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AN1300 Datasheet(PDF) 7 Page - STMicroelectronics |
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AN1300 Datasheet(HTML) 7 Page - STMicroelectronics |
7 / 9 page 7/9 AN1300 APPLICATION NOTE From a graph, the value of Rfstart to produce a +80kHz start frequency or a start frequency of 190KHz, one 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. The selection of the small signal NPN transistor for the 115V AC line should hinge mainly upon thr VCEO of the device. For higher voltage AC lines, the PD and arc-over voltage of the package become more of an issue. The 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 -------------------- = |
Similar Part No. - AN1300 |
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Similar Description - AN1300 |
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