DC to DC Converter Performance Testing

In this application note, we introduce the important performance characteristics of DC to DC converters and testing methods. The testing circuits and operation are also discussed. Note that the importance of each performance characteristic is dependent on the end application. As such, more detailed testing and methods may be warranted. This is left to the user to decide. For specific question regarding your application, you may contact us or simply comment at the bottom.

Oscar Agudelo, Shiraz Macuff, Nesh Basnet, Subash Shrestha, Anil Rajpatei,

Adrian Serrano,Lhakpa Dhondup, Lobsang Lekshey

Dc to dc converters performance testing

When choosing a DC/DC converter, engineers sometimes look at many different brands on the market and compare their specifications. Some specifications are more important depending on the end application. However, the manufacturers’ data about the best and worst case performances can be misleading because the converter may not be needed to operate on these regions in the end application. A description of the different performance characteristics and accepted test methods are discussed to guide users in selecting and testing their DC/DC converter.

Efficiency

Dc to dc converter’s efficiency is one of the most touted performance characteristic in the industry. The efficiency number is the ratio of the output power to the input power and it is normally given in percentage. Manufacturers normally publish the best case scenario, using words such as “It has up to xxx percent efficiency”, In addition, some manufacturers have begun to provide efficiency plots to help designers determine the efficiency in their designs. To do this, the designer has to estimate the load and line conditions in which the instrument would be operating. Then, he can read the efficiency at this point on the efficiency graph. This is normally done for applications where power loss is a major concern, for example in battery operated application.

DC/DC Converter efficiency graph
DC/DC Converter efficiency graph

Testing Efficiency

Testing efficiency of a dc to dc converter requires voltage and current measurement on both the input and the output.  The input and output power is then calculated and the efficiency is calculated by dividing the output by the Input power as shown in the equation below.

 Efficiency=\frac{Pout}{Pin}

The voltage source used at the input of the dc to dc converter should have very low impedance to avoid any voltage drop since it would introduce error due to the high pulse current associated with the switching converter. For a variable load, resistors can be incrementally paralleled to sweep the load from zero to its maximum. The circuit below can also be used given that heating of the output transistor is addressed.  The circuit below is a variable current sink. The current is controlled by changing the voltage at the set point.  The input voltage to the DC/DC converter should be varied from the maximum to the minimum as indicated by the converter’s manufacturer .Also, the load should be changed at each incremental input voltage as stated above and the input and output power should be measured.

Current sink for testing DC to DC converter at different load.
Current sink for testing DC to DC converter at different load.

Table 1 is an example excel sheet that can be used.

Vin (V) Iin (a) Vout (V) I load(A) Calculated Pin (W) Calculated Pout (W)
Steady state and transient Load regulation.
The steady state load regulation is how accurately the output  voltage is maintained at different load conditions.
Transient load regulation is how long it takes for the output to go to its steady state voltage during a transient load change. This specification is important in applications where switching components introduce ripple on the output of the converter. ADC’s and prisi ion analog component can suffer from the high frequency signal introduced on the supply line. Also important is if the converter overshoots and by how much. This performance is rarely published but it is of great importance in end applications having switching components such as digital circuits and switching motor control.
To measure the steady state load regulation, use  the setup mentioned above and measure the voltage at the output at different load and line conditions. To record the full response, the line voltage( input voltage to the converter) should also be varied to record the worst case conditions. For a specific application , this may be varied within the range that the converter will be subjected to in the end application.
Measuring the transient load regulation requires that the load be switched on and off and the output waveform measured on an oscilloscope. The line voltage should again be varied to find the worst case condition. Applying a pulsed load may require using a pulse train and toggling the load on and off as shown in the figure below. The circuit above can be modified such that the VCC supplying the potetiometer be replaced with a pulsed source.
Application note TR Steady stat
Line regulation
Line regulation is how close the output voltage is maintained when the input voltage is varied. Care should taken when considering the line regulation of dc to dc converters. The worst case minimum and maximum input voltage should be the requirement of this specification.  Tolerance of 20 percent should be subtracted from the minimum and added to the maximum. Note that in rechargeable battery system , it’s. not uncommon to have 30 percent higher voltage at full charge.
Testing the line regulation requires the setup mentioned above and varying the input voltage from its minimum to its maximum while recording the output voltage. The line regulation characteristics may  vary based on load conditions so it would be wise to also vary the load as shown in the load regulation sections above
Cross regulations
With dc to dc converter with multiple output, the cross regulation is how well the each output voltage is maintained when one or more of the outputs are loaded.
Output ripple
The output ripple is produced by the switching nature of the dc to dc converter. Some manufacturers have been increasing  power density and decrease cost while compromising on ripple. Reducing output ripple on a converter may be as simple as adding a capacitor at the output of the converter. The value of the capacitor should be recommended  by the manufacturer. Excessive ripple on the output can effect the performance of ADC’s and other sensitive components in the design. To test the output ripple, the unit under test should be powered up and a load connected. The voltage a across the load should be monitored using an oscilloscope. The sweep frequency and volt per division should be incrementally increased and decreased respectively until a repeating waveform is visible. Note that it requires searching for the waveform. For some of the newer oscilloscope, a push on the auto set button can quickly find the waveform. The amplitude and frequency should be considered when the converter is used in systems with ADCs.