What causes tension ripple

DC-DC boost converter¶

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  • DC-DC boost converter

Goal setting¶

Here we will examine an inductive circuit that can produce an output voltage that is higher than the input voltage. This circuit class is referred to as a DC / DC converter or boost regulator. In this experiment the voltage from a \ (1.5 \, V \) supply (battery) is raised to a sufficiently high voltage (\ (\ approx 5 \, V \)) to operate two LEDs in series. Note that the forward voltage of the LED is typically\ (1.8 \, V \)although with some diodes it is up to\ (3.3 \, V \)can be (blue LED).

Remarks¶

In these tutorials we use the terminology from the user manual when it comes to the connections to the Red Pitaya STEMlab board hardware. The oscilloscope and signal generator application is used to generate and observe signals on the circuit. The extension connector pins for the power supply \ (+ 5 \, V \), \ (- 3.3 \, V \) and \ (+ 3.3 \, V \) are shown in the documentation.

Background information¶

Temporarily connect one of your LEDs to the \ (1.5 \, V \) battery. Pay attention to the polarity of the diode so that it faces forward. Is it lit? Probably not, since \ (1.5 \, V \) are generally not enough to turn on an LED. We need a way to raise the \ (1.5 \, V \) to a higher voltage in order to operate a single LED, let alone two LEDs connected in series.

A step-up converter is a DC / DC power converter that increases the voltage (when lowering the current) from its input (supply) to its output (load). It is a class of switched-mode power supplies (SMPS) that contain at least two semiconductors (a diode and a transistor) and at least one energy storage element: a capacitor, an inductor, or a combination of both. In order to reduce the voltage ripple, filters made of capacitors (sometimes in combination with inductors) are usually added to the output (load-side filter) and input (supply-side filter) of such a converter.

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The functionality of the DC-DC boost converter is explained in detail in this Wikipedia article. A brief overview of the theory is recommended before performing the experiment.

The classic DC-DC boost converter circuit is shown in. The inductance \ (L_1 \) must be selected depending on the desired operating (switching) frequency and the maximum rated current. In this experiment a \ (100 \, \ mu H \) power inductance with a \ (1 \, A \) rated current is used for \ (L_1 \). The operating (switching) frequency should be in the range of \ (10-50 \, kHz \). Classic 1N4001 or 1N3064 diodes can be used for the rectifiers \ (D_1 \). We use IRLU120N for the \ (M_1 \) transistor. We chose this power MOSFET transistor because it has a low threshold voltage \ (V_TH \). If you use FET transistors with a high threshold voltage and a low voltage driver signal (gate signal), the switching of the MOSFET may not be optimal. The selected MOSFET already has an integrated bulk diode, so that an external diode D2 is not required.

For the storage capacitor \ (C_1 \) electrolytic high-capacity capacitor can be selected. The choice of this capacitor depends on the rated currents, the switching frequency and the value of the inductivity. But to be on the safe side, values ​​over \ (10 ​​\, \ mu F \) would be sufficient. A DC-DC boost converter used in this experiment is shown in FIG.

DC-DC boost converter

The basic DC-DC boost converter circuit is shown in FIG. A \ (200 \, \ Omega \) load is added to the converter circuit. Either constant load or load regulation is required for stable operation of the DC-DC boost converter. Without regulation, any change in load will affect the output voltage level. Therefore we have set \ (200 \, \ Omega \) load to stabilize the output voltage. In parallel to the load, two LED diodes are added in series with \ (1 \, k \ Omega \) resistors. Note that adding or removing additional LEDs in parallel to the load has no effect on the output voltage, as the current consumption of the LEDs is much less than the current consumption of the \ (200 \, \ Omega \) load. LEDs serve as an indication that our DC battery voltage is being increased from \ (1.5 \, V \) to \ (\ approx 5 \, V \). If the LEDs are off, it means that our battery voltage is below the LED bias voltage (\ (2 \ cdot 1.8 \, V \)), indicating that the DC-DC step-up circuit is not working properly.

Red Pitaya STEMlab outputs can generate voltage signals with a maximum output range of \ (+/- 1 \, V \) (\ (2 \, V_ {pp} \)). Since the higher signal amplitudes are required for MOSFET switching, we have used two NPN transistors in switching mode as an intermediate stage between the OUT1 switching signal and the MOSFET transistor. The square wave signal OUT1 switches the first NPN transistor on and off, causing the collector voltage to fluctuate between \ (0-5 \, V \). This collector voltage then controls the second NPN transistor and its collector voltage, which also fluctuates between \ (0-5 \, V \), then switches the MOSFET transistor on and off. The reason for using two NPN transistors is that the OUT1 and MOSFET gate signals must be in phase. I.e. when OUT1 is high, the signal on the MOSFET gate should also be high. The use of only one transistor leads to a phase shift of \ (180 \, ^ \ circ \). You can see the other more important problem here as well. If we only use an NPN transistor, then if OUT1 is constantly turned off, the MOSFET transistor will turn on constantly and create a short circuit: battery - inductance - mosfet - gnd. The use of two NPN transistors prevents this.

warning

Note that the \ (+ 5 \, V \) voltage rail of the STEMlab is only used for the transistor circuit and not for the load supply. That means the electrical energy flows from the battery to the LOAD and the LEDs.

