Overview
Source: Ali Bazzi, Department of Electrical Engineering, University of Connecticut, Storrs, CT.
DC/DC converters are power electronic converters that convert DC voltages and currents from a certain level to another level. Typically, voltage conversion is the main purpose of DC/DC converters and three main types of conversion exist in a single converter: stepping up, stepping down, and stepping up or down. Among the most common step-up converters are boost converters (Refer to this collections video: DC/DC Boost Converter), while among the most-common step-down converters are buck converters. (Refer to this collections video: DC/DC Buck Converter.) Buck-boost converters are also common to perform both step-up and step-down functionalities, and flyback converters can be considered as special types of buck-boost converters where electrical isolation is achieved between the input and output ports. (Refer to this collections video: Flyback Converter.)
DC/DC converter topologies are numerous, and their control, modeling, and operational improvements (e.g. efficiency, reliability, performance, etc.) are areas of continuous interest. The HiRel Power Pole board presented in this experiment provides a very flexible tool to study and analyze the performance of boost, buck, and flyback converter, all on a single board.
The objective of this experiment is to introduce the major components and capabilities of the Power Pole Board from HiRel systems, which is the board being used in three experiments on DC/DC converters.
Principles
The HiRel Power Pole board has five major sub-circuit areas that are labeled in Fig. 1. (Areas labeled in Fig. 1 are approximate.) The first area (red) includes the primary side which has filter capacitors, a current sensor, and connectors labeled "V1" and "COM," which can connect to a DC voltage source or load. Fig. 2 shows a zoom in at the first area with labeled components.
The second area (yellow) includes the secondary side, which has filter capacitors, a current sensor, and connectors labeled "V2" and "COM," which connect to a DC voltage source or the load shown as a planar power resistor. Fig. 3 shows a zoom in at the second area with labeled components. Either the first or second area can be used to connect to a DC voltage source, e.g. DC power supply, while the other connects to a load. Note that when the second area is connected to a source, the load resistor can be unsoldered from the board or left without having impact on the converter's operation as it would be directly fed from the DC voltage source.
The third area (green) is the power-pole area, where two MOSFETs and two diodes are connected. The first "leg" includes an upper MOSFET and a lower diode, while the second "leg" includes an upper diode and a lower MOSFET. The actual components of the upper MOSFET and diode are mounted on the same heat sink in the green rectangle of Fig. 1 on the top left side, while the lower MOSFET and diode are mounted on the same heat sink on the bottom left side in the green rectangle in Fig. 1. A zoom-in view on that area is shown in Fig. 4. The other small green rectangle includes gate drivers that take a low-power switching pulse, e.g. pulse-width-modulated signal, and convert it to the appropriate voltage levels that can turn the MOSFETs on and off.
The fourth area (blue) has four connecting points where a daughter board that includes a magnetic component can be mounted. Two boards are used with this board for the DC/DC converter experiments: the first board is the BB board, shown in Fig. 5, which includes an approximate 100 µH inductor; and the second board is the flyback board, shown in Fig. 6, which includes a flyback coupled inductor or transformer along with its R-C-Diode snubber circuit. The snubber circuit helps provide a path for the stored energy of the primary transformer side in one of the flyback converter's operating modes.
The fifth area includes low-power electronics that generate switching pulses to the MOSFETs, and provide protection to the board including over-current and over-voltage protection. A separate DC power supply is connected to the bottom left of the board, next to switch "S90" which turns on power to all of the low-power circuits so that the high-power side, i.e. areas 1-4, can function properly. The external DC power supply and its connector that plugs in to the Power Pole board are shown in Fig. 7 and 8, respectively.
Figure 1: HiRel Power Pole Board with Five Major Areas Please click here to view a larger version of this figure.
Figure 2: Zoom-in of Area 1.
Figure 3: Zoom-in of Area 2.
Figure 4: Zoom-in of Area 3.
Figure 5: BB Board.
Figure 6: Flyback Board.
Figure 7: External power supply for the low-power electronics.
Figure 8: External power supply connector.
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Procedure
This procedure mainly focuses on the ability of the Power Pole board to adjust switching pulses to the upper and lower MOSFETs
1. Setup
- Connect the external DC power supply to the Power Pole board.
- Turn on "S90."
- Observe that the green LED turns ON.
- Check the locations of "S90" and the green LED in Fig. 9.
- Place the second sliding switch in the blue switch array on "Int. PWM. Check the location of the sliding switch array in Fig.10.
- Int. PWM" setting means that the switching pulse (PWM: Pulse width modulation) to either MOSFET is generated on the Power Pole board itself.
