ISO5500_15 Datasheet, PDF (30/43 Pages) Texas Instruments – ISO5500 2.5-A Isolated IGBT, MOSFET Gate Driver

ISO5500
SLLSE64D – SEPTEMBER 2011 – REVISED JANUARY 2015
Typical Application (continued)
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1
V_IN+
ISO5500
2
V_IN-
3
V_CC1
µC
4
µC
GND1
5
RESET
6
FAULT
7
NC
8
GND1
1
V_IN+
ISO5500
2
V_IN-
3
V_CC1
4
GND1
5
RESET
6
FAULT
7
NC
8
GND1
Figure 67. Auto Reset with Prior Gate-low Assertion for Non-inverting and Inverting Input Configuration
9.2.2.8 DESAT Pin Protection
Switching inductive loads causes large instantaneous forward voltage transients across the freewheeling diodes
of IGBTs. These transients result in large negative voltage spikes on the DESAT pin which draw substantial
current out of the device. To limit this current below damaging levels, a 100 Ω to 1 kΩ resistor is connected in
series with the DESAT diode. The added resistance neither alters the DESAT threshold nor the DESAT blanking
time.
Further protection is possible through an optional Schottky diode, whose low forward voltage assures clamping of
the DESAT input to V_E potential at low voltage levels.
ISO5500 _VE ¹⁶
V_EE-L ¹⁵
DESAT ¹⁴
V_CC2 ¹³
V_C ¹²
V_OUT ¹¹
V_EE-L ¹⁰
V_EE-P ⁹
100 ⁺
pF
V_FW
-
D_S (opt.)
R_S
D_DESAT
15V
R_g
15V
-
V_FW-inst
+
Figure 68. DESAT Pin Protection with Series Resistor and Optional Schottky Diode
9.2.2.9 DESAT Diode and DESAT Threshold
The DESAT diode’s function is to conduct forward current, allowing sensing of the IGBT’s saturated collector-to-
emitter voltage, V_CESAT, (when the IGBT is "on") and to block high voltages (when the IGBT is "off"). During the
short transition time when the IGBT is switching, there is commonly a high dV_CE/dt voltage ramp rate across the
IGBT. This results in a charging current I_CHARGE = C_D-DESAT x d_VCE/dt, charging the blanking capacitor.
To minimize this current and avoid false DESAT triggering, fast switching diodes with low capacitance are
recommended. As the diode capacitance builds a voltage divider with the blanking capacitor, large collector
voltage transients appear at DESAT attenuated by the ratio of 1+ C_BLANK / C_D-DESAT.
Table 3 lists a number of fast-recovery diodes suitable for the use as DESAT diodes.
Because the sum of the DESAT diode forward-voltage and the IGBT collector-emitter voltage make up the
voltage at the DESAT-pin, V_F + V_CE = V_DESAT, the V_CE level, which triggers a fault condition, can be modified by
adding multiple DESAT diodes in series: V_CE-FAULT(TH) = 7.2 V – n x VF (where n is the number of DESAT
diodes).
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