9/13

AN1660 APPLICATION NOTE

(18)

According to this equation, the minimum voltage Vcmin is obtained as follows:

(19)

The Vcmax can be calculated according to equation (14). The maximum voltage applied to the resonant

capacitor tends to increase as the output current increase.

Calculated Vcmax in this application is 310V at maximum output power.

3.5 Peak value Il1pk of resonant current

Resonant current iL1 is calculated according to equation (16). The peak value of iL1 is

(20)

3.6 Over current protection and over voltage protection

0.4V, The calculated Rsense will be 0.4V/2.1A = 0.19ohm. Choose 0.2ohm/1W resistor as Rsense (R20).

Then the voltage on R20 will be 0.42V at expected OCP point.

By choosing the suitable voltage divider R18 and R17, we can get the 0.4V voltage to OPin+.

In this application we choose R17 = 39Kohm , R18 = 1Kohm.

Over voltage protection is achieved by sense the supply voltage of L6598 --Vs. R4 and R5 is a voltage

divider which connected to Pin8 (EN1). When the voltage of Pin8 is higher than the threshold 0.6V, it forc-

es the device in a latched shut down state.

By choose R4 = 110Kohm and R5 = 5.6Kohm, the OVP is trigged at Vs = 12.4V. That is 10% higher than

the normal operating voltage.

4

EVALUATION RESULT OF DEMOBOARD

4.1 Operating waveforms

See the operating waveform in Fig.5

The measured value : I1 is 1.4A, Vcmax is 300V, IL1pk is 1.9A.

That is coincide with the calculated value.

Iout

2n L

T'r

-------------------------

V

in

nVou t

–

Vcmin

–

()

2

Cr

L

1

------

I

1

2

–

=

Vcmin

Vin

nVo ut

–

I

out

T'

r

2n L

1

Cr

⋅

-----------------------------

⋅





I

1

2

+

Cr

L

1

------





--------------------------------------------------------------

–

=

IL

1pk

Vi n

n Vout

–

Vcmin

–

() Cr

L

1

------

=