LED Power Supply Overview Part 2

“Waste not, want not.” —Old Proverb

In part one of this article (Sign Business November 2006, page 74) I explained how Linear Regulated power supplies work. In this installment I will explain how Switch-mode Regulated or “switching” type power supplies work and summarize the pros and cons of each for end users.

The unit on the left in this photo is a 50W switch-mode power supply that weighs only five ounces. The unit on the right is a 15W linear power supply that weighs over two pounds. That means the unit on the left regulates more than three times as much power while being less than 1/6th the weight of the unit on the right.

First, a brief review of how Linear Regulating power supplies work:

1. The AC line voltage is converted to a lower voltage (often 6 to 48VAC) by a line frequency transformer (see “transformer” and “galvanic isolation” in the glossary included on page 74). The transformer also provides electrical isolation for safety.

2. This lower AC voltage is converted to DC by diodes. This DC voltage is not smooth and steady so it must be filtered by a large electrolytic capacitor.

3. After the capacitor, the voltage is much smoother but not perfectly steady and not regulated to a specific voltage value. It must pass through a device that can vary its resistance electronically. Most often this is a power transistor on a large heat sink.

4. A feedback control circuit drives a pass transistor to vary the amount of power passing through it to maintain the desired output voltage. It does this in response to changes in the AC input voltage or output load current. The “extra” voltage is dissipated as heat. Efficiency is often only 50% or less.

In Part One I used the analogy of a linear regulated supply being like a car without an accelerator. The engine speed is fixed and you control the speed of the car with the brake pedal. This would create a lot of heat and give you very poor gas mileage (or efficiency).

A switch-mode power supply is more like a car with fuel injection. A good fuel injection system puts only a small squirt of gas in each cylinder as needed. It may meter out 4,000 or more of these small squirts a minute. By putting just enough into the engine to maintain speed, you waste less fuel.

In a switch-mode supply, the AC line voltage is rectified (converted to DC) before going through a step-down transformer. It is then filtered by a large Electrolytic Capacitor just like in a linear supply but the voltage is much higher at this point and is not electrically isolated yet.

The typical DC voltage at this point is about 170VDC or 340VDC. (The math: the AC input line voltage 120VAC times the constant 1.414 equals 169.58VAC; i.e.,

120VAC x 1.414 = 169.68VDC and

240VAC x 1.414 = 339.36VDC.

1.414 is the square root of 2 and also the ratio of the RMS (root means square) voltage to peak voltage value for a sine wave.)

You can use a regular voltmeter to measure the AC input voltage, and then calculate the peak value of that voltage. That peak voltage is the value the filter capacitor will charge to.

A transformer cannot work on DC; it needs a voltage that is changing in amplitude over time, which is an AC voltage. We need a transformer for electrical isolation and to scale down the relatively high DC voltage we have created. Since we need AC voltage to make a transformer work, we convert the high DC voltage we have into a form of AC by using transistors that can switch DC voltage on and off very fast. The faster the transistors can switch the voltage on and off, the less power they waste and the higher the frequency at which we can turn them on and off. (In the linear supply, the transistor was on the low voltage side of the transformer and it varied its resistance linearly. It is never fully on or fully off.)

For a given amount of power, the size and weight of a transformer gets smaller the higher the frequency the voltage going into is. For AC power from the wall, the frequency is 60Hz or 60 times per second (50Hz in Europe). In a switch-mode supply, the frequency is always higher than 20,000 times per second or 20KHz. This is so any audible noise is above the range of human hearing since transformers can make some noise.

Most switch-mode supplies today work at higher frequencies (in the 50KHz to 200KHz) range but much higher is possible. Since the size of the transformer decreases proportionally to the frequency increasing, theoretically a 60KHz transformer should be 1,000 times smaller in volume than a 60Hz transformer of the same power rating.

In practice it is not quite that drastic a difference but it is significant. You can easily see why from a size and material cost standpoint you would like to power the transformer at as high a frequency as possible. There are other considerations though and that is why most commercial switch-mode supplies operate in the 50KHz to 200KHz range previously mentioned.

We now have a transistor turning on and off very fast at a high frequency and applying a changing voltage to a high frequency transformer. Let’s say the transistor is on exactly 50% of the time. The voltage coming out of the transformer will be: The DC voltage on the electrolytic capacitor times the transformer turns ratio times 0.5 (for 50% on time). If the capacitor voltage is 170VDC and the turns ratio is 10:1, the transformer output voltage is 170 x 0.1 x 0.5 = 8.5 volts of high frequency AC.

