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This version (30 Oct 2012 15:52) was approved by Doug Mercer.
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Table of Contents
Activity 15. DC-DC Converters I
Objective:
The object of this activity is to explore an inductor based circuit which can produce an output voltage which is higher than the supplied voltage. This class of circuits are referred to as DC to DC converters or boost regulators.
Background Basics:
When the current flowing in an inductor is quickly interrupted a large voltage spike is observed across the inductor. This large voltage spike can in fact be useful in some cases. One example is the DC to DC boost converter, which is a circuit that can create a larger DC voltage from a smaller one with very high efficiency. The basic idea is to combine an inductive spike generator with a rectifier circuit, as shown in figure 1. Whenever the transistor is abruptly turned off the voltage at the drain spikes up, the diode D1 is forward biased and current will flow from the inductor to charge up the high capacitance storage capacitor C2. When the drain voltage subsequently drops below the voltage on the capacitor, the diode is reverse biased and the output voltage remains constant. Just as in the chapter on AC power supplies, the output capacitor must be sized appropriately to minimize the ripple relating to the current flowing in the load. We will just use a small capacitor here and the circuit will not be able to source a large output current.
Materials:
Analog Discovery Lab hardware
Solder-less breadboard
Jumper wires
1 - LM2901 quad comparator (or similar device like a CMOS single supply opamp)
1 - 74HC00 quad CMOS NAND gate (or similar device)
1 - ZVN2110A NMOS FET (2N7000 or power FET device such as IRF510)
1 - 20KΩ resistor
1 - 10KΩ resistor
1 - 4.7KΩ resistor
1 - 1mH inductor
1 - 47uF capacitor
1 - 220uF capacitor
2 - rectifier diodes (1N4001, 1N3064)
Additional Equipment:
Small handheld DMM
+5 V bench power supply or battery holder for 3 AA cells to provide +4.5V
Simple inductor and switch DC/DC Converter:
Build the circuit in figure 1 on your solder-less breadboard. Note that in this inductor based DC to DC converter the spikes of current needed may exceed the limits of the on-board +5V supply of Discovery and cause it to shut down. You should use a standalone wall powered bench supply or batteries. You can use a 1N4001 or a 1N3064 for the rectifier diode. Start with a load resistance of 100kΩ and a switching frequency of 10 kHz which can be supplied by AWG1. What is the DC voltage of the “boosted” output? Record the value for your lab report.
Figure 1 Simple DC to DC converter
Now increase the frequency to 50 kHz. Measure and record the output voltage again. Explain why it has changed? One advantage we have here is that we can control the time period between the peaks of the signal going into the rectifier; in the power supply lab, we were stuck with a 60 Hz source.
Next decrease the load resistance to 10kΩ and again measure and record the output voltage.
Clearly what is needed if we want a constant output voltage is some active regulation to keep the voltage constant when the load resistance changes. A larger output capacitance to filter out the ripple would be a good addition as well. There are a few simple ways to implement the active regulation, and indeed there are a number of other interesting design considerations for DC to DC boost converters that you can read about if you are interested, but our goal here is really just to illustrate the concept, so the circuit shown in figure 1 is not optimal in any practical sense. With more careful design, boost converters can drive a much larger output current at very high conversion efficiency (very little wasted power). Note that the DC to DC converter requires a square-wave generator to drive it. Ordinarily this square-wave generator is part of the circuit and also powered by the input supply, so that the conversion circuit is self-contained. We will discuss the design of oscillators at the end of the lab as optional exercises.
Adding active regulation:
Directions:
The breadboard connections for the regulated version are as shown in figure 2. The DMM should be connected to measure the voltage at VOUT. The +5V bench power supply should be connected to the VIN node. The output of the arbitrary waveform generator, serving as a DC reference voltage, drives the positive input of the comparator at pin 9. The digital clock output drives the second input of the first NAND gate at pin 2. Scope input 2+ (single ended) is connected to the output of the comparator at pin 14. Diode D2 is not required if a power FET, such as the IRF510, is used because devices such as that have the diode built in.
Figure 2 Regulated DC to DC boost converter
Hardware Setup:
The waveform generator should be configured for DC output and 2.5V offset. The digital clock output should be configured for a 50% duty cycle and 100 KHz output frequency. One of the digital outputs from the Analog Discovery could be programed for this or the second AWG output could be used as well. The single ended input of scope channel 1 ( 1+ ) is used to measure the signal seen at the output of the analog voltage comparator.
