Saturday, November 29, 2014

Why do I measure voltage to earth ground on a power supply with a floating output?

Occasionally, one of our power supply users contacts us with a question about voltages measured from one of the power supply output terminals to earth ground (same as chassis ground). All of our power supply outputs are floating with respect to earth ground. See my previous post about this here. In that post, I stated that neither output terminal is connected to earth ground. To be more specific, no output terminal is connected directly to earth ground. We do have internal components, mainly resistors and capacitors, connected from each output terminal to earth ground. These components, especially the caps to ground, help mitigate issues with RFI (radio-frequency interference) and ESD (electrostatic discharge). They help prevent our power supplies from being susceptible to externally generated RFI and ESD, and also help to reduce or eliminate any internally generated RFI from being conducted to wires connected to the output terminals thereby reducing RFI emissions.

So even though our outputs are considered floating with respect to earth ground, there frequently is a DC path from at least one of our output terminals to earth ground. It is typically a very high value resistor, such as several megohms, but could be as low as 0.5 MΩ. This resistor acts as a bleed resistor to discharge any RFI or ESD caps to earth ground that could be charged to a high float voltage.

As an example of a power supply with a resistor to earth ground, the Keysight N6743A has 511 kΩ (~0.5 M) from the minus output terminal to earth ground. This resistor was responsible for the voltage measurements to earth ground observed and questioned by one of our power supply users. He was using this power supply in the configuration shown in Figure 1 and measured 9.7 Vdc from his common reference point to earth ground (again, same as chassis ground).

He understandably did not expect to measure any stable voltage between these points given that the output terminals are floating from earth ground. But once we explained the high impedance DC path from the minus output terminal to earth ground inside each power supply (see Figure 2), and the 10 MΩ input impedance of his DMM, the measurement made sense. The input impedance of the voltmeter (DMM) must be considered to accurately calculate the measured voltage. This is especially true when high impedance resistors are in the circuit to be measured.

Figure 3 shows the equivalent circuit which is just a resistor divider accounting for the 9.7 V measurement. (The exact calculation results in 9.751 V.) Notice that the voltage of the 28 V power supply does not impact this particular voltage measurement (but its resistor to ground does). If the user had measured the voltage from the plus output of the 28 V power supply to earth ground, both the 28 V supply and 20 V supply would have contributed to his measurement which calculates out to be 37.05 V (if you check this yourself, don’t forget to move the 10 MΩ resistor accounting for the different placement of the DMM impedance).

So you can see that even with power supply output terminals that are considered floating, there can still be a DC path to earth ground inside the supply that will cause you to measure voltages from the floating terminals to ground. As one of my colleagues always said, “There are no mysteries in electronics!”

Monday, November 24, 2014

Prewired rack delivers up to 90 kW of single-output DC power

As I have mentioned before, I avoid posting product-only-focused material in this blog since our goal is to educate about power-related items rather than to directly promote Keysight products. But when something new comes out, I like to announcement it here.

A little over a year ago, Keysight (we were Agilent at the time) announced a new power supply family with high-power outputs up to 15 kW per output (see link here). These high-power autoranging DC power supplies with individual outputs up to 1500 V or 510 A can be paralleled for even more power. Paralleling outputs with these supplies is simplified since multiple outputs can be grouped to act as a single output and the current share bus enables multiple outputs to more equally share current. But there are other considerations when paralleling multiple units. Wiring the AC inputs and DC outputs together takes design time and assembly time. Integrating the units together physically in a rack also takes time and effort. Designing the system to ensure the safety of operators is also very important. So Keysight decided to offer a prewired rack to help you overcome the challenges associated with racking high-power supplies.

On November 11, 2014 (less than 2 weeks ago), we announced a prewired rack that combines up to six of the 15 kW N8900 series power supplies for total power up to 90 kW from a single output. You can choose from a variety of DC output combinations with voltages from 80 V to 1500 V and currents from 60 A to 3060 A. That’s a lot of current! The rack’s internal configuration makes the multiple outputs appear as a single output allowing you to communicate with just one power supply through LAN, GPIB, or USB (all standard in the system). Click here for the press release for this new system and here for additional system information.

Perhaps you are working in R&D or manufacturing on EV/HEV, alternative energy (fuel cells, solar, etc.), industrial DC motors, large UPS’s, electroplating, or any of the many other high-power application areas that need DC power up to 90 kW with voltages to 1500 V or currents to 3060 A. If so, and you want to spend your engineering resources on your core competencies instead of racking power supplies together, let Keysight help you with our new N8900 Series Rack System.

Tuesday, November 18, 2014

Why does the response time of OCP vary on the power supply I am using and what can I do about it? Part 1

In a previous posting of mine “Providing effective protection of your DUT against over voltage damage during test”(click here to review), an important consideration for effective protection was to factor in the response time of the over voltage protect (OVP) system. Due to the nature of over voltage damage, the OVP must be reasonably fast. The response time can typically be just a few tens of microseconds for a reasonably fast OVP system on a higher performance system power supply to hundreds of microseconds on a more basic performance system power supply. This response time usually does not vary greatly with the amount of over voltage being experienced.

