4 Ways to Build Your Power Supply Skill Set – Part 1

Today's designs place higher demands on the DC power systems you develop, which often cause design problems.

In this eBook, our experts help you better understand DC power supply issues and provide four ways to build your DC power supply skill set so you can overcome design challenges. Topics covered include:

• How to Set Constant Voltage and Constant Current
• Get a Voltage or Current Boost
• Dealing with Unexpected Temperature Effects
• Dealing with Noise Sensitive DUTs

Tip1: How to Set Constant Voltage and Constant Current

One of the most important concepts for anyone using power supplies is understanding constant voltage (CV) and constant current (CC). Here are some basic rules to help you get started.

The output of a power supply can operate in either CV or CC mode depending on the voltage setting, current limit setting, and load resistance.

• If the load current is low and the current drawn is less than the current setting, the power supply will operate in CV mode. The voltage is regulated, keeping the value constant, with the current being determined by the load.
  
• If the load current is high and the load is trying to draw more current than the current setting, the power supply will limit the current at the current setting value and operate in CC mode. The current is regulated with the voltage being determined by the load.

Most power supplies are designed in such a way that it is optimized for CV operation. This means that the power supply will look at the voltage setting first and adjust all other secondary variables to achieve the programmed voltage setting. Look for a power supply that maintains a constant voltage—even in a dynamic environment—but can also provide a constant current when needed.

Tip 2: Get a Voltage or Current Boost

Connect Multiple Power Supplies for Higher Voltage or Current

There may be situations when you need more voltage or current than your power supply can provide. Here are some simple steps to connect two or more power supplies to meet those needs. For higher voltage, connect power supply outputs in series, and for higher current, connect outputs in parallel.

Set each power supply output independently so that the voltages or current sum to the total desired value.

For higher voltage, first set each output to the maximum desired current limit the load can safely handle. Then, equally distribute the total desired voltage to each power supply output. For example, if you are using three outputs, set each to one third the total desired voltage.

• Never exceed the floating voltage rating (output terminal isolation) of any of the outputs
  
• Never subject any of the power supply outputs to a reverse voltage
• Only connect outputs that have identical voltage and current ratings in series

For higher current equally distribute the total desired current limit to each power supply.

• One output must operate in constant voltage (CV) mode and the other(s) in constant current (CC) mode
• The output load must draw enough current to keep the CC output(s) in CC mode
• Only connect outputs that have identical voltage and current ratings in parallel

Set the voltage of the CC outputs to a value slightly higher than the voltage value of the CV outputs. The actual voltage across CC outputs is determined by the load (see Tip 1). In the parallel setup, the CV output determines the voltage at the load and across the CC outputs. The CV unit will only supply enough current to fulfill the total load demand.

Tip 3: Dealing with Unexpected Temperature Effects

Calculate and Apply Temperature Coefficients

Not only do the devices under test vary with temperature, but so do the instruments you use to make measurements. On a cold, winter day we tested Lithium Ion batteries at room temperature and found that the voltage of the cells drifted up as time progressed, not down as we expected.

The nighttime decrease in room temperature increased cell voltage—an effect stronger than the expected decrease in voltage as a result of cell self-discharge during the day. Since the power supplies that are used to apply power to cells also vary with temperature, you may need to apply the temperature coefficients to properly characterize the output voltage down to microvolts.

Tip 4: Dealing With Noise Sensitive DUTs

Two Ways to Minimize Noise

If your device under test (DUT) is sensitive to noise, you will want to do everything you can to minimize noise on the DC power input. The easiest thing you can do is use a low-noise power supply. But if you don’t have one, here are a couple of other things you can do.

Take a Closer Look at your Connections

The connections between your power supply and the DUT can be susceptible to interference, such as noise arising from inductive or capacitive coupling. There are a number of ways to reduce noise, but the most effective is using shielded two-wire cables for load and sense connections. Make sure you connect the shield to earth ground only at one end. Do not connect the shield to ground on both ends because ground loops can occur. The ground loop current can produce voltage on the cabling that appears as noise to your DUT.

Balance Output to Ground Impedance

Common-mode noise is generated when common-mode current flows from inside a power supply to earth ground and produces voltage on impedances to ground, including cable impedance. To minimize the effect of common-mode current, equalize the impedance to ground from the plus and minus output terminals on the power supply. You should also equalize the impedance from the DUT plus and minus input terminals to ground. Use a common-mode choke in series with the output leads and a shunt capacitor from each lead to ground to accomplish this task.