Just as in the linear example in part one, this is unregulated voltage at this point. The exact voltage will change if the input AC voltage changes or as the load current changes.

It is also high frequency AC voltage and we want DC out of our supply. You already know we can convert AC to DC with some diodes and smooth it out with a capacitor, so we do that next. We also add a small inductor in series with the diodes. A small capacitor and inductor work great together for smoothing out the high frequency pulsating DC coming out of the diodes. Since the capacitor is being re-charged many thousands of times per second, it can be much smaller than the one used in a linear supply.

So now we have isolated low voltage DC that is well filtered but still not regulated. Remember, the linear supply regulates voltage by starting with more voltage than needed and dissipating what is not needed as heat. In a switch-mode supply all you need to do is change the amount of time the fast switching transistor is on each cycle to get regulation.

A control circuit is needed to measure the output voltage and compare it to a reference voltage. It then tells the transistor how long to stay on for each cycle. This is known as the duty cycle and is generally expressed as a percentage of the total time of each cycle. (A cycle is just the time from when the transistor turns on, turns off and then starts to turn on again. For a 60KHz supply the total cycle time is 1/60,000 = 16.67uS [uS = micro-second or 1/1,000,000th of a second].)

If only a small amount of current is needed, the duty cycle will be very small. As the load current increases, so will the duty cycle. This dynamic variation of the duty cycle is known as Pulse-Width Modulation or PWM. By “switching through” only the amount of power needed, a switch-mode supply can be very efficient. Some are greater than 90% efficient but almost all quality supplies are better than 75%. The exact efficiency depends on a number of factors. Generally the higher the output voltage and the higher the total power, the higher the efficiency will be.

The circuit that controls the transistors is constantly measuring the output voltage to determine exactly when to turn the transistors on and off. It constantly adjusts the duty cycle based on that voltage or “feedback”.

This process is known as “closed loop control”. In any closed loop control system, there are always small delays in the system. The output voltage doesn’t change instantly when the transistor turns on and off. It may take less than a millisecond (1/1000th of a second), but it takes some time.

All the filtering that turns pulsing current into smooth DC adds small delays too. In an electronic control circuit, a millisecond delay can be a long time. If the feedback control loop isn’t designed properly, it can over react to changes in line voltage and load current and cause instability in the regulation of the output voltage. That is why a well-designed control circuit is paramount to reliability.

Imagine you are in a car driving on a curvy road. When you turn the steering wheel to stay in your lane, you expect the car to respond instantly to how you turn the steering wheel. Now imagine a three-second delay in the car’s response to any adjustment you make in the steering wheel. You are the feedback loop. You would now have to know exactly how much to turn the steering wheel and do it at exactly three seconds before the car needed to change direction.

Most people would find themselves all over the road. This is the equivalent of a bad closed loop feedback that has caused instability in the output of your power supply. If the road was straight and you hold the steering wheel, everything is fine. It is the need to adjust quickly and accurately to change where the problem comes in.

A fixed output power supply that is specified at 12V is rarely at exactly 12.000VDC. It changes some as the input voltage varies and depending on what the load current is. The output voltage will always change at least a small amount (for at least a few milliseconds) when there is a rapid change in line voltage or the load current changes suddenly (like when a switch turns on or off a lot of LEDs).

How well the output of the supply stays regulated is known as Dynamic Regulation. As previously mentioned, a sudden change in input or load can cause the output voltage to become unstable if the control circuit is not properly designed.

Let’s say you have a 12VDC supply rated at 10 amps. You have a sign with a lot of LEDs and half are on all the time and the other half are flashed on and off once a second. The output current coming from the supply could be changing from 4 amps to 8 amps once a second.

A well-designed control circuit might allow the output voltage to have a momentary change up and down of 3% to 5% of the nominal output voltage and then return to that nominal voltage. An unstable supply might allow 10% or even 50% momentary change in output voltage.

During this unstable time, there are often added stresses on the internal components in the supply which may lead to eventual failure of the supply. This is not something you can see or measure with a regular voltmeter. You would need an oscilloscope and other special test equipment.

Ask your vendor what the dynamic regulation is for the supply and if control loop stability testing was done. Because of the technical nature of this subject, you are really at the mercy of the power supply vendor. The best thing you can do is deal with a reputable company. All major power supply makers do this testing. It is when you are considering a “generic” or unknown brand of supply that you really need to ask the vendor for lots of test data. You may not understand all the data but the fact that they can produce it shows they have at least done their homework.

