uc3843 buck-boost

Thread Starter

mrh586

Joined Dec 5, 2021
66
Hello friends
In the circuit below, what should I do if I want to increase the input voltage to 30V?
I know that I have to put a regulator for the IC and connect the output of the regulator to pin 7 of the IC
But the MOSFET gate voltage is 20 and it is probably damaged.
please guide me
Thankful
 

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Ian0

Joined Aug 7, 2020
5,819
Increase the value of R6, and put a zener (12V to 15V) from pin 7 to ground. Put 100nF||47uF from pin 7 to ground.
(and, obviously, use appropriate voltage ratings oh the other capacitors and MOSFET)
 

Thread Starter

mrh586

Joined Dec 5, 2021
66
Increase the value of R6, and put a zener (12V to 15V) from pin 7 to ground. Put 100nF||47uF from pin 7 to ground.
(and, obviously, use appropriate voltage ratings oh the other capacitors and MOSFET)
Thanks for your guidance
My problem was mostly MOSFET
If I use a 15V zener on pin 7 IC, will the MOSFET gate be protected?
 

Thread Starter

mrh586

Joined Dec 5, 2021
66
Yes. The maximum output voltage is just a bit less than the voltage on pin 7.
Hello
I have done it
But the zener diode became very hot
The input had not yet reached 24 volts
What is the right value for voltage 24 for resistor R6?
I tried 22 ohms
 

Ian0

Joined Aug 7, 2020
5,819
Don’t just guess. work it out:

R=(supply voltage- zener voltage)/(IC supply current+Output current+zener current)

look up the IC supply current in the datasheet. Allow 3 to 5 mA for the zener current.
output current = MOSFET gate charge x frequency
 

Thread Starter

mrh586

Joined Dec 5, 2021
66
Don’t just guess. work it out:

R=(supply voltage- zener voltage)/(IC supply current+Output current+zener current)

look up the IC supply current in the datasheet. Allow 3 to 5 mA for the zener current.
output current = MOSFET gate charge x frequency
Does IC supply current mean the same part shown in the picture?
 

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Thread Starter

mrh586

Joined Dec 5, 2021
66
thank you so much
Actually, I have assembled this circuit on the breadboard
Now with a voltage of 24 volts (3 amps) without load, the MOSFET gets a little hot
And another problem is that I can't get a current above one amp from the circuit and the voltage is close to zero and the MOSFET gets very hot.
Of course, this problem existed before, could it be due to weak connections (breadboard)?
The core I have chosen for the coil has an outer diameter of 40 mm, an inner diameter of 23 mm, and a height of 14.5 mm
I have used a wire with a diameter of 2 mm and the same 10 turns
 

Ian0

Joined Aug 7, 2020
5,819
thank you so much
Actually, I have assembled this circuit on the breadboard
Now with a voltage of 24 volts (3 amps) without load, the MOSFET gets a little hot
And another problem is that I can't get a current above one amp from the circuit and the voltage is close to zero and the MOSFET gets very hot.
Of course, this problem existed before, could it be due to weak connections (breadboard)?
The core I have chosen for the coil has an outer diameter of 40 mm, an inner diameter of 23 mm, and a height of 14.5 mm
I have used a wire with a diameter of 2 mm and the same 10 turns
What is the core made of? Iron, Iron powder, ferrite or what?
 

Thread Starter

mrh586

Joined Dec 5, 2021
66
What? The input is 24V @ 3A and the output is not loaded? Where is the 72 watts going?
When I try with a normal DC motor, it works with a slight voltage drop compared to no load
But when I try a 775 model motor, the motor turns on for a moment and then turns off, and the voltage becomes zero and the MOSFET heats up, although with an input voltage of 12 volts.
When the input voltage is close to 20 volts, the 775 starts working but with a voltage of 1 volt!!
And the MOSFET gets very hot
I checked everything
 

Ian0

Joined Aug 7, 2020
5,819
The core color is yellow and white
I think it is ferrite
It is exactly like the picture below
That's Micrometals #26, or a copy thereof. It's iron powder, not ferrite, and it is exactly what you need for a SEPIC converter. From your previous posts, if it is 40mm outside diameter than it is a T157-26
https://datasheets.micrometals.com/T157-26-DataSheet.pdf
It has a permeance of 100nH, so 10 turns will give you 10uH, which is WAY too small for 95kHz.
You haven't specified the output power, but 500uH is probably more like what you need.
Also, do you have the two windings connected in the correct phase?
 

ronsimpson

Joined Oct 7, 2019
2,246
My thought is that when you increased the voltage 3X you should have increased the inductance 3x to get to the same current in the same amount of time. (17 turns) That is assuming 10uH was right in the first place, at 12V.
 

Ian0

Joined Aug 7, 2020
5,819
My thought is that when you increased the voltage 3X you should have increased the inductance 3x to get to the same current in the same amount of time. (17 turns) That is assuming 10uH was right in the first place, at 12V.
The original said 10 turns, but didn't specify the core. The TS had a core on which 10 turns gave 10uH.
10 turns probably gives the right inductance on a ferrite core, under the false assumption that the double-winding choke was actually a transformer.
 

ronsimpson

Joined Oct 7, 2019
2,246
What is the input voltage? 30V
What is the output voltage? 30V????
What is the current?
Why are you doing this? Speed control?

It is hard to drive a motor like this. A 1A motor might take 10A to start up!
Here is a SPICE version of it working at 5A.
1659185716874.png
This version goes into current limit of 7A with a 3 ohm load.
 

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Ian0

Joined Aug 7, 2020
5,819
If your input voltage is 30V, then why would you want to step the voltage UP for the motor?
If you buy a motor with a rated voltage lower than the power supply, then you would only ever have to step down, and you could use a standard buck converter; or, as it appears to be a brushed DC motor, you could driver it directly with PWM and not need any inductors
 
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