Dear All,

I have been analysing SMPS circuits using the TL494 based on the TI application datasheet. While studying further, I came across current-mode PWM controllers and found the UC3846 controller.

Initially, I considered UC3846 because its supply-voltage capability appears suitable for my application. My input is a 24 V AC transformer output, and after bridge rectification and filtering, I expect approximately 34 V DC under no-load conditions.

I also considered the UC3843, but its maximum VCC rating is around 30 V, so it does not appear suitable for directly powering the controller from a 34 V DC bus. TI specifies UC3843 as a single-ended current-mode controller and lists buck among its supported topologies.

The UC3846, on the other hand, is a current-mode PWM controller with a 40-V-class supply capability, but it is primarily described as a dual-ended controller, with a maximum duty cycle of 50%.

I could not find many practical examples showing the UC3846 configured for a single-ended buck converter.

My question is:

1. Can the UC3846 be used for a single-ended buck converter?
2. If yes, how should the two PWM outputs of the UC3846 be configured for single-ended operation?
3. Is there any disadvantage compared with using a dedicated single-ended current-mode controller?
4. Are there any application notes or reference designs showing UC3846 + single-ended buck operation?
5. For a 34 V DC input, would it be better to use UC3846, or should I use another current-mode PWM controller with a higher VCC rating?

I am particularly interested in a through-hole/DIP solution, if available.

Any guidance, application notes, or practical circuit examples would be greatly appreciated.

Thank you.

 

I can make a schematic but: Look at UC3841,2,3,4,5,6 (US384x series) there are other versions that do not have a 50% limit. I have used the UC2842 many time for buck, but it was not intended for that use.
I know this is not what you want. Look up the LM2596.

Here is another version that includes a voltameter.

 

I can make a schematic but: Look at UC3841,2,3,4,5,6 (US384x series) there are other versions that do not have a 50% limit. I have used the UC2842 many time for buck, but it was not intended for that use.
I know this is not what you want. Look up the LM2596.
View attachment 371001
Here is another version that includes a voltameter.
View attachment 371002

Sir,

Transformer Secondary is 24-0-24V @10Ampere, These buck module have capacity of 3 Ampere.

 

First of all, show the diagram you want to use.
Secondly, if the transformer produces 24VAC, then this controller won't work, because there's no voltage margin left

 

if the transformer produces 24VAC, then this controller won't work, because there's no voltage mar

24 V AC transformer output, and after bridge rectification and filtering, I expect approximately 34 V DC under no-load conditions.

I have used this part years ago. It is through hole and easy to use. The datasheet shows PCB layout and example circuits.

 

Dear All,

I have been analysing SMPS circuits using the TL494 based on the TI application datasheet. While studying further, I came across current-mode PWM controllers and found the UC3846 controller.

Initially, I considered UC3846 because its supply-voltage capability appears suitable for my application. My input is a 24 V AC transformer output, and after bridge rectification and filtering, I expect approximately 34 V DC under no-load conditions.

I also considered the UC3843, but its maximum VCC rating is around 30 V, so it does not appear suitable for directly powering the controller from a 34 V DC bus. TI specifies UC3843 as a single-ended current-mode controller and lists buck among its supported topologies.

The UC3846, on the other hand, is a current-mode PWM controller with a 40-V-class supply capability, but it is primarily described as a dual-ended controller, with a maximum duty cycle of 50%.

I could not find many practical examples showing the UC3846 configured for a single-ended buck converter.

My question is:

1. Can the UC3846 be used for a single-ended buck converter?
2. If yes, how should the two PWM outputs of the UC3846 be configured for single-ended operation?
3. Is there any disadvantage compared with using a dedicated single-ended current-mode controller?
4. Are there any application notes or reference designs showing UC3846 + single-ended buck operation?
5. For a 34 V DC input, would it be better to use UC3846, or should I use another current-mode PWM controller with a higher VCC rating?

I am particularly interested in a through-hole/DIP solution, if available.

Any guidance, application notes, or practical circuit examples would be greatly appreciated.

Thank you.

Hello, I hope you are doing well.

I just have a question, I was just wondering
how you were able to convert a 24V AC transformer input to 34V DC.
(because my intuition is telling me Vrms of a 24V AC should be lower
unless you injected additional energy with an amplifier or used a boost converter)

 

hi Justin,
The 24V transformer is rated in RMS, so Vpeak would be 24V*1.414= ~34Vdc if capacitor smoothing is used.
From that 34Vdc, it would also be reduced by two forward voltage drops, say to 32.6Vdc

Of course the available current would be very limited.
E

Added: basic LTspice circuit

 

Under no load conditions the reservoir capacitor will charge to the peak voltage of the sine wave which is 1.414 x the RMS value.
Les

 

I have used this part years ago. It is through hole and easy to use. The datasheet shows PCB layout and example circuits.
View attachment 371013

Dear sir,

This part number is obsolete now.

 

This part number is obsolete now.

I know but you might be able to find some. There are 700 in Marocco. (LOL) I might have some hiding in a box. Amazon has 5 pieces at $15.00. (I do not know the source.) Most parts now days are surface mount.

 

Here is a through hole part. Digikey.com has 6 in stock. data sheet
This schematic is for 6A so you will need to get MOSFETs with 1/2 the resistance. Change the current monitoring resistor to 0.0045 ohm. Maybe change the inductor.
I did not do much studding the part. There might be other problems.

 

Again about a 40V rated controller working with a 24V transformer. The controller will work, I have no doubt about that, but how reliable will such a setup be?
A 24V transformer, when its secondary current is low, gives about 10-15% higher voltage, so it is about 27V. When that voltage is rectified and filtered, we’ll have about 38 VDC, so theoretically the controller should work.
But the point is that there can be voltage spikes in the power grid, and practically they, less or more often, but they always happen.

Once we made a rectifier with a high-capacity capacitor bank, it was about a 3000 uF 400V bank, powered by a 230VAC mains. In normal mode, the battery is not loaded at all, it only needs to give its charge and disconnect the circuit breaker when the power goes out.
The battery discharge resistor wasn't installed.
The device was powered from the mains,and its output voltage was found about 330VDC.
However, when we checked the output voltage the next day, we found not 330V anymore, but about 420V
The device's power was immediately turned off, and two discharge resistors in parallel (two -for reliability) were immediately installed.After that, the voltage at the output was checked again for several days in a row, but it didn't exceed 330V.
Where did such a big voltage come from - I already mentioned it above--

 

hi Justin,
The 24V transformer is rated in RMS, so Vpeak would be 24V*1.414= ~34Vdc if capacitor smoothing is used.
From that 34Vdc, it would also be reduced by two forward voltage drops, say to 32.6Vdc

Of course the available current would be very limited.
E

Added: basic LTspice circuit
View attachment 371017

Ah silly me, I got too accustomed to having the rms suffix at the unit (and the notation I follow with small signal analysis ingrained treating AC otherwise). 24Vrms does rectify to a higher peak AC voltage 34V, I remember this notation is standard with IEC 60076

Thanks,
Justin