Showing posts with label capacitor. Show all posts
Showing posts with label capacitor. Show all posts

Tuesday, April 28, 2015

Optimizing a Power Supply’s Output Response Speed for Applications Demanding Higher Performance

Most basic performance power supplies are intended for just providing DC power and maintain a stable output for a wide range of load conditions. They often have lower output bandwidth to achieve this, with the following consequences:
  • Internally this means the feedback loop gain rolls off to zero at a lower frequency, providing relatively greater phase margin. Greater phase margin allows the power supply to remain stable for a wider range of loads, especially larger capacitive loads, when operating as a voltage source.
  • Externally this means the output moves slower; both when programming the output to a new voltage setting as well as when recovering from a step change in output load current.

While this is reasonably suited for fairly static DC powering requirements, greater dynamic output performance is often desirable for a number of more demanding applications, such as:
  • High throughput testing where the power supply’s output needs to change values quickly
  • Fast-slewing pulsed current loads where the transient voltage drop needs to be minimized
  • Applications where the power supply is used to generate power ARB waveforms

A number of things need to be done to a power supply so that it will have faster, higher performance output response speed. Primarily however, this is done by increasing its bandwidth, which means increasing its loop gain and pushing the loop gain crossover out to a higher frequency. The consequence of this the power supply’s stability can be more influenced by the load, especially larger capacitive loads.

To better accommodate a wide range of different loads many of our higher performance power supplies feature a programmable bandwidth or programmable output compensation controls. This allows the output to be set for higher output response speed for a given load, while maintaining stable operation at the same time. As one example our N7900A series Advanced Power System (APS) has a programmable output bandwidth control that can be set to Low, for maximum stability, or set to High1, for much greater output voltage response speed. This can be seen in the graph in Figure 1, taken from the APS user’s guide.

Figure 1: N7900A APS small signal resistive loading output voltage response

Low setting provides maximum stability and so it accommodates a wider range of capacitive loading. High 1 setting in comparison is stable for a smaller range of capacitive loading, but allowing greater response bandwidth. This can be seen in table 1 below, for the recommended capacitive loading for the N7900A APS, again taken from the APS user’s guide.

Table 1: N7900A APS recommended maximum capacitive loading

While a maximum capacitive value is shown for each of the different APS models for each of the two settings, this is not altogether as rigid and fixed as it may appear. What is not so obvious is this is based on the load remaining capacitive over a frequency range roughly comparable to the power supply’s response bandwidth or beyond. Because of this the capacitor’s ESR (equivalent series resistance) is an important factor. Beyond the corner frequency determined by the capacitor’s capacitance and ESR, the capacitor looks resistive. If this frequency is considerably lower than the power supply’s response bandwidth, then it has little to no effect on the power supply’s stability. This is the reason why the power supply is able to charge and discharge a super capacitor, even though its value is far greater than the capacitance limit given, and not run into stability problems, for example.

One last consideration for more demanding applications needing fast dynamic output changes, either when changing values or generating ARBs is the current needed for charging and discharging capacitive loads.  Capacitors increasingly become “short-circuits” to higher AC frequencies, requiring the power supply to be able to drive or sink very large currents in order to remain effective as a dynamic voltage source!


Friday, May 31, 2013

Fun at Matt's desk!

I am about to head out for a week long vacation (actually Gary will be there too) so I wanted to do something short and fun for this month’s blog post.  I have been with Agilent 13 years come mid June (man I am getting to be old).  Through the years, I have collected some interesting items on my desk.  I wanted to share some of the more interesting items that I have collected through the years.

Item 1:
What do you think this green object is?

If you answered a 2500 Watt resistor then you’re right!  This particular resistor is rated for 0.8 Ohms. Agilent sells power supplies that are rated all the way up to 6.6 kW so sometimes you need a high power resistive load.  I personally would not put 2500 W through this resistor unless I had a whole lot of ventilation.  Luckily last time I used it I only put like 1500 W through it so it only got mildly toasty.   

Item 2:
How much voltage do you think that this probe can measure?

This is the Agilent 34136A high voltage probe for our DMMs.   Before I acquired this probe, I was used to teeny tiny normal alligator clip probes but this probe can measure up to 40 kV!.  I don’t know about any of you readers but I probably would not want to be anywhere near a 40 kV Voltage myself.  This probe  has banana plugs on it and you can hook it up to our DMM products (34401A, 34410A, etc.).  This probe almost looks like a sword of some sort with the pointy tip and all.

Item 3:
How much capacitance do you think this smallish capacitor is rated for?

This is a 10 F capacitor.  That is right 10 Farads!  When I was in college, a 10 Farad capacitor was unthinkable, now you can find them in these tiny packages.   My colleague Paul used this to research this video:

So this is just a quick tour of some of the neat stuff around my desk.  What kind of neat engineering stuff do you readers have on your desks?  Feel free to share in the comments.

Happy Summer everyone!