Materials¶

  • Red Pitaya STEMlab
  • 1x \ (1 \, k \ Omega \) resistance
  • 3 x \ (470 \, \ Omega \) resistance
  • 1x \ (10 ​​\, k \ Omega \) resistance
  • 1x \ (100 \ mu H \) power inductance
  • 1x \ (47 \ mu F \) capacitor
  • 2x LED (red)
  • 1x 1W \ (200 \, \ Omega \) resistor
  • 1x signal diode (1N4001)
  • 2x small signal NPN transistor (2N3904)
  • 1x power MOS transistor (IRLU120N)
  • 1x AA \ (1.5 \, V \) battery or laboratory power supply
  • 1x solderless breadboard

Procedure¶

  1. Build up the circuit. Follow the instructions above and use the circuit diagram as a guide.

DC - DC Boost Converter on the breadboard

  1. Set the attenuation of the IN1 and IN2 scope probes to x10.
  2. Connect the IN1 scope probe to point 3 and the IN2 scope probe to point 5 on your circuit ().
  3. Start the Oscilloscope & Signal Generator application - OUT1 must be deactivated (switched off)
  4. Set the probe attenuation to x10 in the menu settings IN1 and IN2
  5. In the MEASUREMENTS menu, select the MEAN option for IN1 and IN2.
  6. What are the values ​​of the DC voltage at points 3 and 5 ()?

At this point, if the switching signal OUT1 is deactivated, the DC-DC boost converter is not functional. The transistor \ (M_1 \) is switched off (idle) and the battery voltage is transferred to the load side via the inductance \ (L_1 \) and the diode \ (D_1 \) (point 5 in). With direct current signals (no switching) the \ (L_1 \) inductance behaves like a short circuit, so the output voltage is reduced by the battery voltage by the threshold voltage of the \ (D_1 \) diode: \ (V_ {out} = V_ {battery } - V_ {Diode} \). This condition is shown in the measurements on. As expected, the \ (LED_1 \) and \ (LED_2 \) will not light up because the output voltage is below the forward voltage of the LEDs (\ (2 \ cdot 1.8 \, V \)).

DC-DC boost converter is switched off

  1. In the OUT1 menu settings, set the frequency to \ (10 ​​\, kHz \), the waveform to PWM, the amplitude to \ (0.5 \, V \), the DC offset to \ (0.5 \, V \), uncheck SHOW and choose Enable.
  2. In the MEASUREMENTS menu, select P2P measurements for IN1 and IN2
  3. Set \ (t / div \) value to \ (100 \, us / div \) (you can set \ (t / div \) with horizontal +/- sliders)

At this point, at which the switching signal OUT1 is activated, the DC-DC boost converter is functional and behaves as described above in the theory. The output voltage is raised to approx. \ (5 \, V \) and the LEDs light up. This state is shown in. As we can see from the measurements, there is a residual ripple in the battery and output voltage, which is caused by the battery voltage ripple and transistor \ (M_1 \) circuit. The battery's voltage ripple is due to the fact that the battery is not an ideal voltage source, and when M1 is turned on, the current delivered by the battery causes a voltage drop.

DC-DC boost converter is switched on

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Ripple voltage values ​​are one of the most important parameters of DC-DC converter quality. The lower output ripple corresponds to a better DC-DC boost converter. The capacitor \ (C_1 \) is therefore required to compensate and smooth the switching voltage occurring at the inductance \ (L_1 \) and the diode \ (D_1 \). Try to remove \ (C_1 \) and watch \ (V_ {out} \).

  1. To observe the switching voltages of the \ (M_1 \) -MOS transistor, place the IN1 probe on point 2 () and the IN2 probe on point 4 ().
  2. Set the vertical offset to \ (- 4.0 \, V \) in the IN2 settings menu (in order to be able to see the IN2 signal better).
  3. Select NORMAL in the TRIGGER menu and set the trigger level to \ (3.0 \, V \).
  4. Set \ (t / div \) value to \ (20 \, us / div \) (you can set \ (t / div \) with horizontal +/- sliders)

M1 switching voltages

On the \ (M_1 \) gate and drain signals are shown. From we can see that the gate signal is a switching square wave that controls the transistor. The drain signal corresponds to the "open / closed" states of the transistor \ (M_1 \), but clear oscillations are visible in the "off" state. This is the effect of the inductance \ (L_1 \) as it takes on any change in current through it that affects the drain voltage \ (M_1 \).

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The output voltage value of the DC-DC boost converter is often controlled with the duty cycle of the switching signal (PWM signal).

  1. To observe the effects of the switching signal (OUT1), place the IN1 probe on point 2 () and the IN2 probe on point 5 ().
  2. Set the vertical offset to \ (- 3.0 \, V \) in the IN1 and IN2 settings menu.
  3. Set \ (t / div \) value to \ (50 \, us / div \) (you can set \ (t / div \) with horizontal +/- sliders)
  4. In the settings of the menu OUT1, change the duty cycle from 30% to 80% and observe the results.

Above: output voltage at 40% duty cycle. Bottom: output voltage at 80% duty cycle

warning

From we can observe the influence of the duty cycle on the output voltage. If we go with the duty cycle to 0% or 100%, then we switch off the M1 transistor or short-circuit it. To avoid a short circuit and the associated damage to the circuit, the switch-on times (high) should be limited.

Ask¶

  1. Change the load value to \ (470 \, \ Omega \) and observe the results.
  2. Change the OUT1 frequency to \ (5 - 20 \, kHz \). Measure and record the waveform of the amplified output voltage and output current. Explain what has changed and why?
  3. How would adding an LC filter to the drive output affect voltage ripple?

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