Unterschiede

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--- professoren_webseiten:rebholz:course_a_power_electronics [2025/02/06 15:08] – [Reactive Current] hrebholz
+++ professoren_webseiten:rebholz:course_a_power_electronics [2025/06/27 07:17] (aktuell) – [PWM-Signals] hrebholz
@@ Zeile 4: / Zeile 4: @@
 One main goal of the lab course is to train the usage of measurement tools such as oscilloscopes, current clamps, differential voltage probes, and more.
-====== Simulation ======
+====== Simulation Buck-Converter ======
 The problem often arises that electronic assemblies such as microcontrollers, sensors, small actuators or motors have to be supplied with 5V from a higher-level voltage source, e.g. 20V.
 Since linear regulators, which are nothing more than variable series resistors, generate excessive losses, buck converters are often used. With the use of modern semiconductors, efficiencies of > 98% can be achieved. \\
@@ Zeile 84: / Zeile 84: @@
 {{ :professoren_webseiten:rebholz:irs2890.png?200 |}}
 Gate drivers are often equipped with additional features. Our driver can detect overcurrents and if necessary, will turn off all MOSFETs for safety. Since overcurrents in a simulation tool will not damage your PC or notebook, you can ignore this feature in the simulation. Pull up the RFE output and connect ITRIP to ground. \\
-Datasheet: {{ :professoren_webseiten:rebholz:infineon-irs2890ds-ds-v01_00-en.pdf |}}
+Datasheet: {{ :professoren_webseiten:rebholz:infineon-irs2890ds-ds-v01_00-en.pdf |}} \\
+Download {{ :professoren_webseiten:rebholz:irs2890ds.zip |}}  the model file for LTSpice.
 <WRAP center round todo 80%>
 **Task 4a)**
-Add the driver circuit to your simulation and check the functionality. The general behavior should be the same as before! The simulation time now might significantly increases depending on how fast your computer is. Use the datasheed to complete the circuit.\\
+Add the driver circuit to your simulation and check the functionality. The general behavior should be the same as before! The simulation time now might significantly decreases depending on how fast your computer is. Use the datasheed to complete the circuit.\\
 **Task 4b)**
 Now you can determine the switching time of the drive and the MOSFETs. Measure the required time to turn the MOSFETs on respectively off for every MOSFET.\\
@@ Zeile 117: / Zeile 118: @@
 It might now be clear that the final output voltage does not depend solely on the duty cycle. In a real system, many parameters influence its behavior. This is one of the reasons why we need a control loop, which we will implement later in the lab.\\
-===== Reactive Current =====
+==== Reactive Current ====
 When we think of reactive currents or reactive power, we usually think of AC voltage systems but not of a step-down converter. But let's imagine the following scenario: The load resistor increases, or the load is lost— for example, because you turned it off or due to a broken connection. The DC-DC converter still attempts to regulate the output voltage to 5V with a constant duty cycle. In the half-bridge configuration, the high-side MOSFET will charge the output capacitor, while the low-side MOSFET discharges the capacitor within the same cycle. This creates a circulating current between the output capacitor and the power supply. You can easily demonstrate this behavior with a simulation. However, in the lab, it might be dangerous. Why? During the discharge of the capacitor, the circuit behaves like a boost converter. If the input voltage source cannot handle negative currents, it might get damaged. In many cases, however, the reactive current generates losses, and the bulk capacitor at the input of the buck converter can absorb the reverse current.
@@ Zeile 140: / Zeile 141: @@
 The PCB was originally not only developed for the power electronics lecture but also to drive different motor applications. Therefore, the PCB contains more features than are necessary for your measurements.
-Download: {{ :professoren_webseiten:rebholz:schematic_diy_power_pcb.pdf |}}
+Schematic and PCB files see: [[professoren_webseiten:rebholz:diypowerpcb|]]
 <WRAP center round todo 80%>
@@ Zeile 203: / Zeile 204: @@
 The power PCB is controlled with the help of a Simulink experiment. The microcontroller exchanges data cyclically with the PC via the serial interface.\\
+Dowload the lastes Simulink experiment to control the DIY Power PCB: {{ :professoren_webseiten:rebholz:vorlage_course_a_basis_v01_2021b.zip |Simulink Version 2021b}}
+{{ :professoren_webseiten:rebholz:control_pannel_simulink.png?direct&1000 |}}
-- Connection, COM- Port, Start, Stop
+If you can not establish a connection to the board double check the COM port settings.
