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AN1300 APPLICATION NOTE

should use a 33k

Ω resistor on pin 2.

Design of the Bootstrap Start-up Circuit

The bootstrap start-up circuit is intended to allow a small amount of current from the input voltage tothe control

IC when power is first applied. It is also intended to restart the converter in the event of an output overcurrent

situation. The selected circuit allows a current of about 1mA to charge the IC bypass capacitor should have

enough stored energy to start the converter, after which IC can then be powered directly from the auxiliary wind-

ing on the power transformer. The input start-up current is not high enough to allow sustained operation of the

IC and MOSFET drive. This results in a "hiccup" overcurrent start-up. Another nice operational mode is the input

start-up current is turned off after the converter has started thus saving several watts of power during operation.

transistor that meets the needs of the converter is the SO642.

The start-up resistor (collector Resistor)

R(start) is found by:

The base-bias resistor R(bias) must supply both the base current and the minimum amount of current needed

to operate the zener diode (225mW) at its "knee" voltage:

Make this resistor 360k.

Compensating the feedback Loop:

The bandwidth of the control circuit is being set to 15kHz which produces a very good response time for almost

all switching power supplies.

The resonance frequency of the tank circuit presents a peculiar set of problems for stability. Approaching the

resonance point from below, the gain drastically increses and the gain begins to decrease. Above the resonance

frequency, both the gain and the phase are frequency, both the gain and the phase are decreasing rapidly.

Since the resonance point of the tank circuit and the ESR of the output capacitor are not fixed, it is advisable to

roll-off its gain below to the tank circuit’s resonance frequency. One can do this by calculating the gain of the

open-loop circuit at DC. The gain is essentially the minimum input voltage divided by the change needed in the

control voltage (2V) which produces a +35.5dB gain at DC. By closing the loop, one must have an attenuation

of 35.5dB at 15kHz to produce the desired closed-loop gain and phase. There are of course other methods of

closing the loop with mixed results. This is the most conservative, thus yelding a guaranteed stable system.

To set the frequency of the crossover frequency of the error amplifier one, essentially "substracts" the DC gain

of the control-to-output" characteristic or:

[Eq. 11]

using a 15kHz closed loop bandwidth and solving, results in an error amplifier bandwidth of 252Hz. Then solving

the equation;

[Eq. 12]

This yields a feedback capacitor of 0.066

µF

The final schematic is shown in figure 5.

R

start

120V

12V

–

()

1mA

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

108k or 100k

==

R

bias

120V

12V

–

()

250

µA

I

c

h

FE

----------

+







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

379k

==

35dB

–

10 Log

f

ep

f

xo

-------







=

C

f

1

2

πRf

ep

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

=