ISO5451DW datasheet(26/38 Pages) TI

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(

)

on-max

off-max

OL-WC

INP

CC2

EE2

on-max

off-max

= 0.5

+ R

´ ç

ISO5451

SLLSEO1B – JUNE 2015 – REVISED DECEMBER 2015

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10.2.2.8 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 dVCE/dt voltage ramp rate across the

IGBT. This results in a charging current I(CHARGE) = C(D-DESAT) x dVCE/dt, charging the blanking capacitor. C(D-DESAT)

is the diode capacitance at DESAT.

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).

Because the sum of the DESAT diode forward-voltage and the IGBT collector-emitter voltage make up the

voltage at the DESAT-pin, VF + VCE = V(DESAT), the VCE level, which triggers a fault condition, can be modified by

adding multiple DESAT diodes in series: VCE-FAULT(TH) = 9 V – n x VF (where n is the number of DESAT diodes).

When using two diodes instead of one, diodes with half the required maximum reverse-voltage rating may be

chosen.

10.2.2.9 Determining the Maximum Available, Dynamic Output Power, POD-max

The ISO5451 maximum allowed total power consumption of PD = 251 mW consists of the total input power, PID,

the total output power, POD, and the output power under load, POL:

PD = PID + POD + POL

(1)

With:

PID = VCC1-max × ICC1-max = 5.5 V × 4.5 mA = 24.75 mW

(2)

and:

POD = (VCC2 – VEE2) x ICC2-max = (15V – ( –8V)) × 6 mA = 138 mW

(3)

then:

POL = PD – PID – POD = 251 mW – 24.75 mW – 138 mW = 88.25 mW

(4)

In comparison to POL, the actual dynamic output power under worst case condition, POL-WC, depends on a variety

of parameters:

where

•

fINP = signal frequency at the control input IN+

•

QG = power device gate charge

•

VCC2 = positive output supply with respect to GND2

•

VEE2 = negative output supply with respect to GND2

•

ron-max = worst case output resistance in the on-state: 4Ω

•

roff-max = worst case output resistance in the off-state: 2.5Ω

•

RG = gate resistor

(5)

Once RG is determined, Equation 5 is to be used to verify whether POL-WC < POL. Figure 49 shows a simplified

output stage model for calculating POL-WC.

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