Power Dissipation Considerations

(Continued)

The bootstrap diode power loss is the sum of the forward

bias power loss that occurs while charging the bootstrap

capacitor and the reverse bias power loss that occurs during

reverse recovery. Since each of these events happens once

per cycle, the diode power loss is proportional to frequency.

Larger capacitive loads require more current to recharge the

bootstrap capacitor resulting in more losses. Higher input

voltages (V

recovery losses. The following plot was generated based on

calculations and lab measurements of the diode recovery

time and current under several operating conditions. This

can be useful for approximating the diode power dissipation.

Diode Power Dissipation V

20088806

Diode Power Dissipation V

20088807

The total IC power dissipation can be estimated from the

previous plots by summing the gate drive losses with the

bootstrap diode losses for the intended application. Because

the diode losses can be significant, an external diode placed

in parallel (refer to Figure 4) with the internal bootstrap diode

can be helpful in removing power from the IC. For this to be

effective, the external diode must be placed close to the IC to

minimize series inductance and have a significantly lower

forward voltage drop than the internal diode.

20088808

FIGURE 4. LM5101 Driving MOSFETs Connected in Half-Bridge Configuration

www.national.com

10