Switch power supply ICs usually operate from supply voltages of up to 18V. When the input power for the switchmode converter is available at voltages higher than the maximum voltage rating of the IC, the voltage has to be reduced with a startup circuit
The voltage will be reduced with a startup circuit, when the input power voltage higher than the maximum voltage rating of the IC.
The circuit in Figure 1 shows the MOSFET(N-Channel Depletion-Mode MOSFET) being used to provide the low voltage power supply to be connected to the VCC pin of the Power control IC.
This circuit could be used in the input voltage range from 40VDC to 500VDC. The Figure2. is another example used a power resistor and a Zener diode. The major difference between the two approaches is that the switch consumes negligible power after the SMPS has started, but the resistor will consume power continuously from the input line.
But, the continuous power dissipated in the resistor may be up to 5 watts or even higher.
The Championmicro part #CM6800 specifications relevant to the startup are as follows:
Start-up current 150uA max
Operating current 7.0 mA max
Start-threshold voltage 13.26 volts max
Min. operating voltage after turn-on 12.74 volts max
The start-up current is the biasing current for the IC when the output is not switching. Once the output starts switching, the IC is considered to be in the operating mode and will draw no more than 7.0mA plus the load current being sourced into the gate of the MOSFET. This load current is calculated as fCV, where f is the operating frequency, C is the effective input capacitance, and V is the VCC voltage. The value for the resistor, R, must be small enough such that under the worst case condition of minimum input line voltage, it can source 150uA to the IC and some biasing current for the zener diode, IBIAS. The value of R is calculated as follows:
The worst case power dissipation is determined as follows:
The continuous power dissipation at higher voltages from R increases when the required input operating voltage for the converter is widened. Consider for example a converter required to operate at 90VAC and 264VAC. R is calculated as:
Operating the power supply from rectified 264VAC, R will dissipate:
Figure 3 shows the current paths during start-up and Figure 4 is after the start-up has occurred.
The circuit in Figure 5 shows the Bipolar being used to provide the low voltage power supply to be connected to the VCC pin of the Power control IC.
Figure 5 shows the current paths during start-up and Figure 6 is after the start-up has occurred. Consider for example a converter required to operate at 90VAC and 264VAC.
We used the switch (bipolar) to charge the capacitor to 13V then turned on the Power IC. And the bipolar will be turn off when the VE larger than11.74V. The start-up voltage of CM6800 was 13V, so the bipolar must be turn off after start-up.
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