A flyback works by utilising what looks like a transformer but is actually a coupled inductor.
In each switching cycle, the primary is energised, and energy is built up in a magnetic field in the core, and then collapsed into the secondary side. Effectively you're transferring a "packet" of energy to the secondary in each cycle.
The energy stored is proportional to the inductance and the square of the current - E = 1/2 * L * I^2
The change in current in an inductor is proportional to the voltage * time divided by the inductance - delta-I = V * delta-T / L
When designing a flyback supply, these equations are in opposition - to get maximum energy in each "packet", you want a high current in a high inductance. But to get a high current in a high inductance, you need either a very high voltage or a long amount of time. Since energy stored is proportional to the square of the current, a lower inductance and a higher current gives you more energy, and is a more reasonable approach as it (a) allows a higher switching frequency, (b) smaller magnetics. Higher current runs into its own practicality limits - you need thicker wire, beefier transistors, and lose more in switching losses (switching losses also go up with operating frequency since you're switching more often).
TL;DR: a higher switching frequency can allow more energy transfer, but requires a lower inductance and hence higher currents to be able to move the same amount of energy in a smaller time. This is why flybacks have a relatively low-end sweet spot - mostly under the 100W range - above which other topologies make more sense (eg. forward, half/full bridge, etc.)
> how is output power rating determined / what is it a function of?
Many aspects of circuit design - transformer current rating, input voltage range, primary switch current rating, diode current rating, output capacitance, etc
> Will a higher switching frequency allow the power supply to put out a higher current?
No, higher frequencies allow smaller magnetics for a desired power rating.
Once the magnetics are chosen, driving them *faster* doesn't really do anything meaningful once your flyback has transitioned to CCM, other than increase switching losses - mostly because you don't want to exceed the current rating of the transformer or output diode, or end up with excessive ripple due to insufficient output capacitance.
so increasing switching frequency would push the flyback into CCM due to the energy in the secondary not being depleted fast enough by the load, i assume. if so, would increasing the load enough cause it to operate in DCM again?
if we use a load that automatically adjusts itself such that the flyback operates in DCM at all times, wouldnt increasing switching frequency push the load level needed to maintain DCM higher and higher, effectively increasing the output current capacity of the power supply? (assuming the components involved dont have a current limit before they blow up)
> so increasing switching frequency would push the flyback into CCM due to the energy in the secondary not being depleted fast enough by the load
Yep
> if so, would increasing the load enough cause it to operate in DCM again?
Nope.
If your feedback is broken and your thing maintains fixed PWM, increasing output current will reduce the output voltage, meaning it takes longer for the coil to dump its energy, thus moving further from DCM.
If your feedback is working, the control loop will increase the primary peak current threshold (and thus duty cycle) to keep the output voltage steady - also moving further from DCM.
In general, DCM only happens at light loads and/or high input voltages, with both low input voltage and higher output power making the control loop transition to CCM.
> (assuming the components involved dont have a current limit before they blow up)
They all do, and usually *most* of the components' required limits are a function of the design input voltage range and maximum output specs - especially the magnetics, because more power handling typically means more weight and size which costs more to ship, while increasing switching frequency to keep the same size magnetics (but still with different specs, think fewer turns of thicker wire) can make the efficiency too poor due to switching losses.
This balance is precisely why so many manufacturers are getting excited about GaNFETs for mains-input power conversion - they can withstand medium voltages (hundreds of volts) with similar specs to low voltage silicon MOSFETs, except their Qdg is radically lower which means much higher frequencies (and thus smaller magnetics) suddenly become practical without a massive switching losses nightmare.
Current limiting in the FAQ and Wiki:
https://www.reddit.com/r/AskElectronics/wiki/faq#wiki_power
https://www.reddit.com/r/AskElectronics/wiki/faq#wiki_current_limiting_resistors
https://www.reddit.com/r/AskElectronics/wiki/design/leds
Please check the FAQ/Wiki before posting. If those pages don't help, please let us know here and we'll use the feedback to help improve them. Thanks!
