Offline Switch mode power supplies Flashcards
Why can buck, boost and inverting converters not be used directly from the mains
They require a DC mains where the mains is AC
They do not have isolation between input and output
When creating a flyback converter from a basic circuit what will you have to change
Replace the single inductor with 2 windings on the same core
What happens in the flyback circuit when the switch is closed
current flows through the primary coil and charges the inductor and the diode prevents any current from flowing into the 2nd winding
What happens in the flyback circuit when the switch is open
No current flows through the primary coil but the polarity of the secondary voltage reverses and current flows into the capacitor and load
What is the Vo/Vin equation for a flyback controller
Vout /Vin= -D/(1-D) x (Nout)/(Nin)
For a flyback converter how does the inductor work in relation to the windings
Current only flows in one winding at a time and in only 1 direction so one winding charges inductor and one discharges
What is true about both input and output currents of flyback converters
Both pulsed and require heavy smoothing
What can a Flyback converter be used for
Can be used as the core of a complete power supply unit to give a low DC voltage from AC mains (often called off-line)
In the AC Mains line what does the EMI filter do
Protects against noise
In AC mains line what does the Power factor correction do
Smooth’s the input current
In the AC mains line what regulates the system output voltage
Controlling the PWM Duty Cycle
What do practical products often require protection against
Over load or short circuit
Describe the AC Mains voltage graph
It is an ideal sin wave
Describe the voltage of AC mains after rectification
It is an Ideal but rectified sin wave
Describe the voltage of AC Mains after rectification and smoothing
Capacitor smoothed voltage leaving ripple
Describe the graph of the AC mains current after Rectification and smoothing
Input is not a sign wave and the rectified form only flows around the peak of rectified voltage
What do peaks of current cause
Large voltage drop which clip the peaks of the mains voltage so it is not a perfect sin wave which lead to losses in the mains distribution system
What is passive power correction
Passive power correction uses filters to reduce the higher harmonics and leave the current closer to a sin wave
What is active power correction needed for
More powerful loads
What does active power factor correction involve
Rectifying inputs and feeding into a boost convertor to make it close to a sin wave
What happens to the output from the power factor correction after it has been fed into the boost converter
It enters the flyback converter
What is the main advantage of a switch mode power supply
smaller and lighter than linear supply as the inductor works at switching frequency
What sort of values would you expect for the frequency of a switch mode power supply
MHz