- Ext. PWM" means that the switching pulse to either MOSFET is generated by an external source, e.g. function generator or micro-controller.
- Place the first sliding switch in the blue array on "TOP FET." Only one PWM signal is generated on the Power Pole Board, therefore one of the MOSFETs has to be selected as the receiving pulse. Once a MOSFET is selected, that MOSFET should now be able to switch on and off.
- TOP FET "selection means that the upper MOSFET will be receiving the switching pulse.
- BOT FET" selection means that the lower MOSFET will be receiving the switching pulse.
Figure 9. External Power Supply Connector, Main Switch, and LED Indicator
Figure 10. Slider Switch Array
2. Measurements to Monitor the MOSFET Gate Pulses
- Turn on an oscilloscope.
- Connect a regular 10x probe to the oscilloscope's Channel 1.
- Set up the oscilloscope Channel 1 to be in DC coupling to see the PWM offset.
- Set up Channel 1 to be scaled for a 10x probe.
- Set up Measurements on the oscilloscope to measure the frequency and positive duty cycle of the signal to be measured on Channel 1.
- Hook the probe's measuring clip to the "PWM" pin shown in Fig. 10.
- Connect the probe ground to the "GND" pin shown in Fig. 10.
- On the oscilloscope screen, observe a pulse-train which is the PWM signal going to the upper-switch gate driver.
- To ensure that the upper MOSFET is switching, remove the probe's measuring clip and hook it to the "Gate" pin on the top left of the upper MOSFET shown in Fig. 11. You should observe a similar waveform to that you saw when the PWM pin was being probed.
- To ensure that the lower MOSFET is not switching, remove the probe's measuring clip from the upper "Gate" pin and place it on the lower "Gate" pin shown in Fig. 11. You should observe zero voltage.
- Re-place the probe's clip on the "PWM" pin.
- Adjust the duty cycle of the "PWM" signal by changing the potentiometer's knob shown in Fig. 12. Going clockwise increases the duty cycle from zero to 100%, and going counter-clockwise decreases it.
- Adjust the PWM frequency by turning the potentiometer's screw shown in Fig. 13. Use a small screwdriver to adjust the screw's position.
- Observe that the number of pulses displayed on the oscilloscope screen increases or decreases as the potentiometer is adjusted.
- Repeat the above procedure with the BOT FET selection and check to make sure that the lower MOSFET gate is now seeing a switching pulse
Figure 11: Gate signal pins.
Figure 12: Potentiometer Duty Cycle Adjustment.
Figure 13: Potentiometer for Frequency adjustment
3. Shut down the circuit
- Turn off "S90."
- Disconnect the external DC power supply.
- Disconnect the oscilloscope from both sides.
- Turn off the oscilloscope.
The HiRel Power Pole Board is a tool for studying and analyzing the performance of simple DC-DC converter circuits. DC-DC converters take DC voltage inputs and produce DC voltage outputs with a different value. For example, boost converters step up voltage, while buck converters step down voltage. These converters can be assembled and tested on a bread board, but can be evaluated more simply using a pre-made demonstration board, such as the HiRel Systems Power Pole Board. This video will introduce the major components and capabilities of the Power Pole Board, which is used in experiments with boost, buck, and flyback converters in this collection.
The HiRel Power Pole Board has five major sections. The first is the primary side, which has filter capacitors that are used in the converter circuits, a sensor for measuring current through the circuit, and connectors V1 and COM that connect to a DC voltage source or a load. The second section is the secondary side, which also has filter capacitors and a current sensor. This section has connectors labeled V2 and COM that connect to a DC voltage source or a load. Here the load is shown as a planar power resistor. For the DC-DC converter experiments in this collection, the load is a power potentiometer, which can be adjusted based on the requirements of the circuit and test. Depending on the converter typology, one of these two sections acts as the input side, connected to a DC voltage source, while the other is the output side that is connected to a load. The third section is the power pole, which contains the components at the core of the DC-DC conversion process. The power pole has two metal oxide semiconductor field effect transistors, or MOSFETs, and two diodes. The upper MOSFET and upper diode are mounted back to back on a single heat sink. Similarly, the lower MOSFET and lower diode are mounted on one heat sink. Also included in this section are gate drivers that convert a switching signal to the voltage levels that turn the MOSFETs on and off. The fourth section has connections for a daughter board, which carries a magnetic component, such as an inductor or transformer. Two daughter boards are used for the DC-DC converter experiments: the BB board and the flyback board. The fifth section contains electronics that generate switching pulses for the MOSFETs and provide over-current and over-voltage protection for the circuit. An external DC power supply can be connected to the HiRel Power Pole Board through a DIN connector. Main Switch S90, which is next to the DIN connector, turns on power to all of the low power circuits on the board. Now that we've seen the main sections of the HiRel Power Pole Board, let's set up the board and show how it will be used in DC-DC converter circuits.
