Resonant Inverter: Amplitude to Voltage?

[johnywhy]

I've read the V_out of a resonant inverter is lowered by increasing the oscillator frequency, pushing it above the resonant frequency of the LC tank.

Instead, why not lower V_out by reducing the amplitude of the square input oscillator, (and keep frequency the same)?

[johansen]

The whole point of a resonant converter is to use switches to modulate a system under easy to switch resonant conditions.


Keeping above the resonant frequency is easier because you dont have diode recovery in the body diode of the igbt or mosfet to deal with.

You can decrease the power transfer by droppin below the resonant frequency as well as above.

[temperance]

This should clarify most of your questions concerning resonant power supplies:

https://www.youtube.com/playlist?list=PLMZ8LHWk_2jjYz1gM9x3dpLOb3E8GLVBq

[johnywhy]

This should clarify most of your questions concerning resonant power supplies:

https://www.youtube.com/playlist?list=PLMZ8LHWk_2jjYz1gM9x3dpLOb3E8GLVBq

Sam's great. I've watched a lot of his videos, but i don't always understand them.

Was there a particular video that explains why we don't vary amplitude to adjust V_out?

Thx!

[johnywhy]

The whole point of a resonant converter is to use switches to modulate a system under easy to switch resonant conditions. Keeping above the resonant frequency is easier because you dont have diode recovery in the body diode of the igbt or mosfet to deal with. You can decrease the power transfer by droppin below the resonant frequency as well as above.

Sounds like an answer to me 

You're saying that, reducing pulse amplitude will cause diode recovery in the body diode of the igbt or mosfet?

I understand that raising f_in is preferable, because if you lower f_in, then harmonics of f_in could sweep down into the passband of your resonant LC tank. Correct?

[johansen]

Pulse width stays the same. Amplitude is always the same. The switch is fully on or fully off. Anywhere inbetween is losses

[johnywhy]

Amplitude is always the same. The switch is fully on or fully off. Anywhere inbetween is losses

Right, of course, the switch must be fully on or off.

But you could vary the power supply.

[Weston]

Are you talking about a dc-dc converter using a resonant topology like LLC? Or an inverter with an AC output used for something like induction heating?

Either way, the same point applies:
One of the main goals of a power supply is to regulate the output with a varying input voltage.

You could regulate the input voltage and still use the resonant inverter, presumably to generate AC or to provide power across an isolation barrier. You are entirely correct in your understanding that it could be done.

However, this would require an additional power converter to adjust the input voltage. From a complexity and cost standpoint this is worse than just using the inherent frequency/amplitude characteristics of the resonant inverter.