SI8285 Datasheet, PDF (11/32 Pages) Silicon Laboratories – System Safety Features

Si8285/86 Data Sheet
Applications Information
3.3 Power Dissipation Considerations
Proper system design must assure that the Si828x operates within safe thermal limits across the entire load range. The Si828x total
power dissipation is the sum of the power dissipated by bias supply current, internal parasitic switching losses, and power dissipated by
the series gate resistor and load. Equation 1 shows total Si828x power dissipation.
PD = (VDDA)(IDDA) + (VDDB)(IDDB) +
f
× Q_int
× VDDB +
1
2
(
f
)(^QIGBT
)^(VDDB)
Rp
Rp + RH
+
Rn
Rn + RL
where:
PD is the total Si828x device power dissipation (W).
IDDA is the input-side maximum bias current (5 mA).
IDDB is the driver die maximum bias current (5 mA).
Q_int is the internal parasitic charge (3 nC).
VDDA is the input-side VDD supply voltage (2.7 to 5.5 V).
VDDB is the total driver-side supply voltage (VDDB + VSSB: 12.5 to 30 V).
f is the IGBT switching frequency (Hz).
RH is the VH external gate resistor, RL is the VL external gate resistor. For Si8286, RG works for both RH and RL.
RP is the RDS_(ON) of the driver pull-up switch: (2.6 Ω).
Rn is the RDS_(ON) of the driver pull-down switch: (0.8 Ω).
Equation 1
The maximum power dissipation allowable for the Si828x is a function of the package thermal resistance, ambient temperature, and
maximum allowable junction temperature, as shown in Equation 2:
PDmax
≤
Tjmax − TA
θja
where:
PDmax = Maximum Si828x power dissipation (W).
Tjmax = Si828x maximum junction temperature (150 °C).
TA = Ambient temperature (°C)
θja = Si828x junction-to-air thermal resistance (60 °C/W for four-layer PCB)
f = Si828x switching frequency (Hz)
Equation 2
Substituting values for PDmax Tjmax (150 °C), TA (125 °C), and θja (90 °C/W) into Equation 2 results in a maximum allowable total
power dissipation of 0.42 W.
PDmax
≤
150 − 125
60
= 0.42W
Maximum allowable load is found by substituting this limit and the appropriate data sheet values from Table 4.1 into Equation 1 and
simplifying. The result is Equation 3.
PD = (VDDA)(IDDA) + (VDDB)(IDDB) +
f
× Qint
× VDDB +
1
2
(
f
)(^QL
)^(VDDB)
Rp
Rp + RH
+
Rn
Rn + RL
( )( )( ) ^{PD = (VDDA)(IDDA) + (VDDB)(IDDB) + f}
× Qint
× VDDB +
1
2
f
C_L
VDDB²
Rp
Rp + RH
+
Rn
Rn + RL
( )( )( ) ^{0.42 = (VDDA)(0.005) + (VDDB)(0.004) + f}
×
3
×
10^−9
×
VDDB
+
1
2
f
C_L
VDDB²
2.6
2.6 + 15
+
0.8
0.8 + 10
0.42 − (VDDA + VDDB)5 × 10^−3 − f × 3 × 10^−9 × VDDB = 0.111VDDB² f ₍C_{L )}
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