Output power calculation

This is approximately the power stored in the coil times

the frequency of operation times the efficiency.

Assuming a current of 1.2 amps in a 30µH primary, the

stored energy will be 21.6µJ. The frequency is set by

the time it takes the primary to reach 1.2 amps plus the

1.7µs time allowed to discharge the energy into the

reservoir capacitor. Using 3 volts, the ramp time is

12µs, so the frequency will be 73kHz, giving an input

power of about 1.6 watts. With an efficiency of 75% the

output power will be 1.2 watts.

An 80µF capacitor

charged to 300 volts stores 3.6J, so 1.2 watts will take 3

seconds to charge it. Higher input voltages reduce the

ramp time, the frequency therefore goes up and the

output power is increased, resulting in shorter

charging times.

Output voltage adjustment

The ZXSC440 are adjustable output converters

allowing the end user the maximum flexibility. For

adjustable operation a potential divider network is

connected as follows:

The output voltage is determined by the equation:

In a circuit giving 300 volts, the "1" in the above

equation becomes negligible compared to the ratio

which is around 1000.

It will not be exactly

1000because of the negative input current in the

feedback pin. The resistor values, RA and RB, should

be maximized to improve efficiency and decrease

battery drain. Optimization can be achieved by

of the switching transistor, Q1.

In practice, there will be some stray capacitance across

RA and this will cause a lead in the feedback which can

affect hysteresis (it makes the device shut down too

early) and it is best to swamp this with a capacitor CA

and then use a capacitor CB across RB where CB/CA =

RA/RB.

This is similar to the method used for

compensating oscilloscope probes.

ZXSC440

SEMICO NDUC TORS

DRAFT ISSUE F - MAY 2004

8