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13

RD-0608

If a MOSFET is used for the lower switch, reverse recovery losses are incurred during the dead time after the low

side MOSFET has been on and conducting current. During this dead time the low side MOSFET is off, but the body

diode is freewheeling and providing current to the load. Since the body diode is conducting current, carriers are

present and will eventually need to be recombined leading to reverse recovery delay. When the high side MOSFET

turns on the VS node is very rapidly pulled from COM to VBUS and the low side MOSFET or the freewheeling

diode conducts current from VS to ground due to the reverse recovery effect resulting in power losses. This can

result in overheating of the low side switching component and component stress, as can be seen in Fig. 17 to 18.

Since the power diode has a much shorter reverse recovery time the diode will conduct current for a significantly

shorter period and exhibit lower power losses. At lower frequency and reduced load current, the long recovery time

associated with the MOSFET body diode may not be an issue. For higher frequency higher current applications a

diode could provide lower power losses with respect to a MOSFET.

With a low side MOSFET fitted to the IRPLLED1 evaluation board reverse recovery current peaks 8A were

measured. This resulted in a temperature rise which could be reduced by replacing the low side MOSFET with an

appropriate freewheeling diode. The reverse recovery current peaks were reduced to 4.5A. The frequency was also

selected to keep the diode reverse recovery associated power losses low.

With the inclusion of a freewheeling diode instead of a low side MOSFET comes the need for an alternative means

of pre-charging the bootstrap capacitor CBOOT at startup. RS3 and RS4 provide a current path from the DC bus to

VB and from the output to COM in order to accomplish this. After the IRS2540/1/01/11 begins to switch and the

high side Buck MOSFET switches off after the first cycle, the voltage at VS transitions to 0V/COM due to the

freewheeling action of the lower diode. this allows CBOOT to charge through DBOOT in the normal way and this

operation is repeated every cycle. RS4 represents only a small power loss if the output voltage is below 30V,

however for higher output voltages a low side MOSFET is necessary.

Fig. 17 Using a low side FET, Vin = 100V, Iout = 1.5A, Vout = 17V