Technical Insights > Benchtop

Why Do I Need to Add an External Protection Diode to My Bench Power Supply?

2020-08-25  |  6 min read 

Adding a diode to the output of your power supply will keep your power supply safe while you work with energy-storing devices such as batteries or large capacitors. The diode will prevent current from flowing out of the battery and back into the power supply. A constant current flowing into a Keysight E36200 Series power supply will damage the supply. Insert the diode so that it protects the remote sense connections as well as the primary output connections.

Diode connected to a power supply

Figure 1. Diode added to the output of a power supply

Down-Programming

Why is the protection diode not built into the power supply? While the diode protects the power supply, when the power supply is set to a lower voltage, the diode will cause it to react slowly. Setting a power supply to a lower voltage is known as down-programming. Down-programming also occurs when turning off a bench power supply’s output, as the output is set to 0 V. When down-programming a power supply, it needs to absorb energy from the output capacitor and external parasitic capacitance. Parasitic or stray capacitance occurs in wiring and on printed circuit boards.

Passive down-programmer

Figure 2. Power supply’s output capacitor and external parasitic capacitance

The Keysight E36200 Series power supply can lower its output from full voltage to less than 0.5 V in about 30 ms when fully loaded. If the outputs are open, it takes 100 ms to discharge the output capacitance from full scale to 0.5 V.

Active Down-Programming

While the E36200 Series power supplies work well for bench applications, some applications benefit from an active down-programmer to change voltages faster. Many of our system power supplies use an active down-programmer to quickly change to a lower voltage. Test systems tend to have longer wires and more connections, leading to higher parasitic capacitance.

Applications with higher voltage swings also benefit from an active down-programmer because a large step of 50 V to 0 V takes more time than small step of 10 V to 0 V. When test engineers need to reduce test times, quickly changing voltages helps.

Table 1. N6752A module uses an active down-programmer to quickly change voltages

Down-programming time

No load

1,000 µF

Small step

10 to 0 V

0.3 ms

2.1 ms

Large step

50 to 0 V

1.3 ms

11 ms

The Keysight N6750 Series is an example of a power supply with an active down-programmer. The N6752A can reduce its output voltage from 50 V to 0 V in 1.3 ms and in 11 ms with a 1,000 uF load. It can dissipate 7 W of continuous power and 7 A peak current. Table 2 lists Keysight power supplies with active down-programming. There are two methods to determine down-programming times. In both methods, the outputs are open as the load can influence the results. Method one measures the time from full scale to less than 0.5 V, while method two measures from 90% to 10% of full scale. Method one requires a larger swing and takes more time than method two. In all cases, the down-programing times are rapid.

Table 2. Keysight power supplies with an active down-programmer

N6750 Series

1.3 ms (100% to less than 0.5 V)

N6760 Series

1.3 ms (100% to less than 0.5V)

N6783A-BAT and N6783A-MFG

4.0 ms (100% to less than 0.5 V)

N6900 Series Option 303

0.35 ms (90% to 10%)

N7900 Series

0.35 ms (90% to 10%)

RP7900 Series

0.2 ms (90% to 10%) 

Remember to use a diode when charging a battery or large capacitor because most power supplies cannot dissipate the stored energy. Figure 1 shows the correct placement for the diode as it protects both the power supply’s output and the remote sense circuit. Most loads store very little energy and do not require an external diode. In that case, it is best to remove the diode because the diode prevents current flow into the down-programming circuit and prevents an accurate measure of the voltage at the load. Without a load, the E36200 requires 100 ms to down-program from 50 V to 0.5 V. A power supply with an active down-program can do the same in just over 1.3 ms, 100 times faster.