Blog for students taking ECEN4797/5797 Introduction to Power Electronics, ECEE Department, University of Colorado at Boulder, Fall 2011
Sunday, October 2, 2011
HW 6 Problem 6.8
Was anyone able to figure out how to solve this without knowing Vg? It seems that everything (primary winding voltage, zener voltage, output voltage, etc.) need to be referenced to the input voltage.
Note that the problem is focused on a current-sensing circuit only. Fig.6.44 shows only a switching MOSFET transistor and a current-sensing transformer. The rest of the converter, which is not shown, makes no difference
It seems we have to use the given magnetizing current condition (using the current vs. time plot, and avoid the use of the input voltage, referencing everything in terms of the given quantities), but I am not very sure if this is the right way, yet.
Here is the issue: When current is flowing throughthe MOSFET and the primary of the current sense transformer, there is also current in the secondary of the current sense transformer. This current flows through D1 and then R, where is develops a voltage that can be used to measure the current.
However, the votlage across D1 adn R1 applies a voltage to the secondary and to the magnetizing inductance and generates a magnetizing current. You want a large voltage on R to make the measurement more accurate but that creates more volt-seconds which means more manetizing current and the possibility of saturating the transformer core.
When the MOSFET turns off, the magnetizing current now flows through D2 and Dz creating a votlage that causes the magnetizing current to decrease.
The challenges in designing a transformer isolated current sense circuit are: enough voltage that sensing is accurate, to make sure the transformer is reset (magnetizing current = 0) before the start of the next switching cycle, and that the transformer does not saturate. If the duty cycle is large, that is difficult because there is not much time to reset the transformer. (Question: what does that mean for Dz?)
I think I have some misunderstanding with this problem because I determine that in part b, v(t) = -Vd for 1st interval and 0 for other 2 intervals. This seems like a sign that I've done something wrong. This is stating that the voltage across the resistor is not dependent on other things, even though the point of the sense resistor is to measure a voltage which is proportional to transistor drain current. I came to find v(t) = -Vd based on the loop formed on the secondary side in 1st interval. In 2nd and 3rd intervals, this loop is broken because of diode D1 being off, and no current flowing through R so v(t) = 0V. Any help is greatly appreciated. Thanks.
Never mind my previous question. Professor Maksimovic helped me to clear my confusion there. I went wrong because I was thinking in terms of voltages more than currents. In the 1st interval I should think about v(t) in the way it relates to the current from the primary, after all this is a current-sense circuit.
5 comments:
Note that the problem is focused on a current-sensing circuit only. Fig.6.44 shows only a switching MOSFET transistor and a current-sensing transformer. The rest of the converter, which is not shown, makes no difference
It seems we have to use the given magnetizing current condition (using the current vs. time plot, and avoid the use of the input voltage, referencing everything in terms of the given quantities), but I am not very sure if this is the right way, yet.
Here is the issue: When current is flowing throughthe MOSFET and the primary of the current sense transformer, there is also current in the secondary of the current sense transformer. This current flows through D1 and then R, where is develops a voltage that can be used to measure the current.
However, the votlage across D1 adn R1 applies a voltage to the secondary and to the magnetizing inductance and generates a magnetizing current. You want a large voltage on R to make the measurement more accurate but that creates more volt-seconds which means more manetizing current and the possibility of saturating the transformer core.
When the MOSFET turns off, the magnetizing current now flows through D2 and Dz creating a votlage that causes the magnetizing current to decrease.
The challenges in designing a transformer isolated current sense circuit are: enough voltage that sensing is accurate, to make sure the transformer is reset (magnetizing current = 0) before the start of the next switching cycle, and that the transformer does not saturate. If the duty cycle is large, that is difficult because there is not much time to reset the transformer. (Question: what does that mean for Dz?)
I think I have some misunderstanding with this problem because I determine that in part b, v(t) = -Vd for 1st interval and 0 for other 2 intervals. This seems like a sign that I've done something wrong. This is stating that the voltage across the resistor is not dependent on other things, even though the point of the sense resistor is to measure a voltage which is proportional to transistor drain current. I came to find v(t) = -Vd based on the loop formed on the secondary side in 1st interval. In 2nd and 3rd intervals, this loop is broken because of diode D1 being off, and no current flowing through R so v(t) = 0V. Any help is greatly appreciated. Thanks.
Never mind my previous question. Professor Maksimovic helped me to clear my confusion there. I went wrong because I was thinking in terms of voltages more than currents. In the 1st interval I should think about v(t) in the way it relates to the current from the primary, after all this is a current-sense circuit.
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