BQ2057W: Unexpected battery float voltage in BQ2057WSN charging Circuit and battery not charging

Hello

This post is similar to your other post can the two be combined?

The power dissipation on Q1 will be high during charging and I expect a significant thermal rise.  Power dissipation will be in the 3W to 4W range during charging.  Have you checked voltage drop and current across Q1?  Will reducing charge current or input voltage bring the temp into an acceptable range?

How many devices are showing this problem?

OK I am combining and editing my question here. So, please don't reply on the post #824097. 

In some of the circuits, where I have found the battery float voltage unexpected (mostly 11.7V), the voltage drop across 0.2 ohm Rsense resistor is found as 0V and the circuit is drawing 1.2A (double than the maximum desirable charge current) and within the seconds the Charging  IC burn out. 

In a normal circuit whose battery float voltage is 8.4 (as expected), with the reducing of the input voltage or the charge current, the temperature is reducing to 55- 60 degree Celsius. But as per as our requirement, the input voltage should be minimum 12V and our desired minimum charge current is 600mA. In this case, the chaging is fine, but heat is generating. Discussion regarding the problem of heat is on process with your customer support team. If we want to guide us regarding the heat dissipation, then you are welcome. 

But, almost 30% of our total circuit, is giving unexpected battery float voltage. What could be the possible reason of it?

Hi, 

Earlier I am discussing about the heat dissipation in our BQ2057WSN charging circuit with your customer support team. They advice me to ask the question in this forum. So, I am asking the question below 

:

I have attached a file below regarding thermal calculation. It shows that I need a FET having a thermal resistance of 25 C/W and required board area for this heat dissipation is 9 sq. in. Am I correct? What does the 9 sq. in means? Is this the total area required for the BQ2057 charging circuit or it is the area of the thermal pad which I should have?

I have calculated all this from the datasheet and the pdf files which you have sent regarding the thermal calculation. If I made a mistake in calculation please make me correct?BQ2057 thermal calculation.docx

Hello

Regrading thermal area and "BQ2057 thermal calculation.docx" , this would be the total copper area need.  Some of the factors to consider are, thermal vias, copper thickness, are the vias filled or not.

This will help to keep the junction temp of the pass transistor in a safe range.  Temp of the board will increase and some air flow may be needed. 

Some things to consider:

1.) Verify that the power dissipation across Q1 is only DC as expected.  If the system has some instability oscillation will be present on current and power dissipation could be higher.

2.) Reducing the charge current will reduce power dissipation on Q1 and temp.

3.) Check power dissipation on Q1 to verify that it matches calculations.

4.) Check temp of Q1, is it acceptable for the system. 

5.) It is common for heat sinks to be used to reduce temp.  Also air flow may be needed for cooling.

Regarding the battery float voltage 8.4V vs 11.7V.

The device should go into voltage regulation mode at light load.  Output voltage will be set by voltage at BAT pin.  Check the voltage at CC and SNS to verify Q1 is turning off at light load.

Regarding the battery float voltage 8.4V vs 11.7V.

For the circuit where the float voltage coming 8.4V, the voltage between  CC and SNS pins at light load ( 75mA current is drawing by the battery) are found to be 0.861 which means Q1 is OFF. In the same circuit, the voltage between  CC and SNS pins at Full  load ( 600mA current is drawing by the battery) are found to be 5.2V which means Q1 is ON.

For the circuit where the float voltage coming 11.7 V, the voltage between  CC and SNS pins at light load ( 75mA current is drawing by the battery) are found to be 0.258 which means Q1 is OFF. I am not able to measure the CC and SNS pin voltage in this case when battery is connected as within a second the IC burn out by taking 1.2A load current (double the load current which I set).

What could be the fault?

Dear Sir,

As I have found that in 30-40% circuit, we are getting the battery float voltage is 10.5 to 11.7 V and the battery is not charging. Can I know, what are the possible conditions that the IC fails to charge a 7.4V Li-ion battery.

In reply to Bill Johns:

Hello

Have you tried exchanging parts between good and bad boards to isolate the problem to, PCB, BQ2057C or Q1 (MJD32) ?

Also are you testing with a battery or battery simulator?  A simulator may give better control over the operating conditions.  See BQ24040 EVM Users Guide, it describes a simple design for a battery simulator.

The descriptions sounds like something is going wrong with Q1, have you tried other devices?

Hi,

I have exchanged the BQ2057 IC in many boards and after changing the charging IC, the circuit is working fine. I have also replaced the charging IC in many boards as follows: Suppose I have 2 circuits:

Circuit no 1 and Circuit no 2

Circuit no 1: The float voltage is 11.7V and Battery is not charging.

Circuit no 2: The float voltage is 8.97V and Battery is charging fine.