Before using the Power Pole Board, it must be configured to generate switching pulses for the MOSFETs. First, plug the external DC power supply into the DIN connector. Then, turn on Main Switch S90. The green LED by Switch S90 illuminates to indicate that power is applied to the board. Locate selector switch bank S30 and set the first switch to TOP FET. With this setting, the pulses that turn the MOSFETS on and off control the upper MOSFET. If this switch is set to BOTTOM FET, the pulses control the lower MOSFET. Now, set the second switch to PWM Internal. In this position, pulse with modulated signals generated on the board turn the selected MOSFET on and off. If this switch is set to PWM External, then an external source, like a function generator or microcontroller controls the MOSFET.
Connect a 10X probe to Channel 1 of an oscilloscope. Clip the probe's ground lead to the ground terminal of the board and the probe tip to the PWM terminal. To see the offset of the pulse with modulated signal, set Scope Channel 1 for DC coupling. The oscilloscope screen should show a train of pulses to the driver for the upper MOSFET. Check the control signal directly by removing the probe tip from the PWN terminal and clipping it to the gate terminal by the upper MOSFET. A pulse train should be visible on the scope. Clip the probe tip to the PWM terminal again. The duty ratio of this pulse train determines the on time of the MOSFET as a percentage of the period. This duty ratio is a major control variable because it affects the relationship between a DC-DC converter's input and output voltages. To change the duty ratio of the pulse with modulated signal, adjust potentiometer RV64. The duty ratio may be varied from zero to one. Because a component's maximum operating frequency by type and design, the switching frequency is a critical parameter in the performance of DC-DC converters. In addition, higher switching frequencies typically yield smaller output voltage and current ripples for a given combination of capacitor and inductor. Change the frequency of the pulse with modulated signal by adjusting potentiometer RV60. Observe how the number of pulses on the oscilloscope screen increases or decreases as the potentiometer is adjusted. Next, set the first switch of selector switch bank S30 to BOTTOM FET. Remove the probe tip from the PWM terminal and clip it to the gate terminal by the lower MOSFET. Finally, confirm that the gate of the lower MOSFET receives the switching pulse.
Because of their high efficiency and excellent regulation, DC-DC converters are used in many commercial applications. Three common converters are introduced here and covered in subsequent videos in this collection. Boost converters generate a DC output voltage that is greater than the DC input, therefore boosting up the supply voltage. The video "DC/DC Boost Converter" explains the operation of boost converters, accompanied by experiments using the HiRel Power Pole Board. Buck converters generate a DC output voltage that is less than the input. In other words, buckling down or decreasing the supply voltage. The video "DC/DC Buck Converter" discusses how buck converters work and demonstrates their use with experiments on the HiRel Power Pole Board. Flyback converters generate a DC output voltage that can be either greater than or less than the DC input. Please watch the video "Flyback Converter" to see how they are derived from the joining of a buck converter with a boost converter to obtain the behavior of both.
You've just watched Jove's introduction to the HiRel Power Pole Board. You should now understand the design of the board, how to set it up, and how to use it for experiments with DC-DC converter circuits. Thanks for watching!
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Results
A PWM pulse is expected to be seen on the oscilloscope screen. The duty cycle is a major control variable for DC/DC converter as it adjusts the period during which a MOSFET or any other semiconductor actively-controlled switch is on. All input-output voltage relationships of DC/DC converters rely on the value of this duty ratio, along with some other variable in some converter topologies.
The switching frequency is critical in component selection as the maximum operating frequency of components varies by component type and design. Higher switching frequencies typically yield smaller voltage and current ripples but require larger capacitors and inductors.
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Applications and Summary
DC/DC converters are very common in DC power supplies used to charge electronics, and to supply power to many other electronic circuits. For example, any motor drive will require some smaller DC power supplies to power its low-power electronics, protection circuits, and high-power gate drives. Computer processors and other peripherals and accessories require very well-regulated DC voltages that are provided by DC power supplies. Renewable energy systems, e.g. solar photovoltaic panels, require DC/DC converters to regulate the DC output voltage of the panels, since solar irradiance and ambient temperature vary causing variation in the solar panel's voltage and current outputs. Many more industrial, transportation, military, and other applications use DC/DC converters instead of linear regulators due to their high efficiency, high performance, and excellent regulation.
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