AN232

DS00232A-page 6

 2002 Microchip Technology Inc.

DRIVE CIRCUITRY

One of the most efficient methods to drive the resonant

tank is a class - D drive circuit in either a full or half

bridge mode. Figure 7 shows a typical 1/2 bridge imple-

mentation. One gets very good results with a 1/2

bridge, and it has the advantage of being low cost and

it is easy to implement.

The use of a serial resonant tank circuit becomes

clearer in this section. A high Q serial resonant circuit

has its lowest impedance at resonance. The current

passing through the antenna coil shown in Figure 7

consists mostly of the fundamental current component,

although the drive circuit uses square wave excitation

that has very high harmonic content.

FIGURE 7:

SQUARE WAVE

EXCITATION

A very economical and straightforward drive circuit can

be realized with the circuit shown in Figure 8.

FIGURE 8:

SIMPLIFIED DRIVE

CIRCUIT

Microchip’s high-current MOSFET drivers (such as

TC4421/TC4422) are very suitable for this application.

The device takes care of all the necessary level trans-

lations and deadband control, resulting in a very effi-

cient and fast output, thereby reducing cost and losses.

These devices have the added advantage that they can

be directly driven from logic levels.

Be sure to use high quality capacitors with low toler-

ances (see ‘Calculating the Bandwidth and Q’ section

on tolerances). A good choice is Panasonic polypropy-

lene film capacitors - ECQO(

µ) series, rated at either

400V or 600V as they reduce losses and are relatively

stable over temperature variations.

across the inductor and capacitor. It consumes

0.5 A

the average current consumption is typically 0.25 A

RMS

for Manchester encoding.

The drive signal can be generated directly by the PWM

PIC16F627 microcontroller. For a device operating at

20 MHz, one can obtain a 125 kHz signal by setting the

Timer2 prescaler to 1. A period of 8

µs is then obtained

by setting the PR2 register to 39. To get a 50% duty

cycle output, set CCPR1L to 14 and CCP1CON<5:4>

to <0:0>. These settings will ensure a constant carrier.

To modulate the data, one can turn the drive signal on

and off by setting and clearing the CCP1RIL bit.

There are various transmission formats that can be

used to transfer data, but keep the rise and fall times of

the resonant tank in mind. The response for the circuit

in Figure 8 is shown in Figure 5. The drive circuit was

modulated on and off at 400

µs intervals.

Towards a Faster Response

The rise and fall times are limiting factors to shortening

increase the baud rate, one needs to accelerate the

turn-on and/or turn-off response. Figure 9 shows a

modification of the basic circuit in Figure 7 to accom-

plish both a faster turn-on and turn-off time.

The basic bridge drive circuit shown earlier took about

130

µs to reach 90% of full-scale value. The start-up

time can be decreased by starting the tank in full bridge

mode, then maintaining resonance in 1/2 bridge mode,

once full scale is reached.

This concept can be implemented (see Figure 9) by

using two FET drivers as the two halves of the full

bridge converter. The tank is started up by driving the

two 1/2 bridge drivers 180

° out of phase. This effec-

tively doubles the applied source voltage. As soon as

the full-scale amplitude is reached, the other 1/2 bridge

driver is grounded. A circuit based on two TC442X FET

drivers takes about 40

µs to reach full-scale. That is a

significant speed increase compared to the 120

µs to

reach 90% of full-scale for the standard

1/2 bridge system. Herein lies a warning: if the reso-

nant capacitor voltage rating is not at least double the

1/2 bridge full-scale voltage swing, then care should be

taken to ensure that 1/2 bridge mode is engaged before

the output voltage swing becomes too large. A good

safety backup is to engage the 1/2 bridge after 6 full

bridge cycles, irrespective of the output voltage.

V+

+

-

Vs

Vs =

Q = 25

12V

+12V

TC4422

PWM

5V

10

µF

L = 160

µH

a = 25 mm

wire = 26 AWG

N = 41

drive

signal