NOW, I have replaced the charging IC each other. Now, I have found:

Circuit no 1: The float voltage is 8.97 V and Battery is charging.

Circuit no 2: The float voltage is 11.7V and Battery is not charging.

I have also tested the Changing IC which is giving 11.7V float voltage with not only Q1(MJD32), but also with TIP32AG and FZT788B. What ever the combination of the FET, the charging IC which is giving 11.7 V float voltage with MJD32 is also giving same float voltage with other FET.

I don't think that the issue is with the FET or with the battery or with the PCB. But, only a concern is heating (about 80 degree) , as I told earlier. Is it possible that due to heating the ICs are showing 11.7V float voltage. I also found that in every circuit the float voltage is 8.7-9V (but should be 8.4 hight), but with this circuits the battery is charging. Should it behave like this( 8.7-9V float voltage)?

Hi

The IC should not be heating, the power dissipation should be in Q1 and it will be high.   High thermal rise of the BQ2057 point to a damaged device

In you above test ( Circuit 1 and Circuit 2) the problem followed the IC.  If the IC is heating and has a high Float Voltage this would indicate the device was damaged. 

Float voltage should be regulating at about 8.4V.  But with no battery present this voltage will have a high ripple content and voltage meter could measure 8.7V to 9V.  Recommend checking with a scope.

As I noticed, the heat is basically generated from Q1 itself during charging. As the heat is unable to dissipate to the ambient, it is heating the IC and the other components of the circuit. So the main concern is that how to dissipate the heat generated in Q1 to ambient.

I have also checked the float voltage in scope. The image is attached. Can you please suggest me that how to fix the float voltage between 8.2-8.4V as it is coming 8.7-9.2V. ( However, In the circuit where the float voltage is coming 8.7-9.2V, the battery is charging fine. But how can we fix this float voltage within 8.4V). Is there any drawback in our design? Can you please share us your commanded design file? 

I have tested with a large capacitor (10uF, Electrolytic) in parallel to the battery Port (across BAT_V and GND) and got the the float voltage as expected (8.38V). Thanks for the valuable suggestion. I think a 10uF Electrolytic capacitor will be suitable in this case. More over, Shall I also change the capacitor across VCC (3rd pin of BQ2057) pin and GND? If I need to change, what would be better , a ceramic or electrolytic? Please suggest us a suitable value of capacitor? ( The present schematic is given below).

Due to some requirements and our product specifications,  we cannot lower the charge current or the Input voltage. So, as per as your second advice, we can add a heat sink. A through hole Q1 will be better for us as we can fix a heat sink to it easily. we have TIP32C, through hole transistor. It is still heating, but somewhat less? Can you please suggest us a better Through hole Q1 better than TIP32C? 

Also, we are still unknown about the fact that why in some of the circuit the float voltage is coming as 11.7V ( In this case battery is not charging). What could be possible reason of coming float voltage as 11.7 in some of the circuits?

Thanks for the update, sorry for late reply.

"Shall I also change the capacitor across VCC (3rd pin of BQ2057) pin and GND? If I need to change, what would be better , a ceramic or electrolytic? Please suggest us a suitable value of capacitor? " ---- Bill J -- If the power 12V power supply has some distance from VCC then a bulk filter capacitor would help, 10uF electrolytic would be OK.   If the 12V supply is near by then a 1uF ceramic for high freq noise would be best.

"It is still heating, but somewhat less? Can you please suggest us a better Through hole Q1 better than TIP32C? " ---  Bill J -- The TIP32C is a good device, recommend staying with it.  It will also support a larger heat sink.   Check if the thermal rise is following power dissipation ( V-drop * I ).  Also check voltage and current to verify no oscillation  and all power dissipation is DC.

"Also, we are still unknown about the fact that why in some of the circuit the float voltage is coming as 11.7V ( In this case battery is not charging). What could be possible reason of coming float voltage as 11.7 in some of the circuits?"  --- Bill J - This sounds like a damaged device, the problem will follow the device.  Installing a new part will fix the problem.  Check for handling of the device during assembly or during test of unit.

Thanks for your valuable suggestions.

1. We will definitely place a suitable capacitor near to the VCC pin.

2. After testing with TIP32C having a heat sink, I will let you know the matter about heating.

3." Check for handling of the device during assembly or during test of unit." ------- We often touches the IC with bare hand. Is there any possibility that the IC may damage in this condition due to ESD? If so, is there any solution to protect the IC from ESD? How can we prevent the damage of the IC as we are not able to find the reason of damaging the IC. Please guide us regarding the damaging of the IC.

Handling of the device during assembly is a possible source of ESD damage.  Some info on ESD handling below and additional info can be found online. 

http://www.ti.com/lit/an/ssya010a/ssya010a.pdf