Karteikarten
Energy VIVA
Karten in diesem Stapel (25)
- 1COMPONENTS Explain why the ESR (Equivalent Series Resistance) of the electrolytic capacitors used in the project is important.Leads to additional power loss within the circuit (I^2R). This makes the filtering capabilities of the capacitor ineffective, increases high frequency noise at input and leads to larger ripple output.https://www.avnet.com/wps/portal/abacus/resources/engineers-insight/article/understanding-esr-in-electrolytic-capacitors/
(Add capacitors in parallel to reduce ESR, high value capacitors have lower ESR)
ESR important as increases power loss and leads to larger ripple - 2COMPONENTS Why is it that we don’t need to consider the low frequency input voltage ripple when we design the output filter of the Forward Converter?It is removed by the closed loop circuit control that maintains a constant output voltage within the specified ripple amount.
(Filter in forward converter is designed at the specific frequency (115kHz), to also include lower frequency components much larger capacitors and resistors would be needed) - 3COMPONENTS When operating off the transformer derived DC input- why does the input capacitor voltage reduce when we have the lowest resistance (highest power) load attached.When resistance decreases, more power is drawn and so the voltage drops resulting in the current to increase.
As there less resistance it allow for more voltage to flow thus, the overall voltage stored on the capacitor will be less ??
At the lowest resistance, the current must increase to maintain a constant voltage (V=IR), i=C*dv/dt so increased i -> must decrease dv/dt as C constant. - 4COMPONENTS Why does the PWM chip (and the other ICs in the circuit) need a decoupling capacitor?To avoid voltage and current spikes from affecting other components within the circuit. (parasitic inductance and capacitance)
They are used to suppress high frequency noise that could cause voltage spikes that would affect the circuit (smoothen voltage)
Different sized capacitors are usually added in parallel to cover a wide frequency range. Large capacitors store a lot of energy and decouple low frequencies (usually electrolytic), does not decouple high frequencies due to the high inductance they have.
Smaller capacitors are used for high frequency noise (usually ceramic) must be close to component too. - 5COMPONENTS Why have we decided to use Ferrite in all the magnetic components considered except for the input transformer which is made of silicon steel?Ferrite is used because the frequency is too high to use steel, which would lead to power losses that are too high
can sustain higher frequency/temperatures
Low Eddy current loss
Properties:
Low hysteresis loss.
High permeability.High resistance.
Virtually eliminated ageing
Input transformer is made of silicon steel because the input is at low frequency (mains frequency so like 50Hz) therefore it won’t suffer from Eddy current losses.
We use ferrite due to its high permeability and also its low eddy current loss. For the input transformer, a silicon transformer core is used due to it having a low AC Frequency. Ferrite can only be used/ most effective at high frequencies/ temperature. - 6COMPONENTS Why might it be better to wind the magnetic components with multiple strands rather than a single wire of equivalent (approximately) cross sectional area?Due to the skin effect, multiple wires reduce this effect. Area where almost all the current flows is called skin depth, the radius of the wires used must be smaller than the skin depth so multiple wires can be used to achieve large calculated radius. ESR increases at higher frequencies due to this. This prevents overheating of the wires which could lead to the insulation melting
- 7COMPONENTS Since it is stated in the gate drive transformer document that a Ferrite toroid is a good option for winding small components- why don’t we use a toroidal Ferrite core for the output filter inductor?Inductors need an air-gap for flux to pass through, this way it can store a lot of energy. There is no air gap in the toroidal core. Main transformer needs to step down the voltage whilst the inductor is used just to store energy.
- 8COMPONENTS The PWM IC (UC3524N) has a sawtooth generator which has an offset of 0.8V- why is it that we don’t need to worry about this in the closed loop system using either a Type 1 or Type 3 controller?This is because the closed loop system has feedback, therefore the system will correct the offset voltage. This is done by the integrator that fixes the steady state error caused by this offset voltage.
(For an open loop system the offset would never be corrected and output would be increased by offset) this could be an additional question - 9FORWARD CONVERTER In the original specification document a switching frequency of 75-150 kHz is defined. What are the implications (benefits/drawbacks) of operating at these two extremes?High frequency:
Benefits:
reduce component size, reduced operation voltage, noise controlled easier, smaller ripple
Drawbacks: increases power loss/wastage due to stray capacitance (v high freq, current can by-bass inductor through stray capacitance.), less efficient
Most of the power loss is in the MOSFETs and is caused by charging and discharging the parasitic capacitance of the MOSFET, this requires heatsinks
Low frequency:
Benefits: more efficient,
Drawbacks: larger components used, increased ripple frequency - 10FORWARD CONVERTER Why is the duty cycle of the isolated forward converter limited to <0.5?To avoid saturation of the transformer, the magnetising current will not have time to discharge completely hence the magnetising current increases to dangerous levels.(grows and grows until something blows)
(For a really low duty cycle you will not have enough magnetizing current for the transformer to work) - 11FORWARD CONVERTER What would happen to the current in the MOSFETs if the main transformer (not the input transformer) was saturated? Why does this happen?Essentially, the MOSFETs current will have current spikes. This is due to the transformer saturating, a saturated transformer will have not produce any more magnetic field (or flux) to produce when current is increased so, the coil will have a large current flowing through it leading to the current spike.