Static load/line regulation is something you can check if you have a voltmeter. Load/line regulation just means the maximum change in output voltage you should see for all combinations of input line voltage levels and all output load conditions under a steady load current.

An example is high input voltage (132VAC) with 5% of maximum load current vs. low input voltage (100VAC) at full load current. The output voltage under these two conditions should fall within the static load/line voltage regulation specifications supplied by the manufacturer.

For a fixed output voltage supply, common ranges for this accuracy are ±1% to ±5%. This means if the output load/line regulation is stated as 12.0VDC ±3%, the output should be between 11.64VDC and 12.36VDC. A 1% tolerance does not necessarily mean the supply is more reliable than one with a 5% tolerance. Don’t pay more for a tighter regulation tolerance unless you need it.

NOTE: When you measure the output voltage to check regulation, make it as physically close to the supply as possible. All wire has some voltage drop with current flowing through it and this can make it seem like the supply is not in specification.

As an example, if you were to measure the voltage at the LEDs and there were 3 feet of #22 gauge wire between the supply and the LEDs, there would be a 0.1V drop for every amp of current passing in that wire. For 10 feet of #22 wire with 5 amps of load current, the total voltage drop would be 1.62V.

If you want the voltage at the LEDs to be the same as at the power supply’s output, make sure you use a wire gauge that is large enough to minimize voltage drop. Remember, a 1V drop from 120V is less than a 1% loss but a 1V drop from 12 volts is over an 8% loss.

If you’ve made it this far I’m sure you agree that power switching power supplies are fairly complex devices. Because of this complexity the quality of the design can vary greatly between suppliers. While a linear supply needs to be properly designed to be reliable, it is less complex and many companies have the technical ability to do an adequate design job.

A properly designed switch mode supply can be just as reliable (or more so in some applications)—if properly designed. Like many products though, the more complex and critical the design, the fewer suppliers there are that do a good job at it.

You may ask yourself why bother with switch-mode type supplies at all if they are so complex. Assuming good design for both types of supplies the following is a short comparison of the potential advantages of each.

For low power (under 20 watts) a linear power supply may be lower in cost. Linears are generally electrically very quiet on their inputs and outputs. For powering LEDs this is generally not important like it would be in say a high-end audio system.

Switch-mode supplies are generally superior in all other features. They can be much smaller and lighter. They are more efficient, so they generate less waste heat for a given amount of power. For higher wattage supplies, they are generally lower in cost than a linear supply. There are models available for operation off of a battery. I have a customer who makes solar-powered remote location signs. A switch-mode supply allows the product to get the most out of the battery by being very efficient.

Switch-mode supplies have matured a lot in the last 25 years and have proven themselves to be reliable with good design and quality manufacturing. Don’t let their complexity scare you off. Just ask your vendor lots of questions to be sure you are dealing with the right supplier.

Remember, you can have the best LEDs in the world in your sign but if the power supply is not reliable, great LEDs won’t matter.

Glossary Terms

Regulation – to maintain a constant output level of voltage (or current in some supplies) regardless of input voltage changes and/or output load changes.

Diode – A solid-state device that allows electrical current to pass through it in only one direction. Think of it as a one-way valve for electrons.

Transformer – An electrical component with two or more sets of wire windings separated by some insulation material. The wires 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) (Line Frequency Transformer) or high frequency (greater than 20KHz) in a switch-mode power supply. They provide voltage scaling and galvanic isolation.

Galvanic Isolation - No electrically conductive path from the AC input to the DC output. This is a safety feature to prevent shock if a user could come in contact with say a broken LED in a sign. Isolated low voltage (less than 42V) is not considered to be a shock hazard by safety agencies like UL. It is similar to a 9V or 12V battery in terms of shock hazard. At these low voltages even with wet hand touching the battery terminals directly, you won’t get a shock.

Electrolytic Capacitor - A type of capacitor that has a lot of capacity to store an electrical charge for its size and price. Can work similar to a small battery but can be charged and discharged almost instantly. It contains an electrolyte solution that can dry out over time and that in turn decreases its capacity. For every 10C increase in operating temperature, the rated life of the capacitor is reduced by half. A 20C increase in operating temperature decreases life to a quarter of the original.

(Click here to read Part 1 of this article series.)