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24V to 5V Buck converter

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Blue10

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Hi,
I want to design a simple buck converter that convert a 24V input to a 5V output.
I made a schematic but it doesn't work, it gives a higher output voltage (even when I reduce the duty cycle to 1%) than desired which is confusing me.
Could anyone please tell me where the problem is?

I tried various MOSFET, reduced and increased the duty cycle but nothing helps.

Vin = 24V
L1 = 1µH
C1 = 220µH
Duty cycle = 25%

buck_converter.png

Thank you.
 

You probably didn't consider that at low output currents a buck converter operates in DCM (discontinuous mode) and you don't have a fixed relation between duty cycle and buck voltage ratio. Operating a buck converter without voltage regulation and automatic control of duty cycle is basically a bad idea. For a principle demonstration, you should reduce the load resistance respectively to shift to continuous mode.

You simulation circuit doesn't show the actual gate driver voltage. It should be applied between gate and source, not between gate and ground.
 
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    Blue10

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10 V gate voltage referred to ground is too low, it doesn't turn the MOSFET completely on. Required is 30 to 35V referred to ground, but there's a risk to exceed the maximum Vgs rating.

You need a floating voltage source, no problem in simulation. Less easy to achieve in real hardware.
 

FvM said:
10 V gate voltage referred to ground is too low, it doesn't turn the MOSFET completely on. Required is 30 to 35V referred to ground, but there's a risk to exceed the maximum Vgs rating.
Click to expand...
Thank you for your fast response.
The circuit still doesn't work even after applying higher voltage.
 

I didn't say that inappropriate gate voltage would be the reason for unexpected high output voltage. It's just an additional point needing to be fixed for correct buck converter operation.

If you get all available informations from your simulation circuit (currents and voltages), you can "see" why It's not working. In case of doubt compare with text book waveforms.
 

What could be the reason for the high output voltage?
 

Hi,

Did you read post#2?
About "duty cycle" .... and "regulation"

Klaus
 

Hi,

So why your question in post#7 ... when it is already answered?

Klaus
 

Another info yet missing in your posts is switching frequency
 

Hi

Because that answer doesn't solve my probleme.
Click to expand...
So, did you modify the duty cycle, or did you add a regulation loop...
If YES: You did not give any information about this. Please show the modifed circuit.
If NO: how can you expect a modifed output voltage?

Klaus
 

KlausST said:
Hi


So, did you modify the duty cycle, or did you add a regulation loop...
If YES: You did not give any information about this. Please show the modifed circuit.
If NO: how can you expect a modifed output voltage?

Klaus
Click to expand...

Here is the modified circuit:
buck_feedback.png

The same problem persist.

The multivibrator is 2.2kHz and the duty cycle is 30%.
 
Last edited:

Hi,

At 10k and 5V, load current is 5V/10k=500uA.
Is that your design spec?

Let's start from there.

Also tell us whether you have a controller already. If you do, then we'll have to work with that. If you don't, then we can recommend one. None the less, we have to know your requirements first of all.
 
Last edited:

Blue10, what everyone is telling you is the output capacitor has nowhere significant to discharge to. After one or two pulses, even very short ones, it will charge to almost the input voltage. Think of the MOSFET as a switch, it doesn't have to be closed very long before the output voltage is the same as the input voltage.

These circuits need two things to operate properly:
1. sufficient load current that the MOSFET has something to 'push' against.
2. a feedback circuit that controls the switching to keep the output voltage steady.

What I think you are trying to achieve is a regulated output by controlling the on/off timing (pulse width) of the MOSFET drive signal. What you actually have is a circuit that switches once, goes over-voltage and then shuts down the oscillator.

I'm not sure how you are simulating but note that the diodes you are specifying are unsuitable for all but the very slowest switching speeds, use a small signal diode in the oscillator (1N914 for example) and a Schottky power diode after the MOSFET.

Brian.
 

Blue10 said:
No, I want at least 20mA to turn a LED on.
Click to expand...

So what's the forward voltage of the LED?

Also note that 1n400x series of diodes are general purpose diode and can't switch fast enough for use as an SMPS switch. Change it to a 1N5819
 
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