+For safety reasons (PC protection) please always us an isolation device: [[https://wiki.ei.htwg-konstanz.de/professoren_webseiten/rebholz/diypowerpcb#isolation|Link]]
 ===== Before you start =====
@@ Zeile 245: / Zeile 249: @@
 <WRAP center round todo 80%>
 **Task 9a**
-Use two oscilloscope probes to visualize the output signals for the high-side and low-side MOSFET. \\
+Use two oscilloscope probes to visualize the output signals of the µC for the high-side and low-side MOSFET. \\
 **Task 9b**
 Measure the minimum and maximum of the adjustable deadtime.\\
@@ Zeile 272: / Zeile 276: @@
 **Task 10a**
 Measure how long it takes for the driver to start doing anything when it receives a high signal and turns on the high-side MOSFET\\
+Attention: To measure the gate source voltage of the high-side MOSFET at differential voltage probe is needed. Do you now why? \\
 **Task 10b**
 How long does it take to bring the gate-source voltage to the desired value?\\
 **Task 10c**
-Determine the same values for turning the MOSFET off and for the low-side MOSFET.\\
+Determine the same values for turning the MOSFET off and for the low-side MOSFET. Do you also need a diffential probe? \\
 **Task 10d**
 Now set the frequency to 30 kHz. Does this make sense? What is your conclusion for the minimal duty cycle?
@@ Zeile 375: / Zeile 380: @@
 Please use the following parameters: \\
 Switching Frequency: 20kHz\\
-Inductor: 100µF \\
+Inductor: 100µH \\
 Capacitor: 4700µF \\
 Load: 25W, 5V, 5A \\
@@ Zeile 421: / Zeile 426: @@
 <WRAP center round todo 80%>
 **Task 16a**
-alculate the critical load resistance at which critical conduction mode (CRM) occurs.\\
+Calculate the critical load resistance at which critical conduction mode (CRM) occurs.\\
 **Task 16b**
 Increase the load resistance until you can measure the CRM state. Check what happens if you increase the resistance further.
 </WRAP>
-. Reduce the load current until critical current Mode
+Reactive currents are not always bad. They can be used to eliminate the reverse recovery of the body diode. If we exactly meet the CRM mode, there is even no switching loss because we turn on the low-side MOSFET at zero current. If it is possible to generate a small amount of negative current, we even have no switching losses at the high-side MOSFET, as it operates in zero-voltage switching. This type of power electronic circuit is called a resonant converter, and it is now commonly used in many applications. The theory behind resonant switching converters is part of the Master's course Power Electronic Systems." \\
-. Reduce until discontinuous current mode and observe the output voltage
+The reduction of the reverse recovery effect can be seen by measuring the current through the MOSFETs with a Rogowski coil. If you don't have one, it is also possible to look at the drain-source voltage of the low-side MOSFET. \\
+{{ :professoren_webseiten:rebholz:reverse_recovery1.png?direct&600 |}}
+<WRAP center round todo 80%>
+**Task 17a**
+Consider and discuss with your neighbor why in this state the high side mosfet can be switched on at a voltage of less than 1V. This state is then referred to as ZVS zero voltage switching. \\
+</WRAP>
+====== Simulation Full-Bridge-Converter ======
+A basic buck converter topology, consisting of a single MOSFET and a freewheeling diode, is inherently limited to supplying a unidirectional (positive) output current while maintaining a strictly positive output voltage. This configuration confines the system's operation to the first quadrant of the voltage-current (V-I) plane.
+By extending the topology to a half-bridge configuration, typically involving two active switches (e.g., MOSFETs) and appropriate gate drive circuitry, it becomes possible to control the direction of the output current. This enhancement enables both buck (step-down) and boost (step-up) modes of operation, allowing the system to handle current in both directions while still maintaining a positive output voltage. This enables operation within both the first and second quadrants, depending on current direction.
+To achieve full four-quadrant operation—i.e., the ability to independently control both the direction of the output current and the polarity of the output voltage—the converter must be further expanded to include two half-bridges arranged as a full-bridge (H-bridge) topology. This configuration allows for complete bidirectional power flow and supports both sourcing and sinking current at positive and negative output voltages, thereby enabling operation in all four quadrants of the V-I plane. \\
+{{ :professoren_webseiten:rebholz:dc_motor.png?direct&600 |}}
+A typical application is the control of a DC motor, where the direction of rotation, the speed, or the power flow can be adjusted. In the next experiment, we aim to control the speed and rotational direction of a DC motor using a full-bridge (H-bridge) converter. To achieve this, we apply pulse-width modulation (PWM) to the bridge legs and observe the resulting motor current under different switching schemes—specifically, when both bridge legs are driven simultaneously (synchronous PWM), as well as when the PWM signals are phase-shifted by half a switching period (i.e., T/2 offset). These different modulation strategies influence the RMS current drawn by the motor.
