Dynamic parameters are related to the switching speed and the energy required to gate driver.

The intrinsic capacitances of a MOSFET (C_iss, C_oss, C_rss) decide the switching behavior. Figure 2-1 shows the switching process of a usual Boost device low-side MOSFET and then the role of these capacitances is explained.

Figure 2-1 MOSFET Switching Waveforms

How capacitances affect switching speed: when the MOSFET is turning on, the gate driver must charge C_iss (input capacitance) to turn the MOSFET on first (from t₀ to t₂). As V_GS reaches the plateau voltage, the driver current is consumed to discharge C_rss, which falls down the V_DS (from t₂ to t₃). A larger total gate charge (Q_g) and capacitance require more drive current and time to switch.

Usually there are two negative impacts with larger capacitance:

1. Increased switching losses: slower switching transitions increase the time during which high voltage and high current overlap (t₁ to t₃ and t₆ to t₈ in Figure 2-1), leading to high switching losses (P_sw) and low efficiency.
2. Risk of shoot-through: if the switch turns off too slowly, there is not be enough time to prevent both the high-side and low-side switches from being on simultaneously. This shoot-through condition creates a short circuit across the output voltage rail, and there is a high risk of destroying the MOSFET and the Boost controller.

For controllers with adjustable dead time, the dead time must be set longer than the sum of the turn-off delay of the switch and fall time to avoid shooting-through. While for controllers with fixed dead time like TPS61287, the selected MOSFET must have a sufficiently low Q_g or junction capacitance to verify the switching time is shorter than the fixed dead time.

Figure 2-2 Switching Behavior with Poor Dynamic Characteristic

Figure 2-3 Switching Behavior with Good Dynamic Characteristics

Figure 2-2 and Figure 2-3 clearly demonstrates the negative impact of high dynamic parameters on switching speed and the risk of shooting-through.

However, the MOSFET junction capacitance is not as small as possible. Too small a junction capacitance can cause the switching speed to be too fast, increasing the SW voltage spikes (transient voltage at nanosecond level), causing the MOSFET and chip overvoltage stress. For safety of TPS61287, TI recommends that engineers control the voltage spike of the SW below 36V.

Figure 2-4 SW Voltage Spike Measurement with Low-Inductance Probe.

If the SW voltage spike is too high due to the selection of a MOSFET with a very small junction capacitance, TI recommends series a drive resistor (RDRV) at the gate of the MOSFET to slow the drive speed and thus reduce the SW voltage spike.

Figure 2-5 Gate Drive Resistor to Slow the Drive Speed and Reduce the SW Voltage Spike