- 12MOSFET Why is it that we cannot simulate the gate drives driving an actual MOSFET in PLECS? How could we get around this and verify operation of the gate drive circuit?Simulation: plecs simulates ideally, does not include the resistances(RdsON) and gate source capacitance present in MOSFETSUse a capacitor instead of the MOSFET
- 13MOSFET For the gate drive circuit used in the project- why is it important that C1 is not too large?If it was large it would cause the transformer core to saturate. If capacitor is too large, it will have a lot of voltage and when it discharges it will cause a large spike in current and this will saturate the transformer (confirm this) i=CdV/dt
- 14MOSFET Why do we use a Zener diode rather than a standard diode in the gate drive circuit?For safety issues and voltage regulation, zener diode that will breakdown before the voltage supplied gets too high and destroys the MOSFET. Standard diode has a higher breakdown voltage as it is used for rectification.
Zener can work on lower voltages as it is for voltage regulation. - 15MOSFET Why do we need a gate driver chip such as the TC4425A? Why can’t we drive the MOSFETs directly from the PWM IC?The PWM chips have low voltage outputs of 3-5V and are designed for only driving a few mA. The MOSFETS require higher voltages and currents to drive them, that is why a gate drive circuit is required. The TC4425 chip amplifies voltage and current to levels that can drive the MOSFETs.
- 16MOSFET Why is isolation needed in the gate drive circuit? Since we’re driving both MOSFETs with the same demand (i.e. they are driven to switch on and off at the same time) why can’t we use a single gate drive circuit for both?Isolation is required to separate the system controller from the power devices. It is to prevent high potential difference between the two MOSFETS.
- 17MOSFET For the gate drive transformer- does it matter how you wind the three windings onto the toroidal core?Winding method has an effect on the leakage inductance (check PWE notes). Best to wind the three wires next to each other, so that leakage inductance and magnetic field (sandwiched wires) is reduced
- 18CONTROL Why is feedback important for a power supply?Lets you measure the error in the output (likely to happen if load changes), so that you can adjust the duty cycle to keep Vo constant.
Without control output voltage would change with different loads as duty cycle would not be modified to fix this.
Vo=N2/N1*Vs*d - 19CONTROL The control transfer functions are implemented using Operational Amplifiers- what alternative might the project have used?Through the PWM IC chip. It has error amplifiers that could have been used to implement the control transfer functions.
Microcontrollers/microprocessors could be used as the transfer functions are implemented through them. PID control can be implemented - 20CONTROL What is the purpose of the integrator part of the controller/compensator transfer function (Both type 1 and Type 3)?Removes steady state error (the 0.8V DC offset created by PWM chip).
Gain of Gc needs to be high to do this - 21CONTROL Why is it important to achieve a reasonable phase margin in the design? What would be an appropriate Phase Margin? Why is this?This means that the simulation is more realistic and consequently a better circuit design is possible
Phase margin should be above 60 degrees so that the system is not close to instability
60 degrees is also a magic number because it allows for the fastest settling time when attempting to follow a voltage step input - 22CONTROL Why should we be careful when having a low Gain or Phase Margin?It means that the system is close to instability, in a practical circuit this could lead to an unstable response. (negative)
Having a higher phase or gain margin means that your system has a higher range where it is stable., this is good - 23CONTROL Why is it important to not have significant open loop gain at the converter switching frequency?Speculation: significantly higher gain means less phase margin?
Peak in the magnitude response at the switching frequencyAt this frequency the loop phase shift will be 180 (90 from integrator and 90 from the filter)
Loop gain must be significantly less than unity or else there will be no gain margin - 24CONTROL What effect does the output filter capacitor ESR (Equivalent Series Resistance) have on the Bode plots of the forward converter?Reduces the phase shift back to 90 degrees due to zero created by capacitor ESR
- 25CONTROL Why is important to have a reasonably high open loop gain at 100Hz?
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- ENERGY VIVA Karteikarten am Energy VIVA, erstellt von Amiel Dacanay am 26/03/2019.
- Erstellt von
- Amiel Dacanay
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