# Cool Cap Engineer

## Tutorials: Simple Electronic Load Circuit

Electronic load on bench. Drawing close to 1.20A at 21V

A couple weeks ago, I had a discussion with one of my co-workers about electronic equipment. I complained that alot of the equipment that I want is very expensive. I told him some of the requirements that I’m looking for. After a couple minutes talking, he said to me “Why not build it yourself? The requirements are not that stiff.” “Crap, he’s right”, I said. After couple weeks researching the topic and brushing up on Mosfets, I built a very simple one. There is a alot of refinement that could be done on the circuit, but it demonstrates the topic nevertheless.Today’s post will talk about how this electronic load circuit works and how to build one yourself.

An electronic load is a device design so a power supply can draw a certain amount of current without desipating tons of heat. Electronic loads are useful for testing a power supply’s efficiency, current limit, etc.To understand how the circuit works, I’ll talk about two crucial things involving mosfets and op amps. Let’s talk about the mosfet aspect first.

VDS vs ID characteristics. Shows the mode we want to operate to behave as an electronic load.

Although the picture above looks like a bunch of lines for hobbyists, this VDS vs Id characteristics shows how the mosfet will behave depending on the voltage applied at the gate and source (VGS for short) Mosfets have 3 modes of operations: cutoff, active, and saturation. When the mosfet is in cutoff operation, it does not turn on. This is due to the voltage applied to the gate and source  not high enough to turn it on. Active is a state in which the mosfet’s gate has enough voltage to allow the mosfet to behave as a variable resistor. Finally, saturation is when the mosfet behaves like a switch. For this load to work, we need it to enter active mode since we can set the mosfet to draw a certain amount of current.

Now let’s talk about one common configuration using an op amp: a unity amplifier. The sole purpose of a unity amplifier is to make sure that the voltage seen at the non inverting pin of the op amp (+ pin) appears at the output. Although it sounds simple, these amplifiers are useful to prevent the input source from getting affected from output impedance. However, we will be using this configuration for another reason.

Now let me explain how the circuit works. First the user sets the voltage using a potiemeter (R2 in the schematic) and gets set at the inverting pin (- pin).  Although the op amp looks like its configured as a comparator, it’s actually set as unity amplifier due to the mosfet. To make sure the voltage at the non inverting pin appears at the inverting pin, the op amp must set a certain voltage to the mosfet gate. Depending on the gate voltage, the mosfet will draw a certain current until the voltage at R1 is equal to the voltage at the non-inverting pin.

Unlike my other tutorials, there’s a high possibility you’ll damage the mosfet in this tutorial, if you’re not careful. When looking at the datasheet for a mosfet, remember to pay attention to the drain current (ID) and drain to source (VDS) ratings. ID determines how much current the mosfet can handle before exploding and VDS determines the maximum voltage that can be applied for safe operation.

BUZ11 safe operating area per datasheet

Now for the little known parameter you should REALLY look at when reading mosfets datasheets: the safe operating range (SOA). Keep in mind, mosfets liked to be switched on and off. Rarely do you want the mosfet to work with analog voltages. The SOA tells you the recommended voltage and current that the mosfet can handle before exploding.

The following circuit is designed to handle 20V from 0A-5A. You’ll need the following to build this circuit

1x 10k Poteimeter

1x Buz11 Mosfet

1x 10Watt TO-220 Resistor

1x 741 Op Amp

2x .1uF Capacitors

2x Heatsinks

Misc. wires

1x Cooling Fan (optional)

Consequentially, you’ll need the following tools for this circuit

1x +15V/-15V power supply

1x +32V/5A Power Supply

1x Mulitmeter

Now assemble the circuit as shown in the picture below. Remember to attach the heatsinks to the resistor and mosfets! THEY WILL GET DAMAGED WITHOUT THE HEATSINKS! If you have a cooling fan lying around, turn it on and direct it towards the resistor and mosfet. The colder these components run, the less likely they will get damaged.

Before Turning The Circuit On.
Do not apply 20V to the mosfet just yet.You want to make sure the voltage at the poteimeter is 0V, otherwise your power supply will go into current limit.  Apply power to the op amp/poteimeter. Make sure the voltage at the poteitmeter is 0V. Now apply power to the mosfet and start turning the knob. You should start seeing the circuit consuming a certain amount of current will keeping the voltage of the power supply the same.

## Mini Projects: Boost Converter Experimentation

Since I started working in the power electronics industry, I figured I should spend a little more time building power electronic circuits. I remembered that I understood how non-isolated boost converters worked during my school work and decided to build one for myself.  For those who do not know, a boost converter is a power electronic circuit that converts incoming voltage to a higher voltage. I decided for starting purposes, I would build a 12V to 24V boost converter. Although I plan to write a tutorial which shows how boost converters work,  this post is to talk about what I did in my free time.

12V TO 24V Boost Converter Schematic

After a week relearning important boost converter design parameters, I managed to draw out the basic schematic. Although there are 3 components missing from my schematic, those parts functioned as a way to implement a controller for the boost converter. There are alot of major improvements that could be made, such as protecting Q2 from high voltages, solving the logic inversion caused by Q1, and prevent L1 from causing my power supply to current limit. However, I just want to see the boost converter work.

