LED Power Supply Overview Part 1

“A chain is only as strong as its weakest link.”
—Old Proverb

Several large LED sign makers have contacted me this past year to determine why they are having relatively high field failure rates of their power supplies. This got me to thinking there are likely many sign makers I haven’t heard from who are having the same problems, so I thought an article might be useful. The problem is that this subject cannot be meaningfully addressed in a single short article. This is the first installment on that subject but not the last word.

LEDs can last a very long time in a sign–if they are not over-driven and proper thermal management is designed in. Exactly how long depends on many factors and what you consider to be a useful amount of light output. In some applications a 30 percent decrease in brightness is not acceptable but in many, even a 50 percent decrease in output over many years is still useful. I have a digital weight scale that has been left turned on 24/7 for 20+ years and the red LED digits are still easy to read. They’re not as bright as when I bought it but no segments have failed. On the other hand the power supply for it did die about 10 years ago and had to be replaced. I won’t be surprised if the replacement power adaptor also dies before the LED display is too dim to read.

Given enough time and use, everything ever built will fail but it doesn’t make sense to build a long life sign with LEDs only to have the power supply fail in a few months. Most sign builders don’t give a lot of thought to the power supply they use. Usually they just buy one that puts out enough power and has the lowest cost. They are thought of as generic commodities–until you have a rash of field failures still under warranty. Then suddenly the power supply gets lots of attention. They can cost you your reputation as a quality sign company. My goal in this multi-part article is not to make a power supply designer out of you but to arm you with information that will help you pick the right power supply and to know what questions are key to ask your power supply source before buying. A good understanding of the general workings of power supply is the best starting point.

WHAT IS A POWER SUPPLY?
Power supplies come in more types than Baskin-Robbins has flavors of ice cream but at their core they all serve the same fundamental function–a power supply converts electricity of one form into electricity of another form.

This conversion may include any or all of the following: AC into DC (or DC into AC), change a voltage or current to a higher or lower level, regulate current and/or voltage to a specific value and/or provide electrical isolation for safety. When used for LEDs, a power supply most often changes 120 or 240VAC line voltage into isolated low voltage DC with the output voltage (or current) regulated. To keep this article relevant to LED signage, this is the type of power supply I will refer to in the rest of this article. There are of course many other types of power supplies for other applications.

The term power supply is what I generically use for power conversion devices. Other names I often hear (depending on application) are ballast, switcher, switch mode, linear regulator, converter, driver and Transformer. Each of these terms (when used properly) describes different, specific types of power supply designs. Each type has its own electrical characteristics which need to be understood when specifying a power supply.

LINEAR POWER CONVERSION
In the taxonomy of power supplies, most regulated power supplies can be divided into two main categories–Linear or Switch-Mode type. There are many topologies of each but knowing the main difference of these two groups is important.

The main difference between a Linear and a Switch-Mode type is a Linear power supply starts with more voltage than needed and regulates it to the desired value by linearly varying the drive to a power transistor in series with the output; a Switch-Mode supply switches the voltage on and off at a high frequency (20KHz to hundreds of KHz). The on to off ratio is varied to control the regulation point and a filter averages out all the on/off cycles to make a smooth DC voltage.

A Linear Regulated Power Supply has several key function blocks. The first is a transformer to change 120VAC – 60Hz (or other AC line voltage) to a lower AC voltage (most often between 6VAC to 48VAC). It also provides electrical safety isolation (more properly known as Galvanic Isolation). A true transformer converts voltage through Magnetic coupling of two or more windings placed on a Core of magnetic material. The voltage changes by the ratio of the number of wire turns on the input winding vs. the number of turns of the output winding. If the 120V input winding on a transformer has 1,000 turns of wire and the output winding has only 100 turns, then the output voltage will be 1/10 of the 120VAC input or 12VAC. Keep in mind that this is AC voltage (LEDs work on DC) and it has no method of regulation. If the input voltage drops to 105VAC, then the output voltage of the transformer will drop to 10.5VAC.

