boost converter not providing enough current

I designed a simple boost converter - 555 timer with 437kHz controlling the gate of the Mosfet, a 100muH inductor and 2.2muF capacitor. The duty cycle is 90%. The circuit converts 12v input to 90v output as desired. When I connect a 12w LED light to 120 volt ac, at the LED input (after the simple bridge rectifier and a few resistors to burn off some voltage), the voltage is 103.4v and the current is 123.8mA. When this LED light is connected to the boost converter (the rectifier and resistors were removed, only the bunch of LEDs connected), the LEDs light up but the current to the LED shows only 6mA, not 123mA. I thought may be the capacitor value was low. So I connected the boost converter to a regular filament bulb. I expected to see 6mA consumption but the bulb was drawing about 80mA (though it did not light up). Why is the current to the LED so low? I increased the capacitor to 4.4muF. Still the current drawn was only 6mA. The inductor is reated for 1.5A. Please help.

are you driving the mosfet gate directly from the 555 output with a frequency of 437KHz?

alexan_e said:

are you driving the mosfet gate directly from the 555 output with a frequency of 437KHz?

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Yes, I am driving the gate of Mosfet directly from 555 output at 437KHz. I have attached the circuit. The inductance coil is SRR1208-101YL and tghe MOSFET is AOD1N60. Hope this helps.

I believe your frequency is too high. This gives a very short time for a pulse to build up current in the coil.

The output may read at high volt level, but that is only with light load attached.

Try reducing the frequency. This will give each pulse a longer time for greater current to build in the coil. It will supply a greater load.

BradtheRad said:

I believe your frequency is too high. This gives a very short time for a pulse to build up current in the coil.

The output may read at high volt level, but that is only with light load attached.

Try reducing the frequency. This will give each pulse a longer time for greater current to build in the coil. It will supply a greater load.

Click to expand...

Thanks BradtheRad. How about the duty cycle. Can it be as high as 90% or should I bring it between 50 and 70%?

nsubra2001 said:

Thanks BradtheRad. How about the duty cycle. Can it be as high as 90% or should I bring it between 50 and 70%?

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Optimum results will come from adjusting duty cycle and frequency both.

Here are some more tips about operation:

A boost converter provides current to the load regardless whether the switching device is on or off.

An led will automatically and instantly gobble up whatever charge is on the capacitor, until it is down to the forward threshold of the led.

Attach the load before applying power to a boost converter, so the capacitor charge will stay within reasonable levels.

Whatever watts you want at the load, is the watts that has to come through the coil.

Thus if your load draws 100 mA at 96 V...
it draws 9.6 watts...
therefore your 12V supply must feed the converter an average of 800 mA (really more like 900 mA). This is the current you should be able to read on an ammeter.

In your layout, no matter what is your duty cycle, you must allow sufficient time for coil current to build to a peak of a couple of amps.

A given frequency of operation tends to go with a certain **henry value** of coil. Example, a 100 uH coil is often pulsed at tens of kHz.

An important factor is resistance in your coil, supply and switching device. These must not restrict current through the coil.

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Edited to clarify that 'size' of coil means henry value of coil.

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