8.2.2.4 Power Dissipation

Power dissipation of the gate driver has two portions as shown in Equation 1:

Equation 1.

The DC portion of the power dissipation is P_DC = I_Q × 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, and so forth 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 and so forth). The UCC27611 device features very low quiescent currents (see 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 Equation 2:

Equation 2.

where

• C_LOAD is load capacitor of driver.

There is an equal amount of energy dissipated when the capacitor is charged. This leads to a total power loss given by Equation 3.

Equation 3.

where

• f_SW is the switching frequency.

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 × V_REF, to provide Equation 4 for power:

Equation 4.

This power P_G is dissipated in the resistive elements of the circuit when the MOSFET or 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 Equation 5:

Equation 5.

where

• R_OFF = R_OL and R_ON = 2.7 × R_OL (effective resistance of pullup structure).
• R_GH and R_GL is external gate resistors connect to the OUTH and OUTL pins respective.