Help needed to solve SEPIC converter

[samson.bunty1986]

Hi All,

I have designed a circuit which can give 24V,2.5A output with the input varies from 21-28V. I tested that circuit in LT Spice. Everything is fine. So,I made PCB. But now,at no load my output is good and am getting 24V. Once i connect the Load(24V DC Bulb,40W) my output voltage drastically coming down to 13-14V. Glowing the bulb very low. Kindly anyone help me out.

Thanks in advance.

 

[ronsimpson]

You better send a schematic!
Also I need to know what inductor/transformer you are using.

Probably inductor saturation. Maybe current limit.

 

[Diver300]

Post the circuit that you are using.

 

[dpj]

Your power supply is not able to source the current.
Which topology you are using?
Whats current rating of the switching IC?
Can you post the schematic? Need more details of the circuit.
Regards.

 

[dpj]

Your power supply is not able to source the current.
Which topology you are using?
Whats current rating of the switching IC?
Can you post the schematic? Need more details of the circuit.
Regards.

 

[samson.bunty1986]

Hi evryone,

My power supply source rating is 24V,2.2A. But i have connected the load as 24V DC bulb which can take current 1.6A(40W Bulb). So that time my voltage drastically drop to 8V. Later i tried with resistive load. I have connected 22Ohm,10W & 22Ohm,5W resistors in series at load. It is drawing 270-300mA and my output voltage dropped to 13-14V. Kindly help me out from her.

Here is my circuit.View attachment 63421

 

[samson.bunty1986]

It is SEPIC Converter

 

[dpj]

It seems your battery capacity is not enough to generate 2.A of current.
Refer equation no 2 from datasheet.
A bigger Ah battery should solve the problem.

 

[samson.bunty1986]

I havn't used any battery now. I used powersupply as source. Which equation you are saying. I dint get any eqautin no 2. Kindly specify the equation.

 

[ronsimpson]

Back in post #2 I asked about L1, L2. I need all the information you can find. In spice inductors, typically, can not saturate. In real life they do saturate.

Are you using two inductors or one transformer?

Do you have the correct resistor for R5?

Start with a smaller load! 0.5 amp and test. Then 1.0 then 1.5 and see where it stop regulating.

Is Q1 hot?

C1 is small. That type of capacitor? What type of capacitor for C3,4

 

[WTP Pepper]

How are L1 & L2 wound? Bifilar winding them on the same core will increase effiency. Take care of their sense (i.e. winding direction).

What guage wire are both L1 & L2 wound with? Thicker wire will work better.

Are C2 & C8 low ESR capacitors? Anything else just creates heat and can explode.
Same goes for C3 & C4.

Why not tie /SHDN straight to ground rather than through resistors that may cause problems at start up. No idea what IC you are using.

Be careful running off a bench PSU. They can "crowbar" limit and effect how your circuit starts up.

Other than that what you have is a SEPIC convertor on paper but I believe your problems are implementation problems and not basic design problems.

 

[ronsimpson]

Why not tie /SHDN straight to ground rather than through resistors that may cause problems at start up. No idea what IC you are using.

I think the SHDN pin, used in this mode, will cause the IC to not work until the supply voltage is at some level. (some thing like brown out detect)
This is a good idea because:
24V 2.5A in 24V 2.5A out,
12V 5A in 24V 2.5A out, This bench supply will current limit!

 

[JimB]

Beware of using a lamp as a load.
When the lamp is cold, the resistance is much lower.

The converter could be struggling to provide enough current to heat the lamp filament sufficiently to increase its resistance.

JimB

 

[ronsimpson]

We have a hand full of answers:
The bench supply has current limited.
The input voltage is low causing the input current to go up.
The load is pulling MORE than 2.5 amps because it is cold. (4 amps)
The core is saturated.

 

[samson.bunty1986]

L1 & L2 are through hole inductors from Coil master. I used two inductors. I used R5 exact 0.01Ohm , 1W resistor.