Just as with voltage, system power supplies usually incorporate over current protect (OCP) systems as well. But unlike over voltage damage, which is almost instantaneous once that threshold is reached, over current takes more time to cause damage. It also varies in some proportion to the current level; lower currents taking a lot longer to cause damage. The I2t rating of an electrical fuse is one example that illustrates this effect.

Correspondingly, like OVP, power supply OCP systems also have a response time. And also like OVP, the test engineer needs to take this response time into consideration for effective protection of the DUT.  However, unlike OVP, the response time of an OCP system is quite a bit different. The response time of an OCP system is illustrated in Figure 1.

Figure 1: Example OCP system response time vs. overdrive level

Here in Figure 1 the response time of the OCP system of a Keysight N7951A 20V, 50A power supply was characterized using the companion 14585A software. It compares response times of 6A and 12A loading when the current limit is set to 5A. Including the programmed OCP delay time of 5 milliseconds it was found that the actual total response time was 7 milliseconds for 12A loading and 113 milliseconds for 6A loading.

This is quite different than the response time of an OVP system. Even if the OCP delay time was set to zero, the response is still on the order of milliseconds instead of microseconds for the OVP system. And when the amount of overdrive is small, as is the case for the 6A loading, providing just 1A of overdrive, the total response time is much greater. Why is that?

Unlike the OVP system, which operates totally independent of the voltage limit control system, the OCP system is triggered off the current limit control system. Thus the total response time includes the response time of the current limit as well. The behavior of a current limit is quite different than a simple “go/no go” threshold detector as well. A limit system, or circuit, needs to regulate the power supply’s output at a certain level, making it a feedback control system. Because of this stability of this system is important, both with crossing over from constant voltage operation as well as maintaining a stable output current after crossing over. This leads to the slower and overdrive dependent response characteristics that are typical of current limit systems.

So what can be done about the slower response of an OCP system? Well, early on in this posting I talked about the nature of over current damage. Generally over current damage is much slower by nature and the over drive dependent response time is in keeping with time dependent nature of over current damage. The important thing is understand what the OCP response characteristic is like and what amount of over current your DUT is able to sustain, and you should be able to make effective use of the over current protection capabilities of your system power supply.

If however you are still looking how you might further improve on OCP response speed, look for my follow up to this in my next posting!

Friday, November 7, 2014

Providing effective protection of your DUT against over voltage damage during test

The two most common ways DUTs can be electrically damaged during test are from current-related events or voltage-related events that mange to over-stress the DUT. Sometimes the cause can be an issue with the DUT itself. Other times it can be an issue stemming from the test system. The most common voltage-related damage to a DUT is an over voltage event, beyond a maximum level the DUT can safely tolerate. While there are a number of things that can cause this, most invariably it was an issue with the test system power supply, either from inadvertently being set too high or from an internal failure.

To protect against accidental over voltage damage, test system power supplies incorporate an over voltage protect (OVP) system that quickly shuts down the output upon detecting the voltage has gone above a preset threshold value. More details about OVP have been written about here in a previous posting “Overvoltage protection: some background and history”(click here to review).

The critical thing about over voltage damage is, in most all cases, that it is virtually immediate once the voltage threshold where damage to the DUT occurs is exceeded. It is therefore imperative that you optimize the test set up and settings in order to provide effective protection of your DUT against over voltage damage during test. To start with, the OVP trip threshold needs to be set at a reasonable amount below the threshold where DUT damage occurs and at the same time be set to a reasonable amount above the maximum expected DUT operating voltage. This is depicted in Figure 1.

Figure 1: OVP set point

However, to understand what are “reasonable amounts” above the maximum operating voltage and below the DUT damage voltage levels you need to take into account the dynamic response characteristics of the power supply output and OVP system, as depicted in Figure 2.

Figure 2: Power supply output and OVP dynamic response characteristics

It is important to have adequate margin above the maximum operating voltage to account for transient voltages due to the DUT drawing current from the power supply and resulting voltage response of the power supply in correcting for this loading, in order to prevent false OVP tripping. It is likewise important to adequate margin below the DUT damage threshold as it takes a small amount of time, in the range of 10’s to 100’s of microseconds, for the OVP system to start shutting down the power supply’s output once the OVP trip point has been crossed. At the same time the power supply typically has a maximum rate the output voltage can slew in. In practice these “reasonable amounts” typically need to be a few tenths to several tenths of a volt as a minimum.

Generally these margins are not difficult to manage, except when the DUT’s operating voltage is very small or the DUT operating current is very large producing a correspondingly large voltage drop in the power supply wiring. This is because the OVP is traditionally sensed on power supply’s output power terminals, so that it provides protection regardless of what the status and condition of the remote voltage sense wiring connection is. To improve on this we also provide OVP sensing on the remote sensing wires as an alternative to, or in addition to, the traditional sensing on the output power terminals. More details about this are described in another posting here “Protect your DUT: Use sense leads for over voltage protection (OVP)”(click here to review).

By following these suggestions you should be able to effectively protect your DUT against over voltage damage during test as well!