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.

Perhaps a bit less intuitive are MOSFET and diode switching losses, which arise due to the non-ideality of their switching characteristics. Time is required for the devices to transition from fully off to fully on, and vice versa, and that translates into power consumption as the devices change state.

A simplified plot of drain-to-source voltage (V_DS) and drain-to-source current (I_DS) is usually given to explain the switching loss encountered in MOSFETs. The upper plot in Fig. 3 depicts such waveforms, where non-instantaneous voltage and current transitions occur during t_SWon and t_SWoff due to the charging and discharging of capacitances found in the MOSFET.

As indicated in the plots, full load current (I_D) must be transferred to the MOSFET before its V_DS decreases to its final on-state value (= I_D x R_DSon). Conversely, the turn-off transition demands that V_DS increase to its final off-state value before current is transferred from the MOSFET. These transitions result in overlap of the voltage and current waveforms and lead to the power dissipation seen in the lower plot in Fig. 3.

Switching transition times are more or less constant over frequency, causing switching loss to increase as the frequency of the SMPS is raised. This can be understood by noting that the constant-transition periods consume more of the available switching period as that switching period shrinks.

A switching transition that requires only one-twentieth of the duty cycle will have much less of an effect on efficiency than one that consumes one-tenth of the duty cycle. Due to its frequency dependence, switching loss dominates conduction losses at high frequencies.

MOSFET switching loss (P_SWmosfet) is estimated by applying triangular geometry to Fig. 3 to achieve following equation:

P_SWmosfet 0.5 x V_D x I_D x (t_SWon + t_SWoff) x f_s

where V_D is the drain-source voltage of the MOSFET during off-time, I_D is the channel current during on-time, and t_SWon and t_SWoff are the turn-on and turn-off transition times, respectively. For the stepdown converter, V_IN is applied across the MOSFET during the off-state, and it carries I_OUT while it is on.

To demonstrate the aforementioned MOSFET conduction and switching loss equations, an oscilloscope was used to capture the V_DS and I_DS waveforms of a typical integrated high-side MOSFET in a stepdown converter. The conditions of the circuit were as follows: V_IN = 10 V, V_OUT = 3.3 V, I_OUT = 500 mA, R_DSon = 0.1 Ω, f_S = 1 MHz, and the switching transient (t_ON + t_OFF) is 38 ns.

As can be seen in Fig. 4, switching is not instantaneous, and the overlap in the current and voltage waveforms results in a power loss indicated by the lower waveform. The current waveform is ramped since I_DS follows inductor current for the "on" cycle (Fig. 2), resulting in more switching loss occurring during the "off" transient.

Using the previously mentioned approximations, the total average MOSFET loss is calculated:

P_Tmosfet = P_CONDmosfet + P_SWmosfet

= I_OUT² x R_DSon x (V_OUT/V_IN) + 0.5 x V_IN x I_OUT x (t_SWon + t_SWoff) x f_s

= 0.5² x 0.1 x 0.33 + 0.5 x 10 x 0.5 x (38 x 10^-9) x 1 x 10⁶

8.3 mW + 95 mW

P_Tmosfet = 103.3 mW

The result is consistent with the average value of 117.4 mW of the lower trace. Note that in this case, f_S is high enough that P_SWmosfet dominate conduction losses.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.