4 High Power Continuous Operation

The trend in many applications of bidirectional DC-DC converter is increasing power and output current levels. Multi-phase designs up to 3.6kW are not uncommon which can result in high current in each phase of 75A or higher. There are details in the operation that relate to the gate driver in steady state, high current operation that the designer must pay close attention to.

The timing of the low side MOSFET and high-side MOSFET switching has some dead time to prevent potential cross conduction of the MOSFET half-bridge. During the dead time when the low-side power MOSFET is off and the high-side power MOSFET is off the current flowing in the output inductor conducts through the body diode of the low side power MOSFET. During the dead time the voltage drop across the MOSFET body diode is higher than the voltage drop during the low side MOSFET on conduction which results in the switch node transitioning to a lower negative HS voltage level. This transition when the low side FET turns off and later the high-side MOSFET turns on results in details in the gate driver operation that are important to understand. Refer to the timing shown in Figure 4-1.

Figure 4-1 Detail Timing Diagram of Steady State HS Voltage and Boot Diode Current

The area of concern is the transition of the low side driver turn off, dead time and high-side driver turn on. When the low side driver and low side MOSFET is on the output inductor current is flowing through the MOSFET channel and the HS or switch node voltage is slightly negative based on the inductor current and MOSFET R_DSON. After the low side MOSFET turns off the inductor current flows through the low side MOSFET body diode which results in a more negative HS voltage based on the body diode forward drop. Referring to Figure 4-1, this increase in negative HS voltage causes the boot diode to start conducting to charge the HB capacitor back to the VDD- V_F of the boot diode resulting in boot diode forward current. The boot diode peak current during this time depends on the body diode forward drop and the boot diode dynamic resistance, which for a body diode drop of 1.5V and boot diode resistance of 1Ω can result in a 1.5A boot diode forward current. During the dead time the HO output of the driver transitions high to initiate the turn on of the high-side MOSFET which discharges the HB-HS capacitor to charge the high-side MOSFET Q_G. This causes some voltage drop on the bootstrap capacitor which also results in some forward current in the boot diode to restore the charge on the bootstrap capacitor. These two events cause the boot diode to start conducting current at a time close to when the high-side MOSFET turns on resulting in the switch node transitioning high. If the forward current in the boot diode has not decreased sufficiently to a low level when the switch node transitions high, this results in high reverse recovery current in the boot diode when forced off, possibly causing stress or damage on the internal boot diode.

Figure 5-1 shows the body diode forward current Vs forward drop of a 100V low R_DSON MOSFET to illustrate the shift in negative HS voltage during the dead time. The red marker indicates the maximum voltage at 40A which is at cold temp. One thing to keep in mind is the HS negative voltage shift of interest is the difference between the channel conduction and body diode conduction. You can see in the graph in Figure 5-1 that the body diode drop increases at cold temperature which is also the condition where the R_DSON voltage drop decreases due to the negative temperature coefficient of the MOSFET R_DSON.