Class-E PA Appendix

        On this page we give a little more background information for the design as well as the design process used for the  Class-E Power amplifier.

Below is a list of all the topics we cover in this page. Click on any of these links to go to that section.

background and purpose of the class-e pa

            The class-E Power Amplifier (PA) is a switch mode PA which has a potential theoretical efficiency of approximately 100%. Being a switching mode PA, the Power FET is used as an ON-OFF switch. Using the basic switching mode PA with the circuit topology seen in image (a) below, we can only achieve a maximum efficiency of ~81% due to losses from all other odd harmonics besides the fundamental. To overcome this efficiency limitation, first a series resonator (LC) is added at the output to act like a short at the fundamental frequency, and an open at all other harmonics. Next a capacitor (Cp) placed in parallel with the switch to store charge when the switch is closed and to provide current when it is open. Finally an inductor (Ls) is put in series with the resonator to phase shift the current to ensure it is 0 (zero) Amps at the switch closing point. The resulting Class-E circuit topology is shown in image (b).

Ideal Class-E Circuit Topology

            The Class-E PA was used within our Magnetically Coupled Resonant, Wireless Power Transmission  system (or MCR-WPT system) based on the now obsolete Rezence WPT standard (superseded by Qi). This is mainly due to the high efficiency of the Class-E PA which results in most of the power going to the Transmitter coils of the Rezence system. Also, the fundamental load of the Class E PA is inductive, which is good for driving WPT coils.

            For more detailed information on the basic theory of the Class-E PA, the design methodology used, design specifications and results, check out section 2.3 of the thesis. For more brief overview of the Rezence WPT standard, see section 1.1.1 The thesis can be obtained from the Ryerson University library by clicking the button below.

Design Component Values

Class-E Measurement Results

Parameter Value (experimental value)

Ideal Class Parameters

VDC

36V

Cp-external

68pF

Cp-internal

68pF

Lshift+Lres

15uH

Cres

39pF

Output Matching Network
C2

470pF

C3

620pF (150+470)

L3

500nH

Input Matching Network

C6

2750pF (1600+820+330)

C7

10nF

C8

220pF

L6

500nH

Miscellaneous

Choke Inductors (L1 and L4)

15uH

Input bias voltage

3V

Input Power

2Watts

Input impedance

27Ω

simulation and prototype efficiency differences

        The main source of the efficiency decrease was due to the 39pF Cres capacitor with 2% tolerance used within the prototype. It was found to have a measured capacitance of 40pF. A harmonic balance simulation with this value shows efficiency drops by more than 10%.

Design Flow Step-by-step example walkthrough

        Provided below is a step-by-step example of the Class-E design flow using the FQPF3N25 N-Channel, QFET MOSFET.

Step 1 & 2. Frequency, Efficiency, and Device Selection


Operating Frequency
              6.78MHZ
Target Efficiency                     >=85%

Device                                    FQPF3N25 MOSFET
Datasheet                               located HERE on ON Semiconductor’s website

Step 3. Ideal Class-E & Feasibility and Reliability Check

Ideal Class-E Circuit Topology

Class-E feasibility and reliability check (VDC =36V)

Device Parameter Value Class-E design Parameter Value Reliability Requirement Pass?
Cint/Cdev
25.3pF
Cp
415.4pf
Cp > Cint/Cdev
yes
Vmax
250V
Vpeak (2 x VDC x 3.6)
172.8V
Vpeak < Vmax
yes
Imax
9.2A
Ipeak
1A
Ipeak < Imax
yes

Step 4. Ideal Time Domain and Frequency Domain Simulations

Ideal Class-E Schematic Setup

Ideal Class-E Circuit Schematic Setup

Ideal Time and Frequency Domain Simulation

Ideal Class-E Time and Frequency Domain Simulation Results

Step 5. Output Matching With Internal Parasitics

FQPF3N25 Class-E Output Matching Schematic

FQPF3N25 Class-E Circuit Output Matching Schematic Setup

FQPF3N25 Class-E Output Matching Simulation Results

FQPF3N25 Class-E Circuit Output Matching Simulation Results

Step 6. Input Matching

FQPF3N25 Class-E Input Matching Schematic

FQPF3N25 Class-E Circuit Input Matching Schematic Setup

FQPF3N25 Class-E Input Matching Simulation Results

FQPF3N25 Class-E Circuit Input Matching Simulation Results

Step 7. Final Circuit

FQPF3N25 Class-E Final Circuit Schematic

FQPF3N25 Class-E Final Circuit Schematic Setup

FQPF3N25 Class-E Final Circuit Simulation Results

FQPF3N25 Class-E Circuit Final Circuit Simulation Results