Boost converter circuit soldered onto a perfboard

After buying my parts from Digi-key, I soldered my parts to a perfboard. Why not assemble the circuit on a breadboard? In order for this  boost converter to properly work, I needed to switch the main transistor (Q2 in the schematic) at a high frequency (I based my calculations around a 62.5KHZ switching frequency). Since I’m switching at a high frequency, building the circuit on a breadboard will screw up the signal due to the breadboard’s nature of acting as a capacitor at high switching frequencies.

Finally, to make sure the boost converter functions correctly, I needed to connect a load. I decided to go with a 25 ohm/50W resistor. If the boost converter was not connected to a load, then there’s a high chance the boost converter will go unstable. A high value resistor can be connected at the output to function as a dummy load to prevent the boost converter from going unstable. Also, I was dissipating a lot of heat through this resistor. The resistor got so hot that it melted one of my oscilloscope clips.

Blue waveform is the output at the load, while yellow is the square pulse going into the the Q1.

So I was ready to power up my converter. Since I was using my multimeter as a current meter, I had to setup my crappy Hantek oscilloscope to measure the voltage by measuring the DC level. One thing that surprised me was the actual duty cycle needed to set the voltage to 24V. To get 24V, instead of setting my frequency generator to 62.5KHZ with a 50% duty cycle, I got a 24V output  using the same frequency, but a 25% duty cycle. This usually happens when your boost converter is incredibly inefficient. Brother, my boost converter was the definition of it. According to my calculations, my boost converter efficiency was around 70%.Well…it was a good attempt, but converter needs a lot more work. I will need to investigate the causes of the low efficiency, and rectify it. My first suspicion involves the big bulky inductor. The bigger the inductor, the higher the parasitic resistance.

Thank you guys for reading this post, and if you have any suggestions on how I can improve the efficiency, then leave a comment below!

## Projects: Single Level Inverter Implementation #1

So a couple months ago, I did an REU power electronics experience at University of Tennessee at Knoxville. Despite the initial frustration, the experience was worthwhile and made me realize the importance of power electronics in today’s industry. I spent most of the research at the REU program running ideal simulations of a seven level cascaded that’s powered by a PV module for each level.  While participating in the research, I learned various parameters that must be taken in account when implementing a real multilevel inverter.  After participating in the program, I went to work on designing the circuitry, PCB, and selecting the parts for my own multi-level inverter.

So how does a multilevel cascaded inverter work? First, let me explain how a single inverter level works. A single level of an inverter consists of 4 switches and a DC source connected between them.  A single inverter level must be able to generate 3 different switching states: 0 volts( when only the top two switches turn on), VDC (when the top left and bottom right switches turn on), and -VDC (when just top right and bottom left switches turn on).

Now, let say we connect another inverter level to the original level, but the switching sequence is out of phase of the original. By connecting out of phase inverter levels, you’ll get a stair case waveform, where the number of stairs is just two times the number of inverter levels added by one. By adding an infinite amount of out of phase inverter levels in series with each other, you’ll get a perfect sinusoidal waveform. The easiest way to understand this is to think of the multilevel inverter as those baby stacking toys you use to play with. Figure 1 shows an example of a S level inverter and figure 2 shows the waveform associated with the multilevel inverter.

Figure 1: Multilevel Cascaded Inverter example

Figure 2: Multilevel Inverter Waveforms (n level)

The goal is to successfully implement a single level of the inverter. Not only am I doing this for sheer curiosity, but it can be use for my microcontroller class I will be taking this fall. By implementing the single level correctly, the multi-level inverter can be implemented later. So, there were a couple things I had to take in account when selecting the parts for a single level of a inverter.

Transistor Blocking Voltage:  The blocking voltage has different terminologies for different transistors. For example, the VDS rating is the blocking voltage of the mosfet, while the VCE rating of a BJT is also the blocking voltage. The blocking voltage must be 150% higher than the peak voltage of the inverter. The 150% may seem excessive, but I want to make sure your transistor will not burn out due to the peak voltage of the inverter.

Transistor Current Rating: Since my project will use mosfets, we want to make sure the drain current rating of the mosfet is much higher than the possible peak output current of the inverter. Like the blocking voltage rating, you want to choose a current rating 150% higher than the expected output current of the inverter.

Gate Driver: Let’s get one this straight; transistors are not ideal switches. Transistors have a lot of imperfections and limitations that makes everything I learned from my ideal seven level cascaded inverter simulations absolutely useless.  Among them, transistors cannot instantaneously turn on or off due to the limited rise and fall times associated with them.  The rise and fall times for a mosfet can be further reduced by adding a gate driver to the gate of the mosfet. The gate driver will allow a bigger current to go through the gate of the mosfet, allowing a faster charge time for the mosfet’s gate capacitance.  By increasing the charge time of the mosfet’s gate capacitance, you achieve a faster rise time for the mosfet. Of course, the gate driver has its own rise/fall time as well.

Although I did not select the parts yet, I managed to design the circuitry and the PCB associated with the inverter with Eagle Cad since the parts will keep the same format for the most part. I’ll update you guys on any further progress I make with this project.If you worked on multilevel inverters, and you have a complement, or compliant, let me know by posting a comment! Thank you and have a great day!

Figure 3: Single Level Inverter Schematic

Figure 4: Single Level Inverter PCB Design