SNVSB29C October 2018 – June 2021 LM5143-Q1
The choice of power MOSFETs has significant impact on DC/DC regulator performance. A MOSFET with low on-state resistance, RDS(on), reduces conduction loss, whereas low parasitic capacitances enable faster transition times and reduced switching loss. Normally, the lower the RDS(on) of a MOSFET, the higher the gate charge and output charge (QG and QOSS, respectively), and vice versa. As a result, the product of RDS(on) and QG is commonly specified as a MOSFET figure-of-merit. Low thermal resistance of a given package ensures that the MOSFET power dissipation does not result in excessive MOSFET die temperature.
The main parameters affecting power MOSFET selection in a LM5143-Q1 application are as follows:
The MOSFET-related power losses for one channel are summarized by the equations presented in Table 9-1, where suffixes 1 and 2 represent high-side and low-side MOSFET parameters, respectively. While the influence of inductor ripple current is considered, second-order loss modes, such as those related to parasitic inductances and SW node ringing, are not included. Consult the LM5143-Q1 Quickstart Calculator. The calculator is available for download from the LM5143-Q1 product folder to assist with power loss calculations.
|POWER LOSS MODE||HIGH-SIDE MOSFET||LOW-SIDE MOSFET|
|MOSFET gate drive(1)|
|MOSFET output charge(4)||Negligible|
The high-side (control) MOSFET carries the inductor current during the PWM on-time (or D interval) and typically incurs most of the switching losses, so it is imperative to choose a high-side MOSFET that balances conduction and switching loss contributions. The total power dissipation in the high-side MOSFET is the sum of the following:
The low-side (synchronous) MOSFET carries the inductor current when the high-side MOSFET is off (or 1–D interval). The low-side MOSFET switching loss is negligible as it is switched at zero voltage – current just commutates from the channel to the body diode or vice versa during the transition deadtimes. The LM5143-Q1, with its adaptive gate drive timing, minimizes body diode conduction losses when both MOSFETs are off. Such losses scale directly with switching frequency.
In high step-down ratio applications, the low-side MOSFET carries the current for a large portion of the switching period. Therefore, to attain high efficiency, it is critical to optimize the low-side MOSFET for low RDS(on). In cases where the conduction loss is too high or the target RDS(on) is lower than available in a single MOSFET, connect two low-side MOSFETs in parallel. The total power dissipation of the low-side MOSFET is the sum of the losses due to channel conduction, body diode conduction, and typically one-third of the net loss attributed to body diode reverse recovery. The LM5143-Q1 is well suited to drive TI's portfolio of NexFET™ power MOSFETs.