LM3478 Boost converter stops working when I increase the load current

Thread Starter

josmA

Joined Jun 12, 2019
39
Hello,

I have used Webench to design a boost converter circuit using LM3478 that powers a brushless motor. Parameters below:
Input voltage from a battery: 3.5-4.2 V
Output voltage: 13 V
Output current: 3A
You can find the report attached with the schematic used.
It works at low speed, if I increase the speed or add some load it stops. After some seconds, it recovers and starts rotating again.

I could measure ringing, but I am not sure if that is enough to stop working.
1654776410146.png
Channel 1: Vin / Channel 2: Vout / Channel 3: mosfet gate

I hope you can lead me to where is the fault. Thank you
 

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BobTPH

Joined Jun 5, 2013
5,248
Is your battery (and wiring) capable of the 10.5A required to boost 3.7V to 13V at 3A? Also, is that 3A at full loading? Stall current could be as much as 5X higher.

Bob
 

Papabravo

Joined Feb 24, 2006
18,806
Yes, that is to be expected. The immutable rule of DC-DC conversion schemes is that the Power Out will always be less than the Power In. Sometimes it will be much less. When you boost the voltage, you reduce the available current in such a way that the output power cannot exceed the input power. The calculations by @BobTPH would apply if the conversion scheme was 100% efficient. You must also allow for the impossibility of that proposition. I suggest 80-85% as a reasonable estimate for the efficiency unless you have better information. Your duty cycle of 72% is also approaching a practical maximum making regulation increasingly difficult.

EDIT: After a career spanning half a century, running a motor from a boost converter is probably one of the top WORST ideas I have ever encountered. It has never ended well.
 
Last edited:

Thread Starter

josmA

Joined Jun 12, 2019
39
Is your battery (and wiring) capable of the 10.5A required to boost 3.7V to 13V at 3A? Also, is that 3A at full loading? Stall current could be as much as 5X higher.

Bob
Battery specs says >7A (https://docs.rs-online.com/d6d6/0900766b8163e9ca.pdf). 3A would be at full load. I know increasing the speed will increase the current, so I will adapt the maximum speed depending on the current demand. But it is stopping without any force back, so there is something wrong with the design I guess. I have powered it from a power supply as well, and it doesn't get the max 4A at all.
thanks
 

Thread Starter

josmA

Joined Jun 12, 2019
39
Yes, that is to be expected. The immutable rule of DC-DC conversion schemes is that the Power Out will always be less than the Power In. Sometimes it will be much less. When you boost the voltage, you reduce the available current in such a way that the output power cannot exceed the input power. The calculations by @BobTPH would apply if the conversion scheme was 100% efficient. You must also allow for the impossibility of that proposition. I suggest 80-85% as a reasonable estimate for the efficiency unless you have better information. Your duty cycle of 72% is also approaching a practical maximum making regulation increasingly difficult.

EDIT: After a career spanning half a century, running a motor from a boost converter is probably one of the top WORST ideas I have ever encountered. It has never ended well.
Do you think the high-duty cycle could be the issue? what would you modify, the circuit is already built.
Regarding using a boost for running a motor, what would you do to provide the right voltage?
Thanks!
 

Papabravo

Joined Feb 24, 2006
18,806
Do you think the high-duty cycle could be the issue? what would you modify, the circuit is already built.
Regarding using a boost for running a motor, what would you do to provide the right voltage?
Thanks!
Motors are all about current. Current is the one thing a boost converter is short on. I don't know what you application is, but a combination of deep cycle marine batteries would make more sense if you must run from batteries.
 

BobTPH

Joined Jun 5, 2013
5,248
Battery specs says >7A (https://docs.rs-online.com/d6d6/0900766b8163e9ca.pdf). 3A would be at full load. I know increasing the speed will increase the current, so I will adapt the maximum speed depending on the current demand. But it is stopping without any force back, so there is something wrong with the design I guess. I have powered it from a power supply as well, and it doesn't get the max 4A at all.
thanks
> 7A does not mean 10.5A! It means it can provide at least 7A, but anything above that is only if you are lucky. Your battery is undersized.

Bob
 

Thread Starter

josmA

Joined Jun 12, 2019
39
> 7A does not mean 10.5A! It means it can provide at least 7A, but anything above that is only if you are lucky. Your battery is undersized.

Bob
I agree with you, but I haven't used even 2A from the battery. I am trying to find out where is the problem first, and then I will deal with the battery. Thanks
 
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