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QR PFC'd Flyback can be PWM dimmed and Analog dimmed?

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cupoftea

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Hi,
The NCL300000.... we wish to use it to drive 30W's worth of LEDs from the mains, with good Power factor.
However, we wish to PWM dim it , and also analog dim it.
So, since the Error amp is transconductance type, may we simply put a voltage to the COMP pin (eg from an opamp buffer), to give an analog dimming level like that?

And for our PWM dimming exploits, may we simply then PWM it ON/OFF/ON.... by simply applying a voltage of 1V to the CS pin, by way of pulses to make it go OFF for the OFF state of the PWM dimming?...(eg put a series 1k resistor between the source sense resistor and the CS pin, then switch the 1V onto that resistor, to turn the power stage off temporarily)?

NCL30000

The thing is, we wish to use this chip with LEDs without a significant output capacitor.
Also, it looks like the OVP pin can be used for PWM dimming?
 
Last edited:

Thanks, do you agree, the statement of page 1 of AND8541/5 (concerning NCL30000 CrM flyback ) is totally wrong?....it says...

>>.QUOTE....The maximum switching frequency occurs under conditions of maximum input voltage and maximum output power. The high input voltage charges the primary inductor rapidly and the higher load drains the stored energy quickly.<<UNQUOTE

AND8451/D

Do you agree...it should say ...."max f(sw) occurs at max vin and min load." ?

This is because it is the vout that discharges the secondary L......not the output load, (in the first instance).
 

The inside of the COMP pin is a constant current source, and the voltage of this pin is controlled by the optocoupler. If it is a constant current source, the voltage will also be clamped by the optocoupler. If it is a constant voltage source, will it damage the optocoupler?
 
Thanks, but no this C pin cannot damage the opto......the C pin gets regulated to about 5.5V.....so the TOPswitch will draw whatever current it needs to from the opt in order to keep V(C) at ~5.5V....it will then look at that current, and decide what duty cycle to give, depending on the current....the more current, the lower the duty cycle.
It does all this after strippng out the bias current for the topswitch.
 

The comp pin also has a capacitance to the ground, so the voltage here will be Sawtooth wave wave rather than PWM. Personally, I think a triode with large magnification will be connected in series on the optocoupler, and PWM dimming by the triode may be more in line with your requirements.
 

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