+==== Simulation ====
+To compare the functionality of both control methods, we set up a simulation in which both approaches can be analyzed in parallel. Since we have already examined the operation of a half-bridge in detail, we now focus solely on the control strategy. For simplicity and clarity, the MOSFETs are replaced by ideal switches, and the gate drivers are assumed to be ideal as well. This abstraction allows us to isolate the effects of the switching schemes themselves without being influenced by non-idealities such as switching losses, gate charge dynamics, or dead times. The simulation will thus provide a clear comparison of how the different PWM strategies impact the motor current, voltage waveforms, and overall system behavior.
+In the simulation, the motor is modeled as a simple resistive-inductive load with a resistance of 1 Ω and an inductance of 100 µH.
+{{ :professoren_webseiten:rebholz:full_bridge.png?direct&600 |}}
+When using labels to assign node names in your schematic, all nodes with the same label are electrically connected. This simplifies schematic design but also requires careful attention.
+Incorrect or careless use of labels can lead to unintended connections and, in the worst case, short circuits. Always double-check your labeling to ensure that only intended nodes share the same potential.
+<WRAP center round todo 80%>
+**Task 18a**
+Expand the simulation to include the necessary control so that one part of the circuit operates with synchronous PWM on both sides, while another part operates with a T/2 offset. Verify the functionality at various load points. Does the result match your expectations? \\
+**Task 18b** \\
+Observe the output currents and compare the currents for the different control schemes.\\
+There are a total of three significant reasons to choose T/2 offset clocking. What are they? \\
+Write a few centences about:\\
+. Current Ripple: \\
+. Zero voltage output: \\
+. Heat distribution:
+</WRAP>
+{{ :professoren_webseiten:rebholz:h-bridge_output_current.png?direct&1200 |}}
+The output voltage becomes positive for PWM duty cycles greater than 0.5, and correspondingly negative for values below 0.5. In the case of simultaneous clocking on both sides, the following can be observed: at D = 0.5, the average output voltage is 0 V. As we know, in power electronics, we are almost always concerned with average values. This means that even though the average is 0 V, the RMS value can still be greater than zero. Since we are switching at several kHz, the motor naturally cannot respond quickly enough to the rapid changes in direction. However, if we were to switch at a very low frequency, we would see the motor physically oscillate back and forth in real time. If the effective current is greater than 0 A, this means that power is being converted and continuous losses are introduced into the motor. This is, of course, not efficient and certainly not desirable.\\
+You have probably already discovered yourself that at D = 0.5, the output current becomes exactly zero, without any circulating reactive power if we use the PWMs with T/2 offset.
+==== Experiment ====
+Lets try the full bridge with our DIY board. As a first step, we will look at the signals without the motor connected. Once we are sure that everything is correctly wired, we can operate the motor.\\
+**Hardware requirements**\\
+Make sure, that the driver circuit of the second half bridge V is assembled correctly. Check the PWM signals with the oscilloscope.\\
+**Software requirements**\\
+In order for current to flow through the motor, it is essential to activate the diagonal MOSFET pairs of the full bridge simultaneously—i.e., Q1 together with Q4 or Q2 together with Q3. Control laws typically refer to the high-side switches rather than the low-side ones. To ensure that Q4 is activated in sync with Q1 (or Q3 with Q2), we can apply a simple trick: operate bridge leg V with an inverted control signal relative to bridge leg U. This avoids the need for a 180° phase-shifted triangular carrier in PWM generation. In this case, the duty cycle for leg V becomes:
+Duty_V = 1 − Duty_U
+This inversion effectively ensures complementary switching of the corresponding low-side Mosfet, simplifying the implementation of the control scheme.\\
+Within Simulink just us a simple sum block:
+{{ :professoren_webseiten:rebholz:simulink_t-2.png?direct&400 |}}
+Additionally, using the microcontroller in combination with Simulink, we have the capability to implement a phase shift between PWM signals. For our purposes, the two relevant configurations are 0° and 180° phase shift. As described earlier, the mathematical trick used to compute the duty cycle for phase V by inverting the duty of phase U automatically results in a T/2 (i.e., 180°) phase-shifted switching pattern between the two bridge legs.
+If we wish to revert this automatic phase shift, we can do so by using a slider or parameter in Simulink to apply an additional 180° phase shift, effectively canceling out the original offset and returning to in-phase (0°) operation.
+{{ :professoren_webseiten:rebholz:simulink_phase-shift.png?direct&600 |}}
+<WRAP center round todo 80%>
+**Task 19a**
+Setup Simulink for full bridge operation. Measure the PWM Signals for Q1 and Q4 and check if you can apply synchronous and phase shifted PWM signals. Please do this with no load connected and without power supply.\\
+**Task 19b**\\
+Set the duty cycle to D = 0.5. Connect the load (DC motor) and the power supply. Consider how you can verify that your full bridge is operating correctly.
+</WRAP>
+<color #ffc90e>**Congratulations. Now you are ready and prepared for the exam!**</color>