ADP3629/ADP3630/ADP3631

Rev. 0 | Page 13 of 16

THERMAL CONSIDERATIONS

When designing a power MOSFET gate drive, the maximum

power dissipation in the driver must be considered to avoid

exceeding the maximum junction temperature.

Data on package thermal resistance is provided in Table 3 to

help the designer in this task.

Several equally important aspects must also be considered.

•

Gate charge of the power MOSFET being driven

•

Bias voltage value used to power the driver

•

Maximum switching frequency of operation

•

Value of external gate resistance

•

Maximum ambient (and PCB) temperature

•

Type of package

All of these factors influence and limit the maximum allowable

power dissipated in the driver.

The gate of a power MOSFET has a nonlinear capacitance

characteristic. For this reason, although the input capacitance

useful to calculate power losses.

The total gate charge necessary to turn on a power MOSFET

This parameter varies from a few nanocoulombs (nC) to several

or 4.5 V).

The power necessary to charge and then discharge the gate of a

power MOSFET can be calculated as follows:

where:

by the number of drivers (in this case, 1 or 2) being used in each

charging and discharging the gates of the power MOSFETs.

Not all of this power is dissipated in the gate driver because

The larger the external gate resistor, the smaller the amount of

power that is dissipated in the gate driver.

In modern switching power applications, the value of the gate

resistor is kept at a minimum to increase switching speed and

to minimize switching losses.

In all practical applications where the external resistor is in the

order of a few ohms, the contribution of the external resistor

can be ignored, and the extra loss is assumed to be in the driver,

providing a good guard band for the power loss calculations.

In addition to the gate charge losses, there are also dc bias losses

regardless of the switching frequency.

where n is the number of gates driven.

When the total power loss is calculated, the temperature

increase can be calculated as follows:

Design Example

For example, consider driving two IRFS4310Z MOSFETs with a

ADP3630 in the MSOP package.

The maximum PCB temperature considered for this design is 85°C.

The MSOP thermal resistance is 162.2°C/W (see Table 3).

This estimated junction temperature does not factor in the

power dissipated in the external gate resistor and, therefore,

provides a certain guard band.

If a lower junction temperature is required by the design,

the SOIC_N package, which provides a thermal resistance

of 110.6°C/W, can be used. Using the SOIC_N package, the

maximum junction temperature is

Other options to reduce power dissipation in the driver include

frequency, and choosing a power MOSFET with a smaller gate

charge.