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!

Thursday, May 23, 2013

How much AC power do you need to support your DC output?

You may know the maximum rated output power of your DC power supply, but do you know how much AC input power is needed to support the DC output of the power supply? Probably not. Since no power supply is 100% efficient, you know you will need more input power than the output power produced, but how much more? The answer depends primarily on the efficiency of the power supply (since efficiency here is power out divided by power in).There are 2 main sources of losses contributing to the efficiency, or perhaps I should say “inefficiency”, of a power supply:

  1. Overhead power loss– this is the power consumed by the power supply that is not directly related to the output power conversion. It is the amount of power consumed by the internal circuits that are needed just to provide the basic internal functions of the power supply, such as front panel display control, internal bias supplies, cooling fans, and microprocessor control. This power is dissipated as heat inside the power supply and is therefore not available to flow to the output. Some of this power is consumed even when the output is providing no power.
  2. Power conversion loss – this is the power lost in the power conversion circuitry. All of the output power flows through the power conversion circuitry, but as it does so, some heat is generated. The power lost as heat is not available to flow to the output.

A power supply’s efficiency is typically specified at the maximum output power point and includes losses associated with both the overhead power and power conversion circuitry. Most power supply vendors will publish the maximum expected AC input current, watts, and/or volt-amperes for their products so you should be able to get this information from the vendor’s documentation. But let’s consider an example based just on the output power rating and efficiency.

A power supply rated for 2000 W of output power with an efficiency of 80% will require 2000 W / 0.8 = 2500 W of AC input power. In the United States, the standard AC line voltage is 120 Vac. At a nominal voltage of 120 Vac, the AC input current would be 2500 W / 120 Vac = 20.8 Aac which is more than a standard outlet can provide (15 A maximum is a typical rating for an outlet). If the AC input line voltage sags a little making it lower, the input current would be even higher! To accommodate the AC input of this 2000 W power supply, there are several alternatives:

  1. Use a less-common receptacle (outlet) and plug rated for more than 20 A.
  2. Have an electrician hard-wire the AC input connection to the AC mains ensuring the wires and AC mains branch circuit can handle the higher current (no outlet would be used).
  3. If the power supply is rated for it, power the power supply from a higher AC input voltage, such as 208 Vac or 240 Vac to reduce the current required. This solution will also require a less-common receptacle and plug (middle receptacles in photo).

Many very high-power supplies (a few kW and above) require a 3-phase AC input voltage to accommodate the larger amount of output power (orange receptacle shown at top of photo).

One of my colleagues, Bob Zollo, wrote an article entitled “Do You Have Enough AC For Your DC?” that appeared in Electronic Design on May 7, 2013. For some additional information about this topic, take a look at the article:

Wednesday, May 15, 2013

Power Factor and Active Power Factor Correction for Switched-mode Power Supplies

In my previous posting “More on Early Power Supply Preregulator Circuits” SCRs served to provide basically line frequency switched-mode operation for efficient power conversion and regulation in earlier mixed-topology DC power supply designs. Now that high frequency switched-mode power conversion circuits have long been highly refined, are physically much smaller, and are extremely cost effective they have become the game-changer. They can be used as a preregulator for mixed-topology DC power supply designs, as well as the complete DC power supply from the AC input to the regulated DC output, right? Well almost “yes”. They do bring all those of benefits over line frequency operation. As they can span a much wider range of AC input another benefit they bring is to eliminate the need for a complex AC line switch arrangement for the wide range of AC voltages needed.

It was recognized that one downside of high frequency switched-mode conversion is the AC input suffered from rather low power factor (PF). PF is the ratio of the real power to the apparent power. Low PFs cause increased losses in the AC power distribution system. Not only was it low, it was very non-linear, drawing current having high levels of odd harmonics. It turns out the third harmonic in particular can be additive, causing excessive current through the neutral line of AC power distribution systems. The reason for the low and non-linear PF is that the AC input of a high frequency switched-mode conversion circuit is a diode bridge feeding a large, high voltage, bulk storage capacitor, as shown in Figure 1. This non-linear load draws large peaks of current over short portions of the AC line period.

Figure 1: Non-linear AC load input of a high frequency switch-mode power converter circuit

As more and more electronic equipment was making use of switch-mode DC power supplies, minimum PF standards were established for products above a certain power rating, to avoid causing problems with the AC power distribution system. To meet the standards switch-mode DC power supplies above a certain power rating have had to incorporate power factor correction (PFC) into their AC inputs. While a few different approaches can be taken for adding PFC, most switch-mode DC power supplies incorporate a specialized switched-mode boost converter stage for providing active PFC. The active PFC stage is placed between the input rectifier bridge and bulk storage capacitor as depicted in Figure 2. An active PFC stage is designed to draw AC current in phase and in proportion to the AC voltage, typically providing PFs in a range of 0.95 to 0.99, which is comparable to a nearly purely resistive load!

Figure 2: Active PFC circuit in typical switched-mode DC power supply

While adding active PFC to a switch-mode DC power supply increases complexity, cost, and power loss somewhat, the overall combination of benefits of a switch-mode DC power supply with active PFC, either stand-alone or as a preregulator, is hard to beat!

Friday, May 10, 2013

More on Early Power Supply Preregulator Circuits

In my last posting “Ferroresonant Transformers as Preregulators in Early DC Power Supplies “, I introduced the concept of preregulators as a means of improving the efficiency of power supplies.  While a linear regulator provides excellent performance as a power supply, it has to dissipate all the additional power resulting from the voltage drop across it as it takes up the difference between the output voltage setting and the unregulated DC voltage at its input. This voltage difference becomes quite large for high-line AC input voltage levels, as well as low DC output voltage settings when the power supply has an adjustable output. A linear power supply becomes quite inefficient and physically large, having to dissipate a lot of power in comparison to what it provides at its output.  A preregulator helps to mitigate this disadvantage while still retaining the performance advantages of a linear output stage.

The ferroresonant transformer was a clever device and was an effective means of compensating for variance in the AC input voltage, but its output was fixed so it did not do anything for compensating for low DC output voltage settings when the power supply had an adjustable output.  A far more common type of preregulator circuit often used was an SCR preregulator circuit, depicted in Figure 1.

Figure 1: Constant voltage power supply with SCR preregulator

The SCR is a four layer diode structure. Unlike a conventional diode it does not conduct in the forward direction until a signal current is applied to its gate input. It then latches on and remains conducting in its forward direction. It does so until the forward bias voltage is removed or reversed and it resets. In the reverse direction it is the same as a conventional diode.  By replacing two of the conventional diodes in the full wave diode bridge with SCRs as shown in Figure 1, the DC voltage feeding into the linear regulator output stage can now be preregulated.  The preregulator control circuit senses the voltage across the series linear regulator output stage. For each half cycle of the line frequency it adjusts the firing angle of the SCRs in order to adjust the DC voltage at the input of the linear regulator so that the voltage across the linear regulator remains constant, compensating for the load and output voltage level setting accordingly. Figure 2 shows how changing the firing angle of the SCRs changes the output voltage and current delivered by the SCR preregulator circuit.

Figure 2: SCR firing angle control of the preregulator’s output

In all, an SCR preregulated power supply with a linear output stage provided a good balance of efficiency, performance, and cost making its topology well suited for DC power supplies for a variety of lab and industrial applications for the time.  Still, time marches on and high frequency switching-based topologies have come to dominate for the most part, due to a number of advantages they bring. As a matter of fact it is not uncommon today to find a switching power supply serving as a preregulator as well!

Reference: Agilent Technologies DC Power Supply Handbook, application note AN-90B, part number 5952-4020 “Click here to access”