The next block in a linear supply is made up of a bridge rectifier and a large Electrolytic Filter Capacitor. A bridge rectifier is made by connecting four rectifiers (also known as diodes) in a configuration that converts AC voltage into DC. Rectifiers are like one-way valves for electric current. While the output of the bridge rectifier is DC, it is not the smooth steady kind of DC needed by most electronics. It is more of a Pulsating DC. A large electrolytic filter capacitor is needed to smooth it out. In some ways, a capacitor can be thought of as a very small rechargeable battery. If you charge it with a short “pulse” of electricity, you can then discharge it at a steady low level for a short period of time. This process causes the input charge pulses to change into a smoother, steadier DC output. Think of a bucket of water with a hole near the bottom. If you start with the bucket half full, the flow of water out of the hole is fairly smooth. If you refill the bucket by dumping cups of water into it, you are filling the bucket with “pulses” of water but the output (flow from the hole) stays smooth as long as the hole isn’t too big and you keep filling the bucket with lots of cups of water.

So the first two function blocks give us smooth, low voltage DC that is isolated but not regulated. When powering LEDs we need some regulation to prevent over- drive and to keep the brightness constant. The method of regulation is where a linear power supply gets its name.

Let’s say we want exactly 5VDC as our regulated output. No power supply can convert 100 percent of the input power into the desired output power. Any practical power supply design must also have limits on the range of the input voltage it will work over and the maximum amount of power it can supply. Over-specifying these limits will cost you money, space, weight and may reduce reliability. As I explain how linear regulation works, you will see why over-specifying is not always better for reliability.

GIVE ‘EM A BRAKE
All linear regulation circuits work by taking in a voltage higher than the desired output voltage and dissipating (throwing away) the unneeded extra voltage in the form of heat. Ohm’s Law shows that the greater the current you draw through a given resistance, the higher the voltage drop across that resistance.

If we vary the resistance as the current (or voltage) changes, we can keep the voltage coming out of the variable Resistor constant. Imagine driving a car that doesn’t have a gas pedal. The motor speed is preset to some high rpm. You engage the clutch and the car starts accelerating. When the car gets to the speed you want to go, you apply the brake (increased resistance) enough to keep the car at the desired speed. When going up a hill, you can let off the brake a bit (decrease resistance) because the hill provides part of the needed resistance. Going down a hill, you will need to brake harder (increase resistance to fight gravity).

With a little practice you can “regulate” your speed quite well. A car being a mechanical device, the brakes and clutch will not last long doing this but in its electronic equivalent, it works fine as long as you can get rid of the Waste Heat.

In the linear power supply, a power Transistor is placed in series between the output of the filter capacitor and the power supply’s output. Its resistance is varied electronically based on a signal it gets from a control circuit. The control has a reference voltage that it compares to the output voltage to maintain the desired voltage regulation. (Current can also be regulated to a specified value instead of voltage if needed.) In the car example, you are the control circuit. You look at the speedometer and vary the pressure on the brake to regulate the car’s speed.

THE WASTE HEAT FACTOR
The size, weight and cost of a linear power supply are strongly related to how much total power the supply is rated for. The input operating voltage range can also have a huge impact on size, weight and cost. Let’s get back to our 5VDC example power supply. If we specify the output at 20 amps, that would make it a supply rated for 100 watts of output. Let’s also specify the input voltage range as between 100VAC and 132VAC @ 60Hz.

Now let’s calculate how much waste heat our sample power supply will make and need to dissipate. Remember, heat greatly affects power supply (and LED) reliability. A good starting point is to assume the voltage drop across the regulating power transistor will be at least 3 volts. The output from the filter capacitor is fairly smooth but not perfect. It has some “AC Ripple”. We will assume it is 1 volt peak to peak. This means the average voltage we need to supply to the power transistor will have to be 0.5 volts higher to prevent “dropout”. This is because half of this ripple voltage is above the average voltage and half of it is below the average voltage. The half that falls below the average must be taken into account or the minimum voltage needed for regulation could go just a little too low at that point in the ripple. This could cause the output voltage to “dropout” of regulation at the low point in the ripple voltage.

Let’s work backwards to figure out what the input voltage to the power transistor needs to be.

• Output = 5.0V;

• Regulating power transistor voltage drop = 3.0V; 

• Half of ripple voltage = 0.5V;

Total = 8.5V average VDC.

