Datasheet

( ) ( )
OFF ON
SW g REF SW
OFF GATE ON GATE
R R
P Q V f
R R R R
æ ö
= ´ ´ ´
ç ÷
ç ÷
+ +
è ø
2 2
G LOAD REF SW g REF SW
P C V f Q V f= ´ ´ = ´ ´
2
G LOAD REF SW
P C V f= ´ ´
2
LOAD REF
1
EG C V
2
= ´ ´
DISS DC SW
P P P´ +
UCC27611
www.ti.com SLUSBA5B –DECEMBER 2012
Power Dissipation
Power dissipation of the gate driver has two portions as shown in equation below:
(1)
The DC portion of the power dissipation is P
DC
= I
Q
x V
DD
where I
Q
is the quiescent current for the driver. The
quiescent current is the current consumed by the device to bias all internal circuits such as input stage, reference
voltage, logic circuits, protections etc and also any current associated with switching of internal devices when the
driver output changes state (such as charging and discharging of parasitic capacitances, parasitic shoot-through
etc). The UCC27611 features very low quiescent currents (less than 1 mA, refer ELECTRICAL
CHARACTERISTICS) and contains internal logic to eliminate any shoot-through in the output driver stage. Thus
the effect of the P
DC
on the total power dissipation within the gate driver can be safely assumed to be negligible.
The power dissipated in the gate-driver package during switching (P
SW
) depends on the following factors:
• Gate charge required of the power device (usually a function of the drive voltage V
G
, which is very close to
input bias supply voltage V
REF
due to low V
OH
dropout).
• Switching frequency.
• Use of external gate resistors.
When a driver device is tested with a discrete, capacitive load it is a fairly simple matter to calculate the power
that is required from the bias supply. The energy that must be transferred from the bias supply to charge the
capacitor is given by:
where
• Where C
LOAD
is load capacitor and VDD is bias voltage feeding the driver. (2)
There is an equal amount of energy dissipated when the capacitor is charged. This leads to a total power loss
given by the following:
where
• where ƒ
SW
is the switching frequency. (3)
The switching load presented by a power MOSFET and IGBT can be converted to an equivalent capacitance by
examining the gate charge required to switch the device. This gate charge includes the effects of the input
capacitance plus the added charge needed to swing the drain voltage of the power device as it switches between
the ON and OFF states. Most manufacturers provide specifications of typical and maximum gate charge, in nC,
to switch the device under specified conditions. Using the gate charge Q
g
, one can determine the power that
must be dissipated when charging a capacitor. This is done by using the equation, Q
G
= C
LOAD
x V
DD
, to provide
the following equation for power:
(4)
This power P
G
is dissipated in the resistive elements of the circuit when the MOSFET and IGBT is being turned
on or off. Half of the total power is dissipated when the load capacitor is charged during turnon, and the other
half is dissipated when the load capacitor is discharged during turnoff. When no external gate resistor is
employed between the driver and MOSFET and IGBT, this power is completely dissipated inside the driver
package. With the use of external gate-drive resistors, the power dissipation is shared between the internal
resistance of driver and external gate resistor in accordance to the ratio of the resistances (more power
dissipated in the higher resistance component). Based on this simplified analysis, the driver power dissipation
during switching is calculated as follows:
where
• where R
OFF
= R
OL
and R
ON
(effective resistance of pull-up structure) = 2.7 x R
OL
(5)
Copyright © 2012, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: UCC27611