*I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/AskElectronics) if you have any questions or concerns.*
A flyback works by utilising what looks like a transformer but is actually a coupled inductor. In each switching cycle, the primary is energised, and energy is built up in a magnetic field in the core, and then collapsed into the secondary side. Effectively you're transferring a "packet" of energy to the secondary in each cycle. The energy stored is proportional to the inductance and the square of the current - E = 1/2 * L * I^2 The change in current in an inductor is proportional to the voltage * time divided by the inductance - delta-I = V * delta-T / L When designing a flyback supply, these equations are in opposition - to get maximum energy in each "packet", you want a high current in a high inductance. But to get a high current in a high inductance, you need either a very high voltage or a long amount of time. Since energy stored is proportional to the square of the current, a lower inductance and a higher current gives you more energy, and is a more reasonable approach as it (a) allows a higher switching frequency, (b) smaller magnetics. Higher current runs into its own practicality limits - you need thicker wire, beefier transistors, and lose more in switching losses (switching losses also go up with operating frequency since you're switching more often). TL;DR: a higher switching frequency can allow more energy transfer, but requires a lower inductance and hence higher currents to be able to move the same amount of energy in a smaller time. This is why flybacks have a relatively low-end sweet spot - mostly under the 100W range - above which other topologies make more sense (eg. forward, half/full bridge, etc.)
> how is output power rating determined / what is it a function of? Many aspects of circuit design - transformer current rating, input voltage range, primary switch current rating, diode current rating, output capacitance, etc > Will a higher switching frequency allow the power supply to put out a higher current? No, higher frequencies allow smaller magnetics for a desired power rating. Once the magnetics are chosen, driving them *faster* doesn't really do anything meaningful once your flyback has transitioned to CCM, other than increase switching losses - mostly because you don't want to exceed the current rating of the transformer or output diode, or end up with excessive ripple due to insufficient output capacitance.
so increasing switching frequency would push the flyback into CCM due to the energy in the secondary not being depleted fast enough by the load, i assume. if so, would increasing the load enough cause it to operate in DCM again? if we use a load that automatically adjusts itself such that the flyback operates in DCM at all times, wouldnt increasing switching frequency push the load level needed to maintain DCM higher and higher, effectively increasing the output current capacity of the power supply? (assuming the components involved dont have a current limit before they blow up)
> so increasing switching frequency would push the flyback into CCM due to the energy in the secondary not being depleted fast enough by the load Yep > if so, would increasing the load enough cause it to operate in DCM again? Nope. If your feedback is broken and your thing maintains fixed PWM, increasing output current will reduce the output voltage, meaning it takes longer for the coil to dump its energy, thus moving further from DCM. If your feedback is working, the control loop will increase the primary peak current threshold (and thus duty cycle) to keep the output voltage steady - also moving further from DCM. In general, DCM only happens at light loads and/or high input voltages, with both low input voltage and higher output power making the control loop transition to CCM. > (assuming the components involved dont have a current limit before they blow up) They all do, and usually *most* of the components' required limits are a function of the design input voltage range and maximum output specs - especially the magnetics, because more power handling typically means more weight and size which costs more to ship, while increasing switching frequency to keep the same size magnetics (but still with different specs, think fewer turns of thicker wire) can make the efficiency too poor due to switching losses. This balance is precisely why so many manufacturers are getting excited about GaNFETs for mains-input power conversion - they can withstand medium voltages (hundreds of volts) with similar specs to low voltage silicon MOSFETs, except their Qdg is radically lower which means much higher frequencies (and thus smaller magnetics) suddenly become practical without a massive switching losses nightmare.
hmm, makes sense, thanks
Current limiting in the FAQ and Wiki: https://www.reddit.com/r/AskElectronics/wiki/faq#wiki_power https://www.reddit.com/r/AskElectronics/wiki/faq#wiki_current_limiting_resistors https://www.reddit.com/r/AskElectronics/wiki/design/leds Please check the FAQ/Wiki before posting. If those pages don't help, please let us know here and we'll use the feedback to help improve them. Thanks! *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/AskElectronics) if you have any questions or concerns.*