I checked with resistive load. Two resistors 22 Ohm 10W and 5W in series. It is taking 270 - 300mA. That time my output getting drop to 13-14V. If i connect bulb it is dropping to 8V. The mosfet is getting too hot once i keep load. C3 & C4 are alluminium capacitors.

 

[samson.bunty1986]

L1 & L2 are separately connected. The C2,C8,C3,C4 all have Low ESR and Calculated RMS Current.

My power supply max ratings is 23-26V,2.2A. I have connected resistive load.(By disconnecting the bulb Load). With 22 Ohm,10W & 22 Ohm,5W resistors in series. That time my output droping to 13-14V with current 270-300mA.

If i connect bulb my output dropping to 8V.

Am using LT3758 IC.

 

[ronsimpson]

L1 & L2 are through hole inductors from Coil master. I used two inductors. I used R5 exact 0.01Ohm , 1W resistor.

I checked with resistive load. Two resistors 22 Ohm 10W and 5W in series. It is taking 270 - 300mA. That time my output getting drop to 13-14V. If i connect bulb it is dropping to 8V. The mosfet is getting too hot once i keep load. C3 & C4 are alluminium capacitors.

Why am I talking to you? In post #2, #10 and in #14 I want to know about the inductors. And by post #15 all you will say is "Coil master". Sorry to use strong words. It is 3AM here and you are not helping.

You need to understand that all the power goes through the inductors. The inductor is like a bucket that gets filled with water and then emptied every cycle. If the bucket is not big enough you can not move the water you need. There are numbers and data sheets on the inductor. If you do not know the saturation current and the DC current of the inductor then you are wasting my time. To help you I need to know about the inductor (bucket=how many gallons or liters and you answer 'it is red'). You fill your little tea cup will 10 liters of water walk across the room and ask why do I only have a cup of water. Why are my feet wet?

See attached information. Sepic application note. AND Inductor data sheet.
Look at the data sheet for SER2918h-223L or SER2918h-333L. You have the IC set to current limit at 10 amp if you really have 0.01 ohms not 0.1 ohms. The inductors I chose will stop being inductors at 15 and 9.6 amps. If you had a current probe you will see the current ramp up to the saturation point then go strait up out of control. Look at data sheet page 3 bottom left graph. The inductance goes to hell after 10 amps. Your inductor probably saturates at 1 amp.

What is saturation? It is the point where your inductor becomes a wire.

You have other problems but they will not show up until you solve this problem.

 

[samson.bunty1986]

Dear Ronsimpson,

Am sorry for that.

Here is the Inductor Spec i used:

Inductor Value : 47uH
Inductor saturation current is 4.9A.
Irms : 2 A
DCR is 59mOhm.
Manufacturer : coilmaster
Part number RDR1215-470K-LF

I used 0.01ohm,1W resistor as sense resistor.

Kindly suggest me what to do now.

Thnaks in advance

 

[ronsimpson]

If I understand right; You are only using 300mA from your bench supply. Something is not right.

From attached data sheet. 47uH±10% 0.059 ohms 4.90A saturation 2.00A DC

At 4.9A the inductance will drop some amount that is not stated. Probably -20% or -50%. At this point the core loss is high and the core is hot.

If you passed DC current through the coil it will got too hot at 2A. This is just wire loss. No core loss.

In your application this inductor should not see more than 1A rms because of heating. AC+DC loss. It could be run to 4.9A AC peak. This is dangerous. Your IC is set to current limit at 10 amps.

Can you change the 0.01 ohm current sense resistor to 0.03 to 0.05 to see it the supply will start and work with the 22 ohm load?

What is your switching frequency?
What is the duty cycle when the load is 22 ohms and 13 volts? 30%?
Using spice what is the peak current in the inductors at full load?

 

[samson.bunty1986]

Dear ronsimpson,

Thnks for you reply.

i set the switching frequency as 200KHz.
Correct..at load 22ohms,13V my duty cycle is 30%.
inductor peak current comes nearly 8A in simulation.

I have calculated the sense resistor as in LT3758 datasheet...
If you dnt mine, can you explain me why i need to change the resistor??

Thnks in advance..