Topology selection for the primary coil driver for WPT system

[bhl777]

Hi All, I have one question regarding the topology selection for the primary coil driver for WPT system. I saw in a lot of papers, people use Class E PA to drive the primary power coil, mainly for biomedical applications. While in some industrial products, people use full bridge to drive primary coil, such as https://www.idt.com/products/power-management/wireless-power/wireless-power-transmitters/p9235a-wpc-112-wireless-power-transmitter-ic-tx-a5-and-tx-a11.

Would anyone advise me
(1) what is the considerations in determing the topologies?
(2) if I just want to have the system functional with a DC input voltage, can I say from theory any DC-AC inverter that can generate sine voltage to apply to the coils should work?

Thank you!

 

[CataM]

Output power of standard Class E inverter = 0.58*Vi²/R_load

Output power of standard H bridge inverter with series LC network = 0.8*Vi²/R_load

H bridge topology has 1.4 times more output power capability for the same input voltage and same load resistor than the Class E inverter. Besides that, the H bridge's transistor has to block a maximum voltage of "Vi" while the Class E Inverter's transistor has to block a maximum voltage of 3.5*Vi (for e.g. 60 V input, the transistor has to block 210 V !!!). This is the main reason that the Class E inverter is not used in high power applications, but the H bridge is.

The conclusion is this one:
Class E --> high efficiency, high frequency, low power => useful in biomedical applications
H bridge --> high power, lower frequency => high power applications

 

[bhl777]

Output power of standard Class E inverter = 0.58*Vi²/R_load

Output power of standard H bridge inverter with series LC network = 0.8*Vi²/R_load

H bridge topology has 1.4 times more output power capability for the same input voltage and same load resistor than the Class E inverter. Besides that, the H bridge's transistor has to block a maximum voltage of "Vi" while the Class E Inverter's transistor has to block a maximum voltage of 3.5*Vi (for e.g. 60 V input, the transistor has to block 210 V !!!). This is the main reason that the Class E inverter is not used in high power applications, but the H bridge is.

The conclusion is this one:
Class E --> high efficiency, high frequency, low power => useful in biomedical applications
H bridge --> high power, lower frequency => high power applications

Hi CataM, would you tell me where can I find your equations online? Or would you explain to me briefly how these two equations are derived?
(1) Output power of standard Class E inverter = 0.58*Vi2/Rload
(2) Output power of standard H bridge inverter with series LC network = 0.8*Vi2/Rload

Thank you!

 

[CataM]

From this book.
Class E --> page 255
H bridge --> page 218

 

[mtwieg]

Hi CataM, would you tell me where can I find your equations online? Or would you explain to me briefly how these two equations are derived?
(1) Output power of standard Class E inverter = 0.58*Vi2/Rload

Look for publications by Nathan Sokal, the inventor of the class E amp, for derivations. 0.58 is in the case where your load Q approaches infinity, 0.5 is a more realistic result.

(2) Output power of standard H bridge inverter with series LC network = 0.8*Vi2/Rload

A full bridge produces a square wave with amplitude of Vi. The fundamental component of the square wave has amplitude 4/π*Vi (see the fourier series of a square wave). So the Power associated with the fundamental component is (4/π*Vi²)/2RL, or 0.811*Vi²/RL.

Those equations make the H bridge seems better, except when you consider how many FETs it takes to build each circuit (one for class E, 4 for Hbridge). The choice should mainly come down to: can I hard switch my devices efficiently at my operating efficiency? If so, H bridge is fine. If not, use a soft switching class like E or current mode D.