boost converter drop voltage

[m4xa]

hi all,

i already make a boost converter for driving a load motor with specific : 200 v 9 ampere..
when the load is not connected the voltage can be up 200 v - 400 v but
when the load is connected the voltage output drop until below the input voltage
the output current is about 0.8 -1.1 A, also when i raise the duty cycle the output voltage can't be exceed to the input voltage.

i'm driving the mosfet with PWM from microcontroler
the input votage 15v - 30 v
induktor:100 µH
capasitor : 1µF 450 volt
mosfet :irf460

why the drop voltage si very high? is it because the cuurrent??
if the problem is the current how can i produce a bigger current??

 

[Orson Cart]

Can your input (15-30V) supply the 1800 watts needed for 200V, 9ampere?

 

[m4xa]

sorry i mean the 200v 9 ampere is the for maks output voltage in the motor DC load...

when just connected 7 volt the motor is already spinning, so with the voltage 15-30 v it wil be more fast.
i wantt to make the load spinn with (15-30v) and when i raise the duty cycle the spinnig getting fast and it means the voltage can exceed to the input...

 

[Orson Cart]

How much power is available from your 15-30 volt source?

 

[m4xa]

maksimum power from my source is 150 v 10 A (1500 w)...

 

[Orson Cart]

OK so were you are trying to draw more than 10A from your 15-30V source (150W - 300W), your source will current limit and the o/p volts will fall and therefore the output from your booster will fall... Regards, Orson Cart

 

[leo_o2]

Boost VO/VIN is not strictly following 1/(1-D).
It is related to loading and parasite resistor.
It will lead to VO voltage drop when loading is heavy.

 

[Orson Cart]

Also, is your inductor rated for the peak currents you are drawing through it? Regards, Orson Cart.

 

[m4xa]

okay thank you all,,,, but i still don't understand..
So to increase the current and the voltage it is depend on the inductor rated?? i have to increase it??

 

[Orson Cart]

What is the peak current rating of the inductor? freq of operation?

 

[m4xa]

inductor current rate is 10 A and the frequency i use is about 1 KHz

 

[leo_o2]

Inductor current is not satuated. 10A rating is safe.

 

[Orson Cart]

1 kHz is a bit slow for a 100uH inductor, try 35kHz, Regards, Orson Cart.

 

[m4xa]

when i use 31 KHz the mosfet is always on
i'm using an optocoupler to separate the mikro and boost converter,
the minimum output from optocoupler is always havea voltage to activated the gated so the mosfet always ON,
but if i using a smaller frequency thethe output from the optocoupler have a small voltage..

 

[Orson Cart]

you need a better isolator/gate driver that can handle the 35kHz, at 1kHz there will be far too much current ripple in the inductor and the peak current in the FET will be very high. Regards, Orson Cart.

 

[m4xa]

in datasheet how do we know the best frequency for the mosfet that we used??
for example maybe in irfp460

 

[Orson Cart]

the frequency is related to the inductor and the input voltage (max) and the duty cycle you need to get the boost ratio...to go from 30 to 200volts requires a high duty cycle and this means a very high current ripple in a 100uH choke at 1kHz, to get 200V from 30v requires a step up ratio of 6.6667, and a duty cycle on a std booster of 0.85 (i.e. the on time is 85% and the off time is 15%) at 1kHz this is 850us, 30v across 100uH for 850uS gives a current rise of 255 amps, which likely cannot be supplied by your source and would saturate your inductor - and melt the mosfet.
At 35kHz the duty cycle is still 85%, but this is now 24.3uS, and the di in the choke is now 7.3 amps - much more acceptable (2.55 amps rms approx)

Regards, Orson Cart.