Hot DC-DC Convertor Chips

[itdontgo]

I'm not trying to sell 'em ;o)

I've got two boards with totally different types of DC-DC convertor chips (AP5004 300kHz fixed & a 34063).  They're designed for 12v but I've run both of them at 24v and the chip gets really hot.  I'll admit some slackness here; I haven't really looked into it.  But, at first glance I don't see why they would not work.  Could it be the diode max reverse voltage, inductor too low?  I cant quite see why though (its 40v).  They do suddenly get hot above 20v even though the output current is sub 100mA at the time.
I'm not an idiot (honest) so, yes the caps and chips etc... are rated to 35v.

[Mr Smiley]

 

I'm sure the answer to this question is beyond words.

If you have a car, go and connect it to a 24v lorry battery and see what happens.

Everything in the world is designed to work within it's limitations.

[tom66]

The chip is 10-32V rated.

What is the rating of input cap? Some input cap types - mainly ceramic - decrease capacitance with applied voltage. This could cause destabilisation of the controller.

Also, many converters are inefficient at low load, high input voltage... so it may be normal.

[itdontgo]

I'm sure the answer to this question is beyond words.

If you have a car, go and connect it to a 24v lorry battery and see what happens.

Everything in the world is designed to work within it's limitations.

I told you I'm not an idiot.  Do you know anything about DC-DC convertor or those two chips?

[itdontgo]

I'm sure the answer to this question is beyond words.

If you have a car, go and connect it to a 24v lorry battery and see what happens.

Everything in the world is designed to work within it's limitations.

Auto electronics are designed to be able to withstand 24v for minutes (although the designers will take this to be indefinitely).  So nothing will happen.  Did you have a point?  Would you like me to send you a datasheet from an auto components manufacturer?

From Littlefuse 'Suppression of Transients in an Automotive Environment'

"...the majority of repair vehicles use 24V “battery” jump to start the car.  Automotive specifcations call out an extreme condition of jump start overvoltage application of up to 5 minutes."

[lewis]

I've got two boards...

Are the boards designed for an automotive application?

[peter.mitchell]

Ladies, ladies, calm down.

The chip is rated for 10-32v input and they state they have used appropriately rated components in their implementation. They are curious as to why, even though they are within specification they heat output increases significantly when going from a 12v input to a 24v input.

Appropriate answers would be something like;

What kind of load are you driving with these devices? Is it a high current load? Is it a capacitive load?
Would you be able to provide a circuit diagram of your implementation?
Are you using the correctly spec'd inductors?
Are you using them in an automotive environment, or are you just using that as an example?

[Bored@Work]

Ladies, ladies, calm down.

The chip is rated for 10-32v input and they state they have used appropriately rated components in their implementation.

Yes, and what they haven't stated, and what is their fault, is that they didn't DESIGNED it for 24V. Just because the components are allegedly properly rated doesn't mean it is properly designed for that. And that is why little ladies poke them with their walking sticks. They have no clue what they are doing and need to wake up instead of playing the entitled gods of DC-DC converters.

[itdontgo]

>They have no clue what they are doing and need to wake up instead of playing the entitled gods of DC-DC converters.

What on earth are you going on about?

The lower voltage is the worst case scenario for a DC-DC convertor.  If you design for the lower voltage then you should have no problem at higher voltage.

I investigated the reason the 34063 was cooking at 24v.  I noticed this had a hand soldered inductor on it so I tried another board which had not been reworked; surprise surpsire that had no problems at 24v.  The current input was proportionally lower as you would expect at 24v and no heat.  So I replaced the inductor on the original one and it was fine.

So that was the issue.  It was just unlucky I picked this one to run at 24v.  The coil did look burned and it would not have run for long enough to burn itself.  I dont know the issue with the 5004 board as I'm no longer involved with that product. 

I told you I was not an idiot which meant I was not interested in the 'obviously it's not going to work at 24v if it was designed for 12v' response.  I know the design and it appeared it should work fine at this voltage which indeed it turns out that it does.

When I measured the current in on a few other boards I found a 10% variance in current between them which is concerning and I'm wondering whether these inductors are any good.

[ejeffrey]

>They have no clue what they are doing and need to wake up instead of playing the entitled gods of DC-DC converters.

What on earth are you going on about?

The lower voltage is the worst case scenario for a DC-DC convertor.  If you design for the lower voltage then you should have no problem at higher voltage.

Well, you didn't really provide any useful information in your original post except that you were operating the circuit beyond its design point, so we can only speculate.  My best guess based on your follow up that the temperature goes up sharply at a specific voltage is that you are saturating the inductor.  This would happen if the switch minimum on-time exceeds the V*s rating of the inductor.

I told you I was not an idiot which meant I was not interested in the 'obviously it's not going to work at 24v if it was designed for 12v' response.  I know the design and it appeared it should work fine at this voltage which indeed it turns out that it does.

OK, but you are at least an idiot about asking questions on the internet.  Telling people "trust me I am not an idiot" is not a good way to get people to trust you and believe you aren't an idiot.  You have to demonstrate it, and you honestly sound like you don't really know much about SMPSs, either.  You seem only interested in the max voltage ratings of your components.  That is obviously necessary to get right, but that is not the same thing as designing your circuit to operate at a given voltage.

By the way if I am right about the saturating inductor idea you should be careful.  Swapping the inductor might make a marginal circuit work due to the natural manufacturing variation, but it will still be marginal.  Time, temperature, and load could push it back into saturation. 

[baljemmett]

I told you I was not an idiot which meant I was not interested in the 'obviously it's not going to work at 24v if it was designed for 12v' response.  I know the design and it appeared it should work fine at this voltage which indeed it turns out that it does.

Just a suggestion, but you may want to look the Dunning–Kruger effect up sometime and then reflect upon why people might take declarations like "I'm not an idiot" with a pinch of salt...

[itdontgo]

baljemmett is a dickhead isn't he.

Anyway I did find the problem with this... the v boost capacitor was no longer 100nF at 24V so the RDSon was quite high which heated up the chip.

[Zero999]

You seem only interested in the max voltage ratings of your components.  That is obviously necessary to get right, but that is not the same thing as designing your circuit to operate at a given voltage.

By the way if I am right about the saturating inductor idea you should be careful.  Swapping the inductor might make a marginal circuit work due to the natural manufacturing variation, but it will still be marginal.  Time, temperature, and load could push it back into saturation.

I agree, you're perfectly right just because all the components are rated for the power supply voltage and the circuit can stand it for a short period of time, it doesn't mean it will work reliably. In this case it does sound like core saturation.

[djococaud]

Hi,
In my opinion, the DC/DC is simply running outside of his frequency/duty clycle safe area...

If you want to output 5V from 12V, the duty cycle will be 5/12=42%.
If you want to output 5V from 24V, the duty cycle will be 5/24=21%.

Given the inductor value, 21% could be outside the safe area for the switching mosfet (Ton too short).

To fix this problem, you should try to increase the inductor value (keeping the same amps ratings), or (better) try to increase the timing capacitor (internal osc cap).

[tom66]

baljemmett is a dickhead isn't he.

Anyway I did find the problem with this... the v boost capacitor was no longer 100nF at 24V so the RDSon was quite high which heated up the chip.

Hence, MLCC capacitance decrease with voltage as mentioned above, very important (only for large format X5R/X7R I believe, correct me if I'm wrong.)