10.3.4 Calculating Efficiency, and Junction Temperature

The complete LM2735-Q1 DC/DC converter efficiency (η) can be calculated in the following manner.

Power loss (P_LOSS) is the sum of two types of losses in the converter, switching and conduction. Conduction losses usually dominate at higher output loads, where as switching losses remain relatively fixed and dominate at lower output loads.

Losses in the LM2735-Q1 device:

Conversion ratio of the boost converter with conduction loss elements inserted:

If the loss elements are reduced to zero, the conversion ratio simplifies to:

And this is known:

Therefore:

Calculations for determining the most significant power losses are discussed below. Other losses totaling less than 2% are not discussed.

A simple efficiency calculation that takes into account the conduction losses is shown below:

The diode, NMOS switch, and inductor DCR losses are included in this calculation. Setting any loss element to zero simplifies the equation.

V_D is the forward voltage drop across the Schottky diode. It can be obtained from the manufacturer’s Electrical Characteristics section of the data sheet.

The conduction losses in the diode are calculated as follows:

Depending on the duty cycle, this can be the single most significant power loss in the circuit. Take care to choose a diode that has a low forward voltage drop. Another concern with diode selection is reverse leakage current. Depending on the ambient temperature and the reverse voltage across the diode, the current being drawn from the output to the NMOS switch during time D could be significant, this may increase losses internal to the LM2735-Q1 and reduce the overall efficiency of the application. See the data sheets of the Schottky diode manufacturer for reverse leakage specifications; and, typical applications within this data sheet for diode selections.

Another significant external power loss is the conduction loss in the input inductor. The power loss within the inductor can be simplified to:

The LM2735-Q1 conduction loss is mainly associated with the internal NFET:

The value for should be equal to the resistance at the junction temperature you wish to analyze. As an example, at 125°C and V_IN = 5 V, R_DSON = 250 mΩ (see Typical Characteristics for value).

Switching losses are also associated with the internal NMOS switch. They occur during the switch on and off transition periods, where voltages and currents overlap resulting in power loss.

The simplest means to determine this loss is to empirically measuring the rise and fall times (10% to 90%) of the switch at the switch node:

Quiescent power losses: I_Q is the quiescent operating current, and is typically around 4 mA