Problems when dc-dc converter is connected to h-bridge

Hello All,


I have been having problems when I connect my boost converter to my h-bridge. The problem I am having is that is when I connect it to h-bridge the indicator leds start to flicker,even screen of my laptop!! then I disconnect and everything goes back to normal. Also, I tried connecting a 75W, 120V bulb to my boost converter output which gives 180Vdc across the capacitor of my LC filter when nothing is connected to it but the voltage drops from 180Vdc to about 3Vdc when the bulb is connected and of course the bulb did not light up and my ferrite core transformer begins to make a noise. What could be wrong???

I am using a 10uF capacitor at the output and I am not sure of the value of the inductor or if it is good. I know that the purpose of the LC filter is to reduce the high switching frequency of 50 kHz to a low frequency in the Hz region as it is a low pass filter. The switching frequency being used in the SPWM section with the h-bridge is 16 kHz

Please Help!!!!!!

Here is the circuit diagram:

Let's see .
We know Tungsten coil is ~10x smaller R when cold, resulting in 10x inrush current.

P=V^2/R so R = 120^2/75 = 192 Ohm when hot and 19~20 when cold.

The impedance ratio of core is related to turns ratio, N^2 and voltage ratio = N

If N was 10, the impedance ratio would be 100 more on the output, or 100 smaller on the input.
thus Rin = 20/100 = 0.2 Ohm !!

You got 180V with no load and the source is a center tap 12Vdc bipolar drive source.

You may also be getting some high impedance from the LC filter resonance but without transformer parameters, f and L values, I can only guess the 75W light bulb is the wrong type of load.

SunnySkyguy said:

Let's see .
We know Tungsten coil is ~10x smaller R when cold, resulting in 10x inrush current.

P=V^2/R so R = 120^2/75 = 192 Ohm when hot and 19~20 when cold.

The impedance ratio of core is related to turns ratio, N^2 and voltage ratio = N

If N was 10, the impedance ratio would be 100 more on the output, or 100 smaller on the input.
thus Rin = 20/100 = 0.2 Ohm !!

You got 180V with no load and the source is a center tap 12Vdc bipolar drive source.

You may also be getting some high impedance from the LC filter resonance but without transformer parameters, f and L values, I can only guess the 75W light bulb is the wrong type of load.

Click to expand...

I have generated a 50 KHz pwm signal from the SG3525, the core is a ETD34 turns 3+3 pri and 80 on sec I dont have an inductance meter so not sure of the L value but C value is 10uF

As the input is 12 V and the output is about 200V, you will expect the input side current to be at least 200/12 times higher, is the wiring and transformer primary up to this (20A+?). Are both the transistors working?
Frank

THis looks like an old design https://2.bp.blogspot.com/-IGrcj4RjyYs/UOqa9ISSgXI/AAAAAAAAAYY/PpmE6sGtLck/s1600/circuit+50kHz.png
that is poorly documented.

Try a linear active load use a power transistor on a heat sink. and not a light bulb .
SMPS even if designed for 75W cannot drive a 75W lightbulb due to the 10x surge current as I explained.

SunnySkyguy said:

THis looks like an old design https://2.bp.blogspot.com/-IGrcj4RjyYs/UOqa9ISSgXI/AAAAAAAAAYY/PpmE6sGtLck/s1600/circuit+50kHz.png
that is poorly documented.

Try a linear active load use a power transistor on a heat sink. and not a light bulb .
SMPS even if designed for 75W cannot drive a 75W lightbulb due to the 10x surge current as I explained.

Click to expand...

I used a 20W, 110V bulb and it lit up with no noise or smoke from ferrite core so I guess the current or power is not enough at the output to operate the 75W bulb