So if we supply 8.5 volts minimum into the regulation transistor, we can get 5.0 regulated volts out. Under this condition, a maximum of only 59 percent (5 divided by 8.5 x 100) of all input power is used for powering the load (LEDs in this case). The other 41 percent is converted to waste heat. But wait, it gets worse!

Now let’s see how input voltage range affects waste heat dissipation. The 8.5 volts minimum above must be provided at the lowest specified input voltage. In this example, that is 100VAC. Remember the transformer is a voltage ratio-changing device so the actual voltage output of the transformer will be higher at nominal 120VAC in and higher yet at high line input voltage of 132VAC.

Since the output of the transformer is a simple ratio of input voltage, we can use the same ratio to multiply our needed 8.5VDC to determine what it will be at any input voltage: 100VAC:120VAC = 1.2; and 100:132 = 1.32.

This gives us 8.5 x 1.2 = 10.2VDC for 120VAC input condition and 8.5 x 1.32 = 11.22VDC for 132VAC high line input condition. Using the high line output voltage value, 11.22VDC input and 5.0VDC output we can see that only 45 percent of the input power gets to the load and 55 percent is converted to waste heat! There are other losses in the power supply but they are small by comparison.

Based on the above, at first you may assume that for a 100-watt power supply the maximum waste heat is only about 55 watts (55 percent of 100 watts), but the 45 percent represents the 100 watts of output power. We need to multiply that 100 watts by the ratio of loss to output power. 55:45 = 1.22. 1.2 x 100W = 122 watts of waste heat!

GO FISH
Linear power supplies are relatively simple in circuitry compared to switch-mode type. For low-power supplies they can be cost effective and generally they don’t generate any radio interference like some switch-mode supplies can. They are, on average, larger than a switch-mode and not as efficient and so they make a lot of waste heat which can be a problem if the amount of power needed is large.

In the next article I will try and explain the inner workings of switch-mode power supplies. I know this is not a sexy subject but it is an important one for reliable sign design. You may wish I would just give you a simple recommendation on what to buy but the subject has too many variables to be reduced to a few sound bites. Things are always changing too, so I believe in the “teach a man to fish” approach instead of just solving today’s problem. Have patience learning about this and you will be rewarded with more reliable signs and fewer warranty service calls.

Glossary Terms

Galvanic Isolation — No electrically conductive path from the AC input to the DC output. This is a safety feature to prevent shock should a person come in contact with a broken LED in a sign.

Transformer — An electrical component with two or more sets of wire windings separated by some insulation material. The windings are wound on a magnetic core to magnetically couple energy between the winding. Transformers only work with a changing voltage. They act as a virtual electrical short if you apply Direct Current (DC) to them. They may be designed to work on line frequency AC (50Hz or 60Hz) or high frequency (greater than 20KHz) in a Switch-Mode power supply. They provide voltage scaling and galvanic isolation.

Magnetic Core — a material that can couple changing magnetic fields in a transformer. For AC Line frequency transformers, they are most often made of thin pieces of Silicon Steel stacked to the needed height. For high frequency transformers the core is usually made of ferrite. A material often made from manganese-zinc or manganese nickel alloy.

Electrolytic Capacitor — A type of capacitor that has a lot of capacity for its size and price. It contains an electrolyte than can dry out over time and that in turn decreases its capacity. Every 10C increase in operating temperature reduces the rated life of the capacitor by half. A 20C high temperature decreases life to 1/4.

AC — Alternating Current — An electrical current whose magnitude and direction change cyclically. Direct Current generally stays at a more constant magnitude and does not change direction.

Transistor — The electronic equivalent of an adjustable valve.

Resistor — The electronic equivalent of a fixed valve–available in a wide range of values.

AC Ripple — More correctly just called ripple voltage. When an AC voltage is rectified and filtered by a capacitor, the output waveform is not perfect like the voltage DC from a battery. There is normally a small periodic change in the voltage level superimposed on the average DC voltage. The larger the capacitor, the smaller this ripple will be. In a practical design, a capacitor large enough to limit the ripple voltage to 10% of the average DC voltage is considered a good rule of thumb.

Pulsating DC — A rectified AC voltage without any filter capacitor.

Waste Heat — Electrical energy that is converted to heat as part of the power conversion and regulation process that must be dissipated.

(Click here to read Part 2 of this article series.)