Forward converter magnetizing current.

[paulmdrdo]

Can somebody explain why the magnetizing current starts at a negative value?

The highlighted part below kind of gives me a hard time to comprehend. Does this mean that the voltage across the capacitor is changing from one value to another instantly when S2 closes? I was taught that capacitor voltages can't instantaneously change. Also what does having large value of this cap have to do with the voltage being constant? Thanks!

 

[KlausST]

Hi,

At the very beginning it starts at zero. This is the extreme case at power ON. But during normal operation it repeatedly goes into negative because of the capacitor.

One may explain this with resonance between inductance and capacitance:
If you reduce the circuit to just the inductance and capacitor ... and a switch to push energy into the system. As soon as you switch OFF ... the circuit starts ringing. Voltage as well as currents become positive and negative, repeatedly..
Energy is repeatedly pushed from capacitor to inductance and back.
* When the absolute value of the current is at it's maximum (energy is in the coil), then the absolute value of the voltage is zero (no energy in capacitor)
* When the absolute value of the voltage is at it's maximum (energy is in the capacitor), then the absolute value of the current is zero (no energy in inductor)

Klaus

 

[paulmdrdo]

Hi,

At the very beginning it starts at zero. This is the extreme case at power ON. But during normal operation it repeatedly goes into negative because of the capacitor.

One may explain this with resonance between inductance and capacitance:
If you reduce the circuit to just the inductance and capacitor ... and a switch to push energy into the system. As soon as you switch OFF ... the circuit starts ringing. Voltage as well as currents become positive and negative, repeatedly..
Energy is repeatedly pushed from capacitor to inductance and back.
* When the absolute value of the current is at it's maximum (energy is in the coil), then the absolute value of the voltage is zero (no energy in capacitor)
* When the absolute value of the voltage is at it's maximum (energy is in the capacitor), then the absolute value of the current is zero (no energy in inductor)

Klaus

Thank you for the informative response. What about the second part of my question? The statement seems to me that the capacitor changes voltage instantaneously.

 

[Easy peasy]

it is harder to change the voltage on larger caps - thus the reset cap needs to be large enough such that its voltage don't change by more than about 15% - the pictures show steady state not start up - which is different as the cap is usually obviously zero at cold start - the clamped fwd converter can be a tricky beast if the load has lots of transient changes - so beware ...!

 

[paulmdrdo]

The graph for the magnetizing current Im seems to cross zero at exactly half of the first interval can you explain why is that happening? Same phenomena happens during the second interval it crosses zero at exactly half of that interval

 

[Easy peasy]

otherwise there would be a DC offset in the Imag, this would imply a non zero net applied volts to the pri ....

 

[Akanimo]

The highlighted part below kind of gives me a hard time to comprehend. Does this mean that the voltage across the capacitor is changing from one value to another instantly when S2 closes? I was taught that capacitor voltages can't instantaneously change. Also what does having large value of this cap have to do with the voltage being constant? Thanks!

The voltage across the capacitor does change. It does not change instantly though. At startup, the voltage across the capacitor is 0V. With every passing switching cycle, charges gets dumped into the capacitor and the voltage across it keeps increasing (the increase is in usual continuous piecewise linear change characteristic of smps rather than a step change) until steady state where it is about constant When the capacitor is small, although the voltage across it does not change very fast, the rate at which it changes can be very rapid. Remember that a larger capacitor stores more charges to raise the voltage across it to the same level than that which a smaller capacitor stores. So when discharging, the rate of change of the voltage across a larger capacitor is lower than that for a smaller one for the same discharge restriction (resistance).