Datasheet

( )
OL-WC
4 Ω 2.5 Ω
P = 0.5 20 kHz 650 nC 15 V ( 5V) + = 63 mW
4 Ω + 10Ω 2.5 Ω + 10 Ω
æ ö
´ ´ ´ - - ´
ç ÷
è ø
G
15V ( 5V)
R = = 10 Ω
2A
- -
CC2 EE-P
G
ON-PK
V V
R =
I
-
V
CC2
V
C
V
OUT
V
EE-P
ISO5500
15 V
R
G
C
G
15 V
I
on
V
CC2
- V
EE-P
V
CC2
V
C
V
OUT
V
EE-P
ISO5500
15 V
R
G
C
G
15 V
I
off
V
CC2
- V
EE-P
V
E
V
E
ISO5500
SLLSE64C –SEPTEMBER 2011–REVISED JUNE 2013 www.ti.com
DETERMINING GATE RESISTOR, R
G
The value of the gate resistor determines the peak charge and discharge currents, I
ON-PK
and I
OFF-PK
. Due to the
transient nature of these currents, their peak values only occur during the on-to-off and off-to-on transitions of the
gate voltage. In order to calculate R
G
for the maximum peak current, r
on
and r
off
must be assumed zero. The
resulting charge and discharge models are shown in Figure 70.
Figure 70. Simplified Gate Charge and Discharge Model
Off-to-On Transition
In the off-state, the upper plate of the gate capacitance, C
G
, assumes a steady-state potential of –V
EE-P
with
respect to V
E
. When turning on the power device, V
CC2
is applied to V
OUT
and the voltage drop across R
G
results
in a peak charge current of I
ON-PK
= (V
CC2
– V
EE-P
)/R
G
. Solving for R
G
then provides the necessary resistor value
for a desired on-current via:
(7)
On-to-Off Transition
When turning the power device off, the current and voltage relations are reversed but the equation for calculating
R
G
remains the same.
Once R
G
has been calculated, it is necessary to check whether the resulting, worst-case power consumption,
P
OD-WC
, (derived in Equation 6) is below the calculated maximum, P
OL
= 125 mW (calculated in Equation 5).
Example
The example below considers an IGBT drive with the following parameters:
I
ON-PK
= 2 A, Q
G
= 650 nC, f
INP
= 20 kHz, V
CC2
= 15V, V
EE-P
= –5 V
Applying Equation 7, the value of the gate resistor is calculated with
(8)
Then, calculating the worst-case output power consumption as a function of R
G
, using Equation 6 yields
(9)
Because P
OL-WC
= 63 mW is well below the calculated maximum of P
OL
= 125 mW, the resistor value of R
G
=
10Ω is fully suitable for this application.
32 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links :ISO5500