Datasheet

OFF ON
SW G SW
OFF GATE ON GATE
R R
P 0.5 Q VDD f
R R R R
æ ö
= ´ ´ ´ ´ +
ç ÷
+ +
è ø
2
G LOAD DD SW g DD SW
P C V f Q V f= =
2
G LOAD DD SW
P C V f=
2
G LOAD DD
1
E C V
2
=
DISS DC SW
P P P= +
UCC27511
UCC27512
SLUSAW9E –FEBRUARY 2012–REVISED DECEMBER 2013 www.ti.com
Power Dissipation
Power dissipation of the gate driver has two portions as shown in Equation 1.
(1)
The DC portion of the power dissipation is P
DC
= I
Q
x VDD 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, 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 UCC27511 and UCC27512 features very low quiescent currents (less than 1 mA, refer Figure 7) 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 VDD due to low V
OH
drop-out).
• Switching frequency.
• Use of external gate resistors.
When a driver device is tested with a discrete, capacitive load calculating the power that is required from the bias
supply is fairly simple. The energy that must be transferred from the bias supply to charge the capacitor is given
by Equation 2.
Where
• C
LOAD
is load capacitor
• V
DD
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 Equation 3.
where
• ƒ
SW
is the switching frequency (3)
The switching load presented by a power MOSFET/IGBT is 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 Qg, determine the power that must be dissipated when
charging a capacitor which is calculated using the equation, Q
G
= C
LOAD
x V
DD
, to provide Equation 4 for power.
(4)
This power P
G
is dissipated in the resistive elements of the circuit when the MOSFET/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/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 in Equation 5.
where
• R
OFF
= R
OL
• R
ON
(effective resistance of pull-up structure) = 2.7 x R
OL
(5)
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