Power Supplies for Beginners, Part 4

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Power Supplies for Beginners, Part 4

In part 3 of our series of video tutorials for beginners on power supplies, we looked at testing and using Linear Power Supplies. Here in part 4 of our video tutorial series we will look at testing and using Switching Power Supplies including buck and boost converters which can step-down (buck) or step-up (boost) the output voltage.

Watch the Part 4 video to learn more!

Following Transcript of the Video Tutorial

Time: 0:00sHello I’m Chris Richardson, an electronics engineer who focuses on power supplies. This is the fourth in a series of web seminars for people who like power supplies but who aren’t necessarily trained to be electronics engineers.

So far in this series we have gathered some low cost equipment to test power supplies, both unregulated power supplies, tested various linear regulators, and now its time to test some switching regulators. The modern power supply is what dominates the market today.

The Buck Regulator – Simplest of Switches

buck regulator

Time: 0:25sThe “buck” is the simplest switching regulator, and is the easiest to understand. The control switch on top, a bipolar transistor, or more commonly a MOSFET works together with diode D1 to make a rectangular wave at the point where the switch, diode and inductor connect.

This point is the switching node and it is the most important voltage to probe in the system. The inductor and capacitor form a low pass filter whose output is then mostly DC (Direct Current) with some AC (Alternating Current) ripple. The average value of that output voltage depends upon the input voltage and upon the duty cycle of the rectangular wave.

Time: 0:56sDuty Cycle is equal to TON divided by the sum of TON and TOFF and the higher the duty cycle, the higher the output voltage. This switcher bucks-down the output voltage, hence its name, Buck Regulator.

Much like a linear regulator, the theoretical maximum VOUT is equal to VIN. In practice the maximum VOUT we can achieve is somewhat less than VIN.

Testing a Switching Regulator

Time: 1:17sThe first switching regulator we are going to test today is the buck regulator, and you can see the circuit here. This is the input capacitor, and small capacitors, these are the two control switches. This is a synchronous regulator, meaning that instead of a diode for the low side it has a MOSFET. The power inductor with the loop of wire connected in series is to put a current probe if required, and these are the output capacitors here.

Right now this buck converter is unloaded and I am using the +5 volts from the ATX power supply and here is approximately 5 volts in, and the output I have adjusted to 1.9 volts, approximately. Right now there is no load and these four power resistors here of 8Ω each are connected in parallel to give a load of 2Ω.

When I connect them they give a load of about 1 amp. You can see that in the input voltage drops slightly, and also the output voltage drops slightly but it is still regulating.

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Time: 2:13sI have switched things around to show the high efficiency of the switching regulator here. So now I am measuring input voltage here on the blue multimeter, and input current on the orange multimeter here. So I want you to see that when we power the circuit with the load connected, at 5 volts it draws about 380mA (0.380A).

Now I am using the 12 volt input to power it and you can see 12 volts on the blue multimeter, the load is the same and the output voltage is the same, but now the output current has dropped to 210mA (0.210A).

This is an interesting property of switching converters. As the input voltage goes up, the input current goes down. In fact when you are testing a switching regulator, one of the first basic tests is if you have a variable input supply is to watch and make sure that as you increase the input voltage the input current goes down.

Time: 2:59sThis is the backside of the buck regulator circuit. I am measuring the switching node and output voltage with two oscilloscope probes, there is one amp load being powered by the 12 volt input. We can see the switching node in yellow with its duty cycle and the resulting ripple on the output voltage. About 20 to 30mV peak-to-peak.

Time: 3:23sThe same experiment again except that this time we are powering it by 5 volts at the input, and now we can see that the duty cycle goes much higher, we are in the buck converter and the duty cycle is approximately equal to the output voltage at 1.9 volts divided by the input voltage of 5 volts, and we also have slightly lower ripple maybe somewhere in the range of 10 to 15mV.

The Boost Regulator

boost regulator

Time: 3:45sA boost regulator is little more than a buck regulator operating in reverse. Just imagine that D1 was a MOSFET and that TR1 a diode. Where you see VIN there is always a capacitor even though it is not shown in this schematic.

Boost Regulator is a great circuit for explaining why an inductor is the heart of most switching regulators. To make an output voltage that is higher than the input voltage, the boost regulator stores energy in the inductors magnetic field which develops as the current in L1 increases while TR1 is “ON”.

The control circuit turns TR1 “OFF” while the current is still flowing. The inductor can increase the voltage across it to infinity in theory in order to maintain that current flow. Unfortunately it does not have to go to infinity just increase enough to put D1 ON and into forward bias then the current can flow to the output. In theory the boost regulator itself can increase the output voltage to infinity, but in practice it is limited to about 10 times the input voltage (10*VIN).

One last but important note here. The boost regulator can only increase VOUT with respect to VIN which is like the exact opposite of the buck regulator.

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Time: 4:45sFrom my example boost regulator, I am using a pcb here that actually has two switching regulators. The top one here is an inverting regulator, which is a very interesting topology but we do not have time here to talk about it so it is disabled.

The bottom part is a boost regulator and the parts are quite small. The input capacitor is actually hidden underneath the input leads. This is the power inductor and the output diode is hidden behind this test fixture which I use to make very precise measurements of the switching node while the output capacitor is also hidden beneath this test fixture and that is also there to make precise measurements for the output voltage.

Right now I have +5 volts in coming from my ATX power supply and about 14.7 volts out. You may be wondering why I would have 14.7 volts, and the honest answer is, this circuit like most of the ones I have shown are ones left over from things I did especially for different kinds of customers. 14.7 volts is not a typical voltage, but these are adjustable regulators so you can get just about any voltage you want out of them.