Procedure:
Be sure to start up the waveform generator and digital clock outputs on the Analog Discovery Lab board before turning on the +5V bench supply. The regulated output voltage at node Vout should be observed as the DC offset value of the waveform generator is adjusted. It should be equal to 3 times ( (R1+R2)/R1) ) the DC value of VREF.
Questions:
What is the effect of changing the DC value at the positive input of the comparator?
What is the effect of changing the frequency of the digital clock output?
Is there a minimum? or a maximum?
How much current can the circuit supply to a load?
Is that current affected by the voltage the output is set to?
Is that current affected by the frequency of the digital clock output?
Calculate the conversion efficiency of the circuit. Ratio of output power ( Iout * Vout ) to input power ( Iin * 5V )
Change the value of L1 and redo the above. How have the results changed?
Inverting DC - DC Converter
Objective:
The object of this activity is to explore an inductor based circuit which can produce an output voltage which is negative with respect to ground. This class of circuits are referred to as inverting DC to DC converters.
Materials:
1 - LM2901 quad comparator (or similar device)
1 - 74HC00 quad NAND gate (or similar device)
1 - generic PNP transistor (TIP32 or similar device)
1 - 20KΩ resistor
1 - 10KΩ resistor
1 - 4.7KΩ resistor
1 - 2.2KΩ resistor
1 - 1mH inductor
1 - 47uF capacitor
1 - 220uF capacitor
1 - rectifier diodes (1N4001, 1N3064)
Additional Equipment:
Small handheld DMM
+5 V bench power supply
Directions:
The breadboard connections are as shown in the diagram below. The DMM should be connected to measure the voltage at Vout . The +5V bench power supply should be connected to the Vin node. The output of the function generator drives the negative input of the comparator at pin 8. The digital clock output drives the second input of the NAND gate at pin 2. Scope input 2+ (single ended) is connected to the output of the comparator at pin 14.
Figure 3 Inverting DC - DC Converter
Hardware Setup:
The waveform generator should be configured for DC output and 2.5V offset. The digital clock output should be configured for a 50% duty cycle and 100 KHz output frequency. The Single ended input of scope channel 1 (1+) is used to measure the output of the analog voltage comparator.
Procedure:
Be sure to start up the waveform generator and digital clock outputs on the Analog Discovery Lab board before turning on the +5V bench supply. The regulated output voltage at node VOUT should be observed as the DC offset value of the waveform generator is adjusted. It should be equal to - VREFwhen VREF is set to 2.5V (assuming VIN is +5V).
Questions:
What is the effect of changing the DC value at the negative input of the comparator?
What is the effect of changing the frequency of the digital clock output?
Is there a minimum? or a maximum?
How much current can the circuit supply to a load?
Is that current affected by the voltage the output is set to?
Is that current affected by the frequency of the digital clock output?
Calculate the conversion efficiency of the circuit. Ratio of output power (Iout * Vout) to input power ( Iin * 5V )
Change the value of L1 and redo the above. How have the results changed?
Circuit Additions:
What sort of circuit could you make to generate the 100 KHz square wave other than using the digital clock output on the Discovery Lab board? There are two additional gates in the 74HC00 package. The other two NAND gates along with RC delay network, R4 C4 can be configured into a ring oscillator as shown below. The values for R4 and C4 are approximate for 100 KHz and can be adjusted as needed.
Figure 4 stand-alone gated oscillator
Questions:
What other types of oscillator circuits might be used to generate the 100 KHz square wave?
The DC reference voltage from the waveform generator output of the Analog Discovery Lab board could be replaced by the band-gap reference circuit from Activity 8 in this series. The +5V supply can be connected where reference input is shown in the diagram and R1 and R2 adjusted to produce the desires reference voltage ( where 2+ is shown ) to be used at the plus input of the LM2901 comparator.
Figure 5 Band gap reference from Activity 9
Questions:
What other types of reference circuits might be used to generate DC voltage needed for the plus input of the comparator?
university/courses/electronics/electronics-lab-15.1351608731.txt.gz · Last modified: 30 Oct 2012 15:52 by Doug Mercer