8.2.1.2.5 Calculating Efficiency and Junction Temperature

The complete LM26420-Q1 DC/DC converter efficiency can be estimated in the following manner.

or

The following equations show the calculations for determining the most significant power losses. Other losses totaling less than 2% are not discussed.

Power loss (P_LOSS) is the sum of two basic types of losses in the converter: switching and conduction. Conduction losses usually dominate at higher output loads, whereas switching losses remain relatively fixed and dominate at lower output loads. The first step in determining the losses is to calculate the duty cycle (D):

V_{SW_TOP} is the voltage drop across the internal PFET when it is on, and is equal to:

V_{SW_BOT} is the voltage drop across the internal NFET when it is on, and is equal to:

If the voltage drop across the inductor (V_DCR) is accounted for, the equation becomes: 

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

The LM26420-Q1 conduction loss is mainly associated with the two internal FETs:

If the inductor ripple current is fairly small, the conduction losses can be simplified to:

Switching losses are also associated with the internal FETs. 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 empirically measuring the rise and fall times (10% to 90%) of the switch at the switch node.

Switching Power Loss is calculated as follows:

Another loss is the power required for operation of the internal circuitry:

I_Q is the quiescent operating current, and is typically around 8.4 mA (I_QVINC = 4.7 mA + I_QVIND = 3.7 mA) for the 550-kHz frequency option.

Due to Dead-Time-Control Logic in the converter, there is a small delay (approximately 4 nsec) between the turn ON and OFF of the TOP and BOTTOM FET. During this time, the body diode of the BOTTOM FET is conducting with a voltage drop of V_BDIODE (approximately 0.65 V). This allows the inductor current to circulate to the output, until the BOTTOM FET is turned ON and the inductor current passes through the FET. There is a small amount of power loss due to this body diode conducting and it can be calculated as follows:

Typical Application power losses are:

These calculations assume a junction temperature of 25°C. The R_DSON values are larger due to internal heating; therefore, the internal power loss (P_INTERNAL) must be first calculated to estimate the rise in junction temperature.