Here is the boost converter again, but this time I am measuring input current and output current, so when I switch it “ON”, the 5 volts it is drawing about 900mA (0.9A) at the output and 3.5 amps at the input. Remember that that is the opposite of the buck converter and in the buck converter since the output voltage is lower, the input current is always lower than the output current.

In the boost converter it is the reverse. As power efficiency is high, input power is approximately equal to output power. So since input voltage is lower than output voltage, input current is higher than output current.

Time: 6:25sFor the last boost converter test, I have the circuit here powering the same load at about 900mA and 5 volts in to 14.7 volts out and here we can see the switching node in yellow and the output voltage AC coupled in blue.

There are two important things to notice as differences between this and the buck converter. The switching node voltage goes between zero and the output voltage, and the ripple is much higher. That is always true in a boost converter. In the boost converter the input voltage is much smoother while the output voltage has more ripple, and the opposite is true in a buck converter.

Inverting Buck-boost Regulator

inverting buck-boost regulator

Time: 7:02sThe final topology of the three basics is the Buck-boost Regulator. As the name implies, it can generate an output voltage whose actual value is higher or lower than that of the input voltage. But, and this is a big but, the polarity of the output voltage is reversed with respect to the input.

The number of modern circuit which need negative voltages (-ve) is shrinking. But sensitive amplifiers, sensors and other equipment still use both positive and negative voltages to operate.

Like the boost, the buck-boost uses the amazing ability of the inductor to make that negative voltage. In this case the voltage across the inductor reverses polarity in order to maintain current flow, when TR1 turns “OFF”. If you inspect the transfer function on the left, in theory the output voltage can go to negative infinity. In practice you get to about minus 10 times the input voltage, (-10*VIN).

Time: 7:50sI am sorry to say that I could not find any evaluation board or demo pcb to show off and actual inverting buck-boost regulator. But you can usually take almost any buck regulator and turn it into an inverting buck-boost by changing the polarity and the connection of the output diode and the output inductor. So perhaps I could do something like this in a future video.

The Flyback Regulator

flyback regulator

Time: 8:11sNo discussion of switching regulators would be complete the Flyback regulator. In terms of shear volume, the most common switcher in existence is the buck. Just your mobile phone has about five or ten of them, but the flyback is number two. Just about every AC to DC power supply under 50 watts uses this very flexible topology in one form or another.

The flyback regulator is based on the buck-boost regulator but it has two windings in its inductor. In fact if you made Nps which is the ratio of the two windings equal to one (1:1), the flyback regulator and the buck-boost regulator would have exactly the same transfer function. The ratio of those windings allows VOUT to be equal to VIN, but it could also be much, much higher or much, much lower.

The two windings can also be isolated, and this is great for both electrical safety, as in not electrocuting anyone and also for isolating sensitive circuits from noisy ones. Finally as the flyback output is disconnected by the transformer or coupled-inductor, can be positive or negative.

Time: 9:05sWith all the wires disconnected I can show you the most important parts of this supply. So we have the input capacitors here, a discrete power and MOSFET here on the primary side. This is the transformer or better known as a coupled inductor, the output diode and the output capacitors. Notice also that there is this separation between the primary side and the secondary side so this can be an isolated converter, again that is for electrical safety or to get rid of noise.

On the backside we can see again the separation here and these diodes are actually shorting the ground of the primary to the ground of the secondary. So this particular circuit is not isolated, but it could be. If we wanted to isolate it we would get rid of these resistors and use a device called an opto-coupler to do the feedback for the control of the power supply.

Flyback Power Supply

flyback power supply

Time: 9:54sHere I have a small flyback power supply that is designed to operate from 36 volts upto 72 volts. That is typically known as a telecom range. It’s at the very, very limit of what I can do with my ATX power supply. So to get these 22.4 volts, which should be 24 volts, I am actually running from the negative 12 volts (-12V) up to the positive 12 volts (+12V) and I can do that because the ATX power supply uses the same common ground for both of these two.

But there is a problem. If I was to try and measure with an oscilloscope probe here. Since this is earth, as soon as I connect and touch this here, my supply turns “OFF” as I have caused a short circuit from the output voltage to earth. So what I am going to do is cheat a little bit and actually use what is called an isolated lab power supply to do the rest of the experiments.

Here is the flyback regulator again, but now it is being powered by 48 volts at the input and this is coming from this triple lab DC power supply. I checked on eBay and these typically cost anywhere from about 100 euros to 200 euros. So they are not free, but they are not, lets say a budget buster.

Flyback Power Supply Waveforms

flyback power supply waveforms

In any case, a flyback regulator actually has two switching nodes. It consists really of two inductors that are coupled on the same core. So we are looking at the primary side connected to the power MOSFET in yellow, and the secondary side connected to the diode in blue. Also notice that these voltages are much higher than the ones we have been dealing with so far.

So you have seen me in these videos touching the circuits while they are operating, and that’s fine maybe for circuits operating up to 12 volts or so, but I definitely would not touch this circuit while it is operating as 48 volts is enough to give you a nasty shock.

That concludes part 4 of power supplies for non-EE’s. Stay tuned for part 5 were we will compare and contrast the different types of power supplies so far to see which ones work best in which situations.

On behalf of myself and electronics-tutorials.ws thanks for watching.

End of transcription.

You can find more information and a great tutorial about buck and boost power supplies by following this link about: Switch Mode Power Supply.

In part 5 and final tutorial of our video tutorial series on power supplies for beginners, we will discuss and compare the different types of power supplies including the switching and linear power supply we have looked at over the previous tutorials.


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