Switch-Mode Power Supplies for Beginners: An Efficiency Primer Part 1

May 13, 2008 10:04 AM
By Daniel Wagner, Associate Member of the Technical Staff, Applications, and Roger M. Kenyon, Director, Customer Applications Engineering, Maxim Integrated Products, Chandler, Ariz.

Switching Component Losses

The MOSFET and the diode depicted in Fig. 2 (and in most other basic converter topologies) tend to cause the largest degradation of efficiency due to the nature of these semiconductor devices. Both are victims of two varieties of power loss: conduction loss and switching loss.

The easier of the two to understand is conduction loss. Intuitively, where there is current flowing, there will naturally be an opposition to the current, and energy is expended as a result. Both the MOSFET and the diode act as switches that route current through the circuit when either device is on during each switching interval. Hence, a conduction loss will be generated when that particular device is on, due to the on-resistance of the MOSFET (R_DSon) and the forward voltage of the diode.

Since MOSFET current flows only while it is on, MOSFET conduction loss (P_CONDmosfet) is approximated by the product of R_DSon, duty cycle and the square of on-state current:

P_CONDmosfet = I_{MOSFETon(avg)}² x R_DSon x D

where I_{MOSFETon(avg)} is the average MOSFET current over the on-interval. Referring to the stepdown converter in Fig. 2, the MOSFET conducts output current (I_OUT) while it is on, resulting in an estimated MOSFET conduction loss of:

P_CONDmosfet = I_OUT² x R_DSon x (V_OUT / V_IN)

While MOSFET conduction loss is proportional to the dissipation across its low R_DSon, diode conduction loss is dependant on a comparatively larger forward voltage (V_F). Thus, diodes usually have a larger conduction loss than MOSFETs. Diode conduction loss is proportional to forward current, VF and conduction time. Since the diode will conduct when the MOSFET is off, diode conduction loss (P_CONDdiode) is approximated as:

P_CONDdiode = I_DIODEon(avg) x V_F x (1-D)

where I_DIODEon(avg) is the average diode current over the on-interval. In Fig. 2, average diode forward current is I_OUT during its conduction interval. Therefore, P_CONDdiode for a stepdown converter is estimated as:

P_CONDdiode = I_OUT x V_F x (1 - V_OUT / V_IN)

By observing these equations, it is evident that the longer either device is on during each switching interval, the larger that device’s relative conduction loss. For a stepdown converter, the lower the output voltage is set (for a constant input voltage), the more the diode contributes to power loss, since it conducts for more of the switching interval.

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