate drive resistance
range with respect to these two methods.
2/28
Rev. A, May 2001
2.1 Selection of the Resistance Value Considering the Gate Drive IC Drive
Capability
The value of the gate resistance, R
G
, connected to the gate drive IC output is determined
based on the peak current (I
ON
, I
OFF,PEAK
) which is charged and discharged between the gate
and the emitter. The maximum current which can electrically charge the gate oxide between
the gate and the emitter is determined based on the maximum source current of the gate
drive IC. On the other hand, the maximum current which can discharge the gate oxide is deter-
mined based on the maximum sink current of the gate drive IC. The minimum gate resistance
value, R
G
, is, in turn, determined based on these determined maximum charge and discharge
currents.
Determination of the Minimum R
G
Based on the Maximum Drive Capability of the Gate
Drive IC
If we assume that the equivalent capacitance (C
GE
) between the IGBT emitter and gate has
discharged to the gate drive turn off voltage (V
OL
) to obtain the minimum value of the turn on
resistance R
ON
and that the capacitance has charged to the gate drive turn on voltage (V
OH
)
to obtain the minimum value of the turn off resistance R
OFF
, the following relationships can be
derived from Ohm’s Law. .
–
–
=
----------------------------
¥²¨
=
----------------------------
u
Where,
R
O
MIN
=
Minimum value of the turn on resistance,
MIN
R
OFF
=
Minimum value of the turn off resistance,
V
OH
=
Turn on output maximum voltage of the gate drive IC
V
OL
=
Turn off output minimum voltage of the gate drive IC
MAX
I
SOURCE
=
Maximum output source currnet of the gate drive IC
MAX
I
SINK
=
Maximum output sink current of the gate drive IC
Because the maximum sink current is generally greater than the source current in the gate
drive IC output
, the selected gate resistance must be greater than
if
the turn on and turn off resistances are not separated and used as one.
Calculation of the Power Loss in the Gate Drive IC
The total power loss (P
T
) in the gate drive can be calculated from the following equation. .
=
+
ª
v
=
+
+
where,
P
BIAS
: Power consumed to bias the elements in the IC in normal state
P
SWITCH
: Switching loss associated with turning on and off the IGBT gate
E
SWITCH
: Average loss (
J/cycle)in the IC during one cycle of turning on and off.
f
SWITCH
: Average turn off frequency
V
CC
: + power supply voltage
V
EE
: - power supply voltage
I
CC
: Average + power supply current in normal state
I
EE
: Average - power supply current in normal state
If the gate resistance is lowered, the E
SWITCH
value decreases, and switching speed
increase. If the switching speed is increased, P
SWITCH
value decreases. The total power loss
calculated from the above equation should not exceed the maximum value identified in the
data book.
3/28
Rev. A, May 2001
2.2 Selection of the Resistance value considering the IGBT Drive Conditions
The relationship between the switching loss and the gate resistance
The IGBT is designed such that the MOSFET driven by the gate turns on the output bipolar
TR. Because the determined maximum current between the drain and source of the MOSFET
is proportional to the gate voltage, the IGBT turn on characteristic is affected by the magnitude
of the voltage and current applied to the gate. This turn on characteristic significantly affects
the IGBT turn on loss. Therefore, if the magnitude of the turn on resistance is reduced, the
internal MOSFET turns on quickly, and as a result, the switching loss also reduces. The follow-
ing figure shows each of the waveforms which are turned on when the gate resistance is
changed in the Fairchild IGBT SGP5N60RUFD.
Test Condition: L = 78[
C], V
OH
= 15[V]
V
CC
= 80[V] (20V/DIV), I
C
= 3[A] (1A/DIV)
H], R = 21.2[
], T = 25[
Power Loss
I
C
V
CE
R
ON
= 40[
]
Figure 2.2.1
4/28
Rev. A, May 2001
Power Loss
I
C
V
CE
R
ON
= 80[
]
Figure 2.2.2
Power Loss
I
C
V
CE
R
ON
= 120[
]
Figure 2.2.3
Figure 2.2 Turn on waveforms of SGP5N60RUFD when the gate resistance is changed.
5/28
Rev. A, May 2001
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