L6726A_07 Datasheet, PDF (11/24 Pages) STMicroelectronics – Single phase PWM controller

L6726A
Driver section
5.1
Power dissipation
L6726A embeds high current MOSFET drivers for both high side and low side MOSFETs: it
is then important to consider the power that the device is going to dissipate in driving them
in order to avoid overcoming the maximum junction operative temperature.
Two main terms contribute in the device power dissipation: bias power and drivers power.
● Device Bias Power (P_DC) depends on the static consumption of the device through the
supply pins and it is simply quantifiable as follow (assuming to supply HS and LS
drivers with the same VCC of the device):
P_DC = V_CC ⋅ (I_CC + I_BOOT)
● Drivers power is the power needed by the driver to continuously switch on and off the
external MOSFETs; it is a function of the switching frequency, the voltage supply of the
driver and total gate charge of the selected MOSFETs. It can be quantified considering
that the total power P_SW dissipated to switch the MOSFETs (easy calculable) is
dissipated by three main factors: external gate resistance (when present), intrinsic
MOSFET resistance and intrinsic driver resistance. This last term is the important one
to be determined to calculate the device power dissipation. The total power dissipated
to switch the MOSFETs results:
P_SW = F_SW ⋅ [Q_gHS ⋅ (V_BOOT – V_PHASE) + Q_gLS ⋅ V_CC]
where V_BOOT - V_PHASE is the voltage across the bootstrap capacitor.
External gate resistors helps the device to dissipate the switching power since the same
power P_SW will be shared between the internal driver impedance and the external resistor
resulting in a general cooling of the device.
Figure 4. Soft start (left) and disable (right)
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